Jean Bernard

Keywords: biostratigraphie, Fezouata, Maroc, Ordovicien, paléoécologie, paléoenvironnement, préservation exceptionnelle biostratigraphy, Fezouata, Morocco, Laggerstätte, Ordovician, palaeoecology, palaeoenvironment

Communautés animales du début de l'Ordovicien (env. 480 Ma): études qualitative et quantitative à partir des sites à préservation exceptionnelle des Fezouata, Maroc Résumé : Jusqu'à récemment, l'essentiel de l'information sur les premiers ecosystèmes marins complexes provenait de gisements dits à préservation exceptionnelle, d'âge Cambrien, répartis à travers le globe. Leur caractéristique est de préserver des organismes faiblement sclérotisés, reflétant ainsi fidèlement les faunes de l'époque. Cependant, la découverte de tels sites dans l'Ordovicien inférieur du Maroc permet non seulement d'évaluer pour la première fois avec précision la biodiversité marine de cette époque mais aussi, à qualité de préservation égale, de comparer la vie marine cambrienne et ordovicienne. L'établissement d'un cadre biostratigraphique et envrionnemental précis des niveaux fossilifères a montré que la préservation exceptionnelle était restreinte au sein de deux intervalles stratigraphiques resserrés datant du Trémadocien supérieur et du Floien moyen. Les données sédimentologiques ont mis en évidence un environnement de plateforme peu profond. Ces résultats étaient un prérequis à l'ouverture de fouilles systématiques permettant des analyses paléoécologiques qualitatives et quantitatives à haute résolution. Les résultats préliminaires indiquent l'existence probable de conditions environnementales inhospitalières, du moins temporairement, associées aux niveaux à préservation exceptionnelle. Elles ont également distingué des assemblages in situ (biocénoses) et des associations d'éléments transportés (taphocénoses). Parallèlement, de nouvelles données importantes sur la morphologie, l'écologie, et le mode de vie présumé chez certains groupes ont pu être acquises.

Résumé substantiel en français :

Ce manuscrit de thèse est une synthèse détaillée des études que j'ai menées sur les Schistes des Fezouata dans l'Ordovicien inférieur de l'Anti-Atlas central (Maroc). Ces sites à préservation exceptionnelle ont acquis une renommée internationale suite à la découverte de fossiles abondants et diversifiés d'organismes peu minéralisés qui apportent des informations cruciales sur les dynamiques d'évolution des premiers métazoaires. Ce travail s'articulera autour de trois chapitres : le premier s'attardera sur l'apparition et la diversification des premiers animaux, le deuxième traitera des contextes biostratigraphiques et environnementaux propres aux Schistes des Fezouata, enfin, le troisième chapitre abordera les aspects paléoécologiques, paléontologiques, et taphonomiques de ce site à préservation exceptionnelle.

Dans le premier chapitre, les étapes pionnières de la vie seront détaillées, de son apparition sur Terre jusqu'à l'émergence et la diversification des premiers animaux durant l'Ediacarien et le . En effet, cette période s'avère cruciale dans l'histoire de la biosphère, caractérisée par l'apparence quasi-instantanée d'anatomies et de fonctionnalités complexes sans précédent dans le registre fossile et bouleversant à jamais les écosystèmes marins de l'époque. Les étapes initiales de l'apparition des premiers métazoaires durant le Paléozoïque inférieur, également surnommées l'Explosion Cambrienne, sont particulièrement bien connues, notamment grâce à la présence de nombreux sites à préservation exceptionnelle qui témoignent des écosystèmes de cette époque. Parmi les plus connus, nous pouvons citer les Schistes de Burgess (Canada; Briggs et al., 1994), Chengjiang et Kaili (Chine; Hou et al., 2004;[START_REF] Lin | Bioturbation in Burgess Shale-type Lagerstätten-Case study of trace fossil-body fossil association from the Kaili Biota (Cambrian Series 3), Guizhou, China[END_REF], les Schistes d'Emu Bay (Australie; Paterson et al., 2008), et Sirius Passet (Groenland;[START_REF] Morris | A Burgess Shalelike fauna from the Lower Cambrian of North Greenland[END_REF]. En effet, ces localités ont la particularité de livrer non seulement des fossiles d'organismes minéralisés typiques des faunes cambriennes (à savoir éocrinoïdes, hyolithes, brachiopodes inarticulés et trilobites) mais également des fossiles d'organismes peu ou pas minéralisés, habituellement absents du registre fossile (principalement des arthropodes, des vers et des chordés primitifs).

La grande biodiversification ordovicienne, quant à elle, représente une seconde étape majeure caractérisée par une hausse soutenue et quasi-exponentielle de la biodiversité marine durant une majeure partie de l'Ordovicien. Les faunes cambriennes cèdent alors la place à des faunes dîtes paléozoïques, principalement constituées de brachiopodes articulés, crinoïdes, graptolites, et nautiloïdes. Ces dernières vont dominer les écosystèmes marins jusqu'à la catastrophique extinction marine qui marquera la fin du Permien (Harper, 2006;Servais et al., 2008Servais et al., , 2010)).

Nous verrons ensuite que contrairement à l'Explosion Cambrienne, cette grande biodiversification ordovicienne est presqu'essentiellement connue au travers des restes fossiles d'organismes minéralisés. En effet, seuls quelques sites à préservation exceptionnelle délivrant des organismes peu minéralisés datant de l'Ordovicien ont été découverts. Qui plus est, ils n'ont malheureusement délivré que des faunes peu diversifiées, vivant dans des environnements très restreints ou particuliers. On peut citer pour exemple le niveau à trilobites de Beecher et les sites de Winneshiek (Etats-Unis d'Amérique; Liu et al., 2006;Farrell et al., 2009), de Llanfawr (Royaume Uni; Botting et al., 2011), du Manitoba (Canada; Young et al., 2007) et enfin, les Schistes de Soom (Afrique du Sud; Aldridge et al., 2007).

Les Schistes des Fezouata apparaissent alors comme une exception notable. Leurs cadres stratigraphiques et environnementaux seront détaillés dans le second chapitre de ce manuscrit. Géologiquement parlant, ils sont partie intégrante du groupe des Feijas Externes affleurant notamment dans la partie centrale de l'Anti-Atlas au Sud-Est du Maroc (Van Roy et al., 2010). Au coeur de la plaine du Ternata, située au Nord de la ville de Zagora, la succession géologique de l'Ordovicien inférieure repose de manière discordante sur les grès du Cambrien moyen appartenant au groupe Tabanite. Elle forme une épaisse série homogène, essentiellement composée de siltes et d'argilites (850 mètres pour les Formations inférieure et supérieure des Fezouata) et chapeautée par les grès de la Formation du Zini (Destombes 1962(Destombes , 1971 ; ;Destombes et al., 1985). Depuis le début des années 2000, de nombreux niveaux fossilifères ont été découverts au sein des Schistes des Fezouata. Ces derniers ont livré une faune extrêmement abondante et diversifiée, documentant de manière complète les communautés marine de l'époque. Les communautés des Fezouata comprennent de nombreux individus minéralisés et complètement articulés typiques des faunes paléozoïques (notamment des astérozoaires, brachiopodes articulés, des crinoïdes, des édrioastéroïdes, des gastéropodes, des graptolites, des hyolites, des machaeridiens, des nautiloïdes, des ostracodes, des rhombifères glyptocystides, des stylophores cornutes et mitrates, et des trilobites ; Destombes et al., 1985 ;Vidal 1998 ;Vinther et al., 2008 ;Noailles et al., 2010).

Mais elles comportent également d'abondants et divers restes d'organismes peu minéralisés, dont certains sont typiques de faunes paléozoïques jusqu'alors inconnues aussi bas dans l'Ordovicien (tels les aglaspidides, les arthropodes cheloniellides, les crustacés cirripèdes, et les xiphosures ; Van Roy, 2006 ;Van Roy et al., 2010). De manière plus surprenante, la faune inclut de nombreuses espèces différentes et peu minéralisées que l'on croyait cantonnées aux communautés du Cambrien inférieur et moyen, comme les anomalocaridés, les démosponges, les halkieriidés, les marrellomorphes, les naraoïdes, les paléoscolécides, ou encore les wapkiidés (Van Roy, 2006 ;Botting, 2007 ;Van Roy et al., 2010 ;Van Roy and Briggs, 2011). Dans un tel contexte, les Schistes des Fezouata offrent une opportunité unique de comparer les communautés ordoviciennes et cambriennes à degré de préservation égale.

La cohabitation d'espèces typiquement cambriennes avec des organismes au cachet paléozoïque plus récents suggère une transition relativement douce entre les deux types de faunes. Cependant, la plupart des études basées sur les Schistes des Fezouata se sont essentiellement reposées sur les fossiles à préservation exceptionnelle, sans s'intéresser à leurs positions stratigraphiques précises. Ainsi, deux campagnes de terrain ont été menées en 2012 et 2013 afin d'étudier en détail la stratigraphie des Schistes des Fezouata et surtout la position des différents sites à préservation exceptionnelle au sein de ceux-ci. La présence de passées plus sableuses, d'épaisseurs centimétriques à millimétriques et exhibant des ondulations imprimées par les vagues de tempête, indiquent également que les Schistes des Fezouata se sont déposés dans des conditions relativement peu profondes, autour de la limite d'action des vagues susnommées. Ces données nouvelles et importantes ont fait l'objet de deux publications principales (Martin et al., sous presse a ; Guttiérez-Marco and Martin, soumis).

Enfin, ce nouveau cadre biostratigraphique et paléoenvironnemental défini pour les Schistes des Fezouata permet pour la première fois d'étudier l'évolution des assemblages successifs à travers le temps et l'espace, ce qui fera l'objet du troisième chapitre de ce manuscrit. Pour répondre à ces questionnements, une campagne de fouilles a été initiée dans la localité de Bou Izargane, où affleurent 18 mètres de schistes associés à l'un des deux intervalles à préservation exceptionnelle définis précédemment. Cette première exploration a mis en évidence la présence de trois horizons fossilifères au sein de la section, chacun caractérisé par une faune abondante mais peu diverse. Ces faunes particulières suggèrent des épisodes environnementaux stressants, hypothèse renforcée par la présence de trilobites de tailles anormalement réduites ou la surabondance d'échinodermes stylophores. Ces présomptions ont été confirmées par l'analyse géochimique des différents horizons fossilifères, réalisée dans le cadre d'une étude détaillée des communautés de trilobites au sein des Schistes des Fezouata (Martin et al., soumis). En outre, des organismes peu minéralisés ont été trouvés, ce qui a permis, entre autre, de réaliser une étude des vers paléoscolécides (Martin et al., sous presse b). Reposant sur les nombreuses données récoltées durant cette campagne de fouilles, des études axées sur la taphonomie des Schistes des Fezouata ont également été lancées au sujet notamment des échinodermes stylophores (Martin et al., 2015), d'accumulations massives et monospécifiques, ou encore d'alignements de trilobites en file indienne.

Au cours de ma thèse, j'ai ainsi pu contribuer à l'acquisition d'un abondant jeu de données, aussi bien sur le terrain qu'en laboratoire, qui a permis de nombreuses avancées dans notre connaissance du site à préservation exceptionnelle des Schistes des Fezouata.

(1) un cadre biostratigraphique et environnemental précis a pu être établi (Martin et al., sous presse a ; Lefebvre et al., sous presse b ; Guttiérez-Marco et Martin, soumis ; Vaucher et al., soumis a, b). Avant celui-ci, ni la position et l'étendue stratigraphiques, ni le nombre de niveaux fossilifères à préservation exceptionnelle n'étaient précisément connus. Ainsi, il était suggéré que la préservation exceptionnelle avait lieu dans une grande partie des Schistes des Fezouata, de façon plus ou moins continue du Trémadocien inférieur jusqu'à Floien supérieur (voir Van Roy et al., 2010). L'un des résultats principaux de ce travail de thèse a donc été de replacer toutes les localités fossilifères à préservation exceptionnelle connues sur une même colonne stratigraphique synthétique, ainsi que de fournir un premier cadre biostratigraphique précis à travers l'étude des assemblages successifs de graptolites. L'une des conséquences majeures de ce travail a été de montrer que la préservation exceptionnelle n'était ni continue ni étendue dans les Schistes des Fezouata, mais au contraire restreinte au sein de deux intervalles stratigraphiques resserrés : le premier de 60 mètres d'épaisseur datant du Trémadocien supérieur (zones à A. murrayi et à H. copiosus) et le second de 15 mètres d'épaisseur datant du Floien moyen (Zone à ?B. Jacksoni) où les fossiles à préservation exceptionnelle sont plus rares.

(2) Mes travaux de thèse ouvrent également le chemin à des études plus complètes au niveau des communautés et groupes fossiles. Avant ceux-ci, les conditions environnementales associées aux Schistes des Fezouata restaient floues (voir Vidal, 1996), à tel point que des interprétations très différentes des environnements étaient proposées : d'une tranche d'eau relativement faible (voir [START_REF] Lefebvre | Peltocystis cornuta is also known from the late Tremadocian of Montagne Noire (Ubaghs, 1970; Vizcaïno and Lefebvre[END_REF] à des fonds marins profonds (voir Van Roy, 2010). Or, une meilleure compréhension des environnements s'avère être fondamentale pour interpréter les communautés fossiles et leurs compositions. Ainsi, mes analyses détaillées des communautés d'échinodermes et de trilobites retrouvées dans les Schistes des Fezouata ne sont qu'une prolongation de cette idée directrice (Allaire et al., 2015;Martin et al., 2015;Martin et al., soumis). Ces études répondent à de nombreux aspects concernant leurs taphonomies et qui sont essentiels à la distinction des assemblages in situ (biocénoses) ou d'associations d'éléments transportés et en place (taphocénoses). Dans ce contexte, différents degrés de mélange ont pu être identifiés pour la première avec des échinodermes stylophores, permettant ainsi la description du processus de désarticulation de leurs exosquelettes (Martin et al., 2015). Des analyses géochimiques préliminaires appliquées à de nombreux niveaux fossilifères ont également confirmé l'existence probable de conditions environnementales inhospitalières et dysoxiques, du moins temporairement, associées aux niveaux à préservation exceptionnelle (Martin et al., soumis). Enfin, de nouvelles informations importantes sur la morphologie, l'écologie, et le mode de vie présumé des vers paléoscolécides ont pu être obtenues (Martin et al., sous presse b). Elles sont la première étape d'autres études à mener sur des organismes vermiformes trouvés dans d'autres sites à préservation exceptionnelle.

Bien que réduite dans le temps (quatre semaines) et l'espace (seulement trois niveaux fossilifères au sein d'une section de 18 mètres d'épaisseur), la campagne de fouille a permis de fournir une abondance inégalée de données paléontologiques, sédimentologiques, et géochimiques (bien qu'une large partie des 0.5 tonne de matériel échantillonné reste à étudier). La répétition de telles campagnes, à la fois à Bou Izargane, mais aussi dans d'autres localités et/ou d'autres niveaux fossilifères serait certainement cruciale pour la compréhension globale du contexte général de la faune des Schistes de Fezouata, ne serait-ce qu'au niveau des communautés, de la taphonomie, ou des conditions environnementales. Les premiers résultats préliminaires remettent également en question l'interprétation des faunes comme étant diversifiées et vivant dans des milieux marins francs et 'normaux'. Différents indices semblent suggérer au contraire que la plupart des niveaux à préservation exceptionnelle sont en réalité associés à des conditions environnementales temporairement inhospitalières et dysoxiques (Botting et al., sous presse ; Lefebvre et al., sous presse b ; Martin et al., soumis). Ainsi, à Bou Izargane, tous les sites étudiés jusqu'à présent ont fourni des faunes abondantes mais peu diversifiées avec parfois des spécimens de tailles anormalement réduites (formes naines ?). En conséquence, une hypothèse alternative serait d'interpréter l'apparition massive et isolée d'espèces typiquement cambriennes comme des vagues de recolonisation du plancher marin par des individus opportunistes (stratégie K). Par opposition, la majorité des autres niveaux fossilifères, qui est dominée par des faunes minéralisées ordoviciennes plus classiques (au niveau des brachiopodes, céphalopodes et trilobites), représenterait des communautés plus stables (stratégie R) adaptées à des conditions environnementales bien oxygénées. Pour tester la validité d'un tel scénario (et possiblement déceler un biais taphonomique), il faudrait acquérir davantage de données de terrain, que ce soit au Maroc, mais aussi dans d'autres régions du monde, afin de savoir si les faunes des Schistes de Fezouata sont uniques ou non. Une autre voie de recherche à emprunter concerne la comparaison détaillée des Schistes des Fezouata avec d'autres sites à préservation exceptionnelle du Cambrien afin de mettre en lumière les liens existants entre l'Explosion Cambrienne et la grande biodiversification ordovicienne.

Une autre perspective extrêmement intéressante est offerte par des découvertes secondaires réalisées dans le cadre de la campagne de fouilles. Celles-ci permettent d'améliorer de façon sensible nos connaissances sur les détails anatomiques de nombreux groupes d'invertébrés marins (les appendices de trilobites ou la préservation d'organes chez certains échinodermes ; voir Lefebvre et al., 2013Lefebvre et al., , 2014)). Elles fournissent également de nouveaux renseignements sur l'éthologie de certains groupes permettant des comparaisons avec leurs descendants actuels (alignement en file indienne du trilobite Ampyx priscus), ou sur leur enfouissement (dalle à Platypeltoides magrebiensis). Qui plus est, de nombreuses (si ce n'est la majorité) espèces minéralisées ou non sont en attente d'être décrites, et il est plus que probable que ces descriptions fourniront de précieux détails sur leur écologie ou leur phylogénie. Enfin, un projet de carottage traversant les niveaux à préservation exceptionnelle est en cours de discussion. Cela offrirait une opportunité unique d'acquérir du matériel frais ainsi que de mieux connaitre taphonomie des sites. Cela pourrait également fournir de précieux microfossiles carbonés qui sont, pour l'instant, inconnus dans les Schistes des Fezouata à cause de leur trop forte altération (Nowak et al., sous presse).

Introduction

This PhD thesis is a detailed synthesis of the studies I led on the Lower Ordovician Fezouata Shale (Central Anti-Atlas, Morocco) from 2012 to 2016. The Fezouata Shale is a famous Konservat-Lagerstätte, which yielded abundant, diverse, soft-bodied, lightly sclerotized and shelly faunas and provided critical information on the evolution dynamics of the first animals. This manuscript will be divided in three main parts: the general context of the diversification of metazoans, the general framework and previous work on the Fezouata Shale and the major results obtained during my PhD concerning the age, palaeoenvironment, palaeoecology and taphonomy of this exceptional Lagerstätte.

The first part of this manuscript will detail the apparition of life up to the emergence and diversification of the first animals during the Ediacaran and Cambro-Ordovician (ca. . Indeed, this time period is pivotal in the history of the biosphere, with the relatively sudden appearance of unprecedentedly complex anatomies and functionalities, associated with dramatic and irreversible changes in marine ecosystems: e.g., trophic escalation, Cambrian agronomic revolution and the Ordovician planktonic revolution. The initial stages of the Early Palaeozoic diversification of metazoans (the so-called "Cambrian Explosion") are particularly well-documented based on several Konservat-Lagerstätten: e.g., the Burgess Shale (Canada; Briggs et al., 1994), Chengjiang and Kaili (China;Hou et al., 2004;Zhao et al., 2010), the Emu Bay Shale (Australia; Paterson et al., 2008), and Sirius Passet (Greenland; Peel and Ineson, 2011). These localities have yielded abundant remains of nonbiomineralized organisms, usually not preserved in the fossil record (e.g., a huge diversity of arthropods, various kinds of worms, primitive chordates), together with well-preserved elements typical of the Cambrian Evolutionary Fauna (e.g., eocrinoids, hyolithids, inarticulate brachiopods, trilobites).

The Great Ordovician Biodiversification Event (GOBE) represents the second major burst in marine biodiversity in Early Palaeozoic times. It is characterized by a sustained, almost exponential diversification rate during most of the Ordovician period and the replacement of the Cambrian Evolutionary Fauna by the Palaeozoic Evolutionary Fauna (e.g., articulate brachiopods, crinoids, graptolites, nautiloid cephalopods), that dominated marine ecosystems until the end-Permian mass extinction (Harper, 2006;Servais et al., 2008Servais et al., , 2010)). The possible extrinsic and intrinsic triggers of these events will be extensively discussed and include: ecological escalation, land colonization, snowball Earth, substrate revolution, or superplumes… We will see that, in contrast to the Cambrian Explosion, the GOBE is almost exclusively documented based on the shelly fossil record. Indeed, only a few Ordovician Konservat-Lagerstätten with soft-tissue preservation were described so far, and most of them have yielded low-diversity assemblages, associated with restricted marine environments: e.g., Beecher's Trilobite Bed and Winneshiek (USA; Liu et al., 2006;Farrell et al., 2009), Llanfawr (UK; Botting et al., 2011), Manitoba (Canada; Young et al., 2007) and the Soom Shale (South Africa; Aldridge et al., 2001). A noticeable exception is the Fezouata Biota. Its stratigraphical and environmental framework will be detailed in the second chapter. The Fezouata Shale belongs to the Outer Feijas Group of the central Anti-Atlas, southeastern Morocco (Van Roy et al., 2010). In the Ternata plain, north of Zagora, the Lower Ordovician succession lies unconformably over the middle Cambrian sandstones of the Tabanite Group, and consists of an extremely thick series of more or less silty and micaceous argillites (ca. 700 m; Lower and Upper Fezouata formations) overlain by the sandstones of the Zini Formation (Destombes 1962(Destombes , 1971;;Destombes et al., 1985). Since the early 2000s, several fossiliferous horizons within the Lower Fezouata Formation (Tremadocian) and the conformably overlying Upper Fezouata (Floian) have yielded extremely abundant and diverse remains of fully marine assemblages. The Fezouata assemblages comprise numerous exquisitely preserved remains of shelly taxa typical of post-Cambrian faunas: e.g., articulate brachiopods, asterozoans, bivalves, crinoids, edrioasteroids, gastropods, glyptocystitid rhombiferans, graptolites, hyolithids, machaeridians, mitrate stylophorans, nautiloid cephalopods, ostracods, and trilobites (Destombes et al., 1985;Vidal, 1998a, b;Vinther et al., 2008;Noailles et al., 2010). However, the Fezouata Biota became famous with the discovery of abundant and diverse remains of non-biomineralized organisms. Exceptionally preserved soft-bodied taxa comprise several representatives of the Palaeozoic Evolutionary Fauna (most of them previously known only from younger deposits): e.g., aglaspidid and cheloniellid arthropods, cirriped crustaceans, xiphosurans (Van Roy, 2006;Van Roy et al., 2010). More strikingly, the Fezouata Biota also contains extremely numerous and diverse nonbiomineralized taxa typical of early to middle Cambrian Burgess Shale type assemblages: e.g., anomalocaridids, demosponges, halkieriids, marrellomorphs, naraoids, palaeoscolecids, wapkiids (Van Roy, 2006;Botting, 2007;Van Roy et al., 2010;Van Roy and Briggs, 2011).

Consequently, the Fezouata Biota offers a unique opportunity to compare similarly preserved early-mid Cambrian assemblages with Early Ordovician ones. The Fezouata Biota thus fills a gap of knowledge on the biodiversity of marine communities after the Cambrian Explosion and before the GOBE, prodiving a chance to test the scenario established by Sepkoski (1981) of a rather abrupt replacement of Cambrian Evolutionary Fauna by the Palaeozoic one.

However, most recent studies focusing on the Fezouata Biota mainly concentrated on exceptionally preserved fossils, without precise knowledge of their exact stratigraphic positions. In 2012 and 2013, two field campaigns logged in detail the whole Lower Fezouata Formation, as well as the lower half of the Upper Fezouata Formation, and identified the precise stratigraphic positions of all main fossiliferous levels yielding soft-bodied preservation. The occurrence of mm-to cm-thick silty or sandy layers displaying storm-wave influence (wave ripples) points to relatively shallow offshore conditions for the deposition of the Fezouata argillites. All these data resulted in two main publications (Martin et al., in press a; Gutiérrez-Marco and Martin, submitted).

This new biostratigraphic and palaeoenvironmental framework for the Fezouata Shale provided essential information to study the evolution of successive assemblages through time, as well as their associated palaeoenvironmental conditions, which are detailed in the third part of the memoir. To address this question, a long-term systematic excavation was initiated at Bou Izargane, a locality exposing a 18-m-thick section within one of the two stratigraphic intervals with exceptional preservation recently identified in the Fezouata Shale.

This first campaign has shown that exceptional preservation occurs in three discrete horizons within the Bou Izargane section, each preserving an abundant but low-diversity assemblage. These characteristics, along the presence of abnormally small trilobites and the local abundance of dwarfed echinoderms are suggestive of episodes of environmental stress (Lefebvre et al., in press b). These presumptions are confirmed by preliminary geochemical analyses performed in parallel with the detailed study of trilobite communities of the Fezouata Shale (Martin et al., submitted). Additionally, the anatomy and palaeoecology of palaeoscolecid worms, a group of soft-bodied taxa occurring in the Fezouata Shale, were also investigated (Martin et al., in press b). Based on the diverse data obtained during this campaign, some taphonomic studies were also initiated and focused, by instance, on two distinct groups of echinoderms (eocrinoids and stylophorans; Allaire et al., 2015;Martin et al., 2015), and on trilobites (massive monospecific occurences and linear clusters shedding a new light of their possible behaviours).

Chapter I The Rise of Animal Life

Three questions have always haunted the subconscious of the human kind and were beautifully depicted in Paul Gauguin's painting entitled "Where Do We Come From? What Are We? Where Are We Going?" (1897-1898, Fig. 1). According to the times and society, man tried to answer these crucial questions through myths, religions, philosophy, arts, or sciences.

This thesis will humbly focus on the first of these questions "Where do we come from?" in a state of the art of our current knowledge on the rise of animal life. Nevertheless, reconstructing the History of Life on Earth is challenging, as direct evidence of the earliest forms of life is scarce and hard to interpret. The older they are, the greater chance is that their preservation and their geological history are confusing. By instance, it was widely accepted that the oldest evidence for life on this planet has been microfossils in the 3.5 Ga Apex Chert in Western Australia [START_REF] Schopf | Early Archean (3.3-billion to 3.5-billion-year-old) microfossils from Warrawoona Group, Australia[END_REF]. However, more recent investigations have shown that these traces had an abiotic origin and were pseudofossils [START_REF] Wacey | Solving the Controversy of Earth's Oldest Fossils Using Electron Microscopy[END_REF].

Moreover, many conflicts arise when results from different disciplines are confronted. For example, since the theory of molecular clock emerged in the early 60's [START_REF] Zuckerkandl | Molecular disease, evolution, and genetic heterogeneity in[END_REF], the morphology-based palaeontology and the DNA-based biology have seemed to be dedicated to deeply contradict each other. During decades, molecular clock suggested that the first metazoans appeared much earlier than they did in the fossil record, feeding fierce debates on the fidelity of the fossil record [START_REF] Cooper | Evolutionary explosions and the phylogenetic fuse[END_REF] and on the biases of the molecular clock data [START_REF] Pagel | Inferring the historical patterns of biological evolution[END_REF][START_REF] Bromham | The power of relative rates tests depends on the data[END_REF]Rodriguez-Trelles et al., 2002). Despite all the difficulties inherent to studies in deep times, we will first give a definition of metazoans (=animals), then browse in a chronological order the most important steps leading to their rise and radiation. A peculiar emphasis will be put on three main palaeontological events: the Ediacaran Biota, the Cambrian Explosion, and the Great Ordovician Biodiversification Event (GOBE).

What are animals?

Before entering in the detail of the emergence and diversification of the first animals, a clear definition of what is considered as an animal must be given.

The phylogeny of animals

Phylogenetically speaking, animals are part of the domain of the Eukaryota (Fig. 2A), which are distinct from the other two domains of life (the Archea and the Bacteria) in having cells with a nucleus enclosing their genetic material [START_REF] Gupta | The origin of the eukaryotic cell[END_REF]. They belong to the opisthokont clade within the domain of eukaryotes (Fig. 2B, [START_REF] Cavalier-Smith | The origin of Fungi and Pseudofungi[END_REF], i.e. a group characterized by a single posterior flagellum at least in one life-cycle stage [START_REF] Patterson | The Diversity of Eukaryotes[END_REF][START_REF] Cavalier-Smith | Phylogeny of choanozoa, apusozoa, and other protozoa and early eukaryote megaevolution[END_REF], a haloarchaeal-type tyrosyl tRNA [START_REF] Huang | The presence of a haloarchaeal type tyrosyl-tRNA synthetase marks the opisthokonts as monophyletic[END_REF] and an insertion of 12 amino acids in an otherwise highly conserved region of the elongation factor 1-α (EF-1α) protein [START_REF] Baldauf | Animals and fungi are each other's closest relatives: congruent evidence from multiple proteins[END_REF]. Though challenged by [START_REF] Philip | The Opisthokonta and the Ecdysozoa may not be clades: stronger support for the grouping of plant and animal than for animal and Fungi and stronger support for the Coelomata than Ecdysozoa[END_REF], its monophyly has been confirmed by several molecular phylogenetic studies [START_REF] Cavalier-Smith | The opalozoan Apusomonas is related to the common ancestor of animals, fungi, and choanoflagellates[END_REF][START_REF] Lang | The Closest Unicellular Relatives of Animals[END_REF][START_REF] Medina | Phylogeny of Opisthokonta and the evolution of multicellularity and complexity in Fungi and Metazoa[END_REF][START_REF] Ruiz-Trillo | Capsaspora owczarzaki is an independent opisthokont lineage[END_REF], 2006, 2008;[START_REF] Steenkamp | The protistan origins of animals and fungi[END_REF]; for a recent review, see [START_REF] Paps | Animals and their unicellular ancestors[END_REF]. Other important members of this opisthokont clade are the Fungi, the slime moulds, the choanoflagellates, and some other minor groups [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF].

Since the second half of the 19 th century [START_REF] Haeckel | Generelle Morphologie der Organismen: Allgemeine Grundzüge der organischen Formen-Wissenschaft, mechanisch begründet durch die von Charles Darwin reformierte Descendenz-Theorie[END_REF], animals have been united in the phylogenetic kingdom of Animalia (= Metazoa). They are defined as multicellular heterotrophs capable of complex and relatively rapid movement, acquiring food by ingestion and digesting it in an internal cavity. Their cells lack rigid cell walls, and all except sponges are made up of cells organized into specialized tissues, which are mostly further organized into specialized organs. Most are diploid and reproduce sexually by means of differentiated eggs and sperm. Animal development is characterized by distinctive stages including a zygote, blastula, and gastrula [START_REF] Baldauf | The tree of life -An overview, in: assembling the tree of life[END_REF]. All living animals belong to one of five clades (Fig. 2C), phylogenetic relationships of which remain unclear (for two different visions, see [START_REF] Philippe | Phylogenomics Revives Traditional Views on Deep Animal Relationships[END_REF][START_REF] Ryan | The genome of the ctenophore Mnemiopsis leidyi and Its implications for cell type evolution[END_REF]: Bilateria (most animals, including humans), Cnidaria (corals, medusae, and their relatives), Ctenophora (comb jellies), Placozoa (a small group of creeping marine animals), or Porifera (sponges).

The monophyly of the Animal kingdom (=Metazoa) has been challenged for a long time, especially concerning sponges [START_REF] Sperling | Poriferan paraphyly and its implications for Precambrian palaeobiology[END_REF], until further analyses indicated that they did not arise independently and are part of the Porifera (see [START_REF] Müller | Molecular phylogeny of Metazoa (animals): monophyletic origin[END_REF][START_REF] Philippe | Phylogenomics Revives Traditional Views on Deep Animal Relationships[END_REF][START_REF] Philippe | Resolving difficult phylogenetic questions: why more sequences are not enough[END_REF][START_REF] Budd | At the origin of animals: the revolutionary Cambrian fossil record[END_REF][START_REF] Nosenko | Deep metazoan phylogeny: when different genes tell different stories[END_REF]. Some studies still suggested that Porifera is a paraphyletic clade [START_REF] Sperling | Phylogenetic-signal dissection of nuclear housekeeping genes supports the paraphyly of sponges and the monophyly of Eumetazoa[END_REF], but these commonly considered a relatively small number of nuclear genes [START_REF] Dunn | Animal phylogeny and Its evolutionary implications[END_REF]. Morphological evidence for monophyly consists of many derived attributes that co-occur with the origin of multicellularity at the base of Metazoa, including the presence not only of multicellularity but also of the extracellular matrix [START_REF] Morris | The developmental role of the extracellular matrix suggests a monophyletic origin of the kingdom Animalia[END_REF] and septate junctions (Nielsen, 2001), as well as reproductive features such as eggs with polar bodies and spermatozoa [START_REF] Eernisse | Chapter 13: The history of animals[END_REF]. Furthermore, the molecular support extends beyond SSU rDNA (e.g., [START_REF] Wainright | Monophyletic origins of the metazoa: an evolutionary link with fungi[END_REF] to include combined SSU rDNA and large subunit (LSU, or 28S) rDNA [START_REF] Medina | Evaluating hypotheses of basal animal phylogeny using complete sequences of large and small subunit rRNA[END_REF], heatshock protein HSP70 [START_REF] Borchiellini | Phylogenetic analysis of the Hsp70 sequences reveals the monophyly of Metazoa and specific phylogenetic relationships between animals and fungi[END_REF][START_REF] Snell | Hsp70 sequences indicate that choanoflagellates are closely related to animals[END_REF], the largest subunit of RNA polymerase II [START_REF] Stiller | Are red algae plants? A critical evaluation of three key molecular data sets[END_REF][START_REF] Stiller | Evolution of the RNA polymerase II C-terminal domain[END_REF], and EF-2 and β-tubulin proteins [START_REF] King | A receptor tyrosine kinase from choanoflagellates: Molecular insights into early animal evolution[END_REF]. Main synapomomorphies of animals include: collagen, oogenesis, spermatogenesis, special sperm structure, and mitochondrial genome reduction [START_REF] Dunn | Animal phylogeny and Its evolutionary implications[END_REF]. Although animal monophyly is now firmly established, controversies still remain, especially concerning characteristics feature only shared among Metazoa (for discussion, see [START_REF] Eernisse | Chapter 13: The history of animals[END_REF]. the root of the animal tree with Ctenophora as sister group to other animals [START_REF] Ryan | The genome of the ctenophore Mnemiopsis leidyi and Its implications for cell type evolution[END_REF]. The organism silhouettes were illustrated by Noah Schlottman and submitted to PhyloPic (www.phylopic.org). They are available for reuse under the Creative Commons Attribution-ShareAlike 3.0 Unported license.

Reconstructing the characteristics of earliest animal

Though the genetic composition of extinct taxa cannot directly be assessed, the phylogenetic distribution of developmental genes in related living species can be used to make inferences about the genetic capacities of their last common ancestors (Carroll et al. 2001). In this regard, quite powerful inferences about the genetic capacities of animals that lived at the Precambrian/Cambrian transition can be made [START_REF] Carroll | From DNA to Diversity[END_REF]. The main hyptotheses of the apparition of such characters are presented in the figure 3. Details are given in the following text. The immediate sister group to the animals appears to be the Choanoflagellata (Figs. 2B and 3), a group of unicellular protozoa [START_REF] Carr | Molecular phylogeny of choanoflagellates, the sister group to Metazoa[END_REF][START_REF] King | The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans[END_REF], followed by the Filasterea (Fig. 2B), a group of relatively little-known parasites [START_REF] Cavalier-Smith | Megaphylogeny, cell body plans, adaptive zones: causes and timing of eukaryote basal radiations[END_REF].

Although some information about the filastereans is available, including the presence of signalling pathways [START_REF] Shalchian-Tabrizi | Multigene phylogeny of Choanozoa and the origin of animals[END_REF], most insights about the origins of the animals come from the better-known choanoflagellates and their genome [START_REF] King | The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans[END_REF].

Rather surprisingly, the clade grouping Metazoa and Choanoflagellata does not have a formal name, by instance [START_REF] Fairclough | Premetazoan genome evolution and the regulation of cell differentiation in the choanoflagellate Salpingoeca rosetta[END_REF] use the informal name 'Choanimal'. In this contribution, the term Apoikozoa will be used (Fig. 3). This name was proposed by [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF] and means 'colony-animals', reflecting the widespread, and possibly basal, ability of organisms within this clade to form colonies.

Choanoflagellata focused the attention because of their resemblance to the choanocytes of sponges, so that many authors still consider both as being in some sense homologous (e.g. [START_REF] Maldonado | Choanoflagellates, choanocytes, and animal multicellularity[END_REF]; but see also Mah et al., 2014 and below). Originally, they were split by Kent (1882) into three families: the Codonosigidae and the Salpingoecidae, that are still used today, and the Phalansteriidae, that are no longer included in the Choanoflagellatea (Cavalier- Smith et al. 2004). Another third family, the Acanthoecidae, was later added by [START_REF] Norris | Neustonic Marine Craspedomonadales (Choanofiagellates) from Washington and California[END_REF]. However, a recent phylogeny only supports the Acanthoecidae [START_REF] Carr | Molecular phylogeny of choanoflagellates, the sister group to Metazoa[END_REF], while the Codonosigidae and Salpingoecidae are grouped together in a second clade, the thecate choanoflagellates (Fig. 3). The acanthoecidans, or loricate choanoflagellates, are notable for the presence of a siliceous basket or lorica that is assembled from small costal strips [START_REF] Nitsche | Higher level taxonomy and molecular phylogenetics of the Choanoflagellatea[END_REF]. These siliceous strips ares built around an inner glycoprotein core [START_REF] Gong | Biosilicification of loricate choanoflagellate: organic composition of the nanotubular siliceous costal strips of Stephanoeca diplocostata[END_REF], which is somewhat reminiscent of the spicules of siliceous sponges. Nevertheless, recent discovery of a diatom-like silicic acid transport system in loricate choanoflagellates [START_REF] Marron | A family of diatom-like silicon transporters in the siliceous loricate choanoflagellates[END_REF] questions this potential homology.

Indeed, this suggests that the ability to synthesise siliceous structures was acquired either by the loricate choanoflagellates by lateral gene transfer from diatoms, or vice versa.

Furthermore, although a different silicic acid transport system has been reported from a demosponge [START_REF] Schröder | Silica transport in the demosponge Suberites domuncula : fluorescence emission analysis using the PDMPO probe and cloning of a potential transporter[END_REF], none of the relevant mechanisms are particularly well known. As a result, it remains unclear whether silica production is homologous or not in choanoflagellates and metazoans (sponges) (Fig. 3, [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. One further point is that members of the Ediacaran biota do not seem to preserve evidence for silica spicules or laths, and if some of these are stem-group, then one would expect these structures to have persisted in the stem group. Although some Ediacaran organisms, such as Fedomia [START_REF] Serezhnikova | Fedomia mikhaili-A new spicule-bearing organism of sponge grade from the Vendian (Ediacaran) of the White Sea, Russia[END_REF], Coronacollina [START_REF] Clites | The advent of hard-part structural support among the Ediacara biota: Ediacaran harbinger of a Cambrian mode of body construction[END_REF] and

Palaeophragmodictya [START_REF] Gehling | Long expected sponges from the Neoproterozoic Ediacara fauna of South Australia[END_REF][START_REF] Serezhnikova | Palaeophragmodictya spinosa sp. nov., a bilateral benthic organism from the Vendian of the Southeastern White Sea Region[END_REF], have been interpreted

as being spiculate, this is yet to be documented convincingly (see also [START_REF] Antcliffe | Giving the early fossil record of sponges a squeeze: The early fossil record of sponges[END_REF] for a general review of supposed Precambrian sponges including their spicules; for recent documentation of crown-group demosponges in the Cambrian, see Botting et al., 2015a).

The choanoflagellates also have the ability to form facultative colonies [START_REF] Fairclough | Multicellular development in a choanoflagellate[END_REF], which are all planktonic. They produce a jelly-like extracellular matrix (ECM) to embed themselves in [START_REF] Dayel | Cell differentiation and morphogenesis in the colony-forming choanoflagellate Salpingoeca rosetta[END_REF]. It is thus likely that early metazoans were characterised by a substantial collagenous ECM that would have provided support for large colonies (Fig. 3; [START_REF] Adams | Extracellular matrix evolution: an overview[END_REF]. Cells in colonies of Salpingoeca rosetta (and other choanoflagellates) can remain connected in various ways including via the extracellular matrix, filopodia, and intercellular bridges [START_REF] Dayel | Cell differentiation and morphogenesis in the colony-forming choanoflagellate Salpingoeca rosetta[END_REF]. The intercellular bridges are reported to resemble the products of incomplete cytokinesis seen in metazoans (e.g. in germ cysts; [START_REF] Ong | Germline cyst formation and incomplete cytokinesis during Drosophila melanogaster oogenesis[END_REF] and perhaps also the plugged cytoplasmic bridges seen in hexactinellids [START_REF] Leys | The significance of syncytial tissues for the position of the Hexactinellida in the Metazoa[END_REF]. It is therefore possible that the last common ancestor of animals and choanoflagellates was able to form colonies (Fig. 3; see [START_REF] Carr | Molecular phylogeny of choanoflagellates, the sister group to Metazoa[END_REF], who argue that coloniality must have appeared early in choanoflagellate evolution, and cannot be ruled out as a possible synapomorphy of choanoflagellates and metazoans).

Sexual reproduction is generally present in eukaryotes and in particular in animals [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. Even though it has never been observed directly in choanoflagellates, it is likely that they reproduced sexually as supported by the presence of meiosis genes [START_REF] Carr | Conserved meiotic genes point to sex in the choanoflagellates[END_REF]. As a consequence, these stem animals and eumetazoans were also likely to reproduce sexually, and thus to have at least a mobile sperm stage (Fig. 3). In some sponges, this is the only motile life stage, but others also have motile larval and juvenile stages [START_REF] Maldonado | Choanoflagellates, choanocytes, and animal multicellularity[END_REF][START_REF] Maldonado | The ecology of the sponge larva[END_REF] and adult sponges are known to be capable of slow movement [START_REF] Bond | Locomotion of sponges and its physical mechanism[END_REF]. The peculiar mode of fertilization in some sponges, whereby the sperm is captured by a choanocyte, which in turn ferries it to the oocyte (Simpson, 1984;[START_REF] Maldonado | Reproduction in Porifera: a synoptic overview[END_REF], is interpreted as reproductive strategies in now-extinct morebasal taxa [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. In this regard, mobility at some level was present as a plesiomorphy for Metazoa, and indeed Apoikozoa (Fig. 3).

Moreover, the relatively complex lifestyles of even unicellular opisthokonts potentially induce a set of signalling pathways that preceded the canonical eight developed in metazoans and especially bilaterians (Fig. 3; [START_REF] Pires-Dasilva | The evolution of signalling pathways in animal development[END_REF][START_REF] King | The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans[END_REF][START_REF] Shalchian-Tabrizi | Multigene phylogeny of Choanozoa and the origin of animals[END_REF][START_REF] Sebé-Pedrós | Premetazoan origin of the Hippo signaling pathway[END_REF][START_REF] Fairclough | Premetazoan genome evolution and the regulation of cell differentiation in the choanoflagellate Salpingoeca rosetta[END_REF]. However, it is also clear that considerable evolution of signalling pathways has taken place independently in choanoflagellates and metazoans (e.g. [START_REF] Suga | Ancient divergence of animal protein tyrosine kinase genes demonstrated by a gene family tree including choanoflagellate genes[END_REF], so that the suggestion that choanoflagellates essentially possess animal-like pathways would be misleading.

As it seems that no consensus have emerged yet from the phylogenetic studies alone, another possibility to look at the history and characteristics of early animals is to search for their representatives in the fossil record. The folowing part of this chapter will thus focus on the main phenomena in the history of life and early animal observed through the prism of palaeontology (fig. 4). Indeed, though DNA sequences are obviously not available for fossils, extinct forms are important in revealing lost morphologies that may be intermediates between living taxa and fundamental to determining their relationships (Briggs, 2015).

Fossils are also critical to calibrate the age of branching events in a phylogeny and, by extrapolation, to compute a molecular clock [START_REF] Lee | Morphological phylogenetics in the genomic age[END_REF]see 3.6.1).

The Precambrian Life: a slow motion from bacteria to the complexe Ediacara

Biota

Oldest hints pointing to the presence of life come from fractionations observed in carbon istopes from 3.8-billion-year-old sediments of the Isua Belt in West Greenland, origin of which should be biological (Fig. 5A; [START_REF] Schidlowski | Carbon isotopes as biogeochemical recorders of life over 3.8 Ga of Earth history: evolution of a concept[END_REF].

Until recently, direct evidence of the earliest known life in the Precambrian came from records of microfossils in cherty rocks and stromatolites (Fig. 5B) in Archaean rocks, some 3500 million years old, located at North Pole in the Pilbara Craton, Western Australia (Shu, 2008). Although those reports were widely accepted [START_REF] Goecke & Evers | The oldest fossils and what they mean[END_REF], a recent study showed that the so-called microfossils of Apex Chert actually were pseudofossils made by carbon adsorption on phyllosilicates grains [START_REF] Wacey | Solving the Controversy of Earth's Oldest Fossils Using Electron Microscopy[END_REF]. Then, the oldest fossils, authenticity of which seems valid, would be organic-walled microfossils found in 3.2-billion-year-old shallow-marine siliciclastic deposits of the Moodies group in South Africa (Fig. 5C; Javaux et al., 2009). The next significant step in the history of life is the discovery of the first eukaryote fossils found in Palaeoproterozoic rocks (about 2100 Ma) from the Negaunee Iron Formation, Michigan [START_REF] Han | Megascopic eukaryotic algae from the 2.1-billion-year-old negaunee iron-formation[END_REF][START_REF] Runnegar | Proterozoic eukaryotes: Evidence from biology and geology[END_REF][START_REF] Sun | Early multicellular fossils[END_REF][START_REF] Xiao | Precambrian eukaryote fossils[END_REF]. They look like coiled carbonaceous ribbons, having a shape close to Grypania spiralis, a possible eukariotyc alga found in the Mesoproterozoic (1600-1000 Ma) (Fig. 5D; [START_REF] Walter | Coiled carbonaceousmegafossils from the Middle Proterozoic of Jixian (Tianjin) and Montana[END_REF].

Contemporaneous potential multicellular or colonial organisms are also reported from the 2.1-billion-year-old Francevillian B Formation in Gabon (Fig. 5E; [START_REF] Albani | Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago[END_REF][START_REF] Albani | The 2.1 Ga Old Francevillian Biota: Biogenicity, Taphonomy and Biodiversity[END_REF], but their real nature remains subject to debate [START_REF] Donoghue | Early life: Origins of multicellularity[END_REF][START_REF] Garwood | Patterns In Palaeontology: The first 3 billion years of evolution[END_REF].

In the 1.2-billion-year-old Hunting Formation (Canada), the fossil Bangiomorpha pubescens is a red alga exhibiting differential spore/gamete formation, which indicates it was sexually reproducing (Fig. 5F; [START_REF] Butterfield | Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes[END_REF]: a key step in the evolution of eukaryotes. Later on, the oldest heterotrophic protists are identified in the 750-million-year-old Chuar Group in Grand Canyon (Fig. 5G; [START_REF] Porter | Vase-shaped microfossils from the Neoproterozoic Chuar Group, Grand Canyon: a classification guided by modern testate amoebae[END_REF].

Then, the 680-million-year-old Doushantuo Formation of South China has yielded a suite of fossilized, multicellular structures of diverse morphology (Fig. 5H; Erwin et al., 2011), which have been interpreted by some as the early cleavage states of metazoan embryos (Xiao et al, 1998). Although some of these forms have been assigned to bilaterian clades [START_REF] Chen | Small Bilaterian Fossils from 40 to 55 Million Years before the Cambrian[END_REF] or described as metazoan resting stages [START_REF] Cohen | Large spinose microfossils in Ediacaran rocks as resting stages of early animals[END_REF], it is likely that few (if any) actually represent crown-group metazoans, especially given the absence of any evidence for gastrulation, a metazoan-specific feature [START_REF] Hagadorn | Cellular and subcellular structure of neoproterozoic animal embryos[END_REF].

However, the most solid evidence for reliable multicellular organisms is restricted to the terminal Precambrian referred to as the Vendian (the Eastern European usage proposed for the latest Precambrian by Boris Sokolov in 1952), the Sinian (referred to a similar but a slightly shorter period, covering significant finds in South China) [START_REF] Xing | The Sinian of China[END_REF], or the Ediacaran (applied to the similar period, on account of the important fossil finds in South Australia). The last term now becomes the official usage as decided at the International Geological Conference at Florence, Italy in 2004 (Shu, 2008). This period hosts the Ediacaran Biota (Fig. 6), which represents the first of the three crucial steps (together with the Cambrian Explosion and the GOBE) leading to the rise and prosperity of metazoans. Photography from [START_REF] Javaux | Organic-walled microfossils in 3.2-billionyear-old shallow-marine siliciclastic deposits[END_REF]. D, detailed view of a 2.1-billion-year-old slab covered of Grypania from North Minesota, USA (sample 20 269972, Musée des confluences, Lyon, France).

Photography by Pierre Thomas. E, controversial oldest pyritised multicellular organisms from the 2.1-billion-year-old Francevilian biota of Gabon (Africa). Photography from El [START_REF] Albani | The 2.1 Ga Old Francevillian Biota: Biogenicity, Taphonomy and Biodiversity[END_REF]. F, detailed view of Bangiomorpha pubescens (specimen HUPC 62995 (HUST-1Q, O-45); Collections of Harvard University, Cambridge, USA), the oldest fossil showing sexual reproduction from the ca.

1200 -Ma Hunting Formation (Somerset Island, arctic Canada). Photography from [START_REF] Butterfield | Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes[END_REF].

G, Vase-shaped microfossils attached at the aperture, from the Neoproterozoic Uinta Mountain Group, Utah. Photography from [START_REF] Porter | Vase-shaped microfossils from the Neoproterozoic Chuar Group, Grand Canyon: a classification guided by modern testate amoebae[END_REF]. H, controversial oldest phophatized embryos of Parapandorina raphospissa from the 600-million-year-old Doushantuo Formation of China.

Photography from Xiao and Knoll (2000). Scale bars: 1 cm for A, D, and E; 0,5 cm for B; 100 μm C, and H; 60 μm for G; 10 μm for F.

3 The Ediacaran Biota: the earliest metazoans?

Historical background

The history of the discovery and interpretation of the Edicaran Biota (Figs. 6A andB; 579 to 541 Ma) is quite complexe. According to [START_REF] Gehling | The first named Ediacaran body fossil, Aspidella terranovica[END_REF], Aspidella terranovica is considered to be the earliest described Ediacara-type soft-bodied fossil, as it was published in 1872 by E. Billings using material from Newfoundland. Megafossils of this age were also discovered as early as 1877 in the Charnwood Forest of England, but they were initially dismissed as inorganic concretions [START_REF] Hill | The Precarboniferous Rocks of Charnwood Forest.-Part I[END_REF][START_REF] Watts | The geology of the ancient rocks of Charnwood Forest[END_REF]. These fossils were later recognized and described as Charnia masoni and Charniodiscus concentricus [START_REF] Ford | Pre-Cambrian Fossils from Charnwood Forest[END_REF]. However, no contemporaneous diversifed fauna was known until 1946 and the discovery of R. Sprigg, an Assistant Government Geologist [START_REF] Sprigg | Early Cambrian (?) jellyfish from the Flinders Ranges, South Australia[END_REF][START_REF] Sprigg | On the 1946 discovery of the Precambrian Ediacara fossil fauna in South Australia[END_REF][START_REF] Mccall | The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion[END_REF]. He found a locality yielding abundant and diversified fossils, the future Ediacaran Biota, in the Ediacara Hills, a range of low hills in the northern part of the Flinders Ranges from South Australia, 600 km north of Adelaide [START_REF] Glaessner | The late Precambrian fossils from Ediacara, South Australia[END_REF].

A wide distribution

Since then, half a century of extensive investigations have shown that the soft-bodied fossils of the Ediacaran Period are widely distributed all over the world, at the exception of the Antarctica (Fig. 7). [START_REF] Mccall | The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion[END_REF] provides an exhaustive synthesis of all those localities in the world, the most important being: Charnwood Forest, Leicestershire of England [START_REF] Ford | Pre-Cambrian Fossils from Charnwood Forest[END_REF][START_REF] Brasier | On the late Precambrian-Early Cambrian Hartshill Formation of Warwickshire[END_REF]; Namibia, SouthWest Africa [START_REF] Richter | Die altesten Fossilien Sud-Afrikas[END_REF][START_REF] Pflug | Neue Fossilreste aus den Nama-Schichten in Suedwest-Afrika (New fossil remains from the Nama beds of South-west Africa)[END_REF]; White Sea and Urals, Siberia of Russia and Ukraine [START_REF] Sokolov | The Vendian System[END_REF][START_REF] Sokolov | The vendian system[END_REF]; Newfoundland, Northwest Canada [START_REF] Hofmann | First record of a late Proterozoic faunal assemblage in the North American Cordillera[END_REF][START_REF] Narbonne | Ediacaran fossils from the Sekwi Brook and Mackenzie Mountains, Yukon, Canada[END_REF]; as well as other places in Australia: Amadeus Basin, Northern Territory [START_REF] Glaessner | New Precambrian fossils from the Arumbera Sandstone, Northern Territory, Australia[END_REF]; Kimberleys [START_REF] Grey | King Leopold and Halls Creek Orogens: local sedimentary succession related to the orogens[END_REF] ) and possibly the Stirling Range [START_REF] Cruse | Geological note: the discovery of Ediacaran trace and body fossils in the Stirling Range Formation, Western Australia. Implications for sedimentation and deformation during the 'Pan-African' orogenic cycle[END_REF]; and many localities in China: Yangtze Gorges, Liaoning Province, Anhui, Weng'an in Guizhou Province, South Shaanxi and Heilongjiang Province [START_REF] Xiao | A uniquely preserved Ediacaran fossil with direct evidence for a quilted bodyplan[END_REF][START_REF] Yuan | Radiation of Neopproterozoic Doushantuo Eukaryotes[END_REF]. Locations from [START_REF] Mccall | The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion[END_REF]. Curved map from Dave Goldberg and John Richard Gott.

Diverse communities through space and time

All those localities cover a wide array of palaoenvironnments, so that up to six types of communities are distinguished in the Ediacarian fauna (Shu, 2008): (1) Avalon-type in deep marine habitats, (2) Ediacara-type/microbial biofilms in shallow marine pro-delta settings, (3) infaunal Nama-type in distributary mouth bar shoals, (4) Doushantuo-type in shallow-water platform with preservation in phosphorite of microalgae and micrometazoans, (5) Miaohetype in deep-quiet-water facies dominated by macroscopic benthic algae, and (6) biomineralized Cloudina-type in carbonate facies [START_REF] Ding | SinianMiaohe Biota of China[END_REF][START_REF] Grazhdankin | Patterns of distribution in Ediacaran biotas and facies versus biogeography and evolution[END_REF][START_REF] Yuan | Radiation of Neopproterozoic Doushantuo Eukaryotes[END_REF][START_REF] Hua | Origin of skeletonization in Meatzoa[END_REF].

As the stratigraphic range covered by Ediacaran organisms is extended (579 to 541 Ma):

three distinct biostratigraphic zones have been recognized and share similarity with the ecological communities [START_REF] Waggoner | The Ediacaran biotas in space and time[END_REF][START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF]. The Avalon assemblage (579 to ~560 Ma; Fig. 8) is largely found in Newfoundland and England. This fauna is dominated by the Rangeomorpha [START_REF] Narbonne | Modular construction of early Ediacaran complex life forms[END_REF], a clade of modular organisms built from repetitively branched ("fractal") units, which also includes potential macroscopic sponges [START_REF] Sperling | Rangeomorphs, Thectardis (Porifera?) and dissolved organic carbon in the Ediacaran oceans: Rangeomorphs, Thectardis and DOC[END_REF]. The White Sea assemblage (~560 to ~550 Ma; Fig. 9) is widespread and diverse with more than three times the genera of the Avalon assemblage, marking an expansion in ecospace occupation [START_REF] Bush | Ecospace Utilization During the Ediacaran Radiation and the Cambrian Eco-explosion[END_REF] and behavioral complexity as reflected by diverse trace fossils. The youngest Nama assemblage (~550 to 541 Ma; Fig. 10), is dominated by the Erniettomorpha and includes evidence of predation boreholes in the oldest undisputed macroscopic biomineralizing organisms [START_REF] Bengtson | Predatorial Borings in Late Precambrian Mineralized Exoskeletons[END_REF]. Collectively, these three faunas show that assemblages expanded and diversified through the Ediacaran (Erwin et al., 2011). Thectardis, I: unformally nicknamed 'Xmas tree' by [START_REF] Clapham | Ediacaran epifaunal tiering[END_REF]. All except for C and H are rangeomorphs. Scale bars: 2 cm for A, B, and D-G; 0,25 cm for C. ). B, Kimberella, a possible stem-group mollusc. C, Spriggina, a possible stemgroup annelid or arthropod. D, Medusinites, probably the holdfast disc of a frond (South Australia Museum, P13785-6). E, Charniodiscus, a probable cnidarian (South Australia Museum, P19690). F, holotype of the serial fossil Yelovichnus, possibly a colonial alga or protist (Palaeontological Institute, Moscow, 39931309). G, the probable rangeomorph frond

Charnia (Palaeontological Institute,Moscow,. H, Arkarua, a possible stem-group echinoderm. I, holotype of Tribrachidium, a triradiate fossil of uncertain affinities (South Australia Museum,P12898). J, Helminthoidichnites, a probable bilaterian burrow (Geological Survey of Canada 95924). Scale bars: 1 cm for A-C; 2 cm for D, and F-J; 5 cm for E.

A commonly coarse preservation

Classical Ediacaran fossils from Australia are characterized by a high relief preservation [START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF], which reflects molding of the nonmineralized organisms and colonies by microbial filaments to form a "death mask" that maintained the shape of the specimen and hastened lithification of the overlying bed [START_REF] Gehling | Microbial mats in terminal Proterozoic siliciclastics; Ediacaran death masks[END_REF]. Further studies expanded the microbial role in the preservation to different type of Ediacaran fossils on carbonaceous sheets and sheet-like intraclasts (Narbonne, 1998). Supplementary support comes from traces of mat-grazing organisms [START_REF] Seilacher | Biomat-related lifestyles in the Precambrian[END_REF] and preserved cyanobacterial filaments within the mat [START_REF] Steiner | Evidence of organic structures in Ediacara-type fossils and associated microbial mats[END_REF]. [START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF] provided a detailed survey of Ediacaran-type preservations. He distinguished four distinct styles of preservation: the Conception-style, the Fermeuse-style, the Flinders-style, and the Nama-style.

The Conception-style preservation (Figs. 11A, andB) preserves the organisms under a bed of volcanic ash. It is the predominant mode of preservation in the Conception Group of Newfoundland [START_REF] Seilacher | Vendobontia and Psammocorallia: lost constructions of Precambrian evolution[END_REF] and the Charnwood succession in England [START_REF] Ford | Pre-Cambrian Fossils from Charnwood Forest[END_REF].

Preservation began when volcanic ash instantaneously covered the colonies of organisms and molded their upper surfaces. Preservation of the tops of holdfasts and stems rather than their bases implies that lithification proceded more rapidly in volcanic ash than in the sandstones of the Flinders-style preservation, presumably owing to early diagenesis of labile minerals in the ash. However, less resistant organic materials such as spindles collapsed or decomposed almost immediately, and the still-unlithified ash settled to preserve their impressions on the microbially coated deep-sea floor. Avalonian volcanic ashes weather recessively to reveal spectacular tennis-court-sized surfaces of silicified mudstone, each with the fossil impressions of the original biological community preserved in place on its upper surface [START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF].

The Fermeuse-style preservation (Fig. 11C) typically preserves only trace fossils and the bases of holdfasts. It is the characteristic mode of preservation in outer shelf and slope assemblages, such as the Fermeuse Formation of Newfoundland [START_REF] Gehling | The first named Ediacaran body fossil, Aspidella terranovica[END_REF], the

Windermere Supergroup of northwestern Canada [START_REF] Narbonne | Ediacaran biota of the Wernecke Mountains, Yukon, Canada[END_REF][START_REF] Narbonne | Ediacaran fossils from the Sekwi Brook and Mackenzie Mountains, Yukon, Canada[END_REF], and the Innerelv Member of Finnmark in northern Norway [START_REF] Farmer | Ediacaran fossils from the Innerelv Member (late Proterozoic) of the Tanafjorden area, northeastern Finnmark[END_REF], and also may be responsible for beds with only discs and burrows in the otherwise richly fossiliferous sections at Ediacara and in the White Sea. [START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF] hypothesized that the microbial mat was less developed than in Flinders-style preservation or that it consisted of heterotrophic or sulfur-oxidizing bacteria, which did not produce the rapid mineralization essential to lithify the sole of the overlying bed and preserve epifaunal organisms and colonies. Holdfasts and burrows were well preserved because they were partly buried in the substrate and thus did not require a microbial coating for preservation.

The Flinders-style preservation (Fig. 11D) exhibits a topologically complex surface, with impressions of the bases of holdfasts in the underlying mud (typically preserved as raised features on the bed soles), the tops of resistant epifaunal organisms such as Dickinsonia that were lying on the muds (typically preserved as negative features on soles), the bases of nonresistant epifaunal organisms such as frond "leaves" (typically preserved as positive impressions on soles), and abundant burrows (preserved in both positive and negative relief on bed soles). It is the characteristic mode of preservation of the most fossiliferous beds in the Ediacara Member in the Flinders Ranges of Australia [START_REF] Gehling | Microbial mats in terminal Proterozoic siliciclastics; Ediacaran death masks[END_REF] and is also well described from the interstratified sandstone and shale facies of the White Sea [START_REF] Grazhdankin | Patterns of distribution in Ediacaran biotas and facies versus biogeography and evolution[END_REF]. Flinders-style preservation was best developed in shallow, sunlit environments between wave base and storm wave base, and it involved event deposition of storm beds on sea bottoms with a well-developed microbial mat. This lithified the sole of the overlying bed rapidly and, when a resistant organism colony decayed, unlithified mud from the underlying bed moved upward to fill the impression of the organismcolony [START_REF] Gehling | Microbial mats in terminal Proterozoic siliciclastics; Ediacaran death masks[END_REF]. Lessresistant structures, such as frond leaves, were molded from beneath and cast by sand following decay of the colony of organisms, and structures that were partly buried in the substrate (frond holdfasts and burrows) were preserved when sand from the overlying event bed filled the open hole following departure-decay of the organism [START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF].

Finally, the Nama-style preservation (Figs. 11E, andF) preserves fossils as molds and threedimensional casts within beds of storm-or turbidite-deposited sand, rather than at the base of the event bed as in the other three preservational modes [START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF]. The best-known examples are from the Kuibis and Schwarzrand subgroups of Namibia in southern Africa [START_REF] Grazhdankin | Underground Vendobionta From Namibia[END_REF] that bear monospecific assemblages consisting of abundant Pteridinium, Rangea, Ernietta, or Swartpuntia; similar assemblages occur at isolated levels in Australia [START_REF] Glaessner | The late Precambrian fossils from Ediacara, South Australia[END_REF], California/Nevada (Hagadorn and Waggoner, 2000), the White Sea [START_REF] Grazhdankin | Patterns of distribution in Ediacaran biotas and facies versus biogeography and evolution[END_REF], and Newfoundland [START_REF] Narbonne | Modular construction of early Ediacaran complex life forms[END_REF]. [START_REF] Grazhdankin | Underground Vendobionta From Namibia[END_REF] regarded these assemblages representing infaunal Ediacara-type organisms, but the unimodal orientation of the fossils in many of these occurrences implies that they lived epifaunally and were rapidly buried under the influence of a current or combined wave-current flow [START_REF] Jenkins | The enigmatic Ediacaran (late Precambrian) genus Rangea and related forms[END_REF][START_REF] Narbonne | The Youngest Ediacaran Fossils from Southern Africa[END_REF][START_REF] Narbonne | Modular construction of early Ediacaran complex life forms[END_REF]. Specimens in all of these deposits are preserved three dimensionally within the sandstone beds, with superb preservation of multifoliate structures not readily visible in Ediacara-type fossils preserved on bed soles. There is no evidence of microbial molding of the fossils, and in some instances this has led to partial degradation of the exterior of the colonies of organisms to reveal resistant organic internal structures not seen in normal occurrences of the Ediacara biota [START_REF] Dzik | Organic membranous skeleton of the Precambrian metazoans from Namibia[END_REF][START_REF] Narbonne | Modular construction of early Ediacaran complex life forms[END_REF]. As a consequence, the shape of the fossil in a within-bed preservation may have little resemblance to the external form of the original living organism [START_REF] Dzik | Organic membranous skeleton of the Precambrian metazoans from Namibia[END_REF]. Most of the taxa typical of Nama-style preservation are reported only rarely from more common preservations of the Ediacara biota, where they may be described under different taxonomic names [START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF].

However, the great majority of Ediacaran-type fossils are relatively poorly preserved in coarse sand-siltstones, so that few reliable anatomical details can be observed. Sometimes, the preservation is even so poor that it becomes difficult to determine if some Ediacaranfossils are soft-bodied remains or trace fossils (Shu, 2008). To get around the problem of coarse preservation, one of the ideal routes is to search for their well-preserved counterparts in the extraordinary soft-bodied fossil from the Cambrian and younger strata. Very good examples are Thaumaptilon from the middle Cambrian Burgess Shale (Fig. 12A; Conway Morris, 1993a), or Stromatoveris and Chengjiangopenna from the early Cambrian Chengjiang (Figs. 12B and C;Shu et al., 2006;Shu and Conway Morris, 2006).

The fossil record of Ediacaran biotas, although very sparse (like that of all poorly preservable taxa), is potentially quite widespread in the Cambrian (Fig. 12E; [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF].

Taxa purportedly from this assemblage have been reported from rocks of Cambrian age on various continents, such as Australia, Baltica, south China, or North America (e.g. Conway Morris, 1993a;[START_REF] Jensen | Ediacara-type fossils in Cambrian sediments[END_REF][START_REF] Crimes | A biota of Ediacaran aspect from lower Cambrian strata on the Digermul Peninsula, Arctic Norway[END_REF]Hagadorn et al., 2000;[START_REF] Zhang | New extraordinarily preserved enigmatic fossils, possibly with Ediacaran affinities, from the Lower Cambrian of Yunnan, China[END_REF]Shu et al., 2006;[START_REF] Babcock | Frondose fossil from the Conasauga Formation (Cambrian: Drumian stage) of Georgia, USA[END_REF]. Nevertheless, such

Cambrian survivors are admittedly controversial (for a more skeptical view, see [START_REF] Laflamme | The end of the Ediacara biota: Extinction, biotic replacement, or Cheshire Cat?[END_REF], and Ediacaran taxa are commonly considered to be extinct by the Cambrian [START_REF] Jensen | Ediacara-type fossils in Cambrian sediments[END_REF][START_REF] Kouchinsky | Chronology of early Cambrian biomineralization[END_REF][START_REF] Amthor | Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman[END_REF][START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF].

Even though, sometimes, there is still a good chance to preserve excellent details in Ediacarian fossils. For example, [START_REF] Zhang | A Fresh Look at Dickinsonia: Removing it from Vendobionta[END_REF] reexamined well-preserved specimens of Dickinsonia from the White Sea locality (Russia; Fig. 12D). Thanks to their exceptional preservation, these controversial organisms exhibited relationship to ctenophores and were thus removed from Vendobionta. Other key evidence may come from the possible microscopic metazoans and embryos that were recently discovered in China: the Weng'an locality, Guizhou Province and the Three-Gorges of Hubei Province (Xiao et al., 1998(Xiao et al., , 2000;;[START_REF] Yin | New observation on spicule-like structure from Doushantuo phosphorites at Weng'an, Guizhou Province[END_REF][START_REF] Yuan | Doushantuo Fossils: Life on the Eve of Animal Radiation[END_REF][START_REF] Chen | The Dawn of Animal World[END_REF][START_REF] Bengtson | Comments on "Small Bilaterian Fossils from 40 to 55 Million Years before the Cambrian[END_REF][START_REF] Chen | Small Bilaterian Fossils from 40 to 55 Million Years before the Cambrian[END_REF][START_REF] Xiao | Precambrian eukaryote fossils[END_REF][START_REF] Yuan | Radiation of Neopproterozoic Doushantuo Eukaryotes[END_REF][START_REF] Yin | Doushantuo embryos preserved inside diapause egg cysts[END_REF]. In this context, the biological nature of Ediacaran organisms in themselves remains a conundrum.

Fig. 11. The preservation styles of the Ediacara biota adapted from [START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF]. A and B, the Conception-style preservation. A, spindle-shaped rangeomorphs (arrow) preserved on an upper bedding surface of silicified mudstone beneath a thick bed of volcanic ash, Mistaken Point, Newfoundland. B, preservation of "spindles" and a "feather duster" in positive relief on an upper surface of the Mistaken Point Formation of Newfoundland. The surface is overlain by a thin bed of volcanic ash (visible as dark bands at the top and bottom of the photograph and as the grey speckled grains between and even within the fossils). C, the Fermeuse-style preservation of abundant specimens of Aspidella on the sole of a turbidite sandstone from the Fermeuse Formation of Newfoundland (Geological Survey of Canada 116842). D, the Flinders-style preservation of two specimens of Dickinsonia in negative relief on the sole of a storm sandstone from the Ediacara Member of Australia. Note the pustular nature of the surface, probably reflecting a microbial mat that covered the surface (South Australia Museum P13760). E and F, the Nama-style preservation. E, Aspidella (disc indicated with an arrow) preserved on the sole of a thin turbidite, Sheepbed Formation, northwestern Canada. F, a specimen of Pteridinium within a sandstone bed in the Kuibis Subgroup of Namibia. The scale bar in all figures is 5 cm.

Fig. 12.

Exceptionnally well-preserved taxa from the Ediacaran Biota and their putative occurences 565 Ma rocks [START_REF] Liu | First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation[END_REF], but otherwise most are found in rocks younger than 560 Ma [START_REF] Jensen | Trace fossil preservation and the early evolution of animals[END_REF][START_REF] Jensen | A Critical Look at the Ediacaran Trace Fossil Record[END_REF]. Trace fossils increase in diversity and complexity toward the Cambrian, when the oldest vertical burrows reveal the presence of a hydrostatically resistant coelom in an organism larger than ~1 cm in diameter. This would seem to provide a strong constraint on the evolution of larger bilaterians [START_REF] Budd | A critical reappraisal of the fossil record of the bilaterian phyla[END_REF][START_REF] Valentine | Interpreting great developmental experiments: the fossil record[END_REF]. Discoveries of a variety of Ediacaran fossils now let us see the first large diploblastic radiation, which produced the successful cnidarian bodyplan among a wide range of body forms (Shu, 2008). Valentine proposed a hypothesis of early evolution of metazoans based on the Hox and ParaHox clusters in eumetazoans (2001,2004). However, a morphological tree with real historical data is still needed. It is quite possible that the continuing investigations of early fossils would reveal more stemgroup diploblasts, and maybe more sorts of stem-group bilaterians in the Precambrian (Shu, 2008). represented by stem groups (ancestors lacking one or more diagnostic characters of the modern phylum), or in red where fossils are represented by crown groups (ancestors exhibiting all of the diagnostic characters of the modern phylum) in addition to stem groups. Molecular phylogeny of animals from Knoll and Carrol (1999).Evolutionary level adapted from [START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF].

Phylogenetic positions of Ediacaran taxa adapted from [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. The organism silhouettes were submitted to PhyloPic (www.phylopic.org). They are available for reuse under the Creative Commons Attribution-ShareAlike 3.0 Unported license.

A prelude to the Cambrian explosion?

In the last five million years of the Ediacaran, the first lightly skeletonized forms appear [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF], including the relatively simple multicone-shaped Cloudina (Fig. 14A; [START_REF] Grant | Shell structure and distribution of Cloudina a potential index fossil for the terminal Proterozoic[END_REF][START_REF] Hua | Skeletogenesis and asexual reproduction in the earliest biomineralizing animal Cloudina[END_REF] and the rather more complex, but still small, Namacalathus and its relatives (Figs. 14B and C;[START_REF] Grotzinger | Calcified metazoans in thrombolitestromatolite reefs of the terminal Proterozoic Nama Group, Namibia[END_REF]. This opened the path to the rapid diversification of metazoans in the Early Cambrian, the Cambrian Explosion, which is largely witnessed by at least three aspects of the fossil record (Conway Morris, 1993b): the sudden appearance of skeletal faunas (Bengtson, 1992a;[START_REF] Qian | Palaeontology and biostratigraphy of the early Cambrian Meishucun stage in Yunnan province, South china[END_REF], a striking increase in the diversity and complexity of trace fossils [START_REF] Crimes | The record of trace fossils across the Proterozoic-Cambrian boundary[END_REF][START_REF] Fortey | The trace fossil Cruziana semiplicata and the trilobite that made it[END_REF], and the remarkably well-preserved faunas (Conway [START_REF] Morris | The Burgess shale fauna and the Cambrian explosion[END_REF]Briggs et al., 1994;Shu et al., 1996c;[START_REF] Chen | Biology of the Chengjiang fauna[END_REF]Hou et al., 1999;Shu, 2005a;2006).

Relationships between the Ediacara Biota and later Cambrian explosion are not clear, though the first may have had a decisive impact on the latter.

Molecular clock data and the fossil record

Before the modern dating techniques emerged, it was widely considered that the Ediacaran biota considerably pre-dated the Cambrian [START_REF] Laflamme | The end of the Ediacara biota: Extinction, biotic replacement, or Cheshire Cat?[END_REF][START_REF] Macgabhann | There is no such thing as the "Ediacara Biota[END_REF]. Even after the precise dating of these two biotas, which replaced them in close temporal proximity, there has been a tendency to regard them as being separated by a mass extinction (e.g. [START_REF] Amthor | Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman[END_REF][START_REF] Narbonne | The Ediacara biota: Neoproterozoic origin of animals and their ecosystems[END_REF][START_REF] Darroch | Biotic replacement and mass extinction of the Ediacara biota[END_REF]. Given that bilaterian animals, so characteristic of Cambrian biotas, are inferred to have existed in some form since at least the latest Ediacaran (through the trace fossil record), a model of biotic replacement that was perhaps mediated by a mass extinction has often seemed attractive (e.g. [START_REF] Seilacher | Late Precambrian and Early Cambrian Metazoa: Preservational or Real Extinctions?[END_REF][START_REF] Laflamme | The end of the Ediacara biota: Extinction, biotic replacement, or Cheshire Cat?[END_REF]. Perhaps this assumption is influenced by the analogy with subordinate mammals supposedly only being free to radiate into newly vacated niches after extinction of the dinosaurs at the end of the Cretaceous [START_REF] Archibald | Extinction and radiation: how the fall of dinosaurs led to the rise of mammals[END_REF][START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. This analogy is challenged by [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. Indeed, as both dinosaurs and mammals are members of the same clade (the Amniota), there is no increase in disparity, nor a significant change in diversity as a consequence of the end-Cretaceous extinction [START_REF] Mcghee | Ecological ranking of Phanerozoic biodiversity crises: ecological and taxonomic severities are decoupled[END_REF]. The only changes observed are a replacement of various diapsid clades (dinosaurs, pterosaurs, ichthyosaurs, plesiosaurs, etc.) with an ecologically and morphologically equivalent sister clade: the synapsids (=mammals). This is in sharp contrast to the Cambrian Explosion [START_REF] Bowring | Calibrating rates of Early Cambrian evolution[END_REF][START_REF] Foote | Origination and extinction through the Phanerozoic: a new approach[END_REF], where there is a dramatic increase in both disparity (the difference in species morphology) and diversity (the number of species).

The extinction of the Ediacarans has also been considered as a consequence of the substrate revolution, with the newly evolving sediment-churning bilaterians radically changing the substrate to exclude them [START_REF] Laflamme | The end of the Ediacara biota: Extinction, biotic replacement, or Cheshire Cat?[END_REF]. One final possibility, favoured by [START_REF] Laflamme | The end of the Ediacara biota: Extinction, biotic replacement, or Cheshire Cat?[END_REF], is that the metazoans were simply more efficient at occupying benthic space, and thus indirectly outcompeted the Ediacarans.

In any cases, the available fossil record now shows that some sorts of metazoans existed in the Ediacaran Period [START_REF] Sprigg | On the 1946 discovery of the Precambrian Ediacara fossil fauna in South Australia[END_REF][START_REF] Fedonkin | Vendian faunas and the early evolution of theMetazoa[END_REF][START_REF] Fedonkin | Vendian body fossils and trace fossils[END_REF][START_REF] Fedonkin | The late Precambrian fossil Kimberella is a mollusc-like trilaterian organism[END_REF]Xiao et al., 1998;[START_REF] Chen | The Dawn of Animal World[END_REF], not least because of the presence of trace fossils.

Without doubt, the Cambrian faunas stemmed from some members of Ediacaran life.

However, Ediacaran creatures and Cambrian faunas are very distinct from each other, and any interpretation of Ediacaran organisms as possible forerunners to the Phanerozoic metazoans is controversial (Shu, 2008). Nevertheless, to try to detect the first ripple of the Cambrian Explosion in the Ediacaran times must be central if we wish to understand better the earliest evolutionary history of the TOA. In this regard, recognition of survivors of some Edicaran organisms in the Cambrian is informative (Conway Morris, 1993a,b;Shu et al., 2006;Shu and Conway Morris, 2006).

Teasing apart the mechanisms underlying the Cambrian explosion requires disentangling evolutionary origins from geological first appearances (Erwinn et al., 2011).

This question of the hidden history prior to this remarkable bioradiation of metazoan is thus essential. Though imperfect [START_REF] Donoghue | Telling the Evolutionary Time: Molecular Clocks and the Fossil Record[END_REF]dos Reis et al., 2015), the molecular clock (Runnegar, 1982a;[START_REF] Erwin | Molecular clocks, molecular phylogenies and the origin of phyla[END_REF][START_REF] Ayala | Origin of the metazoan phyla: molecular clocks confirm paleontological estimates[END_REF] is a powerful technique that established itself since the early 60's [START_REF] Zuckerkandl | Molecular disease, evolution, and genetic heterogeneity in[END_REF] to bring an answer. The obtained results, however, remain contradictory and very controversial, depending on the assumptions made [START_REF] Peterson | Estimating metazoan divergence times with a molecular clock[END_REF]. Though the majority of age estimates (750-1000 Ma) are much older than the estimates from the fossil record, some studies, especially those that recognize that different clades evolve at different rates, the importance of concatenating collected sequence data, and the use of multiple and better refined fossil calibration points, place the divergence of the bilaterian phyla within the Ediacaran [START_REF] Aris-Brosou | Bayesian models of episodic evolution support a late Precambrian explosive diversification of the Metazoa[END_REF]Yang 2003, Peterson et al. 2004; see Fig. 15), and most likely after the last Snowball Earth. Range, Western Australia [START_REF] Rasmussen | Discoidal impressions and trace-like fossils more than 1200 million years old[END_REF]. If confirmed in terms of their nature and age, the history of the stem group metazoans (but not, of course, the crown-group metazoans) could be extended back to as early as the Mesoproterozoic. However, until now, no reliable body-fossil record of metazoans has been known in pre-Ediacaran times (Shu, 2008). To explain this lack, the meiofauna hypothesis suggests that the pre-Ediacaran animals were so tiny, only a few millimeters long and quite similar to modern larvae, that they could not be preserved as fossils at all (Giere, 1993;Conway Morris, 1993b, 1998a).

So, while certainly still contentious [START_REF] Graur | Reading the entrails of chickens: molecular timescales of evolution and the illusion of precision[END_REF]dos Reis et al., 2015)), the fossil record and molecular estimates of the times of origin of the animal phyla may well be in general accord, given that most phyla have at least some Precambrian history [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF].

Burrowing as a trigger of the Cambrian Explosion

For example, the emergence of burrowing abilities during the Ediacarian enables early bilaterians to have access to new feeding sources, such as material lost to the sediment by death, or excretion. The suggestion that organisms were driven to burrow by predation pressure (e.g. [START_REF] Dzik | The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian Cambrian transition[END_REF] implies the presence of mobile predators -themselves likely to be the product of the bilaterian radiation, unless the predatorial pressure came from cnidarians, adding yet another level of speculation as this would require inferences about early cnidarian feeding modes [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF].

Burrowing is very expensive energetically, even if it seems that the slow rates at which organisms burrow mitigate the cost per unit time [START_REF] Dorgan | Energetics of burrowing by the cirratulid polychaete Cirriformia moorei[END_REF]. Given the high energy demands, such a lifestyle may not have been possible without resource concentration, such as the extension of microbial mats during the Ediacarian [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. Burrowing, then triggers an ecological cascade, with innovations such as infaunal predation becoming possible. This predation, in turn triggered higher levels of burrowing, and eventually led to the slow transformation of the environment in the so-called 'Cambrian substrate revolution' [START_REF] Bottjer | The Cambrian substrate revolution[END_REF].

Nevertheless, much of the attention focused on the microbial mat that acted as the food source, on which this process depended (e.g. [START_REF] Stanley | An ecological theory for the sudden origin of multicellular life in the late Precambrian[END_REF][START_REF] Gingras | Possible evolution of mobile animals in association with microbial mats[END_REF]. Indeed, experiments have shown that in the absence of bioturbation microbial mats will develop on the sea floor (e.g. [START_REF] Fenchel | Formation of laminated cyanobacterial mats in the absence of benthic fauna[END_REF]. It is thus reasonable to suggest that such mats would have been well developed on the sea floor prior to the advent of bioturbation [START_REF] Seilacher | From biomats to benthic agriculture: A biohistoric revolution[END_REF][START_REF] Schieber | Atlas of microbial mat features preserved within the siliciclastic rock record[END_REF]. This step probably accounted for the very early stages of animal evolution, but the developpment of multicelluarity is more heavily dependent on environmental heterogeneity rather than merely on resource availability. An analogy is drawn by [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF] with the potential role of resource heterogeneity in the savannah for early hominid evolution. Modern savannah environments are highly structured with trees scattered across plains in proximity to other habitats such as hills (Fig. 16A). This heterogeneity leads to a high diversity of taxa and favors their mobility. For instance, a well-known hypothesis of human evolution suggests that bipedality evolved so that humans could move freely between the scattered resource hotspots [START_REF] Rodman | Bioenergetics and the origin of hominid bipedalism[END_REF][START_REF] Isbell | The evolution of bipedalism in hominids and reduced group size in chimpanzees: alternative responses to decreasing resource availability[END_REF].

When applied to the Ediacarian (Fig. 16B), this environmental heterogeneity might have been reached by the death of large stationary organisms on the sea floor [START_REF] Seilacher | Ediacaran biota: the dawn of animal life in the shadow of giant protists[END_REF][START_REF] Antcliffe | Testing the protozoan hypothesis for Ediacaran fossils: a developmental analysis of Palaeopascichnus[END_REF]. This would have concentrated nutrients in the surrounding sediment as well as providing diversity hotspots, partly because of the increased vertical space made available by their corpses to at least the epifauna and probably the infauna as well. This small-scale highly heterogeneous environment with scattered hotspot of ressources would have stimulated the adaptive radiation of the mobile bilaterians and their burrowing habit.

A transition from early bilaterians living largely on the surface and feeding offmats that surrounded the Ediacara-style biota to true burrowing may have been facilitated by partial burial of such mats, with their indigenous fauna continuing to feed on them even when below the surface. The remnants of this early ecological framework may have persisted well into the Cambrian, where, for example, beds containing both trilobites and Ediacara-style taxa seem to be known from both Laurentia (Conway [START_REF] Morris | The developmental role of the extracellular matrix suggests a monophyletic origin of the kingdom Animalia[END_REF]Hagadorn et al., 2000) and South China (Shu et al. , 2006). As Cambrian taxa diversified, they would have rapidly ceased reliance on Ediacara-style taxa to provide spatial heterogeneity, and perhaps this may have been one feature in the ultimate demise of the Ediacara-style taxa. This analysis largely ignores environmental effects such as changes in sea water composition [START_REF] Riding | Calcified cyanobacteria[END_REF], increased nutrient supply [START_REF] Derry | Sr and C isotopes in Lower Cambrian carbonates from the Siberian craton: A paleoenvironmental record during the "Cambrian explosion[END_REF] and changing oxygen levels [START_REF] Mills | Oxygen and animal evolution: Did a rise of atmospheric oxygen "trigger" the origin of animals?: Insights &amp[END_REF], each possibly associated with tectonic events (e.g. [START_REF] Dalziel | Cambrian transgression and radiation linked to an Iapetus-Pacific oceanic connection?[END_REF].

But in this context, the Ediacaran taxa actually provided a highly structured and resourceheterogeneous environment that was an essential framework within which the bilaterians could radiate, enabling the Cambrian Explosion.

The Cambrian Explosion

Definition

Traditionnaly, the Cambrian Explosion corresponds to the sudden appearance of essentially all the readily fossilizable bilaterian animal phyla in the early Cambrian period (Fig. 17; [START_REF] Qian | Palaeontology and biostratigraphy of the early Cambrian Meishucun stage in Yunnan province, South china[END_REF][START_REF] Bengtson | Early Cambrian fossils from South Australia[END_REF][START_REF] Bengtson | Fossilized metazoan embryos from the earliest Cambrian[END_REF][START_REF] Peel | Functional morphology of the class Helcionelloida nov., and the early evolution of the 12Mollusca[END_REF][START_REF] Rozanov | The Lower Cambrian fossil Record of the Soviet Union[END_REF]Bengtson, 1992a;b;[START_REF] Jiang | The Lower Cambrian Fossil Record of China[END_REF][START_REF] Popov | The Cambrian Radiation of Brachiopods[END_REF][START_REF] Landing | Lower Cambrian of Southeastern Newfoundland: Epeirogeny and Lazarus Faunas, Lithofacies-Biofacies Linkages, and theMyth of a Global Chronostratigraphy[END_REF][START_REF] Balavoine | One or Three Cambrian Radiations?[END_REF][START_REF] Feng | Microstructure of Early Cambrian Ramenta from China[END_REF][START_REF] Valentine | Prelude to the Cambrian explosion[END_REF][START_REF] Feng | Phosphate replicated and replaced microstructure of molluscan shells from the earliest Cambrian of China[END_REF][START_REF] Li | Micrina and Tannuolina (Tannuolidae) from the Lower Cambrian of eastern Yunnan, south China[END_REF][START_REF] Li | Explosive Radiation of early Cambrian skeletal faunas in South China[END_REF][START_REF] Feng | Earliest Evolutionary Lineage of Early Cambrian Univalved Mollusks[END_REF], as well as a striking increase in the diversity and complexity of the contemporary trace fossils [START_REF] Crimes | Trace fossils[END_REF]1992). The absent remaining extant phyla are largely represented by tiny organisms and/or parasites, which, with very few exceptions, have not left any fossil record in the rest of the Phanerozoic [START_REF] Erwin | A comparative study of diversification events: the early Paleozoic versus the Mesozoic[END_REF][START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF], not to mention some additional extinct phyla, such as Vetulicolia [START_REF] Li | Explosive Radiation of early Cambrian skeletal faunas in South China[END_REF].

As each phylum represents (for the most part) a fundamentally different mode of organization, the rate of accumulation of disparity during this geological event is unprecedented and unique. Though Cambrian diversity was considerably lower than most of the rest of the Phanerozoic [START_REF] Sepkoski | Biodiversity: past, present, and future[END_REF], Cambrian rates of per genus origination and extinction were on average much higher and more volatile [START_REF] Bowring | Calibrating rates of Early Cambrian evolution[END_REF][START_REF] Foote | Origination and extinction through the Phanerozoic: a new approach[END_REF] than the rest of the Phanerozoic, It can be compared to a Big Bang of life, giving birth to the entire morphological Tree Of Animals (or metazoans), in short the TOA [START_REF] Valentine | The Cambrian Explosion[END_REF][START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. By instance, even after the largest mass extinction of the Phanerozoic, the end-Permian mass extinction, no new phyla appeared.

This rapid diversification of metazoans in the Early Cambrian is largely witnessed by at least three aspects of the fossil record (Conway Morris, 1993b): the sudden appearance of skeletal faunas (Bengtson, 1992a;[START_REF] Qian | Palaeontology and biostratigraphy of the early Cambrian Meishucun stage in Yunnan province, South china[END_REF], a striking increase in the diversity and complexity of trace fossils [START_REF] Crimes | The record of trace fossils across the Proterozoic-Cambrian boundary[END_REF][START_REF] Fortey | The trace fossil Cruziana semiplicata and the trilobite that made it[END_REF], and the remarkably well-preserved faunas (Conway [START_REF] Morris | The Burgess shale fauna and the Cambrian explosion[END_REF]Briggs et al., 1994;Shu et al., 1996c;[START_REF] Chen | Biology of the Chengjiang fauna[END_REF]Hou et al., 1999;Shu, 2005a;2006).

It is important to remember that we see the Cambrian "explosion" through the windows permitted by the fossil and geological records [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. However, more than a half century of extensive and intensive investigations on the Precambrian-Phanerozoic boundary have shown that the Cambrian Explosion or Bioradiation was a real biological event and not an artefact due to some taphonomic bias [START_REF] Glaessner | New fossils from the base of the Cambrian in South Australia (preliminary account)[END_REF]1984;[START_REF] Seilacher | Vendozoa: organismic construction in the Proterozoic biosphere[END_REF][START_REF] Mcmenamin | Vendian and Ediacaran[END_REF][START_REF] Mcmenamin | The Emergence of Animal[END_REF][START_REF] Bengtson | Early Cambrian fossils from South Australia[END_REF]Bergström, 1990;[START_REF] Fedonkin | Vendian faunas and the early evolution of theMetazoa[END_REF][START_REF] Runnegar | Evolution of the earliest animals[END_REF][START_REF] Seilacher | Vendobontia and Psammocorallia: lost constructions of Precambrian evolution[END_REF]Conway Morris, 1993b;[START_REF] Fedonkin | Vendian body fossils and trace fossils[END_REF][START_REF] Runnegar | Proterozoic eukaryotes: Evidence from biology and geology[END_REF][START_REF] Grotzinger | Biostratigraphic and geochronologic constraints on early animal evolution[END_REF][START_REF] Morris | preserved articulated body remains of variety of taxa in the early lowest metazoans can be observed, but also many soft-bodied fossils in all the three subkingdoms of the Eumetazoa[END_REF][START_REF] Collins | Evaluating multiple alternative hypotheses for the origin of Bilateria[END_REF][START_REF] Runnegar | Understanding the Ediacaran organisms; how to proceed?[END_REF][START_REF] Knoll | On the age of the Doushantuo Formation[END_REF][START_REF] Erwin | The origin of bodyplans[END_REF]Conway Morris, 2000;2003;[START_REF] Erwin | The last common bilaterian ancestor[END_REF][START_REF] Valentine | Prelude to the Cambrian explosion[END_REF][START_REF] Valentine | On the Origin of Phyla[END_REF][START_REF] Waggoner | The Ediacaran biotas in space and time[END_REF][START_REF] Benton | Dating the tree of life[END_REF]. Indeed, except for a few skeletalized fossils, such as Cloudina [START_REF] Glaessner | The Dawn of Animal Life: A Biohistorical Study[END_REF][START_REF] Grant | Shell structure and distribution of Cloudina a potential index fossil for the terminal Proterozoic[END_REF][START_REF] Hua | Borings in Cloudina shells -complex predator-prey dynamics in the terminal Neoproterozoic[END_REF], Namacalathus [START_REF] Grotzinger | Calcified metazoans in thrombolitestromatolite reefs of the terminal Proterozoic Nama Group, Namibia[END_REF], and Conotubus [START_REF] Hua | Origin of skeletonization in Meatzoa[END_REF], nearly all the skeletal creatures from the early Cambrian have never been reported in the Ediacaran.

It should be clear that if there had been any early Cambrian SSFs-like animals existing in the Precambrian, they should have easily been preserved in the fossil record. Such an obvious and big gap between life in the Precambrian and Cambrian has forced us to draw the conclusion that the Cambrian Explosion is a genuine evolutionary event (Shu, 2008).

When looking at the proceedings of the Cambrian Explosion in detail, two or three major episodes can be recognized (Fig. 17), if the Ediacaran fauna is considered as being part of the Cambrian Explosion [START_REF] Balavoine | One or Three Cambrian Radiations?[END_REF]. These three steps would be responsible for the earliest radiations of the three subkingdoms of animals respectively. The observed Ediacaran fauna was dominated by diploblasts (the "trunk" of the TOA) with only a few possible stem-group triploblasts. Then, two other episodes happened in the early Early Cambrian, when almost all the major triploblastic crown-branches appear: one represented by the small skeletal fossils "SSFs" at the base of the Cambrian, which yielded the extremely diverse protostomes in the Meishucunian Age, and the other represented by the succeeding Chengjiang and contemporaneous Sirius Passet faunas, a nearly entire lineage of early deuterostomes including even its most derived member -the earliest true vertebrates (Shu, 2008). Endless debates on the tempo of this event occured, whether it was a real explosion or a slowly burning fuse (Conway Morris, 2000a,b;[START_REF] Fortey | The Cambrian explosion exploded?[END_REF][START_REF] Fortey | Phylogenetic fuses and evolutionary explosions; conflicting evidence and critical tests[END_REF]. On one hand, [START_REF] Shu | Birth and early evolution of metazoans[END_REF] noted that even with taphonomic bias, the Cambrian Explosion sensu stricto was the rapid bilaterian phenotype diversification, which occurred in the first 20 million years of the Early Cambrian, and peaked in the Cambrian Stage 3. If the first abundant trace fossils are taken into account, then the radiation ran for some 65 million years [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF].

Historical Background and questions left

The sudden appearance of early animals was recognized at least as early as the time of William Buckland, Oxford's first principal incumbent of palaeontology in 1830s (Fig. 18A; [START_REF] Buckland | Geology and mineralogy considered with reference to natural theology[END_REF]from Conway Morris, 1998b). By 1859 this problem had been even more clearly articulated by Charles Darwin in his famous "On the Origin of species" (Fig. 18B).

This extraordinary event subsequently was widely realized as amongst the most striking of episodes in the history of life, and has been aptly nicknamed the " Cambrian Explosion" (Fig. 18C; [START_REF] Cloud | Some problem and patterns of evolution exemplified by fossil invertebrates[END_REF]. This explosive evolutionary event was recognized as being so significant that it served as the first-rank landmark dividing Earth history into two major divisions (eons), the time of visible life, that is the Phanerozoic and the preceding Precambrian [START_REF] Chadwick | Subdivision of geologic time[END_REF].

Some may say that discussion of "Cambrian Explosion" started when this unique biological event was the cover story, titled "Evolution's Big Bang," of the American news magazine Time at the end of the year 1995 (Nash; Fig. 18D). Time's story probably reflected the worldwide influence of the best seller "Wonderful Life" (1989) by the late S.J. Gould (Fig. 18E). Nowadays, the term Cambrian Explosion is so famous that its use goes far beyond the frontier of palaeontology, for example in management (Fig. 18F; Hammel, 1998;[START_REF] Keith Denton | Gaining competitiveness through innovation[END_REF], or in robotics (Costa et al., 1999;[START_REF] Pratt | Is a Cambrian Explosion Coming for Robotics[END_REF]. (1859). C, [START_REF] Cloud | Some problem and patterns of evolution exemplified by fossil invertebrates[END_REF]. D, [START_REF] Nash | When Life Exploded[END_REF]. E, Gould (1989). F, [START_REF] Pratt | Is a Cambrian Explosion Coming for Robotics[END_REF].

Though known and studied since the early 19 th century, the Cambrian Explosion raises several questions that still remain controversial today. Most of the disagreements (Runnegar, 1982a;b;[START_REF] Signor | Origin and early radiation of the Metazoa[END_REF][START_REF] Valentine | On the Origin of Phyla[END_REF]Shu, 2008) center on: the length of the hidden history of metazoans, i.e before their appearance in the fossil record, the triggers initiating the Cambrian Explosion, and its relationship (together with the Ediacara Biota) with the formation of the T0A.

Additionnaly, there are five major components of the Cambrian Explosion that require explanations [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]: (a) the spectacular increase in animal disparity, (b) the rise in animal diversity, (c) why the time of onset of the explosion was some 543-542 mya, (d) why the duration of the explosion was some tens of million years long, and (e) why this event appears unique.

4.3

The fossil record

Trace fossils

The impact of bioturbation in deep time has recently received increasing interest, in particular in relationship to the Ediacaran-Cambrian transition [START_REF] Mcilroy | The Impact of Bioturbation on Infaunal Ecology and Evolution during the Proterozoic-Cambrian Transition[END_REF]. Indeed, burrowing abilities are probably one of the distinguishing features of the bilaterian clade [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. Although some diploblastic cnidarians can burrow [START_REF] Collins | Modern mucociliary creeping trails and the bodyplans of Neoproterozoic trace-makers[END_REF], especially ceriantharian [START_REF] Jensen | Cerianthus vogti Danielssen, 1890 (Anthozoa: Ceriantharia). A species inhabiting an extended tube system deeply buried in deep-sea sediments off Norway[END_REF] and actinarians [START_REF] Durden | The hemisessile lifestyle and feeding strategies of Iosactis vagabunda (Actiniaria, Iosactiidae), a dominant megafaunal species of the Porcupine Abyssal Plain[END_REF], they generally make simple vertical structures at the exception of potential complex cnidarian burrows claimed by [START_REF] Bradley | Scolicia and Phycodes trace fossils of Renilla (Pennatulacea)[END_REF]1981), which are controversial and were refuted by [START_REF] Lewy | Distinguishing trace fossils and similar-shaped body fossils using sedimentological criteria -an example from the Miocene of New Zealand[END_REF]. Moreover, complex and large burrows generally reflect elaborate triploblastic anatomy, including a coelom or other hydrostatic body cavity and a concentration of sensory organs (e.g. [START_REF] Budd | A critical reappraisal of the fossil record of the bilaterian phyla[END_REF][START_REF] Droser | Trace fossils and substrates of the terminal Proterozoic-Cambrian transition: implications for the record of early bilaterians and sediment mixing[END_REF]. In this regard, radical difference between typical Ediacaran and Cambrian trace fossils, both in size and complexity (Fig. 19A; [START_REF] Droser | Trace fossils and substrates of the terminal Proterozoic-Cambrian transition: implications for the record of early bilaterians and sediment mixing[END_REF], is informative about bilaterian evolution, which seems to be irreconcilable with deep divergence times for the major bilaterian clades [START_REF] Budd | A critical reappraisal of the fossil record of the bilaterian phyla[END_REF]2003). Moreover, the evidence available has shown that the diversification of trace fossils basically parallels but does not precede the diversification of shelly fossils [START_REF] Crimes | Trace fossils[END_REF]1992). The first complex three-dimensional traces, that reflect vertical as well as horizontal movements in the substrate [START_REF] Jensen | A Critical Look at the Ediacaran Trace Fossil Record[END_REF], belong to Treptichnus pedum [START_REF] Droser | When the worm turned: concordance of Early Cambrian ichnofabric and trace-fossil record in siliciclastic rocks of South Australia[END_REF]. The oldest occurrence of the taxon is in Newfoundland and lies just a short distance below the formally defined base of the Cambrian [START_REF] Gehling | Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland[END_REF],

probably around 545 to 543 Ma [START_REF] Jensen | The Proterozoic and Earliest Cambrian Trace Fossil Record; Patterns, Problems and Perspectives[END_REF][START_REF] Mángano | Decoupling of body-plan diversification and ecological structuring during the Ediacaran-Cambrian transition: evolutionary and geobiological feedbacks[END_REF]. This would be thus the latest time that at least stem-group bilaterians could have emerged. Oldest potential trace fossils come from the 1.2-billion-year-old Stirling Range, Western Australia [START_REF] Rasmussen | Discoidal impressions and trace-like fossils more than 1200 million years old[END_REF] but they remain controversial. If confirmed in terms of their nature and age, the history of the stem group metazoans (but not, of course, the crown-group metazoans) could be extended back to as early as the Mesoproterozoic.

Bioturbation would have had profound taphonomic and ecological repercussions (Fig. 19B), for example on fossil preservation [START_REF] Brasier | Evolutionary Trends in Remarkable Fossil Preservation Across the Ediacaran-Cambrian Transition and the Impact of Metazoan Mixing[END_REF], as well as on oxygen and phosphorous cycles [START_REF] Boyle | Stabilization of the coupled oxygen and phosphorus cycles by the evolution of bioturbation[END_REF]. Before burrowing emerged, microbial mats used to cover large surface of the sea floor (see 2.5), as supported by the prevalence in Precambrian siliciclastic sediments of sedimentary structures that have been interpreted as being induced by or related to such biomats [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. The suggestion has been made that the early evolution of animals would have been more or less intimately related to these microbial mats either as anchoring substrates or as a food source (e.g. [START_REF] Stanley | An ecological theory for the sudden origin of multicellular life in the late Precambrian[END_REF][START_REF] Gingras | Possible evolution of mobile animals in association with microbial mats[END_REF][START_REF] Meyer | Interactions between Ediacaran animals and microbial mats: Insights from Lamonte trevallis, a new trace fossil from the Dengying Formation of South China[END_REF]. The sealing effect of these mats, made especially effective by the production of large amounts of mucilage by cyanobacteria (Stal, 2012, p. 115) would have led to much sharper geochemical gradients in the sea floor compared to those in most modern sea floors. Such gradients expand, and become spatially more complex, with the introduction of bioturbation, in turn making deeper sediments of greater interest to deposit-feeders [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. The induced change from a 'matground' to a 'mix-ground' has been referred to as an agronomic revolution [START_REF] Seilacher | From biomats to benthic agriculture: A biohistoric revolution[END_REF], and has been seen as an early example of ecosystem engineering (e.g. [START_REF] Meysman | Bioturbation: a fresh look at Darwin's last idea[END_REF][START_REF] Marenco | 8 Ecosystem engineering in the fossil record: Early examples from the Cambrian Period[END_REF]. Although there is a continuous record of discrete trace fossils from at least 560Ma, firm evidence for effective sediment mixing only appears well into the lower Cambrian strata (e.g. [START_REF] Mángano | Decoupling of body-plan diversification and ecological structuring during the Ediacaran-Cambrian transition: evolutionary and geobiological feedbacks[END_REF]. Other potential well documented example of extensive latest Ediacaran sediment mixing has been reported from the Khatyspyt Formation of Siberia [START_REF] Rogov | The oldest evidence of bioturbation on Earth[END_REF], but remains under discussion as an alternative interpretation of body fossils must be considered [START_REF] Brasier | The oldest evidence of bioturbation on Earth: COMMENT[END_REF][START_REF] Vintaned | The oldest evidence of bioturbation on Earth: COMMENT[END_REF]Rogov et al, 2013a;b). [START_REF] Mángano | Decoupling of body-plan diversification and ecological structuring during the Ediacaran-Cambrian transition: evolutionary and geobiological feedbacks[END_REF]. Note deepening of the redox discontinuity surface (RDS) and complex feedback loops.

Cambrian communities

a) The Small Shelly Fauna The earliest Cambrian faunas were dominated by microscopic fossils, many of which are hard to diagnose, even at the phylum level. Thus, discussions of the body fossil record of the Cambrian must start with these largely problematic fossils. Indeed, though some of the described genera belong to known phyla such as Brachiopoda and Mollusca, many are problematics, including the cambroclaves, coeloscleritopherans, cribricyatheans, machaeridians, tommotiids, as well as a diverse array of incertae sedis [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF].

The earliest Cambrian, corresponding to the early Terreneuvian or Fortunian stage (equivalent to the Manykaian/Nemakit-Daldynian ; 541 to 530 Ma), which until relatively recently was informally considered part of the Precambrian [START_REF] Landing | Precambrian-Cambrian boundary global stratotype ratified and a new perspective of Cambrian time[END_REF], is marked by the first appearance of an array of plates, spines, shells, and other skeletal elements of bilaterian affinity [START_REF] Maloof | The earliest Cambrian record of animals and ocean geochemical change[END_REF][START_REF] Kouchinsky | Chronology of early Cambrian biomineralization[END_REF]. By the succeeding Late [START_REF] Morris | Articulated halkieriids from the Lower Cambrian of north Greenland and their role in early protostome evolution[END_REF]. They reveal a fauna with considerable morphologic and phylogenetic diversity and are collectively referred to as the "small shelly fauna" (SSF; Fig. 20), as defined by S. Matthews and V. V. Missarzhevsky (1975). The earliest SSF are largely of lophotrochozoan affinities [START_REF] Kouchinsky | Chronology of early Cambrian biomineralization[END_REF].

Many of the SSF elements are preserved as phosphate minerals, and their diversity peak is reached in abundant phosphate deposits [START_REF] Porter | Closing the Phosphatization Window: Testing for the Influence of Taphonomic Megabias on the Pattern of Small Shelly Fossil Decline[END_REF]. Although Ediacaran phosphate deposits are common, they lack SSF, suggesting that bilaterian clades acquired skeletons during the Cambrian (Erwin et al., 2011).

b) The macrofossils and the importance of Lagerstätten It is not until the Cambrian Series 2 Stage 3 (commonly refered to as the Atdabanian; 521 to 514 Ma) that we find the first abundant body fossils that most would immediately recognize as belonging to typical Phanerozoic taxa, such as the first trilobites and echinoderms [START_REF] Zhuravlev | The Ecology of the Cambrian Radiation[END_REF]. The first window of exceptional preservation, known as the Chengjiang biota (Yunnan, China), occured in this stage [START_REF] Hou | Composition and preservation of the Chengjiang fauna-a Lower Cambrian soft-bodied biota[END_REF]2004). Another famous fossiliferous locality, known as the Burgess Shale (Whittington, 1985;Briggs et al. 1994), occurs in the Cambrian Series 3 Stage 5 (509 to 504.5 Ma) of Canada. However, at that time, the increases in disparity and diversity that characterize the Cambrian Explosion are largely over [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. Nevertheless, many crown group higher taxa are first found in the Middle and Late Cambrian and beyond [START_REF] Budd | A critical reappraisal of the fossil record of the bilaterian phyla[END_REF].

Cambrian marine communities were dominated by arthropods, including trilobites, and by sponges, which together accounted for more than half of the species. Other invertebrates included annelids (polychaetes), brachiopods, hemichordates and priapulids.

Rare chordates were also present, including some interpreted as potential fish (Briggs, 2015a). This Cambrian world has become familiar through reconstructions of sea floor communities by instance in the book of Stephen Jay Gould (1989), entitled Wonderful Life, which referred to Wiwaxia, Hallucigenia, Opabinia, and Anomalocaris.

Our picture of marine diversity through geologic time is based mainly on the relatively continuous fossil record of animals with shells. Though, shelly fossils reveal the steep rise in diversity that is the Cambrian explosion, the full cast of characters is seen only in settings that preserve evidence of soft-parts (Briggs, 2015a). These peculiar localities, where not only hard but also soft parts are preserved, were named Konservat-Lagerstätten by Seilacher (1970). They will be termed Lagerstätten below.

The Cambrian is remarkable compared to younger geological periods for its unusually high number-more than 50 (Fig. 21; Briggs, 2015a)-of marine Lagerstätten [START_REF] Allison | Exceptional fossil record: Distribution of soft-tissue preservation through the Phanerozoic[END_REF]Butterfield, 1995). The abundance of exceptional preservations was a conundrum until investigations of the chemistry of the associated sedimentary rocks provided a possible explanation (see [START_REF] Gaines | Burgess Shale-type preservation and its distribution in space and time[END_REF]. It is generally accepted that the Cambrian seawater is characterized by a low sulfate concentration of ca. 5-12 mM [START_REF] Brennan | Seawater chemistry and the advent of biocalcification[END_REF][START_REF] Hough | A major sulphur isotope event at c. 510 Ma: a possible anoxia-extinction-volcanism connection during the Early-Middle Cambrian transition?[END_REF][START_REF] Gill | Parallel, high-resolution carbon and sulfur isotope records of the evolving Paleozoic marine sulfur reservoir[END_REF][START_REF] Hurtgen | Evaluating the relationship between the carbon and sulfur cycles in the later Cambrian ocean: An example from the Port au Port Group, western Newfoundland, Canada[END_REF][START_REF] Wotte | Paired δ34S data from carbonate-associated sulfate and chromium-reducible sulfur across the traditional Lower-Middle Cambrian boundary of W-Gondwana[END_REF], probably suppressing decay bacteria in sediments (Gaines et al., 2005). More importantly, perhaps, Cambrian seawater was unusually high in alkalis, which promoted early consolidation of the sediment, sealing off buried carcasses (Gaines et al., 2012a,b;Peters and Gaines, 2012;[START_REF] Gaines | Burgess Shale-type preservation and its distribution in space and time[END_REF].

The chemistry of the Cambrian oceans apparently favored exceptional preservation across the globe (Briggs, 2015a). Morris, 1990;[START_REF] Hagadorn | Burgess Shale-type localities: the global picture[END_REF][START_REF] Retallack | Exceptional fossil preservation during CO2 greenhouse crises? Palaeogeography[END_REF]. Locations are indicated by red stars.

These fortunate circumstances mean that we have an unusually complete window on the marine forms that evolved at this critical juncture in the history of animal life -during the Cambrian explosion (Fig. 22; Conway [START_REF] Morris | [END_REF]. Indeed, some 19 (~60%) of the ~33 phyla recognized today lack readily preserved hard parts and are soft-bodied (Erwinn et al., 2011;Briggs, 2015b). Cambrian Lagerstätten, such as the famous Burgess Shale, yield examples of 14 of the 19 soft-bodied phyla, the other five being potentially too fragile to be preserved (Conway Morris, 2000;Briggs, 2015a,b). It is estimated from these that in the early metazoan communities only about one tenth of the total taxa possessed hard skeletons or mineralized shells (Conway- Morris, 1986). (Smith and Ortega, 2014;Smith and Caron, 2015) as a possible stemgroup onychophoran with pointed sclerites on its back. F, anomalocaridid (juvenile) showing prehensile frontal appendages. G, Waptia fieldensis, an arthropod with a flexible abdomen and wellpreserved head and trunk appendages (Vannier et al. in preparation and Caron and Vannier 2016). H-J, Sidneyia inexpectans, an arthropod in general view with details of its gut contents (e.g. trilobite elements) and trunk appendage (see gnathobase) (from [START_REF] Zacaï | Reconstructing the diet of a 505-millionyear-old arthropod: Sidneyia inexpectans from the Burgess Shale fauna[END_REF] Erdtmann, 1991;[START_REF] Briggs | Wonderful Strife: Systematics, Stem Groups, and the Phylogenetic Signal of the Cambrian Radiation[END_REF][START_REF] Smith | Primitive soft-bodied cephalopods from the Cambrian[END_REF][START_REF] Morris | Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia[END_REF]. It is not thus surprising that since its discovery more than a century ago in

4.4

Potential triggers of the Cambrian Explosion

Developing a coherent explanation for the Cambrian "explosion" faces several challenges.

First, most workers interested in the Cambrian "explosion" study this multidisciplinary problem through the pirsm of their own fields of activity (i.e.

palaeontology, geology, geochemistry, ecology, climate modeling, developmental biology, etc…). This difficulty to give a balanced answer is exacerbated by the fact that different subdisciplines use various approaches for exploring the unknown [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. For example, explanations that flow from the geological record typically focus on events that occurred just before or at the Precambrian/Cambrian boundary, and thus tend to be correlative rather than causal in nature. Indeed, such approaches are unable to explain the causal relationships between the proposed triggering events or changes and the observed radiation [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. On the other hand, ecological or developmental explanations are more obviously evolutionary in nature and so typically give a plausible causal relation between them and the biological radiation seen in the fossil record. However, these explanations are usually disconnected from the geologic record and thus from the field observations [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. Part of the difficulty in developing an explanation for the Cambrian Explosion also stems from the fact that it results from a complex interaction of feedbacks. Indeed, each successive step not only flows from the conditions established in the previous steps but also is shaped by interactions at the current step of the unfolding event [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. For example, the cropping pinciple [START_REF] Stanley | An ecological theory for the sudden origin of multicellular life in the late Precambrian[END_REF][START_REF] Stanley | Ideas on the timing of metazoan diversification[END_REF] suggests in its general form that the introduction of predation into a low-diversity ecosystem will create a self-propagating feedback system of diversification [START_REF] Bengtson | Origins and early evolution of predation[END_REF], such as an enormous selective pressure on the acquisition of skeletons [START_REF] Mcmenamin | The garden of Ediacara[END_REF][START_REF] Morris | preserved articulated body remains of variety of taxa in the early lowest metazoans can be observed, but also many soft-bodied fossils in all the three subkingdoms of the Eumetazoa[END_REF].

In this way, a great number of causes, or at least necessary conditions to explain the Cambrian Explosion have been suggested (Fig. 24; see Zhang et al., 2014 for a comprehensive review). To investigate them, Signor and Lipps (1992) asked 14 questions, which were later summarized in 6 points by [START_REF] Mccall | The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion[END_REF]: the timing and reason of the emergence of the first Metazoa, the time lapse between the appearance of Monerans (Bacteria and Archeaa) and the appearance of the Metazoa, the potential hidden history of the Metazoa due to environmental or taphonomic factors, the physical conditions in the early Earth and their consequences on the evolution of Metazoa, a possible change in the tempo of evolution concurrent with the evolution of the Metazoa, and finally, the nature of evolution as a directional or a steady-state process. To answer those questions, the former authors tabulated respectively the extrinsic factors and intrinsic factors, which had been suggested by various authorities (Shu, 2008). Typically, explanations center on one of the following factors:

changes in the abiotic environment, changes in the genetic or developmental capacity of the taxa involved, or changes in the biotic environment, i.e., in ecology [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF].

Nevertheless, it is most probable that several of those factors were interrelated (Shu, 2008).

Consequently, most authors agree with the fact that the drivers behind the Cambrian Explosion were varied and complex, so that there is no simple explanation of either the timing or ecological mechanism involved (Briggs, 2015). Here, we will review briefly the main factors, which could have together participated to this exceptional and unique event in the history of life. G, Guzhangian; HSP, heat shock protein; PDA, protostome-deuterostome ancestor.

Extrinsic causes

The extrinsic causes mainly include main environmental changes, such as lack of sufficient oxygen, increased carbonate and change in ocean chemistry, changes in palaeogeography, glaciations, or sea level variation (as summarized by [START_REF] Signor | Origin and early radiation of the Metazoa[END_REF][START_REF] Mccall | The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion[END_REF].

According to [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF], this class of explanation benefited from a renewed interest, partly owing to increased interest in the Cambrian Explosion within the wider geoscience community. He underlined two subclasses of environmental explanation. First, those that point to the fact that the environment had to be favorable to complex animal life so that the latter could emerge. The second class of environmental explanations is often motivated by the discovery of some unusual, large-scale, environmental change that occurred at about the time of the initiation of the radiation.

In this context, very large geochemical changes have been documented through the Cryogenian and Ediacaran [START_REF] Grotzinger | Enigmatic origin of the largest-known carbon isotope excursion in Earth's history[END_REF][START_REF] Shields-Zhou | The case for a Neoproterozoic oxygenation event: geochemical evidence and biological consequences[END_REF][START_REF] Fike | Oxidation of the Ediacaran ocean[END_REF], which have been interpreted as indicating substantial changes in redox conditions. For example, changes in molybdenum abundance in black shales [START_REF] Scott | Tracing the stepwise oxygenation of the Proterozoic ocean[END_REF], the iron chemistry of deep-water sediments [START_REF] Canfield | Late-Neoproterozoic deep-ocean oxygenation and the rise of animal life[END_REF], and potentially other proxies [START_REF] Shields-Zhou | The case for a Neoproterozoic oxygenation event: geochemical evidence and biological consequences[END_REF] have been interpreted as a global signal of increased oxygenation during the Ediacaran. The extent to which these signals are truly global, as well as the magnitude of oxidation, remains uncertain (Erwin et al., 2011). However, the global presence of an unusually large negative carbon isotopic anomaly, of between 7 to 9 per mil, at the Neoproterozoic/Cambrian boundary (see fig. 1 in [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF] points to an environmental disturbance at a critical time in Earth history, possibly playing a role in the Cambrian Explosion [START_REF] Knoll | Early animal evolution: emerging views from comparative biology and geology[END_REF][START_REF] Amthor | Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman[END_REF]. While there is no generally accepted explanation for the cause of the anomaly (but see Rothman et al. 2003), it is has been assumed that it represents a sufficiently large environmental disturbance that it caused a mass extinction of the Ediacaran biota, and in doing so laid the foundation for Cambrian radiation (Knoll andCarroll 1999, Amthor et al. 2003 ; see the 3.6.1 for the analogy with the fall of dinosaurs and rise of mammals). In fact, the real question is whether the carbon isotopic anomaly might be independent of the unfolding Cambrian Explosion, or could be one of its consequence [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. When the diversity of animals was still relatively low in the Early Cambrian, the carbon isotopic values show an unusual volatility (see Fig. 1 in [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. Interestingly, a similar volatility is also seen in the Early Triassic (Payne et al. 2004), a time period also characterized by a low diversity because of the end-Permian mass extinction. A controversial attempt to explain this unusual carbon isotopic record of the Cambrian derived from the hypothesis of Cambrian true polar wander [START_REF] Kirschvink | Evidence for a large-scale reorganization of Early Cambrian continental masses by inertial interchange true polar wander[END_REF]. [START_REF] Kirschvink | A methane fuse for the Cambrian explosion: carbon cycles and true polar wander[END_REF] have linked this hypothesis to the unusual carbon isotopic record of the Cambrian, proposing that the rapid movement of the continents (on geological timescales) caused huge methane releases, possibly triggering the Cambrian Explosion. Indeed, the resulting increase in temperatures would have driven the observed increase in diversity, but does not explain the contemporaneous increase in disparity [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF].

Another important point is the oxygen level in the early atmosphere. It seems that the early atmosphere of Earth lacked available oxygen until at least the early Palaeoproterozoic [START_REF] Goecke & Evers | The oldest fossils and what they mean[END_REF]. This question is crucial, since available oxygen is considered as one of the necessary conditions for large and complexe animals to emerge [START_REF] Cloud | Pre-Metazoan evolution and the origins of the Metazoa[END_REF]Runnegar, 1982c;[START_REF] Knoll | Early animal evolution: emerging views from comparative biology and geology[END_REF][START_REF] Budd | A critical reappraisal of the fossil record of the bilaterian phyla[END_REF][START_REF] Knoll | The multiple origins of complex multicellularity[END_REF]. In this regard, the Cambrian Explosion would have been triggered by the accumulation of enough oxygen to support large animals [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. This scenario is supported by interpretations of the Proterozoic sedimentary record, and particularly by some recent data, which demonstrate that stem group metazoans were components of earliest Ediacaran ecosystems (see 3.5.2), later expanding markedly, perhaps in association with rising oxygen level [START_REF] Knoll | Early animal evolution: emerging views from comparative biology and geology[END_REF][START_REF] Canfield | Late Neoproterozoic deepocean oxygenation and the rise of animal life[END_REF][START_REF] Fike | Oxidation of the Ediacaran ocean[END_REF][START_REF] Yin | Doushantuo embryos preserved inside diapause egg cysts[END_REF]. Nevertheless, this hypothesis is hard to test, especially because the minimum oxygen requirements of the first animals and the values of Proterozoic oxygen levels are not firmly constrained [START_REF] Canfield | The early history of atmospheric oxygen: homage to Robert M. Garrels[END_REF]. It is commonly accepted though, that there was sufficient oxygen to support large animals well before the Cambrian Explosion [START_REF] Knoll | Early animal evolution: emerging views from comparative biology and geology[END_REF]. If this is true, then the question still remains concerning the intrasic or extrinsic nature of the appreciable 30-million-years gap between the emergence of the Ediacaran taxa and the first skeletonized organisms, which only appeared lately, mostly at the beginning the Cambrian. Without taking into account the fact that it is still an open question as to whether there was a single oxygen threshold that needed to be crossed for the emergence of the bilaterian phyla, or whether there was a series of thresholds that had to be crossed [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. However, a recent study of polychaete feeding strategies in modern environments suggests that there may be a direct link between critical oxygen thresholds, the rise of larger carnivores, and the evolution of complex food webs [START_REF] Sperling | Oxygen, ecology, and the Cambrian radiation of animals[END_REF][START_REF] Jumars | Diet of worms emended: an update of polychaete feeding guilds[END_REF]. Indeed, the abundance and diversity of carnivorous polychaetes today is greater where oxygen levels are higher [START_REF] Sperling | Oxygen, ecology, and the Cambrian radiation of animals[END_REF]Briggs, 2015).

Other environmental causes that could have triggered the Cambrian Explosion stem from the significant global deep glaciations during the late Neoproterozoic, known as the Snowball Earths [START_REF] Kirschvink | Late Proterozoic low-latitude global glaciation: the snowball Earth[END_REF][START_REF] Hoffman | A Neoproterozoic snowball Earth[END_REF]see discussion in Peterson et al., 2005).

Indeed, those events happened during the Marinoan (635 Ma) and the Gaskiers (580 Ma), a bit before the first unambiguous and continuous fossil record of animals (Fig. 17). However, though this coincidence is certainly suggestive, these events occured too early to explain the Cambrian Explosion on their own. Moreover, the causality between major environmental catastrophes and new levels of developmental and morphological organization is far from being obvious. However, repeated meteorite impacts have been invoked to explain the Ordovician biodiversification [START_REF] Schmitz | Asteroid breakup linked to the Great Ordovician Biodiversification Event[END_REF], as well as the Acraman impact event (c.

580 Ma) has been suggested as being significant in the evolution of the early metazoans [START_REF] Grey | Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact?[END_REF]. A potential explanation is that the significant sea level drop induced by the glaciations would have exposed large landmasses, which could have provided large amount of nutrients through erosion processes due to the break up of the supercontinent Rodinia [START_REF] Valentine | Global tectonics and the fossil record[END_REF][START_REF] Maruyama | Models on Snowball Earth and Cambrian explosion: A synopsis[END_REF]. This increase in nutrient supply could have stimulated the Cambrian Explosion.

In summary, it is unclear how the time of the perturbation (Snowball Earths or onset of the unsettled carbon cycle in the Cambrian) relates to the Cambrian Explosion, and how it might explain either its duration or uniqueness. Moreover, while it is clear that the environment needed to be hospitable for the Cambrian Explosion to proceed, none of the environmental explanations addressed above can explain why it would have necessarily led to the evolution of complex animal life.

Intrinsic causes

On the other hand, the intrinsic causes mainly include developmental causes and ecological escalation, such as genetic causes, acquisition of a sedentary habit with resulting competition, induced development of large and morphologically complex organisms, invasion of habits where preservation could occur, or advent of predators/croppers (as summarized by [START_REF] Signor | Origin and early radiation of the Metazoa[END_REF][START_REF] Mccall | The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion[END_REF].

Without doubt, the internal evolutionary processes may well have been more important in initiating the Cambrian Explosion than environmental causes (Shu, 2008), as animals could not have evolved if the genes for making them were not yet in place [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. A

probably key innovation in the evolution of Metazoa was the evolution of regulatory genes or the evolution of complex genetic mechanisms to regulate development [START_REF] Morris | preserved articulated body remains of variety of taxa in the early lowest metazoans can be observed, but also many soft-bodied fossils in all the three subkingdoms of the Eumetazoa[END_REF][START_REF] Holland | Major transitions in animal evolution: a developmental genetic perspective[END_REF][START_REF] Holland | Did homeobox gene duplications contribute to the Cambrian explosion[END_REF][START_REF] Valentine | On the Origin of Phyla[END_REF].

There is an ongoing revolution in our understanding of the genetic basis of morphological form [START_REF] Carroll | From DNA to Diversity[END_REF][START_REF] Davidson | Genomic regulatory systems: development and evolution[END_REF]. First, whole-genome sequencing of dozens of metazoans has demonstrated that any animal requires only about 20,000 protein-coding genes for the production of its essential morphologic architecture [START_REF] Erwin | Early origin of the bilaterian developmental toolkit[END_REF]. Second, much of this protein-coding repertoire-especially the developmental toolkit-is conserved throughout all metazoans and is even found today among single-celled opisthokonts [START_REF] Knoll | The multiple origins of complex multicellularity[END_REF][START_REF] Sebé-Pedrós | Unexpected repertoire of metazoan transcription factors in the unicellular holozoan Capsaspora owczarzaki[END_REF][START_REF] Richards | The dawn of developmental signaling in the Metazoa[END_REF][START_REF] Srivastava | The Amphimedon queenslandica genome and the evolution of animal complexity[END_REF]. Indeed, comparative development genetics suggest that while the animal phyla might be much different morphogically, they are fundamentally comparable genetically (Erwin et al., 2011). As the genomes of diploblasts and sponges are explored, it is also becoming clear that many developpmental genes predate the first bilaterians (e.g., [START_REF] Erwin | The last common bilaterian ancestor[END_REF][START_REF] Finnerty | Origins of bilateral symmetry: Hox and dpp expression in a sea anemone[END_REF]. The distribution of these genes in extant organisms implies that this toolkit evolved in a two-step pattern: an initial diversification occurring at the base of the Metazoa before the split between sponges and eumetazoans deep in the Cryogenian (and possibly earlier), followed by a pronounced expansion at least in some families at the base of the Eumetazoa during the late Cryogenian [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. Thus, the last common ancestor of metazoans, and especially eumetazoans, was a genetically complex animal possessing all of the families of protein-coding genes used during development, save for the potential absence of Hox complex genes [START_REF] Ryan | The cnidarianbilaterian ancestor possessed at least 56 homeoboxes: evidence from the starlet sea anemone, Nematostella vectensis[END_REF] needed to build the plethora of morphological structures found throughout the crown group. Indeed, at the core of these networks are extremely conserved, highly refractory and recursively wired suites of genes that are crucial for the specification of many of the characteristic morphologies of major clades [START_REF] Davidson | Gene regulatory networks and the evolution of animal body plans[END_REF]2009), and ultimately defining the "developmental morphospace" [START_REF] Davidson | An integrated view of Precambrian eumetazoan evolution[END_REF] accessible to a clade. [START_REF] Valentine | Fossil record of the origin of Baupläne and its implications[END_REF], among many others, argued that early in the Phanerozoic, development was less constrained, or canalized, than it is now. A similar notion flows from [START_REF] Wimsatt | Developmental constraints, generative entrenchment, and the innateacquired distinction[END_REF] discussion of the developmental lock. The common thread in this literature (see [START_REF] Raff | The shape of life: genes, development, and the evolution of animal form[END_REF] for a deeply informed discussion) is the idea that as evolution proceeds, the developmental stages responsible for laying down the body plans are overlain with down-stream genetic pathways, making it progressively harder to modify those earlier pathways; the developmental stages when the phylum-level body plans are laid down become entrenched and the body plans become frozen in place [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. However, it is difficult to know if this increasing genetic constraint is not a consequence of a growing ecological and functional specialisation [START_REF] Hughes | The stability of thoracic segmentation in trilobites: a case study in developmental and ecological constraints[END_REF].

In this context, the origin of this bilaterian developmental system, including the origin of the Hox genes, is commonly identified as the primary cause of the Cambrian Explosion (e.g., [START_REF] Peterson | Regulatory evolution and the origin of the bilaterians[END_REF] but see [START_REF] Peterson | Tempo and mode of early animal evolution: inferences from rocks, Hox and molecular clocks[END_REF][START_REF] Erwin | The last common bilaterian ancestor[END_REF].

However, how this system originated remains poorly known, but clearly gene duplication was a major trigger [START_REF] Carroll | From DNA to Diversity[END_REF] enabling the origin of the Hox cluster from the paraHox cluster of genes. Probably also important in terms of morphological innovation is the combinatorial nature of the animal developmental system [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. Indeed, changes in the way the genes are wired is a major source of morphological innovation (e.g., see [START_REF] Averof | Crustacean appendage evolution associated with changes in Hox gene expression[END_REF][START_REF] Carroll | From DNA to Diversity[END_REF][START_REF] Davidson | Genomic regulatory systems: development and evolution[END_REF]. For example, Solé et al.

( 2003), in a reference to the Cambrian Explosion, show how small increases in the complexity of a combinatorial developmental system can lead to an extraordinary range of stable spatial patterns of gene expression, and thus to rich morphogenetic potential.

This conceptualization is reflected in [START_REF] Wolfram | A new kind of science[END_REF] by the idea that much of biological complexity probably arises from the operation of relatively simple rules within large numbers of cellular automata (=cells). In addition, because the signaling pathways and transcription factors important for bilaterian development first appeared among basal metazoan clades that originated in the Cryogenian, the advent of elements of the metazoan developmental toolkit was a necessary but not sufficient component of the Cambrian explosion [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF].

The temporal lag between the initial construction of this debeloppmental toolkit and the eventual appearance of bilaterian fossils suggests that the solution to the dilemma of the Cambrian explosion lies not solely within this genomic and developmental potential, but instead must also be found in the ecology of the Cambrian Explosion itself [START_REF] Budd | Arthropod body-plan evolution in the Cambrian with an example from anomalocaridid muscle[END_REF](Budd, , 2001)). Indeed, the Cambrian explosion involved the construction of historically unique, and

uniquely complex feedbacks between biological potential and eco-environmental context, including the oxygenation of the ocean's waters [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. These feedbacks relied on networks of gene regulatory interaction that were established long before the construction of metazoan ecosystems. Because of this long lag between the origin and eventual ecological dominance of clades, data on taxonomic occurrences alone are insufficient to understand evolutionary dynamics and must be accompanied by data on abundances and ecological impact, in addition to accurate and precise estimates of both evolutionary origin and geological first appearances [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF].

Macroevolutionary lags such as that which preceded the Cambrian explosion were not unique to animals, as similar dynamics seem to underlie plant evolution as well [START_REF] Knoll | The multiple origins of complex multicellularity[END_REF]. In this regard, a promising avenue of research may lie within the theoretical model of landscape fitness (for a comprehensive review see [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. However satisfactory ecological explanations (e.g., [START_REF] Zhuravlev | Biotic diversity and structure during the Neoproterozoic-Ordovician transition[END_REF] have been overshadowed in the recent literature by the developmental and environmental explanations [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. This probably reflects a domination of the Court Jester hypothesis, which regards physical-environmental perturbations as the most important drivers in evolutionary change [START_REF] Barnosky | Distinguishing the effects of the Red Queen and Court Jester on Miocene mammal evolution in the northern Rocky Mountains[END_REF], in the scientific community.

Ecological-based explanations of radiations are often based on the invasion of preexisting empty theoretical niche space [START_REF] Valentine | Determinants of diversity in higher taxonomic categories[END_REF][START_REF] Valentine | Diversity trends within a model taxonomic hierarchy[END_REF]. In this relatively simple model, at the initiation of a radiation, many higher taxa should appear to occupy the very different ecological niches available [START_REF] Valentine | Determinants of diversity in higher taxonomic categories[END_REF]1995). Afterward, the ecosystem's niches are filling up and it will be progressively harder for new body plans to become established as the niche spaces become saturated [START_REF] Bambach | Ecospace utilization and guilds in marine communities through the Phanerozoic[END_REF]. For example, if no new phyla appeared after the end-Permian mass extinction, it is because the relatively few Triassic survivors still occupied most of the niche space [START_REF] Erwin | A comparative study of diversification events: the early Paleozoic versus the Mesozoic[END_REF]. This model implies that there is a wide range of other potential phyla out there, and that it is the lack of ecological opportunity that keeps them from becoming manifest since the Cambrian Explosion [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. Indeed, estimates have shown that the ecospace filled during the Cambrian Explosion was not very different to that occupied by marine life today (Bambach et al., 2007), which means that the potential range of form was rapidly settled during the Cambrian and prevented the apparition of other new body plans [START_REF] Valentine | Why no new phyla after the Cambrian? Genome and ecospace hypotheses revisited[END_REF][START_REF] Bottjer | Comparative evolutionary palaeoecology: assessing the changing ecology of the past[END_REF].

However, the transition from the Ediacaran consisting of microbial mats and algae, and phytoplanktonic communities with no zooplankton into a Cambrian world with thriving benthic and planktonic animal communities involved far-reaching changes in benthic and neritic ecosystems (Fig. 23B; Erwin and Valentine, 2013;[START_REF] Butterfield | Plankton ecology and the Proterozoic-Phanerozoic transition[END_REF][START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. Erwin et al. (2011) identified a suite of processes that facilitated the construction of biodiversity through positive feedback: ecosystem engineering of the environment and the formation of new ecological linkages [START_REF] Butterfield | Plankton ecology and the Proterozoic-Phanerozoic transition[END_REF]. Ecosystem engineering occurs when the activities of one or more species modify the physical and/or chemical environment(s), affecting the flow of energy, nutrients, and other resources through a network of species [START_REF] Jones | Positive and negative effects of organisms as physical ecosystem engineers[END_REF][START_REF] Wright | The concept of organisms as ecosystem engineers ten years on: progress, limitations, and challenges[END_REF]. This often has important ecological and evolutionary consequences [START_REF] Erwin | Macroevolution of ecosystem engineering, niche construction and diversity[END_REF], especially when resource availability are affected. An example of ecosystem engineering stems from Cryogenian-Ediacaran sponges and later burrowing bilaterians (Erwin et al., 2011). Indeed, sponges remove dissolved organic matter and bacteria from the water column [START_REF] Sperling | Rangeomorphs, Thectardis (Porifera?) and dissolved organic carbon in the Ediacaran oceans: Rangeomorphs, Thectardis and DOC[END_REF].

If they are abundant, they can transfer large volumes of carbon to the sediment, thus changing the geochemistry of the water column. Moreover, the advent of vertical burrowing in the early Cambrian enhanced the oxygenation of the sediment and microbial primary productivity, providing food for benthic metazoans [START_REF] Lohrer | Bioturbators enhance ecosystem function through complex biogeochemical interactions[END_REF].

Compared with Phanerozoic animals, the Ediacaran biota are remarkable by their lack of macroscopic sensory organs, and the virtual absence of macroscopic organs for interacting with other organisms or the environment [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. They show no signs of predation, except for bore holes in some of the earliest skeletonized fossils, Cloudina, from the latest Ediacaran [START_REF] Hua | Borings in Cloudina shells -complex predator-prey dynamics in the terminal Neoproterozoic[END_REF]. This apparently peaceful environment was even compared to the Garden of Eden and have been nicknamed the Garden of Ediacara [START_REF] Mcmenamin | The Garden of Ediacara: Discovering the First Complex Life[END_REF]. It seems that interactions between adult organisms were minimal, so that palaeoecologists consider that fully developed ecosystems are far from being realized in the Ediacaran (e.g., see chapters in part II of [START_REF] Zhuravlev | The Ecology of the Cambrian Radiation[END_REF]. The emergence of predation also probably had an important impact on the growth of these ecological networks [START_REF] Evans | The sudden appearance of the Cambrian fauna[END_REF][START_REF] Hutchinson | The biologist poses some problems[END_REF][START_REF] Vermeij | The origin of skeletons[END_REF]. Indeed, the first appearance of predatory traces, and body fossils of predators, occurs near the Ediacaran-Cambrian transition [START_REF] Bengtson | Origins and early evolution of predation[END_REF]. This new selective pressure potentially favored animals developing novel defensive mechanisms, especially skeletonization [START_REF] Vermeij | The origin of skeletons[END_REF], or new structures and capabilities that allowed movement into new habitats, such as invasion of the water column by small animals to avoid predation on the sea floor [START_REF] Signor | The plankton and the benthos: origins and early history of an evolving relationship[END_REF][START_REF] Peterson | Tempo and mode of early animal evolution: inferences from rocks, Hox and molecular clocks[END_REF]. By instance, pelagic algae evolved antipredator defenses in the Early Cambrian [START_REF] Butterfield | Plankton ecology and the Proterozoic-Phanerozoic transition[END_REF][START_REF] Butterfield | Ecology and evolution of Cambrian plankton[END_REF]. A phenomenon often called the 'arms race' [START_REF] Vermeij | Evolution and escalation: an ecological history of life[END_REF][START_REF] Vermeij | Nature: an economic history[END_REF], one of the many manifestations of the Red Queen paradox [START_REF] Van Valen | A new evolutionary law[END_REF], which places biotic interactions at the heart of evolutionary changes.

One of the main drawbacks of these types of explanation is that they seem to presuppose at least some of the evolutionary events that they are supposed to explain [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF]. Indeed, if some animal innovations are invoked to explain the Cambrian Explosion, the cause of this favoured innovation is left unexplained. For example, if the evolution of the planktonic arthropod mesoplankton had an important role in driving the Cambrian explosion [START_REF] Butterfield | Plankton ecology and the Proterozoic-Phanerozoic transition[END_REF], all the evolution that needs to take place before the arthropod mesoplankton evolved must be taken into account, especially given that they emerged from the arthropod crustacean benthos (Rigby and Milsom, 1996). The same principle applies to the evolutions of grazing [START_REF] Stanley | An ecological theory for the sudden origin of multicellular life in the late Precambrian[END_REF][START_REF] Stanley | Ideas on the timing of metazoan diversification[END_REF], predation (e.g. [START_REF] Evans | The sudden appearance of the Cambrian fauna[END_REF], or vision [START_REF] Parker | Colour in Burgess Shale animals and the effect of light on evolution in the Cambrian[END_REF][START_REF] Parker | In the Blink of an Eye: How Vision Sparked the Big Bang of Evolution[END_REF] that are also presented as potential causes of the Cambrian Explosion.

These sorts of explanations all fall into the same problematic category as other 'key innovation' explanations [START_REF] Budd | Arthropod body-plan evolution in the Cambrian with an example from anomalocaridid muscle[END_REF] in pushing back, rather than solving the problem at hand. That is not to deny, of course, that the innovations mentioned above were without ecological significance, but their overall explanatory significance must be considered to be limited by their necessarily circumscribed nature. Rather, hypotheses are required that rely only on events that have already occurred and not on implied prescience of future conditions [START_REF] Budd | The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution: Early evolution of the animals[END_REF].

Synthesis

It seems obvious that there is not a single cause, which is sufficient to explain all the aspects of Cambrian Explosion (Fig. 25). Indeed, it is clear that environmental, developmental, and ecological factors must have played their parts. The real question is about the extant and importance of each in order to bring out a possible hierachy among all these potential triggers, as well as their possible interactions. [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF] brought up six points that especially deserve attention. First, extrinsic causes, and among them environmental explanations, are not sufficient to explain the establishement of the animal phyla observed during the Cambrian Explosion. Second, among intrinsic causes, both developmental and ecological factors seemed to have a major role in explaining the origin of animal body plans, but little is known about their interactions. Thirds, most proposed theories do not explicitly account for the rise in diversity through the Cambrian Explosion. Indeed, this increase in diversity seems to be considered as a simple consequence of the rise of disparity. Fourth, it seems that the only factors being responsible for the time of onset of the Cambrian Explosion are extrinsic and enironmental, at least because they enabled the conditions necessary to the emergence of animals. Nevertheless, they cannot explain the lag between their establishment and the first metazoan observed in the fossil record. Fifth, no explanation satisfactorily address the question of the duration of the Cambrian Explosion. Sixth, ecological and developmental explanations are the only one that account for the uniqueness of the event. In summation, explaining the Cambrian Explosion of bilaterian animals will remain a rich field of enquiry for quite some time to come [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]! of the Cambrian Explosion to account for the major aspects that need to be explained [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF]. **1, event too early to be immediately causal, but may have permitted evolution of Ediacaran biota, a necessary prelude to the Cambrian Explosion. 2, if the stem lineages were both small and unskeletonized, then we would not expect to see them in the fossil record. These animals will have been small if there was limited oxygen, or if the controversial developmental set-aside cell theory (Davidson et al., 1995; but see [START_REF] Peterson | Tempo and mode of early animal evolution: inferences from rocks, Hox and molecular clocks[END_REF] for alternate view) holds, or if large size was a consequence of predation. 3, according to [START_REF] Marshall | Explaining the Cambrian "explosion" of animals[END_REF], the origin of bilaterian developmental system alone might not be enough to account for disparity increase. 4, in the absence of a combinatorial developmental system, we might not expect a burst in disparity. 5, explanation certainly supports the uniqueness of skeletonization event, but it is not as obvious why body plans should only appear during the initial escalatory radiation.

Following the Cambrian Explosion and the appearance in the fossil record of most animal phyla and body plans (see section 4), the Ordovician period witnessed a staggering increase in disparity and biodiversity at the specific, generic, and family levels (Harper, 2006). This peculiar event in the history of Earth was thus called the Great Ordovician Biodiversification Event (GOBE; Webby et al., 2004a) or Ordovician Radiation (Droser et al., 1996). It is considered as the last step leading to the radiation of metazoans.

The Great Ordovician Biodiversification Event (GOBE)

Although the five major mass extinctions (especially, the Permian-Triassic and the Cretaceous-Tertiary events) have been extensively documented, until recently, the major biodiversifications and radiations of life on Earth have attracted much less attention (Fig. 26; [START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF]. In addition, among these biodiversifications, the Cambrian Explosion completely overshadowed the Ordovician and Mesozoic-Cenozoic radiations of marine invertebrates (Droser and Finnegan, 2003;[START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF][START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF].

Historical Background of research

The identification of the GOBE is intimately tied to the history of quantification of past life.

A year after the release of 'On the Origin of species' (Fig. 18B; Darwin, 1859), Phillips

(1860) published the first attempts of semi-quantitative analyses applied to the Phanerozoic biodiversity [START_REF] Sepkoski | Ten years in the library: new data confirm paleontological patterns[END_REF][START_REF] Harper | The Great Ordovician Biodiversification Event: Reviewing two decades of research on diversity's big bang illustrated by mainly brachiopod data. Palaeoworld, Geologic and biotic events and their relationships during the Early to Middle[END_REF]. In this work, three stratigraphically different fauna were recognized. Among them, the first corresponded to the 'Palaeozoic Life'. This field of research has largely been deserted since, even though numerical compilations of fossil range data were made during the 1950's [START_REF] Newell | Periodicity in invertebrate evolution[END_REF]. However, it was not until the 1980's that the first detailed analysis of diversity emerged, mainly through the impetus given by Jack John [START_REF] Sepkoski | A kinetic model of Phanerozoic taxonomic diversity I. Analysis of marine orders[END_REF]Sepkoski ( , 1979Sepkoski ( , 1981[START_REF] Sepkoski | A model of onshore-offshore change in faunal diversity[END_REF][START_REF] Sepkoski | Ten years in the library: new data confirm paleontological patterns[END_REF][START_REF] Sepkoski | The Ordovician radiations: diversification and extinction shown by global genus-level taxonomic data, in: Ordovician Odyssey: Short Papers for the Seventh International Symposium on the Ordovician System[END_REF][START_REF] Sepkoski | Biodiversity: past, present, and future[END_REF][START_REF] Sepkoski | A compendium of fossil marine animal genera[END_REF], who created a global database. These new data strongly confirmed the foreseen major radiations of marine diversity during the Palaeozoic, including the Cambrian Explosion and Ordovician Radiation (Fig. 26). They also evidenced dramatic palaeoecological changes, which were later confirmed by further studies [START_REF] Sheehan | A new look at Ecologic Evolutionary Units (EEUs)[END_REF]Droser and Sheehan, 1997;Harper, 2006;[START_REF] Miller | The Ordovician Radiation: macroevolutionary crossroads of the Phanerozoic[END_REF]. The palaeobiodiversity data are now being subjected to intense and rigorous numerical scrutiny, revealing the clear impact of the GOBE (for example, see [START_REF] Alroy | Phanerozoic trends in the global diversity of marine invertebrates[END_REF]; red curve in fig. 26). In recent years, the GOBE has drawn much attention from the international geological community [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF][START_REF] Harper | The Great Ordovician Biodiversification Event: Reviewing two decades of research on diversity's big bang illustrated by mainly brachiopod data. Palaeoworld, Geologic and biotic events and their relationships during the Early to Middle[END_REF], with the organization of three successive international research projects. The first one was the International Geoscience Programme (IGCP) Project 410, entitled "The Great Ordovician Biodiversification Event" (1997)(1998)(1999)(2000)(2001)(2002) [START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF]. It resulted in several review papers and special issues, especially on Lower Palaeozoic environments (Servais and Owen, 2010), climate and sea level [START_REF] Munnecke | Ordovician and Silurian seawater chemistry, sea level, and climate: Aasynopsis[END_REF], as well as a synthesis on Early Palaeozoic biogeography and palaeogeography [START_REF] Harper | Chapter 1 Early Palaeozoic biogeography and palaeogeography: towards a modern synthesis[END_REF].

The next IGCP project 653 (2016-2020), entitled "The onset of the Great Ordovician Biodiversification Event" will explore the Cambrian roots of the GOBE, and will focus on the identification of the putative triggers leading to this major burst in metazoan diversity.

Caracterization

In the marine realm

Though the Cambrian Explosion resulted in a range of new and spectacular animal body plans, the Ordovician Radiation generated few new higher taxa, but witnessed a staggering increase in disparity and biodiversity (Harper, 2006). Indeed, while all phyla were established during the Cambrian Explosion [controversial bryozoans were recently reported from the upper Cambrian of Mexico by [START_REF] Landing | Cambrian origin of all skeletalized metazoan phyla-Discovery of Earth's oldest bryozoans (Upper Cambrian, southern Mexico)[END_REF] but they were reinterpreted as octocorals [START_REF] Taylor | Reinterpretation of the Cambrian "Bryozoan" Pywackia as an Octocoral[END_REF]], taxonomic increases during the Ordovician were manifest at lower taxonomic levels, even though ordinal level diversity doubled (Droser and Finnegan, 2003).

Marine family diversity tripled and within clade diversity increases occurred at the genus and species levels, with diversification rates at least four times faster than those in the Mesozoic radiation [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF]. This radiation is more and more considered to be rooted in the 545-530 Ma Cambrian explosion because only 40 to 80 Ma passed for diversity of the new Cambrian phyla to "explode" at the order, family, genus, and species level (Droser and Finnegan, 2003;[START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF]Van Roy et al., 2010;[START_REF] Harper | The Great Ordovician Biodiversification Event: Reviewing two decades of research on diversity's big bang illustrated by mainly brachiopod data. Palaeoworld, Geologic and biotic events and their relationships during the Early to Middle[END_REF]Martin et al., in press a). The term "Great Ordovician Biodiversification Event" (GOBE) has been introduced to designate what is arguably the most important increase of biodiversity of marine life during Earth's history [START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF], which laid the foundation for the basic community structure of marine organisms in the remainder of the Palaeozoic Era (Harper, 2006).

However, the precise duration of the GOBE is subject to debate. By instance, according to [START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF] (Webby et al., 2004b). For several fossil groups, the increase in diversity at the order, family, genus and species level occurred during a relatively short time span of 10 to 20 myr, and hence, the GOBE is considered by some authors as the most rapid diversification in marine life during Earth history (Servais et al., 2010).

The global signal of the radiation represents a combination of many regional diversity changes across a range of fossil groups (Webby et al., 2004a;Harper, 2006;[START_REF] Harper | The Great Ordovician Biodiversification Event: Reviewing two decades of research on diversity's big bang illustrated by mainly brachiopod data. Palaeoworld, Geologic and biotic events and their relationships during the Early to Middle[END_REF]. The Ordovician biodiversification is thus seen both in terms of individual clades and regional biodiversity curves (e.g. Miller, 1997aMiller, , 1997b[START_REF] Miller | The Ordovician Radiation: towards a new global synthesis[END_REF]. [START_REF] Hammer | Biodiversity curves for the Ordovician of Baltoscandia[END_REF] for example calculated the total mean standing diversity of all fossil groups recorded from Baltica (the most extensively studied palaeocontinent in terms of Ordovician faunas) indicating a clear diversity increase with highest values reached at the base of the Upper Ordovician. An obvious Ordovician biodiversification is also recorded for all fossil groups from the South Chinese Yangtze Platform (e.g. [START_REF] Chen | Ordovician graptolite evolutionary radiation: a review[END_REF].

Nevertheless, diagrams of changes in global or regional biodiversity during the GOBE only give a crude representation of the timing and pace of the faunal change [START_REF] Sepkoski | The Ordovician radiations: diversification and extinction shown by global genus-level taxonomic data, in: Ordovician Odyssey: Short Papers for the Seventh International Symposium on the Ordovician System[END_REF][START_REF] Hammer | Biodiversity curves for the Ordovician of Baltoscandia[END_REF][START_REF] Schmitz | Asteroid breakup linked to the Great Ordovician Biodiversification Event[END_REF]. According to [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF], [START_REF] Sepkoski | The Ordovician radiations: diversification and extinction shown by global genus-level taxonomic data, in: Ordovician Odyssey: Short Papers for the Seventh International Symposium on the Ordovician System[END_REF] found that the timing of radiation differed greatly, although all clades rapidly increased in biodiversity during the Ordovician (Fig. 27). For example, bivalves [START_REF] Cope | Bivalves and rostroconchs[END_REF], brachiopods [START_REF] Harper | Brachiopods[END_REF]Harper, 2006;[START_REF] Harper | The Great Ordovician Biodiversification Event: Reviewing two decades of research on diversity's big bang illustrated by mainly brachiopod data. Palaeoworld, Geologic and biotic events and their relationships during the Early to Middle[END_REF], graptolites [START_REF] Cooper | Measures of biodiversity[END_REF], or trilobites (Westrop andAdrain, 1998a, 1998b;[START_REF] Adrain | Post-Cambrian trilobite diversity and Evolutionary Faunas[END_REF][START_REF] Adrain | Trilobites[END_REF] show a dramatic increase of diversity during a relatively short time interval in the Floian-Darriwilian (Early and Middle Ordovician). Others such as bryozoans [START_REF] Taylor | Bryozoans[END_REF], or stromatoporoid sclerosponges (Webby et al., 2004c) did not diversified before the Darriwilian and Sandbian stages, in the Middle and Late Ordovician. In contrast, other groups show a continuous and long-term biodiversification that in some instances started in the Cambrian (Servais et al., 2010). Such groups include acritarchs (e.g. Servais et al., 2008) and blastozoan echinoderms (e.g. Lefebvre and Fatka, 2003;[START_REF] Nardin | La radiation des échinodermes au Paléozoïque inférieur, l'exemple des blastozoaires[END_REF][START_REF] Nardin | Unravelling extrinsic and intrinsic factors of the early Palaeozoic diversification of blastozoan echinoderms[END_REF]. The timing of radiation seems especially different, when the fossils belong to distinct ecotypes [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF]. Indeed, planktonic organisms display similar to identical patterns in different regions, indicating that they were driven by global factors, whereas benthic organisms show considerable regional differentiation, implying that they were strongly affected by regional factors like latitude and climate.

Moreover, these contrasted diversity curves reflect different macroevolutive trajectories [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF]. For example, stomatoporoid corals show an exponential increase in biodiversity, whereas brachiopods display a stepwise increase, and graptolites reached their first diversity acme in the early Middle Ordovician, and then decreased progressively (Fig. 27). Cope , 2004), brachiopods (redrawn from [START_REF] Harper | Brachiopods[END_REF], graptolites (redrawn from Cooper et al., 2004), and trilobites (redrawn from [START_REF] Adrain | Trilobites[END_REF]. Groups diversifying during the Middle and Late Ordovician include bryozoans (redrawn from [START_REF] Taylor | Bryozoans[END_REF]) and stromatoporoids (redrawn from Webby et al., 2004c). Groups showing a sustained increase during the whole Ordovician include acritarchs (based on Servais et al., 2010) and blastozoan echinoderms [START_REF] Nardin | Unravelling extrinsic and intrinsic factors of the early Palaeozoic diversification of blastozoan echinoderms[END_REF].

The invasion of Land: an overview

Though most authors studying the GOBE mainly focus on marine ecosystems, an outstanding revolution also occurred on land during the Ordovician (Fig. 28). It seems that bacteria, followed by algae, probably had already invaded the land during the Precambrian [START_REF] Barton | Evolution, Chapter 10, Diversification of Plants and Animals[END_REF]. Indeed, some 1,000 to 1,200 million-year-old palaeosols and non-marine, microbially induced sedimentary structures occur in the Precambrian of Scotland [START_REF] Prave | Life on land in the Proterozoic: evidence from the Torridonian rocks of northwest Scotland[END_REF]. This is coherent with the marine record, where shallow sea floors at that time were extensively covered by microbial mats, as evidenced by the abundance of stromatolites and microbial carbonates [START_REF] Riding | Microbial carbonate abundance compared with fluctuations in metazoan diversity over geological time[END_REF]. By the late Cambrian, land was probably invaded by green algae, mosses, and primitive plants, which formed only sparse and patchy coverage [START_REF] Masuda | A great revolution of the earth-surface environment: linking the bio-invasion onto the land and the Ordovician Radiation of marine organisms[END_REF]. Moreover, fragments of fossil spores have been found from upper

Cambrian strata (ca. 520 Mya; Yang et al., 2004). Surprisingly, first freshwater arthropods are reported as soon as the latest Cambrian of Argentina [START_REF] Vaccari | Cambrian origins and affinities of an enigmatic fossil group of arthropods[END_REF]) and activities on land including trackways of such large arthropods (6-10 cm in width) in dune sands were already present in the late Cambrian to Early Ordovician of Ontario, Canada [START_REF] Macnaughton | First steps on land: arthropod trackways in Cambrian-Ordovician eolian sandstone, southeastern Ontario, Canada[END_REF]. Up to the Early Ordovician, the surface of the land was probably covered in places by microbiotic crusts, a community of fungi, bacteria, and algae that today can be found in arid regions, where they form a soft, spongy mat on barren clays and silts [START_REF] Prothero | Evolution of the earth[END_REF]. Colonization of the land by plants most likely occurred in a stepwise fashion starting in the Mid-Ordovician [START_REF] Steemans | Origin and radiation of the earliest vascular land plants[END_REF]. Indeed, spores containing undoubted land plant fragments have also been recovered from the non-marine Ordovician (440-470 Mya) of Oman [START_REF] Wellman | Fragments of the earliest land plants[END_REF][START_REF] Kenrick | Palaeobotany: fishing for the first plants[END_REF], while fossil spores of more advanced plants are known from 460 million years ago (Middle Ordovician; [START_REF] Strother | New evidence for land plants from the lower Middle Ordovician of Saudi Arabia[END_REF][START_REF] Edwards | Embryophytes on land: the Ordovician to Lochkovian (Lower Devonian) record[END_REF][START_REF] Prothero | Evolution of the earth[END_REF]Wills and McElwain, 2002;[START_REF] Steemans | Miospores and the emergence of land plants[END_REF][START_REF] Edwards | The earliest vascular plant or a later rooting system? Pinnatiramosus qianensis from the marine Lower Silurian Xiushan Formation, Guizhou Province, China[END_REF]. These dates are in good accordance with molecular-clock studies, which suggest that land plants appeared either about 700 million years ago [START_REF] Heckman | Molecular evidence for the early colonization of land by fungi and plants[END_REF][START_REF] Pennisi | A molecular approach to mushroom hunting[END_REF] or between 425 to 490 million years ago [START_REF] Sanderson | Molecular data from 27 proteins do not support a Precambrian origin of land plants[END_REF].

The first plant-induced palaeosols were reported from the Late Ordovician (440 Mya) fluvial deposits, together with traces of arthropods [START_REF] Retallack | Scoyenia burrows from Ordovician palaeosols of the Juniata Formation in Pennsylvania[END_REF]. Fossils of land plants with fibrovascular bundle-like tissue date from 420-430 million years old strata (Middle Silurian; [START_REF] Edwards | A late Wenlock flora from Co. Tipperary, Ireland[END_REF][START_REF] Rickards | The age of the earliest club mosses: the Silurian Baragwanathia flora in Victoria, Australia[END_REF], while the earliest vascular land plants with an epidermis and conducting tissue are known from 440 million years ago. The first wood appeared in the Middle Devonian (370 to 380 Mya), and large forests had already been formed ca. 360 million years ago [START_REF] Prothero | Evolution of the earth[END_REF][START_REF] Stein | Giant cladoxylopsid trees resolve the enigma of the Earth's earliest forest stumps at Gilboa[END_REF][START_REF] Barton | Evolution, Chapter 10, Diversification of Plants and Animals[END_REF].

In summary, the scenario of land invasion appears to be the following [START_REF] Masuda | A great revolution of the earth-surface environment: linking the bio-invasion onto the land and the Ordovician Radiation of marine organisms[END_REF]: terrestrial animals invaded the land after terrestrial plants, which followed pioneer invaders such as protista, fungi, and prokaryotes (e.g. cyanobacteria). Plants perhaps succeeded in their invasion of the land using the assistance of microbial mats formed by bacteria and fungi, then by algae [START_REF] Prothero | Evolution of the earth[END_REF][START_REF] Barton | Evolution, Chapter 10, Diversification of Plants and Animals[END_REF]. As a result, lichens and mosses (bryophytes), followed by ferns (pteridophytes), and then by seed plants (spermatophytes) probably invaded the land successively. The first forests were made up of vascular plants (tracheophytes), including club mosses, horsetails, true ferns, and seed ferns [START_REF] Prothero | Evolution of the earth[END_REF].

However, until recently, the early fossil record of land plant was poorly known, so that no diversity curve could be studied during the Ordovician and compared to the signals obtained in marine envrionments. Accumulated data now enable the identification of three successive 'planetary' floras [START_REF] Knoll | Character diversification and patterns of evolution in early vascular plants[END_REF][START_REF] Niklas | Patterns in vascular plant diversification: an analysis at the species level[END_REF][START_REF] Knoll | Patterns of change in plant communities through geological time[END_REF][START_REF] Knoll | Adaptation, plant evolution, and the fossil record[END_REF], namely the Palaeophytic, Mesophytic, and Cenophytic Floras, which are the counterparts of the EF defined by Sepkoski (1981). The planetary flora corresponding to the Ordovician Radiation is the Palaeophytic one. It has been further subdivided into three Evolutionary Land Floras (Gray, 1995): the Eoembryophytic (mid-Ordovician to pre-latest Early Silurian), Eotracheophytic (latest Early Silurian through early Lochkovian), and Eutracheophytic (late Lochkovian to mid-Permian) floras.

The Eoembryophytic flora is established on the basis of plant microfossil assemblages, especially tetrad spores (Gray, 1984[START_REF] Gray | The microfossil record of early land plants: Advances in understanding of early terrestrialization[END_REF][START_REF] Gray | Tetrahedraletes, Nodospora, and the 'cross' tetrad: An accretion of myth[END_REF]. It corresponds closely to the major biostratigraphic unit defined by Gray (1984[START_REF] Gray | The microfossil record of early land plants: Advances in understanding of early terrestrialization[END_REF] as Microfossil Assemblage Zone I, extending from the early Darriwilian to the Katian. The initial appearance of the spore tetrads, on which this unit is based, does not correspond to any major marine bio-event (Gray, 1995). The subsequent adaptive radiation of spore tetrad taxa in the late Ordovician transgresses the Ordovician/Silurian boundary without major change in taxa [START_REF] Gray | Land plant spores and the Ordovician-Silurian boundary[END_REF], thus suggesting the continuity of the Eoembryophytic flora, while a major extinction occured among marine biota at this time [START_REF] Boucot | Phanerozoic extinctions: How similar are they to each other?[END_REF]. In the absence of evidence prior to the Darriwilian, the Eoembryophytic marks the first major adaptive radiation of embryophytic plants, and seems to be decoupled from events in the marine record (Gray, 1995).

An ecological breakthrough

The GOBE marks a real ecological shift, with deep modifications of the former Cambrian ecosystems (Figs. 29A andB). Indeed, although these ecosystems -which used to be dominated by sessile and mobile benthos with a few planktonic organisms -had already reached a relatively high ecological diversity (Figs. 23 B and C), they were largely replaced during the Ordovician, so that only a limited number of epibenthic and planktonic taxa (trilobites) survived to the end of Cambrian. This faunistic turnover was highlighted as soon as the 1980's by Sepkoski and his concept of evolutionary faunas. Later, other ecological concepts, such as evolutionary units and ecospace utilization, shed a new light on this revolution in marine ecosystems.

The concept of Evolutionary Faunas

In his compendium of marine families, Sepkoski (1979) provided the first patterns of diversity for marine families during the Phanerozoic (Fig. 26). In Early Palaeozoic times, a threefold increase in family-level diversity was evidenced between the late Cambrian and the Late Ordovician (Harper, 2006). This major diversification began immediately after an apparent late Cambrian diversity plateau. The GOBE was followed by a longterm diversity plateau (despite significant short-term fluctuations) from the Silurian to the catastrophic end-Permian extinction (Droser and Finnegan, 2003). Based on multivariate statistical analyses, Sepkoski (e.g., 1981Sepkoski (e.g., , 1984Sepkoski (e.g., , 1991) ) (Sepkoski, 1981).

Ecological Evolutionary Units (EEU)

In addition to the EFs, other authors have defined Ecological-Evolutionary Units (EEUs), which represent long intervals of Phanerozoic time characterized by stable ecological structures [START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF]. Initially, 12 EEUs were defined by [START_REF] Boucot | Does evolution take place in an ecological vacuum? II[END_REF] but they were reduced to 9 by [START_REF] Sheehan | A new look at Ecologic Evolutionary Units (EEUs)[END_REF]Sheehan ( , 2001a)). Indeed three of them were considered as community recovery after mass exintintions [START_REF] Sheehan | A new look at Ecologic Evolutionary Units (EEUs)[END_REF]. The Palaeozoic EF comprises four of them, two of which are recognized in the Ordovician (Fig. 26). They can be distinguished on the basis of taxonomic diversity, morphological disparity, and ecological change [START_REF] Sheehan | A new look at Ecologic Evolutionary Units (EEUs)[END_REF]Droser et al., 1997;Droser and Sheehan, 1997). [START_REF] Sheehan | A new look at Ecologic Evolutionary Units (EEUs)[END_REF] identified three EEU boundaries: one in the mid Cambrian, one at the Cambro-Ordovician boundary, and one in the Ordovician. These boundaries do not correspond to any major extinction events. However, the mid-Cambrian EEU boundary coincides with a significant extinction of archaeocyathids at the end of the lower Cambrian [START_REF] Debrenne | Extinction of the Archaeocyatha[END_REF]. The Cambrian-Ordovician boundary separates two EEUs: the Early-Middle Ordovician time interval represents a transitional period, characterized by the decline of the CEF and the onset of the PEF. This long-term ecological turnover started in onshore environments and spread into offshore settings [START_REF] Sepkoski | Diversification, faunal change, and community replacement during the Ordovician radiation[END_REF]. Changes in ecologic associations during the Ordovician Radiation might have resembled changes during recovery intervals following extinction events [START_REF] Sheehan | A new look at Ecologic Evolutionary Units (EEUs)[END_REF]. Bambach et al. (2007) subdivided the marine ecospace into 216 potential modes of life, based on ecological tiering, feeding habit and motility (each of these factors being divided into six categories), with each corresponding to a guild of species. They found that ecospace occupation and its use by shelly organisms changed significantly through geologic time. By instance, the skeletonized organisms of the Ediacaran Fauna utilized only two modes of life, while nineteen were used in the Cambrian, and thirty in the Ordovician. In comparision, sixty-two modes of life are filled by skeletonized organisms in present-day marine ecosystems (Fig. 29C; [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF]. The Ordovician was therefore not only a time of increasing biodiversity, but also one of rapidly expanding ecospace utilization.

Ecospace Utilization

.

The Emergence of modern marine ecosystems

The GOBE resulted in a much greater ecological complexity, including several types of swimmers, floaters, attached filter feeders, and burrowers exploiting different levels above and below the sea floor, as well as new types of predators [START_REF] Masuda | A great revolution of the earth-surface environment: linking the bio-invasion onto the land and the Ordovician Radiation of marine organisms[END_REF].

During the Ordovician, brachiopods, cnidarians, molluscs, echinoderms, bryozoans and hemichordates (graptolites), as well as chitinozoans and acritarchs, significantly increased in biodiversity (Servais et al., 2010).

These fossil groups constitute almost all marine ecotypes represented in the Ordovician:

burrowing, sessile benthos, motile benthos, nekton, and plankton. They occupied a wide range of biotopes: from pelagic to benthic, from nearshore shallow-water to offshore deepwater, and acquired a global distribution [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF].

Among those ecotypes, sessile benthic organisms were predominant. In terms of feeding strategies, the most important aspect of Ordovician marine ecosystems is the striking abundance of filter and suspension feeders [START_REF] Droser | The nature of diversification and paleoecology of the Ordovician Radiation with evidence from the Great Basin[END_REF][START_REF] Bambach | Ecospace utilization and guilds in marine communities through the Phanerozoic[END_REF]Guensburg and Sprinkle, 2004). These organisms sorted out suspended organic matter, such as microplankton, as their food. Representative filter and suspension feeders are diversified and include bivalves, rostroconchs, both calcareous and phosphatic brachiopods (feeding near the sea floor), corals and bryozoans (feeding at different heights above the sea floor), crinoids and cystoids (feeding at yet higher positions of 25-100 cm above sea floor, owing to the development of long stalks), and graptolites (feeding in the upper water column in open and pelagic seas), forming a complicated tiering above the sea floor [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF].

Meanwhile, existing deposit feeders continued to flourish during the Ordovician, including trilobites, gastropods, and ostracods [START_REF] Babin | The initial Ordovician bivalve mollusc Radiations on the Western Gondwanan shelves[END_REF][START_REF] Droser | The nature of diversification and paleoecology of the Ordovician Radiation with evidence from the Great Basin[END_REF]Bambach et al., 2007).

The appearance of many predators in the open ocean and on the sea floor [START_REF] Prothero | Evolution of the earth[END_REF], and the rapid increase of potential preys greatly enhanced the complexity of trophic structure of the Ordovician marine ecosystem (Fig. 29B). This complexity is reflected by the utilization of the ecospace during the Ordovician radiation [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF].

The resulting palaeoecological changes

Shifts in abundance, dominance, and other features that define ecological structure and complexity cannot be evaluated from taxonomic diversity studies alone (Droser and Finnegan, 2003). Nevertheless, palaeoecological shifts, and in particular those involving structural changes, cannot be easily quantified (Maddy and Brew, 1995). A possible solution to avoid this problem is to settle a scale for palaeoecological changes, as some changes in ecological structures are far more important than others. A hierarchy of severity of palaeoecological events has been developed for Phanerozoic marine life (Droser et al., 1997;[START_REF] Bottjer | The ecological architecture of major events in the Phanerozoic history of marine life[END_REF]. It comprises four integrated levels from ecosystems to communities (see Fig. 30). All of these palaeoeological levels were impacted during the GOBE (Fig. 30, Bottjer et al., 2001;Harper, 2006).

For example, the colonization of land by the first nonvascular bryophyte-like plants [START_REF] Steemans | Miospores and the emergence of land plants[END_REF] and the appearance of deep water benthos [START_REF] Bottjer | The ecological architecture of major events in the Phanerozoic history of marine life[END_REF] both represent the emergence of new ecosystems (First Level). Changes in dominance, such as the transition from trilobite to brachiopod-dominated communities on soft substrates [START_REF] Droser | The nature of diversification and paleoecology of the Ordovician Radiation with evidence from the Great Basin[END_REF], and from echinoderms to bryozoans on hard substrates (Wilson et al., 1992), the evolution of deep-mobile burrowers and stromatoporoid reefs (Harper, 2006) are cited in support of second level changes. The appearance of new Bambachian megaguilds (groups of organisms with mutually similar adaptive strategies ; [START_REF] Droser | The nature of diversification and paleoecology of the Ordovician Radiation with evidence from the Great Basin[END_REF] and the supplementation of existing megaguilds with new taxa together with the arrival of new community types (receptaculitid/macluritid-dominated, orthid-dominated and bivalve-trilobite combinations ; [START_REF] Bambach | Ecospace utilization and guilds in marine communities through the Phanerozoic[END_REF]Harper, 2006) together with a marked increase in tiering complexity [START_REF] Droser | Ordovician increase in extent and depth of bioturbation: Implications for understanding early Paleozoic ecospace utilization[END_REF], correspond to third level changes.

Finally, the development of many new palaeocommunities is the hallmark of fourth level changes. These types of changes had a profound effect on the composition and structure of Phanerozoic life (Harper, 2006).

Fig. 30. Definitions of the palaeoecological levels (Droser et al., 1997;[START_REF] Bottjer | The ecological architecture of major events in the Phanerozoic history of marine life[END_REF]Harper, 2006) and their illustrated examples during the GOBE. The Russian doll's silhouettes symbolize the imbrication of the palaeoecological levels from the first to the fourth.

Potential causes triggering the GOBE

As for the Cambrian Explosion, most authors think that the GOBE resulted from a complex combination of intrinsic (biological) and extrinsic (physical) causes (Fig. 31; [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF][START_REF] Harper | The Great Ordovician Biodiversification Event: Reviewing two decades of research on diversity's big bang illustrated by mainly brachiopod data. Palaeoworld, Geologic and biotic events and their relationships during the Early to Middle[END_REF].

The intrinsic factors a) Evolutionary Faunas

The last two Evolutionary Faunas defined by Sepkoski (see 5.3.1) are characterized by a slower origination rate but a higher equilibrium diversity than its preceding fauna (Sepkoski, 1981). Classes belonging to a same EF have generally similar diversity histories and turnover rates, although there is considerable variation within them (Droser and Finnegan, 2003). In this context, Sepkoski viewed the Ordovician radiation as inevitable,

given the initial appearance of the PEF in the Cambrian (Sepkoski, 1979[START_REF] Sepkoski | Evolutionary faunas[END_REF]. The late Cambrian ''plateau'' would then correspond to an equilibrium diversity reached by the Cambrian Fauna, while the Ordovician radiation would result from the accelerated diversification of the PEF following its slow expansion during the Cambrian. He suggested that the successively higher equilibrium diversities of the three Evolutionary Faunas could be understood in terms of increasing niche partitioning, with the ''generalist'' CEF being replaced by the relatively ''specialized'' PEF in the Ordovician [START_REF] Sepkoski | Evolutionary faunas[END_REF]. Thus, he considered the Ordovician Radiation as the predictable outcome of diversity-dependent interactions between an already-diverse CEF and a diversifying PEF. However, this model cannot be ruled with our current understanding of patterns of diversification, as resulting from a complex mix of intrinsic biological and extrinsic physical factors (Droser and Finnegan, 2003). Moreover, although fundamental changes in ecosystems occurred concomittantly with the rise of the PEF, their relationships with the GOBE as causes or effects are unclear [START_REF] Botting | Unravelling causal components of the Ordovician Radiation: the Builth Inlier (central Wales) as a case study[END_REF].

b) Ecological Escalation

The possibility of ecological escalation providing a large part of the increase in diversity resolves one major problem: the permanent high diversity following the radiation [START_REF] Vermeij | Evolution and escalation: an ecological history of life[END_REF]. Indeed the appearance of one species may allow other ones to colonize an otherwise hostile environment. For example, nautiloid shells may provide a suitable substrate for bryozoans, worm tubes or 'articulate' brachiopods [START_REF] Botting | Unravelling causal components of the Ordovician Radiation: the Builth Inlier (central Wales) as a case study[END_REF], and some graptoloids can act as an attachment site for partly pseudoplanktic brachiopods [START_REF] Botting | A pseudoplanktonic inarticulate brachiopod attached to graptolites and algae[END_REF]. Once there is a sufficient diversity of organisms in a community, increasing complexity of biologically defined habitats will offer niche opportunities for further specialized organisms to arise, which in turn can lead to additional niches. When the increasing number of utilized niches balances the population requirements of the fauna for a particular environment, the ecosystem should become more stable [START_REF] Botting | Unravelling causal components of the Ordovician Radiation: the Builth Inlier (central Wales) as a case study[END_REF].

c) Nutrient supply: phytoplankton and early terrestrial plant

In addition, Servais et al. (2008) documented that the Early Palaeozoic diversification of the phytoplankton (acritarchs; Fig. 27) slightly preceded -and probably caused-major, ecological turnovers within the water-colum, such as the radiation of zooplanktonic and nektonic groups, the diversification of benthic suspension feeders, and the origination of planktotrophic larval development [START_REF] Signor | The plankton and the benthos: origins and early history of an evolving relationship[END_REF][START_REF] Nützel | Origin of planktotrophy-evidence from early molluscs[END_REF].

According to Nowak et al. (2015a), acritarch diversity started to increase in the 'lower'

Cambrian Series 2, mirroring the 'Cambrian Explosion' of metazoans. However, this radiation was followed by a low taxonomic diversity in the upper Series 3 and lower This major increase in phytoplankton diversity seems to be correlated with the Steptoean Positive Carbon Isotope Excursion δ13Ccarb (SPICE) event (Servais et al., in press). Given that most of the organisms that diversified in the Ordovician radiation were filter or suspension feeders [START_REF] Droser | The nature of diversification and paleoecology of the Ordovician Radiation with evidence from the Great Basin[END_REF][START_REF] Bambach | Ecospace utilization and guilds in marine communities through the Phanerozoic[END_REF]Guensburg and Sprinkle, 2004), the expansion of phytoplankton may imply that trophic structure played an important role in the Ordovician radiation (Servais et al., 2008;Servais et al., in press).

Following the same reasoning, [START_REF] Masuda | A great revolution of the earth-surface environment: linking the bio-invasion onto the land and the Ordovician Radiation of marine organisms[END_REF] proposed that colonization of the land by pioneer organisms could also account for the radiation. The invasion of land by plant is thought to have begun around the Cambro-Ordovician transition (see section 5.2.2). This timing is inferred from a marked inflection point, from an increasing to a decreasing trend, in the strontium isotopic ratio of the oceans [START_REF] Nakano | Chemical evolution of oceans: Strontium isotope stratigraphy[END_REF]. First bio-invasion of the land would have started a chain reaction including an attenuation of weathering on land [START_REF] Algeo | Late Devonian oceanic anoxic events and biotic crises:"rooted" in the evolution of vascular land plants[END_REF][START_REF] Gyssels | Impact of plant roots on the resistance of soils to erosion by water: a review[END_REF], changes in the outflow patterns of fluvial floods [START_REF] Schumm | Speculations concerning paleohydrologic controls of terrestrial sedimentation[END_REF][START_REF] Battin | Contributions of microbial biofilms to ecosystem processes in stream mesocosms[END_REF][START_REF] Murray | Modelling the effect of vegetation on channel pattern in bedload rivers[END_REF] and the circulation patterns of groundwater (Wood et al., 2007;[START_REF] Li | Connecting ecohydrology and hydropedology in desert shrubs: stemflow as a source of preferential flow in soils[END_REF], marked regional differentiation of coastal environments [START_REF] Marques | Diversity distribution and floristic differentiation of the coastal lowland vegetation: implications for the conservation of the Brazilian Atlantic Forest[END_REF], formation of soil layers [START_REF] Belnap | Biological soil crusts: characteristics and distribution[END_REF], and finally the eutrophication of estuaries by nutrient salts of terrestrial-biosphere origin [START_REF] Tappan | The paleobiology of plant protists[END_REF][START_REF] Tappan | Extinction or survival: Selectivity and causes of Phanerozoic crises[END_REF][START_REF] Scheckler | Floras of the Devonian-Mississippian transition[END_REF][START_REF] Algeo | Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events[END_REF]. This phenomenon would have also provided the nutrients necessary to stimulate phytoplanktonic blooms, as in many modern coastal and restricted marine envrionments [START_REF] Kuparinen | Inorganic nutrient and carbon controlled bacterioplankton growth in the Baltic sea[END_REF][START_REF] Lyons | Evidence for large pre-industrial perturbations of the Black Sea chemocline[END_REF][START_REF] Turner | Coastal eutrophication near the Mississippi river delta[END_REF][START_REF] Masuda | A great revolution of the earth-surface environment: linking the bio-invasion onto the land and the Ordovician Radiation of marine organisms[END_REF].

The extrinsic factors a) Palaeogeography, sea level and shelf area

The Ordovician is a particular period in terms of palaeogeography. The Ordovician had the greatest continental dispersal of the Palaeozoic (Fig. 32A) and was also a time of rapid seafloor spreading [START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF]. Several separate continents had emerged and separated after the break-up of the Proterozoic Rodinia supercontinent, including Gondwana, South China, Laurentia, Baltica, and Siberia (Cocks andTorsvik, 2002, 2006). Rifting of the margins of Gondwana gave birth to a several terranes and microcontinents, such as Avalonia, that drifted rapidly away. Magmatic and tectonic processes generated a number of archipelagos, such as those of the Celtic province [START_REF] Harper | The palaeogeography of early Ordovician Iapetus terranes: an integration of faunal and palaeomagnetic constraints[END_REF]. Relationships between largescale palaeogeographical changes and faunic radiations have long been noticed (e.g., Crame and Owen, 2002; but see also [START_REF] Alroy | Phanerozoic trends in the global diversity of marine invertebrates[END_REF], who considered that Mesozoic-Cenzoic diversifications culminated in the mid-Cretaceous). A parallel was drawn by Valentine and Moore (1972) between the Cambro-Ordovician and the Mesozoic-Cenozoic radiations in regards to the break-up of their respective supercontinent (Rodinia for the former and Pangea for the latter). Indeed, when diversity curves are compared with palaeogeographical settings through time (Figs. 26 and32B), a correlation seems to emerge between time of continental dispersion and radiation (Fig. 31. D; Servais et al., 2008). The highest continental dispersion is reached during the Middle Ordovician (Figs. 32A andB).

This separation of continent masses covering a wide range of palaeolatitudes probably drove major ocean circulation currents. This probably induced one of the greatest geographical differentiation of faunas on Earth (Servais et al., 2008). While the description of bioprovinces was originally limited to benthic fossil groups (see Fortey and Cocks, 2003), faunal and microfloral provinces have subsequently also been recognized in many planktonic and nektonic organisms [START_REF] Servais | Ordovician organic-walled microphytoplankton (acritarch) distribution: the global scenario[END_REF][START_REF] Servais | The importance of plankton and nekton distributions in Ordovician palaeogeographical reconstructions[END_REF]. By the late Ordovician, several of these crustal blocks were moving towards each other again, with a consequent loss of biogeographical identity. Baltica and Laurentia, together with Avalonia, formed Laurussia during the Silurian. In the Devonian, Gondwana started to collide with Laurussia, and the Carboniferous amalgamation of all the continents led to the supercontinent Pangea, with a consequent reduction of flooded continental shelf areas (Cocks andTorsvik, 2002, 2006;Torsvik and Cocks, 2013).

The Ordovician was also a greenhouse period with warm oceanic temperatures, which were higher or similar to the present equatorial sea surface temperatures [START_REF] Shields | Sr, C, and O isotope geochemistry of Ordovician brachiopods: a major isotopic event around the Middle-Late Ordovician transition[END_REF]Basset et al., 2007;Trotter et al., 2008). These high temperatures were reached because of a high carbon dioxide level reaching 8-18 PAL until a reduction at the end of Ordovician [START_REF] Berner | Geocarb II: a revised model of atmospheric CO2 over Phanerozoic time[END_REF][START_REF] Berner | Geocarb III: a revised model of atmospheric CO2 over Phanerozoic time[END_REF]Sheehan, 2001b;[START_REF] Zhang | Eustatic sea level cure for the Ashgillian-Llandovery derived from conodont community analysis, Anticosti Island, Quebec. Palaeogeogr., Palaeoclimatol[END_REF]. As a consequence, the Ordovician was characterized by the highest sea levels of the Phanerozoic (Hallam, 1992;[START_REF] Miller | The Phanerozoic record of global sea-level change[END_REF][START_REF] Haq | A Chronology of Paleozoic sea-level changes[END_REF][START_REF] Munnecke | Ordovician and Silurian seawater chemistry, sea level, and climate: Aasynopsis[END_REF], at least 100-150 m above the present one, with the development of warm and saline deep waters in low-to mid-latitude, and a sluggish circulation of surface and deep ocean water [START_REF] Railsback | Palaeontological and isotope evidence for warm saline deep waters in Ordovician oceans[END_REF]. In this regard, it is also interesting to compare the Ordovician oceanic distribution with that of the modern-day oceans (Fig. 32C). Indeed, today, the centers of marine diversification are located in the tropical shelf sea areas of southeastern Asia and, to a lesser extent, in the Caribbean Sea [START_REF] Mironov | Centers of marine fauna redistribution[END_REF][START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF]. Thus, the Ordovician Radiation probably occurred because this time was not only marked by the greatest separation of continental masses, but also because it yielded the largest tropical shelf areas in Earth's history (Fig. 31D). The extent of global shelves increased from the early Cambrian to a maximum value during the Middle Ordovician, and then decreased to its lowest levels at the Permian-Triassic boundary [START_REF] Walker | Continental drift and Phanerozoic carbonate accumulation in shallow-shelf and deep-marine settings[END_REF]. Alhough a similar extent of global shelves was reached in the Late Cretaceous, it was not as wide in the tropics as in the Middle

Ordovician [START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF]. The sea-level rise and the accompanying expansion of flooded continental shelf area would have expanded marine habitats and thus contributed to the increase of biodiversity (Figs. 32C-D; [START_REF] Albanesi | Conodonts: Lower to Middle Ordovician Record[END_REF][START_REF] Barnes | The pattern of global bio-events during the Ordovician Period[END_REF]Cooper et al., 2004). In summary, all the optimum environmental conditions to trigger a radiation were gathered during the Ordovician: a faunal provincialism enhanced by the maximum separation of continental masses, as well as a rapid sea-floor spreading coincident with warm climates, high sea levels, and the largest tropical shelf area of the Phanerozoic.

b) Tectonics and volcanism

The dramatic Ordovician radiation of all major groups, including producers and consumers, was very likely promoted by a marked increase in both nutrients availability and primary production, both probably closely linked to tectonics, volcanic activity, and/or climate change [START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF][START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF]. The influence of a putative superplume was also proposed to account for the high Ordovician sea level and the biotic radiation [START_REF] Barnes | Was there an Ordovician Superplume event?[END_REF].

One hypothesis is that the radiation was tied to increasing global tectonic activity, because related studies showed that the diversity of major benthic organisms in foreland basins or in areas of significant siliciclastic influx was much higher than that of areas farther away from such sediment input [START_REF] Miller | Association of orogenic activity with the Ordovician radiation of marine life[END_REF]Miller and Conolly, 2001). In addition, as discussed above, the rifting of the margins of Gondwana gave birth to a number of terranes and microcontinents, such as Avalonia, that drifted rapidly away due to the closure of the Iapetus Ocean and the opening of the Rheic Ocean [START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF]. Their migration through various latitudes very likely modified the circulation of oceanic currents, which may have triggered the differentiation of faunas. They also promoted the origination of endemic groups and served as the source of biotic dispersal (Harper, 2006).

c) Asteroid

A more spectacular but controversial proposal is that the GOBE is related to several asteroid impacts, which happened 470 Ma ago [START_REF] Schmitz | Asteroid breakup linked to the Great Ordovician Biodiversification Event[END_REF]. The L-chondrite parent body is the largest documented asteroid breakup that occured in the past few billion years [START_REF] Heymann | On the origin of hypersthene chondrites: Ages and shock effects of black chondrites[END_REF][START_REF] Keil | Catastrophic fragmentation of asteroids: evidence from meteorites[END_REF]. The resulting high influx of meteorites would have formed several

Middle Ordovician megabreccias on several palaeoplates and terranes (Parnell, 1999). [START_REF] Schmitz | Asteroid breakup linked to the Great Ordovician Biodiversification Event[END_REF] noted that where accurate faunal data are available, for example, from western Russia [START_REF] Rasmussen | Baltica: A mid Ordovician diversity hotspot[END_REF], there is a precise match between the main phase of the GOBE and an enhanced extraterrestrial flux. According to them, impact related environmental perturbations might have accelerated a process driven also by intrinsic biological mechanisms. In this context, minor and persistent impacts could generate diversity by creating a range of new niches across a mosaic of more heterogeneous environments.

Indeed, such diversity increases are predicted by the well-established intermediate disturbance hypothesis, initially applied to diversity changes in coral reefs and tropical rain forests [START_REF] Connell | Diversity in tropical rain forests and coral reefs[END_REF]. Frequent impacts may also have destabilized ecological communities, allowing invasive species to take over and displace incumbent communities [START_REF] Schmitz | Asteroid breakup linked to the Great Ordovician Biodiversification Event[END_REF].

d) Substrate Revolution

Substrate change has been regarded as a possible controlling factor of the Ordovician radiation [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF]. A 'Substrate Revolution' has been proposed for the Cambrian, from the microbial mat substrate of the Proterozoic to a mixed and soft substrate of the Phanerozoic [START_REF] Bottjer | The Cambrian substrate revolution[END_REF][START_REF] Chen | The Dawn of Animal World[END_REF]).

The Cambrian substrate was largely mixed. Only organisms adapted to either microbial mat or soft substrate occurred alternatively or together. Trace fossil evidence indicates that firm substrates were dominant in the early Cambrian. These sea-floors lacked infaunal organisms and vertical burrowers (see section 4.3.1). However, the substrate changed greatly in the Ordovician when soft substrates became predominant [START_REF] Orr | Colonization of the deep-marine environment during the early Phanerozoic: the ichnofaunal record[END_REF][START_REF] Buatois | The changing face of the deep: Colonization of the Early Ordovician deep-sea floor, Puna, northwest Argentina[END_REF], along with the disappearance of microbial mats, and the increase of burrowing organisms [START_REF] Bottjer | Phanerozoic development of tiering in soft substrata suspension-feeding communities[END_REF], especially vertical burrowers [START_REF] Sheehan | The trace fossil Thalassinoides from the Upper Ordovician of the eastern Great Basin: deep burrowing in the Early Paleozoic[END_REF]. On the sea floor, a mixed layer of sediments and water was formed by the increased suspension of particles, making the substrate soft and the water turbid [START_REF] Fang | Ordovician bivalve radiation in Southern China, with a discussion on the causes of diversification[END_REF]. This new substrate promoted the expansion of tiering above and within the substrate, and provided food resources for the flourishing and radiation of filter feeders [START_REF] Ausich | Tiering in suspension-feeding communities on soft substrata throughout the Phanerozoic[END_REF][START_REF] Bottjer | Phanerozoic development of tiering in soft substrata suspension-feeding communities[END_REF][START_REF] Bottjer | The ecological architecture of major events in the Phanerozoic history of marine life[END_REF]Harper, 2006;Servais et al., 2010). Other specialists have documented the abundance of hardgrounds in the Ordovician, along with the turnover from aragonite seas to calcite seas (Wilson and Palmer, 1992;[START_REF] Palmer | Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas[END_REF]. The extensive development of hardgrounds in the Ordovician may have promoted the diversification of calcareous organisms (e.g. crinoids, bryozoans and brachiopods) and the sharp rise of hardground-based communities [START_REF] Rozhnov | Evolution of the hardground community[END_REF][START_REF] Wilson | The Ordovician bioerosion revolution[END_REF][START_REF] Palmer | Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas[END_REF]Harper, 2006).

A complex phenomenon

Although many possible triggers, physical or biological, have been proposed (see above), none of them would have allowed such a sustained rise of diversification alone (Webby et al., 2004a[START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF][START_REF] Harper | The Great Ordovician Biodiversification Event: Reviewing two decades of research on diversity's big bang illustrated by mainly brachiopod data. Palaeoworld, Geologic and biotic events and their relationships during the Early to Middle[END_REF]. The GOBE more probably results from the subtle and complex interplay of various different triggers. The interaction of various parameters to explain the observed changes in diversity is well-illustrated in regional case studies, as for exemple in the Ordovician series of the Builth-Llandrindod Inlier ('Builth Inlier'), an area of around 50 km² in central Wales [START_REF] Botting | Unravelling causal components of the Ordovician Radiation: the Builth Inlier (central Wales) as a case study[END_REF]. This study demonstrates that the patterns of causal relationship and diversification are complex even at very local scales, and that, at our current level of knowledge, we should not anticipate a clear correlation of global diversity with any single factor (see Fig. 33).

Different biases affecting the GOBE

Although biodiversity diagrams (Fig. 26) show the broad outline of change, at a higher resolution they suffer from different bias, including the effects of poor correlation and poor preservation of the faunas, "monographic bursts", and data binning [START_REF] Alroy | Phanerozoic trends in the global diversity of marine invertebrates[END_REF][START_REF] Schmitz | Asteroid breakup linked to the Great Ordovician Biodiversification Event[END_REF]. For example, [START_REF] Raup | Taxonomic diversity during the Phanerozoic[END_REF] cited some nine important filters that could bias the fossil record and later [START_REF] Raup | Taxonomic diversity estimation using rarefaction[END_REF] noted that the Phanerozoic diversity curve may reflect merely the amount of available rock volume for any time interval (see also McGowan and Smith, 2011) or the efforts of taxonomists on selected intervals (Servais et al., 2010). [START_REF] Raup | Taxonomic diversity estimation using rarefaction[END_REF] thus concluded that the Phanerozoic biodiversity was probably saturated and in a state of equilibrium for much of its duration.

However, three potential explanations have been developped to explain the link between diversity and available rock volume [START_REF] Smith | Marine diversity in the geological record and its relationship to surviving bedrock area, lithofacies diversity, and original marine shelf area[END_REF]. The first is a sampling artifact acting through a species/area effect [START_REF] Raup | Species diversity in the Phanerozoic: An Interpretation[END_REF][START_REF] Smith | Large-scale heterogeneity of the fossil record: implications for Phanerozoic biodiversity studies[END_REF]. The second is that diversity and rock area respond independently to the same common driver. [START_REF] Peters | Geologic constraints on the macroevolutionary history of marine animals[END_REF] has suggested that there might be a common cause, instead of concluding that the Phanerozoic record is a mere artefact of geological processes. By instance, plate tectonic processes and eustatic changes in sea level for example would help accelerate diversity during intervals of high sea level, when platforms were flooded also providing amuch larger volume of sedimentary rocks, generating a true biological signal. This is particularly relevant for the GOBE, indicating that the accelerating diversity trends are real and not simply a function of an increased area and volume of Ordovician rocks (Servais et al., 2010). The third potential explanation is based on redundancy, i.e., a reversed causality hypothesis in which species diversity drives the number of fossil localities [START_REF] Benton | Assessing the quality of the fossil record: insights from vertebrates[END_REF].

Because these potential drivers are probably acting in concert, disentangling biological signal from sampling bias in the fossil record has proven to be very difficult (Servais et al., 2010).

Large-scale sea-level cycles have been shown to likely drive shallow-marine biodiversity [START_REF] Hannisdal | Phanerozoic Earth system evolution and marine biodiversity[END_REF]. However, comparing the land-based and deep-sea microfossil records has shown that the relative availability of rock sections also shapes recorded diversity (Lloyd et al., 2012a(Lloyd et al., , 2012b)). Furthermore, no simple link exists between sea level, shallowmarine shelf area, and marine sedimentary rock area surviving as outcrop [START_REF] Holland | Sea level change and the area of shallow-marine habitat: implications for marine biodiversity[END_REF].

Thus precisely how marine rock area, habitat diversity, and the fossil record of biodiversity are interrelated remains poorly understood [START_REF] Smith | Marine diversity in the geological record and its relationship to surviving bedrock area, lithofacies diversity, and original marine shelf area[END_REF].

By contrast, the curves of [START_REF] Alroy | Phanerozoic trends in the global diversity of marine invertebrates[END_REF] are based on locality-based counts stored in the Palaeobiology Database. [START_REF] Hammer | Biodiversity curves for the Ordovician of Baltoscandia[END_REF] applied a similar strategy in the assembly of data from the Ordovician of the Baltic province, demonstrating the unambiguous track of the event, while deconstructing the curve into patterns and trends for individual taxonomic groups. All these curves, however, are not free from error (Servais et al., 2010). The global Phanerozoic curves developed by [START_REF] Alroy | Phanerozoic trends in the global diversity of marine invertebrates[END_REF] are, for example, subject to sampling bias. Localities in older formations are possibly less numerous and thus the high levels of diversity seen in the Modern Evolutionary Fauna may simply reflect that localities within this unit are more accessible and numerous (Servais et al., 2010). Nevertheless, when the curves are standardized against sample size, the GOBE stands out as a massive hike in biodiversity rarely equalled again throughout the rest of the Phanerozoic [START_REF] Servais | Understanding the Great Ordovician Biodiversification Event (GOBE): influences of paleogeography, paleoclimate, or paleoecology[END_REF].

In most cases, when we compare diversity patterns, we must be careful to compare equivalent entities [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF]. There are at least three aspects to consider: the compared area [START_REF] Peters | Determinants of extinction in the fossil record[END_REF], duration [START_REF] Cooper | Measures of biodiversity[END_REF], and facies. For example, North America is much larger than Precordillera Argentina. If we compare the biodiversity of these two areas directly, without any consideration on sample size, it may bias the pattern [START_REF] Peters | Determinants of extinction in the fossil record[END_REF], especially when the diversity of the former is higher than that of the latter [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF]. Likewise, comparison of different time intervals within the same region must be precautious. By instance, the Tremadocian is about ten million years long, whereas the Hirnantian is only two million years long. A direct comparison of the total diversities of these two intervals is thus less valuable. Refining the time interval, such as time units or biozones may produce more easily comparable data [START_REF] Cooper | Measures of biodiversity[END_REF]. Finally, if two regions have distinct lithofacies and biofacies, a direct comparison of their biodiversity patterns will be difficult to interpret [START_REF] Best | Bivalve taphonomy in tropical mixed siliciclasticcarbonate settings. I. Environmental variation in shell condition[END_REF][START_REF] Smith | Marine diversity in the geological record and its relationship to surviving bedrock area, lithofacies diversity, and original marine shelf area[END_REF]. For example, it would be problematic to explain the diversity differences, if any, between graptolitic shale in Australia and platform facies in the Baltic region [START_REF] Zhang | Diversity history of Ordovician graptolites and its relationship with environmental change[END_REF], which could result from regional differences (such as latitude and oceanographic effects), as well as differences in lithofacies due to taphonomic specifity [START_REF] Peters | Evenness of Cambrian-Ordovician benthic marine communities in North America[END_REF] or substrate affinities [START_REF] Miller | Substrate affinities of higher taxa and the Ordovician Radiation[END_REF].

Thus, the fossil record should be treated with caution [START_REF] Holland | The quality of the fossil record: a sequence stratigraphic perspective[END_REF][START_REF] Smith | Sea-level change and rock-record bias in the Cretaceous: a problem for extinction and biodiversity studies[END_REF]. For example, rapid sea-level rise commonly results in the disappearance of shallow-water sediments and their faunas. The sudden disappearance of a shallow-water fauna could lead to the mis-recognition of an extinction event in the region, which is thus completely wrong [START_REF] Zhang | Principal aspects of the Ordovician biotic radiation[END_REF]. In this context, the dynamic processes producing variation in the sedimentary rock record, such as plate movement and sea-level changes, may also simultaneously shape evolutionary history [START_REF] Peters | Geologic constraints on the macroevolutionary history of marine animals[END_REF]. At larger scales, sedimentary facies may reflect more about the features of the environments in which organisms lived than that of the taphonomic history [START_REF] Ellingsen | Soft-sediment benthic biodiversity on the continental shelf in relation to environmental variability[END_REF][START_REF] Peters | Evenness of Cambrian-Ordovician benthic marine communities in North America[END_REF]. Accordingly, the parallel histories of the fossil record and sedimentary facies may imply largely the dependence of specific fossils on particular living environments, such as latitude, water depth, and ocean currents [START_REF] Zhang | Diversity history of Ordovician graptolites and its relationship with environmental change[END_REF]).

The GOBE is clearly one of the major events in the history of life. Nevertheless, it is so complex that it is still poorly understood, especially its putative causes [START_REF] Harper | The Great Ordovician Biodiversification Event: Reviewing two decades of research on diversity's big bang illustrated by mainly brachiopod data. Palaeoworld, Geologic and biotic events and their relationships during the Early to Middle[END_REF].

In order to truly test the possible influence of extrinsic or intrinsic controls, a number of data still need to be collected. Regional patterns, such as those preliminarily identified for the Builth-Llandrindod Inlier area, are needed from a larger variety of areas [START_REF] Botting | Unravelling causal components of the Ordovician Radiation: the Builth Inlier (central Wales) as a case study[END_REF]. Geochemical data have proven to be instrumental to our understanding of the events surrounding the Precambrian-Cambrian explosion and may yield important insights to the possible extrinsic controls of the Ordovician radiation but only few data are yet available [START_REF] Barnes | Was there an Ordovician Superplume event?[END_REF]. Finally, the better understanding of the biogeographic and palaeogeographic patterns will provide a perspective on the global nature of the event [START_REF] Harper | Chapter 1 Early Palaeozoic biogeography and palaeogeography: towards a modern synthesis[END_REF]. How global is the pattern and how much of the pattern is a compilation of individual clades and regions is a key question.

Chapter II The Fezouata Shale : temporal, stratigraphical, and envrionmental contexts 1 The Fezouata Lagerstätte: a new light on the rise of animal life

As seen in the previous chapter, the rise of animal life can be summarized in three main evolutionary steps: the Ediacaran fauna, the Cambrian Explosion, and the Great Ordovician Biodiversification Event. Though each event begins to be extensively documented, neither their causes, nor their relationships have been elucidated (see chapter I). It seems more and more plausible that these three events are an artefact of the discontinuity of the fossil record (see below). They probably are just part of a same continuous event, which were repeated at lower cladistics scale in the history of life (see for example the rise of mammals, where the different morphological groups first appeared and then diversified [START_REF] Archibald | Extinction and radiation: how the fall of dinosaurs led to the rise of mammals[END_REF].

One main hardship to overcome is that the Cambrian Explosion and the GOBE are not based on the same kind of data. Cambrian Explosion is extensively documented because of the unusually high numbers of soft-bodied fauna during the Cambrian (see chapter 1, section 4.3.2.2). On the other hand, the GOBE is mainly highlighted by the shelly fossil record (see chapter 1, section 5.2.1). However, under normal taphonomic conditions, up to 80% of taxa in a living marine community are not preserved (e.g. [START_REF] Dorjes | Georgia coastal region, Sapelo Island, USA: sedimentology and biology. VIII. Distribution and zonation of macrobenthic animals[END_REF][START_REF] Driscoll | Diversity and structure of epifaunal communities on mollusc valves, Buzzards Bay, Massachusetts[END_REF]. The scarcity of Lagerstätten in the Ordovician had as-yet considerably limited the use of the high-resolution data for the study of the GOBE (Lefebvre et al., in press a).

1.1

The temporal gap of Cambro-Ordovician Lagerstätten Until recently, the late Cambrian and the Early Ordovician, appeared to be nearly barren of any diverse soft-bodied fauna (Fig. 34), see the main macrofossil Lagerstätten list in appendix for references). Before 2010, only five diverse exceptional sites, all of Middle to Late Ordovician age, were known: the Darriwilian Winneshiek Lagerstätte from Iowa (Liu et al., 2006), Beecher's Trilobite Bed (Briggs et al., 1991) and adjacent localities from the Sandbian of New York (Farrell et al., 2009), the Katian Manitoba Lagerstätten of William Lake and Airport Cove (Young et al., 2007), and the Soom Shale from the Hirnantian to Rhuddanian (Vandenbroucke et al., 2009) of South Africa (Aldridge et al., 2001). Moreover, they represented low-diversity communities inhabiting atypical environments. Winneshiek represents a near-shore marginal environment, interpreted to be the result of a meteorite impact [START_REF] Liu | The Winneshiek Lagerstätte and its depositional environments [in Chinese with English summary[END_REF][START_REF] Mckay | Preservation of the Middle Ordovician Winneshiek Shale in a probable impact crater[END_REF] Ordovician of Wales (Botting et al., 2015b), Morocco (Van Roy et al., 2010), China [START_REF] Botting | Echinoderm diversity and environmental distribution in the Ordovician of the Builth Inlier[END_REF][START_REF] Balinski | Fenxiang biota: a new Early Ordovician shallow-water fauna with soft-part preservation from China[END_REF] and Argentina [START_REF] Aris | Primer registro de una fauna Ordovícica 'tipo Burgess Shale' en Argentina y Sudamérica[END_REF]. Moreover, one new Lagerstätte was found in the late Cambrian of China: the Guole Konservat-Lagerstätte [START_REF] Zhu | Furongian (upper Cambrian) Guole Konservat-Lagerstätte from South China[END_REF]. Though, there are still a lot of ongoing studies, these new findings already dramatically altered our vision of the rise of animal life. Many taxa that were considered extinct since the middle Cambrian could be documented in the fossil record more than 20 million years after (Fig. 35). The Fezouata Shale may be one of the most diversified and best studied Lagerstätten, recently found in the Tremadocian and Floian of Morocco [START_REF] Van Roy | fig. S2c) is closely related to both Cornuta indet. A from the late Cambrian of Korea[END_REF], 2015). In the next part of this contribution, we will thus focus on how our knowledge on this soft-bodied fauna of Morocco was critically improved in order to bring crucial information on the relationships between the Cambrian Explosion and the GOBE 2013). C, brachiopod with soft tissue from the Floian Fenxiang Lagerstätte (from [START_REF] Balinski | Fenxiang biota: a new Early Ordovician shallow-water fauna with soft-part preservation from China[END_REF]. D, undescribed mollisoniid-like arthropod from the Tremadocian Afon Gam biota (from Botting et al., 2015b). E, Diania-like lobopodian from the Fezouata Shale (courtesy of P. Van Roy).

F, Tremaglaspis vanroyi from the Guzhangian Weeks Formation (from Lerosey-Aubril et al., 2013).

G, Wronascolex iacoborum from the early Cambrian Emu Bay Shale (from García-Bellido et al., 2013). H, Brachiopod with soft tissue from the early Cambrian Chengjiang biota (from Hou et al., 1999). I, Mollisonia sinica from the middle Cambrian Kaili biota (from Zhang X. G. et al., 2002). J, Diania cactiformis from early Cambrian Chengjiang biota [START_REF] Liu | An armoured Cambrian lobopodian from China with arthropod-like appendages[END_REF]. Scale bars: 1 cm for A, E-F, and J; 5 mm for C-D and H-I; 100 μm for B, and G. C). In this context, four successive field trips were organised to the Lower Ordovician of the Zagora area from 2002 to 2004, where several hundreds of fully articulated echinoderms were collected (Lefebvre, 2007;Lefebvre and Botting, 2007;Lefebvre and Ware, 2007;Van Roy et al., 2010;Sumrall and Zamora, 2011;Allaire et al., 2015;Martin et al., 2015) but also extensive samples of the associated fauna, including both shelly and exceptionally-preserved fossils (e.g. demosponges, and the first marrellomorph specimen). However, over the next couple of years, several additional soft-bodied taxa, including marrellomorphs and problematica, were discovered (Van Roy, 2006;[START_REF] Van Roy | A spinose appendage fragment of a problematic arthropod from the Early Ordovician of Morocco[END_REF] but overall, exceptional preservation remained scarce. The study of components of the shelly faunas started to reveal a much greater diversity than initially assessed (e.g. Lefebvre and Fatka, 2003;Lefebvre and Botting, 2007;Fortey, 2009Fortey, , 2011[START_REF] Fortey | The first known complete Lichakephalid trilobite, lower Ordovician of Morocco[END_REF]Van Roy et al., 2010, 2015a;Sumrall and Zamora, 2011;Kröger and Lefebvre, 2012;Ebbestad and Lefebvre, 2015). Two major finds were made in December 2006 with the discovery of the oldest horseshoe crabs (Van Roy et al., 2010, 2015a) and the first machaeridian specimen preserving soft tissues (Vinther et al., 2008). This revived the interest for the biota, and in September 2008, a successful application was made to the National Geographic Society for a Research and Exploration grant to allow extensive fieldwork in the area. The funded field season of April yielded an unexpectedly large diversity of nonbiomineralised organisms from a host of sites (Lefebvre et al., in press a). Importantly, the recovered faunas include essentially all classical Burgess Shale-type taxa, including giant anomalocaridids, together with significantly more advanced, typically post-Cambrian forms.

This newly collected material prompted the re-evaluation of the Fezouata Shale as one of the rare (and probably the most diverse) Early Ordovician Lagerstätten documenting the initial stage of the GOBE (Fig. 37; Van Roy et al., 2010). First, the exquisite preservation of soft part anatomy in various taxa of the Fezouata Biota allows more complete, accurate reconstructions of their biology, phylogenetic affinities and ecology than has hitherto been possible (e.g. Van Roy, 2006;[START_REF] Van Roy | A spinose appendage fragment of a problematic arthropod from the Early Ordovician of Morocco[END_REF]Van Roy and Briggs, 2011;Botting, 2007;Vinther et al., 2008;Fatka et al., 2013;Van Roy et al., 2015b;Ortega-Hernández et al., in press).

An utmost scientific importance

More importantly, over 160 different taxa, represented by several thousand specimens, are known from the Fezouata Biota (Van Roy et al., 2015b). The assemblages comprise numerous exquisitely preserved remains of shelly taxa typical of post-Cambrian faunas: e.g., articulate brachiopods, asterozoans, bivalves, cornute and mitrate stylophorans, crinoids, edrioasteroids, gastropods, glyptocystitid rhombiferans, graptolites, hyolithids, machaeridians, nautiloid cephalopods, ostracods, and trilobites (Destombes et al., 1985;Vidal, 1998;Vinther et al., 2008;Noailles et al., 2010), together with exceptionally preserved soft-bodied taxa comprising several representatives of the Palaeozoic Evolutionary Fauna: e.g., aglaspidid and cheloniellid arthropods, cirriped crustaceans, xiphosurans (Van Roy, 2006;[START_REF] Van Roy | fig. S2c) is closely related to both Cornuta indet. A from the late Cambrian of Korea[END_REF]. More strikingly, the Fezouata Biota also contains extremely numerous and diverse non-biomineralized taxa typical of early to middle Cambrian Burgess Shale type assemblages: e.g., anomalocaridids, demosponges, halkieriids, marrellomorphs, naraoids, palaeoscolecids, wapkiids (Van Roy, 2006;Botting, 2007;Van Roy et al., 2010;Van Roy and Briggs, 2011;Martin et al., in press b).

This incredible diversity of the Fezouata Biota shed a new light on the context of the rise of animal life. Indeed, the Fezouata Biota contains most of the emblematic components of Cambrian Burgess Shale-type faunas (Figs. 35 B and F;Van Roy et al., 2010, 2015a,b;Van Roy and Briggs, 2011) together with considerably more advanced forms (Figs. 35 A andC), typical of post-Cambrian strata. Until this discovery, it was believed that Burgess Shale-type taxa had essentially disappeared after the middle Cambrian. (Lefebvre et al., in press a). The persistence of typical Burgess Shale-type taxa into the Ordovician indicates that the sudden replacement of the CEF by the PEF depicted by the shelly fossil record might be an artefact due to the lack of soft-bodied and lightly-sclerotised organisms. Indeed, soft-bodied organisms reveal that the decline of typical Cambrian organisms, including those once regarded as 'weird wonders' (Gould, 1989), was slow, gradual, and all along accompanied with the evolution of components of the Palaeozoic Evolutionary Fauna (Martin et al., 2013).

Actually, the stratigraphical range of several Burgess Shale-type resurgences is known to extend well after the Ordovician: up to the Silurian for naraoiids [START_REF] Caron | A new Late Silurian (Pridolian) Naraoiid (Euarthropoda: Nektaspida) from the Bertie Formation of Southern Ontario Canada: delayed fallout from the Cambrian Explosion[END_REF], the Early Devonian for marrellomorphs [START_REF] Kühl | Morphology, paleobiology and phylogenetic position of Vachonisia rogeri (Arthropoda) from the Lower Devonian HunsrückSlate (Germany)[END_REF][START_REF] Kühl | Re-investigation of Mimetaster hexagonalis: a marrellomorph arthropod from the Lower Devonian Hunsrück Slate (Germany)[END_REF], radiodont arthropods (Kühl et al., 2009) and eldonioids [START_REF] Friend | Middle Cambrian Eldonia from the Siberian Platform[END_REF], and the Carboniferous for some Cambrian-type lobopodians [START_REF] Haug | A Carboniferous non-onychophoran lobopodian reveals long-term survival of a Cambrian morphotype[END_REF].

On the other hand, the presence of a significant number of advanced, typically post-Cambrian soft-bodied taxa alongside Burgess Shale-type elements in the Fezouata Biota indicates that significant evolutionary developments must have occurred well before the Tremadocian. This implies that the GOBE was considerably more advanced at this time than has been appreciated, hinting at Cambrian origins and diversification for several typically post-Cambrian non-biomineralised groups (e.g. chelicerates). This ultimately leads to question whether the Cambrian and Ordovician radiations were discrete events, or rather the initial and latest stages of a same major biodiversification (Droser and Finnegan, 2003;Van Roy et al., 2010, 2015a). However, the detailed study of the Fezouata communities was hampered by several impediments. Indeed, soft-bodied fossils were collected from several localities scattered in an area of about 600 km 2 , mainly North and Northeast of Zagora (Fig. 38). All those localities lacked a well-constrained spatio-temporal frame, which will be provided in the next part of this chapter.

The geological context of the Fezouata Shale

The Geological structure of Morocco

The geological structure of Morocco results from three consecutive orogenic events: the Panafrican (685-560 Ma; [START_REF] Leblanc | Interprétation géodynamique du domaine pan-africain (Précambrien terminal) de l'Anti-Atlas (Maroc) à partir de données géologiques et géochronologiques[END_REF][START_REF] Ennih | The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton[END_REF], the Hercynian (380-280 Ma;[START_REF] Piqué | Histoire géologique du Maroc, découverte et itinéraires[END_REF]Burkhard et al., 2006), and the Alpide (120-0 Ma; [START_REF] Rosenbaum | Relative motions of Africa, Iberia and Europe during Alpine orogeny[END_REF]. These different events separated Morocco into five structural domains (Figs. 39 A and B;Choubert 1952 ;[START_REF] Michard | Elements de géologie marocaine[END_REF][START_REF] Piqué | Moroccan Hercynides; a synopsis; the Paleozoic sedimentary and tectonic evolution at the northern margin of West Africa[END_REF]): the Rif and Tell, the Meseta, the Atlas, the Anti-Atlas, and the Reguibat arch. , 1976, 2005;[START_REF] Villeneuve | Structure, evolution and palaeogeography of the West African craton and bordering belts during the Neoproterozoic[END_REF][START_REF] Ennih | The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton[END_REF][START_REF] Villeneuve | Événements panafricains dans l'Adrar Souttouf (Sahara marocain)[END_REF][START_REF] Schofield | Timing and kinematics of Eburnean tectonics in the central Reguibat Shield, Mauritania[END_REF][START_REF] Schofield | A tectonic interpretation of "Eburnean terrane" outliers in the Reguibat Shield, Mauritania[END_REF]. Location of the oceanic margin anomaly after [START_REF] Sahabi | Un nouveau point de départ pour l'histoire de l'Atlantique central[END_REF]. SAF: South Atlas fault. B, Landsat image of Morocco (orange frame and arrow on A).

2.2

The Anti-Atlas

Historical background of discovery

A comprehensive historical review of the Anti-Atlas study has been recently published (Lefebvre et al., in press a). Only the main historical step will thus be presented herein.

Although inhabited since at least the late Palaeolotihic, the Anti-Atlas remains largely unstudied until the late 19 th (de Foucauld, 1888, Gentil, 1929, Despujols, 1933, Choubert, 1952a, Willefert, 2004 and Destombes, 2006a). [START_REF] Jackson | An account of the empire of Marocco and the district of Suse, compiled from miscellaneous observations made during a long residence in, and various journies through, these countries[END_REF] was the first to mention and illustrate the occurrence of a long mountain range (South Atlas mountains) extending along the southern side of the High Atlas (North Atlas mountains), from the Atlantic ocean, south of the Sous plain (Suse, in the SW) to the Tafilalt (Bled Fillely, in the NE; Lefebvre et al., in press a). In 1878, the term Anti-Atlas was officially coined by Hooker and Ball for this extensive mountain range. Indeed, during a scientific exploration of the western High-Atlas in 1871, the British geologist George Maw (1832Maw ( -1912)), accompanied by the botanist Joseph Hooker (1817Hooker ( -1911) ) and the geographer John Ball (1818-1889) could observe from the summit of Jbel Tezah, that this long mountain range was running in front of (Anti-), and more or less parallel to the High Atlas (Fig. 40). Then, the Scottish geologist Joseph Thomson (1858Thomson ( -1895) ) published the first detailed map of the Anti-Atlas in 1888, though he misdated the range as Cretaceous in age [START_REF] Thomson | Geology of southern Morocco and the Atlas mountains[END_REF]. Many precise scientific expeditions were lead in the Atlas from 1862 to 1884 (see Lefebvre et al., in press a for details). All these newly acquired geographic data were summarised on the maps produced by Schnell ( 1892) and de Flotte de Roquevaire ( 1897), resulting in extremely detailed and accurate representations.

Geology of the Anti-Atlas

The Anti-Atlas range is located between the Reguibat arch at the South and the High Atlas at the North, being delimited by the South-Atlas fault filled by Cretaceous and Tertiary deposits. This fault continues southeast in the Ougarta and up to the Mauritanian Zemmour, and East in the Béchar basin. The Anti-Atlas range is about 150-km wide and 700-km long, being oriented ENE-WSW.

Structurally speaking, the Anti-Atlas is a large anticlinal resulting from the Hercynian orogeny (Fig. 41; [START_REF] Piqué | Moroccan Hercynides; a synopsis; the Paleozoic sedimentary and tectonic evolution at the northern margin of West Africa[END_REF]. Its base is Precambrian and has been affected by the Panafrican orogeny [START_REF] Ennih | The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton[END_REF]. It is covered by Late Proterozoic and Palaeozoic deposits, the latter being especially developped in the southern part, where they form the Jbel Bani and the northern border of the Tindouf basin. Along the axis of the range, the anticline is eroded, so that Precambrian deposits regularly crop out. The largest occurrences of such Precambrian rocks are the low Draa, Ifni, Kerdous, Igherm, Zenaga, Bou Azzer, Saghro, and Ougnate.

Outcropping conditions in the Anti-Atlas are exceptionally good, as Palaeozoic series are almost intact. Indeed, the area was mostly spared from the several orogenic events that occurred during the Phanerozoic. The Alpine orogeny was mainly accomodated by the High-Atlas and the Rif in northern Morocco, which acted like a protector shield. The Hercynian orogeny, however, lightly affected the Anti-Atlas, resulting in kilometric folds of high wavelength. This enabled a very good exposure of geological outcrops along three main axis, which defined three subdivisions of the Anti-Atlas (Choubert, 1952) to delimit each part, so that they vary according to authors (Vidal, 1996). Due to these exceptional outcropping conditions, the Anti-Atlas was a favored area to establish the Lower Palaeozoic stratigraphy (Choubert, 1952), especially as far as the Ordovician is concerned (Destombes, 1960(Destombes, , 1971;;Destombes et al. 1985 ;Gutierrez-Marco et al., 2003 ;[START_REF] Ghienne | The Cambrian-Ordovician succession in the Ougarta Range (western Algeria, North Africa) and interference of the Late Ordovician glaciation on the development of the Lower Palaeozoic transgression on northern Gondwana[END_REF]. Marante, 2008 andHollard et al., 1985). Lower and Middle Ordovician deposits (in orange) are the thickest in the area of Zagora. Ordovician series can reach a thickness of up to 2.500-m in the Western Anti-Atlas. They are mainly characterized by an alternation of silty shales and sandstone bars, first formally described in 1931 (Bigot and Dubois). After that, the Ordovician series were divided into four lithostratigraphic groups (Fig. 42;Choubert, 1942;Choubert et al., 1947), which are in the chronological order: the Outer Feijas Shale, the First Bani Sandstone, the Ktaoua Clay and Sandstone, and the Second Bani Sandstone.

The Lower Ordovician stratigraphy of the Anti-Atlas

The first and third groups are mainly composed of tender silty shales, resulting in vast depressions in the landscape, while the second and the fourth groups are sandstonedominated, forming two big cuestas (hence the name of First and Second Bani).

These stratigraphic groups will be later subdivided into several formations (Fig. 42), mainly through the highly detailed studies lead by Jacques Destombes since 1959 during his intensive cartography campaigns (Destombes 1960(Destombes , 1971(Destombes , 2006a-j;-j;Destombes et al. 1985).

The stratigraphy of the Outer Feijas Group will be discussed below in greater detail, as it comprises the Lower Ordovician Fezouata Lagerstätte, which is the subject of this contribution.

The Outer Feijas Group was extensively studied in the central Anti-Atlas, especially around the town of Zagora, because of the thickness of Lower Ordovician deposits in this area (Fig. 41). Four formations were distinguished by Destombes (1960Destombes ( , 1971Destombes ( , 2006a-j;-j;Destombes et al. 1985): the Lower and Upper Fezouata Shale, the Zini Sandstone and Quartzite, and the Tachilla Shale formations .

The Lower Fezouata Shale Formation (Tremadocian) consists of transgressive deposits, unconformably deposited over the sandstones of the underlying middle Cambrian Tabanite Group. Apart from its (locally) conglomeratic base, this formation is entirely constituted by blue-green silty shales and includes several glauconitic layers. Its thickness is relatively constant around 300-400 m in the central Anti-Atlas. Its Tremadocian dating was mainly based on macro-and micro-fossils (Destombes et al., 1985;Elaouad-Debbaj 1987, 1988) The overlying Upper Fezouata Shale Formation (Floian) reaches 600-700 m of thickness in the central Anti-Atlas, around Zagora. As its base does not correspond to any specific bed, it is in direct sedimentary continuity avec the underlying Lower Fezouata Shale Formation. It is mostly composed of green-yellow silty shales,. containing concretions and rare glauconitic layers, especially in the lower and middle part of the Upper Fezouata Shale Formation. In its upper part, deposits are much coarser, becoming more and more silty and sandy. This part delivered huge carbonated lenses. Many fossils, especially trilobites, were collected by Destombes in this formation. Again, its dating was mainly based on macro-and micro-fossils (Destombes et al., 1985;Elaouad-Debbaj 1987, 1988).

Both the Lower and Upper Fezouata Shale formations were defined by Destombes (1970) in the Fezouata plain, located near Zagora. However, as no real boundary can be evidenced between the two formations in the central part of the Anti-Atlas, it was suggested to reunite them into a single lithologic unit termed the Fezouata Shale (Martin et al., in press a). The more precise dating of the different parts of the Fezouata Shale (Tremadocian -Floian) was obtained recently, based on various fossil groups, such as graptolites (Martin et al., in press a; Gutiérrez-Marco and Martin, submitted), trilobites (Martin et al., submitted), conodonts (Lehnert et al., in press), and palynomorphs (Nowak et al., 2015, in press).

The Fezouata Shale is conformably overlain by the Zini Sandstone and Quartzite Formation (late Floian), which was originally defined in the southwestern Anti-Atlas. There, it corresponds to a massive succession of quartzites and sandstones intercalated by tigilittes (e.g. at Jbel Bani). This formation is considered as a diachronic facies laterally equivalent to the middle and upper parts of the Fezouata Shale. The transition from silty shales to sandstones is progressive and occurs at different stratigraphic levels depending on the area. In the central Anti-Atlas, the silty shales facies is dominant, so that the Zini Sandstone and Quartzite Formation is represented by rare and thin sandstone lenses (e.g. at Jbel Tibasksoutine, South-East of Zagora). This formation is poorly fossiliferous and has been considered as a lithologic equivalent to the Armorican Quartzite (Armorican Massif, Portugal, Spain) and the "intermediate sandstone" of the Algerian Sahara (Destombes et al., 1985).

Finally, the Tachilla Shale Formation (Dapingian-Darriwilian) is the last unit belonging to the Outer Feijas Shale Group. Being Middle Ordovician in age, this formation will not be studied in details in this contribution.

2.3

The biostratigraphic and environmental frameworks of the Fezouata Shale Until 2014, most preliminary palaeontological investigations of soft-bodied taxa occurring in the Fezouata Shale lacked sound data on the stratigraphic positions and datings of the different fossiliferous localities (Martin et al., 2013). In this context, and under the auspices

Introduction

The Cambrian-Ordovician time interval (c. 541-443 Ma) is characterised by the evolution and radiation of animals that led to irreversible changes in marine ecosystems (Bambach et al., 2007;[START_REF] Vannier | Early Cambrian origin of modern food webs: evidence from predator arrow worms[END_REF][START_REF] Butterfield | Animals and the invention of the Phanerozoic Earth system[END_REF]Erwin and Valentine, 2013). Triggered by complex interacting factors [START_REF] Smith | Causes of the Cambrian explosion[END_REF][START_REF] Maruyama | Initiation of leaking Earth: an ultimate trigger of the Cambrian explosion[END_REF][START_REF] Santosh | The Cambrian explosion: plumedriven birth of the second ecosystem on Earth[END_REF]Zhang et al., 2014), the Cambrian Explosion is the initial visible phase of animal diversification and is defined by the appearance in the fossil record of a wide spectrum of new anatomies and functionalities, and by complex ecological systems that have no counterparts in the Precambrian (Erwin et al., 2011;[START_REF] Shu | Birth and early evolution of metazoans[END_REF]. This evolutionary and ecological revolution is evidenced by several early to middle Cambrian Konservat-Lagerstätten (sensu Seilacher, 1970; termed "Lagerstätten" below; Fig. 1), which have yielded abundant and diverse remains of both shelly and non-biomineralised organisms such as those of Chengjiang (Hou et al., 2004;[START_REF] Zhao | Quantitative analysis of taphofacies and paleocommunities in the Early Cambrian Chengjiang Lagerstätte[END_REF], Sirius Passet (Peel and Ineson, 2011), Emu Bay [START_REF] Paterson | The "great appendage" arthropod Tanglangia: biogeographic connections between early Cambrian biotas of Australia and South China[END_REF], or the Burgess Shale [START_REF] Caron | Paleoecology of the Greater Phyllopod Bed community, Burgess Shale[END_REF]. They provide detailed information on the anatomy, lifestyles, and behaviours of early animals and also on their interactions within the evolving trophic web (Dunne et al., 2008;[START_REF] Vannier | Gut contents as direct indicators for trophic relationships in the Cambrian marine ecosystem[END_REF]. Postdating the Cambrian Explosion, the Great Ordovician Biodiversification Event (GOBE) represents the second major burst in marine biodiversity. During most of the Ordovician Period, a sustained exponential diversification took place within almost all animal phyla (Harper, 2006;Servais et al., 2008Servais et al., , 2010)). Unlike the Cambrian Explosion, the GOBE is primarily recognised from diversity analyses based on the fossil record of shelly faunas and microfossils. Indeed, Ordovician Lagerstätten are rare and most of them occur in the Upper Ordovician, long after the initial stages of the GOBE (Fig. 1). Moreover, they typically contain low diversity faunas that lived in restricted marine environments (e.g. Beecher's Trilobite Bed from the Lorraine Group, [START_REF] Farrell | Paleoredox and pyritization of soft-bodied fossils in the Ordovician Frankfort Shale of New York[END_REF]Soom Shale, Gabbott, 1998; William Lake Biota of Manitoba, Young et al., 2007).

The recent discovery of a Burgess Shale-type Lagerstätte in the Lower Ordovician of Morocco (Zagora area, Central Anti-Atlas, Fig. 2A; Van Roy et al., 2010) provided unexpected and unprecedented possibilities to obtain high-resolution information on both the aftermath of the Cambrian Explosion and the very beginning of the Ordovician radiation event. Although still largely undescribed, the diverse fossil assemblages of the Fezouata Biota (Fig. 3) are characterised by the cooccurrence of organisms typical of Cambrian Burgess Shale-type biotas (Van Roy et al., 2010), such as anomalocaridids (Van Roy and Briggs, 2011;Gaines et al., 2012a;Van Roy et al., 2015) and some demosponges (Botting, 2007) with classical Ordovician faunal elements. The biomineralised component of the Fezouata Biota is dominated by trilobites (e.g. Destombes, 1972;[START_REF] Rábano | Trilobites del Museo Geominero. I. Platypeltoides magrebiensis n. sp. (Asaphina, Nileidae), del Ordovícico inferior del Anti-Atlas central (Marruecos)[END_REF]Henry et al., 1992;Vidal, 1998a,b;Chatterton and Fortey, 2008;Fortey, 2009Fortey, , 2011a,b) ,b) and echinoderms (Ubaghs, 1963;Chauvel, 1966Chauvel, , 1969Chauvel, , 1971aChauvel, ,b, 1978;;Chauvel and Régnault, 1986;Donovan and Savill, 1988;Lefebvre and Botting, 2007;Noailles et al., 2010;Sumrall and Zamora, 2011). The fauna also contains a great abundance of xiphosurid arthropods (Van Roy et al., 2010), palaeoscolecidan worms, annelids (machaeridians, Vinther et al., 2008), molluscs (Babin and Destombes, 1990;Horný, 1997;Kröger and Lefebvre, 2012), hyolithids (Marek, 1983;Valent et al., 2013), brachiopods (Havliček, 1971;Mergl, 1981Mergl, , 1988)), and conularids (Destombes et al., 1985). Graptolites are also abundant but have not been studied in detail so far (Destombes and Willefert, 1959;Willefert in Destombes et al., 1985;Aceñolaza et al., 1996). Despite this remarkable diversity, two main obstacles have so far limited the impact of the Fezouata Lagerstätte. First, most fossils were collected from isolated excavations lacking an accurate positioning within the lithological succession. This led Van Roy et al. ( 2010) to hypothesise that the Fezouata Lagerstätte could span a time period of almost 10 Myr (late Tremadocian to late Floian). Second, major uncertainties remained concerning the environmental setting of the Fezouata Biota, especially in terms of bathymetry, energy and oxygenation. The Lower Ordovician succession in the Zagora area has remained virtually unstudied since the pioneering work of Destombes et al. (1985). Based on extensive fieldwork in the Zagora area, new fossil studies, and facies recognition, we here provide clarification on these two crucial chronological and environmental issues, making it possible for the first time 1) to establish precise comparisons with other Lower Palaeozoic Lagerstätten and 2) to test the potential of the Fezouata Lagerstätte in resolving the pace and amplitude of the animal diversification in the Early Palaeozoic.

Geological setting

In the Anti-Atlas of south-eastern Morocco, the Cambrian-Ordovician transition is exposed in the Ternata plain, 30 km north of Zagora. The Guzhangian (Cambrian series 3) sandstones of the Tabanite Group are unconformably overlain by the Tremadocian to Floian (Lower Ordovician) deposits of the Lower Fezouata, the Upper Fezouata, and the Zini formations, which, together with the Tachilla Formation (Middle Ordovician), form the Outer Feijas Group (Destombes et al., 1985;Geyer and Landing, 2006). The Lower and Upper Fezouata formations constitute a thick (up to 1200 m) and rather monotonous sequence dominated by silty and micaceous argillites. A glauconitic and ferruginous horizon has often been used as a possible lithological marker for the boundary between the two formations, which have been regarded as Tremadocian and Floian in age respectively (Destombes et al., 1985). However, this horizon is missing in the Zagora area, where the Fezouata Lagerstätte is located. Consequently, the Lower and Upper Fezouata formations are here considered together as a single lithological unit: the Fezouata Shale, which is topped by the sandstone-dominated Zini Formation (Destombes, 1962;Destombes et al., 1985).

During the Early Ordovician, the Anti-Atlas was located on the southern margin of Gondwana and situated in the southern hemisphere at high latitudes (c. 65°S), relatively close to the South Pole (Fig. 2B, Cocks and Torsvik, 2004;[START_REF] Torsvik | BugPlates: Linking Biogeography and Palaeogeography[END_REF]Cocks, 2011, 2013). The Lower Ordovician sediments were deposited in a rifting context associated with the northward drift of Avalonia away from the Gondwanan palaeocontinent (Cocks and Torsvik, 2004).

Material and methods

Exceptional preservation

The Fezouata Lagerstätte is a Konservat-Lagerstätte that is by definition characterised by the occurrence of exceptionally well-preserved fossils (EPF). In the Fezouata Lagerstätte, EPF were soft-bodied and lightly sclerotised organisms (e.g. worms, sponges, arthropods) that are usually preserved via pyritisation and subsequent weathering to iron oxides, giving the fossils a reddish colour. EPF typically display more or less complete anatomical (e.g. digestive structures) and exoskeletal features (e.g. arthropod appendages).

Fieldwork

The whole Lower Ordovician succession exposed in the Zagora area (Central Anti-Atlas, Morocco; Fig. 2C) was logged at a decimetre scale from fourteen different sections (black and red crosses on Fig. 2C). Data were compiled in a single, synthetic c. one thousand-metre-thick stratigraphic column. All localities yielding EPF mentioned in the original contribution of Van Roy et al. (2010) were included in this study, except one located c. 15 km west of the main outcrop area (blue asterisk on Fig. 2C). EPF localities were carefully located and positioned along the stratigraphic column based on their lithological characteristics (colour, composition, marker beds) and faunal elements (mostly graptolites, trilobites, and to a lesser extent echinoderms). Graptolites were collected throughout the succession, but particular attention was paid to horizons yielding EPF. Changes in lithology and sedimentary structures were observed throughout the succession and were used to reconstruct short-term and long-term relative sea-level changes. EPF and graptolites were photographed using a Nikon D3X camera (with a Micro-Nikkor AF 60 mm f/2.8D macrolens) and a Leica MZ12.5 microscope, equipped with a Leica integrated digital camera. When necessary, fossils were immersed under dilute ethanol in order to enhance contrast and avoid reflective spots. All fossil specimens are deposited in the collections of the Cadi Ayyad University, Marrakesh (Faculté des Sciences et Techniques, Guéliz; sample number prefix AA), except for one, which is deposited in the palaeontological collections of Lyon 1 University, Villeurbanne (sample number prefix FSL).

Results and interpretations

Stratigraphic distribution and age of the Fezouata Biota

The lithology of the Fezouata Shale Formation is fairly homogeneous and largely dominated by fine-grained siliciclastic deposits, except in its lowermost and uppermost parts. The lower part of the Fezouata Shale is characterised by a transgression from a foreshore environment, whereas its upper part records a regressive trend. The distribution of EPF throughout the thousand-metre-thick Fezouata Shale shows that exceptional preservation occurs within a restricted interval in the lower part of the succession. Fig. 4 summarises the stratigraphic distribution of graptolites across this interval and its facial characteristics (colour, lithology, sedimentary structures). About three hundred well-preserved graptolite specimens were collected, identified, and used for dating the lower part of the Fezouata Shale. Graptolites also occur locally as monospecific mass-occurrences of Araneograptus murrayi (Hall) and "Tetragraptus" bulmani Thomas. The facies evolution within the entire succession and the structural context of the Lower Ordovician in the area of Zagora are beyond the scope of this contribution and will be the subjects of a separate study.

The studied succession is rather homogeneous, being composed of mainly thinly-bedded argillites with frequent intercalations of silts and millimetre-to centimetre-thick silty to sandy beds. Facies variations consist of slight differences in argillite composition (e.g. presence of silts or micas), or in diagenesis (e.g. induration, bedding). EPF are restricted to a 70-metre-thick interval developed between 260 and 330 m above the basal contact with the Guzhangian Tabanite Group and are usually found in relatively fine facies, the beds overlying them being coarser. They occur within two distinct intervals of c. 25 (interval 1) and 15 (interval 2) metres in thickness respectively, separated by a c. 30-metre-thick interval where only biomineralised remains are found (e.g. brachiopod shells, echinoderm skeletons, trilobite carapaces).

Eleven graptolite species have been recognised through the studied succession (Fig. 5). These include the index species of the A. murrayi and Hunnegraptus copiosus graptolite biozones, which are the uppermost graptolite zones of the Tremadocian (Loydell, 2012). A. murrayi occurs in great abundance from 240 to 295 m above the base of the Fezouata Shale. EPF-bearing Interval 1 falls within the A. murrayi Biozone. EPFbearing Interval 2 mainly belongs to the upper part of the A. murrayi Biozone. Only the uppermost part of this interval corresponds to the following H. copiosus Biozone, as indicated by the first occurrence of H. copiosus. EPF-bearing intervals 1 and 2 are thus both of late Tremadocian age and correlate with the Tr3 substage of Bergström et al. (2009).

Environmental setting of the Fezouata Biota

Ripple marks, hummocky cross-stratification (HCS), and normallygraded tempestites are repeatedly found throughout the section, including in the two intervals with EPF (Fig. 6A-D). These sedimentary structures are interpreted as being induced by storm wave action on the sea floor [START_REF] Harms | Depositional Environments as Interpreted from Primary Sedimentary Structures and Stratification Sequences[END_REF]Duke, 1985;Duke et al., 1991;[START_REF] Gupta | Primordial storms: an overview of depositional environments in Mid-Late Proterozoic platforms of India[END_REF] and suggest a depositional environment at or above storm wave-base. However, HCS observed in the studied succession are only at a millimetre-to centimetre-scale, whereas they can reach several decimetres elsewhere in the succession (e.g., near the contact with the Zini Formation). These small-scale HCS suggest a relatively shallow depositional environment, most probably located within the offshore to lower shoreface (as defined by McLane, 1995; Fig. 6E).

Three long-term cycles of relative sea-level fluctuations are recognised in the entire Fezouata Shale, but only the first and the second of them are documented in the presented section (Fig. 4). All are composed of a transgressive phase marked by an evolution from siltdominated to clay-dominated intervals and a regressive phase marked by the opposite trend. Exceptional preservation occurs at the end of the regressive phase of the first cycle and at the beginning of the transgressive phase of the second cycle (Fig. 4). Additionally, facies changes towards more argillaceous sedimentary deposits and fewer silty tempestites, or towards more silty argillites interrupted by more frequent silty or fine-sandy tempestites are interpreted as transgressive and regressive phases of short-term cycles respectively. Two of them are associated with the preservation of EPF.

Discussion

Exceptional preservation: stratigraphic distribution and environmental factors

The distribution of EPF within the Fezouata Shale is restricted to two narrow stratigraphic intervals, which contradicts earlier assessments (Van Roy et al., 2010) that exceptional preservation occurs throughout the Fezouata Shale. This is a common point shared with most major Cambrian and Ordovician Lagerstätten in which exceptional preservation is limited to a relatively narrow stratigraphic interval, ranging from a few metrese.g. Sinsk (Russia; [START_REF] Ivantsov | Palaeoecology of the Early Cambrian Sinsk biota from the Siberian Platform[END_REF], Emu Bay (Australia; [START_REF] Gehling | The geological context of the Lower Cambrian (Series 2) Emu Bay Shale Lagerstätte and adjacent stratigraphic units, Kangaroo Island, South Australia[END_REF], the Spence Shale (USA; [START_REF] Liddell | Sequence Stratigraphy and Paleoecology of the Middle Cambrian Spence Shale in Northern Utah and Southern Idaho[END_REF], the Burgess Shale (Canada; [START_REF] Collom | Reinterpretation of "Middle" Cambrian stratigraphy of the rifted western Laurentian margin: Burgess Shale Formation and contiguous units (Sauk II megasequence), Rocky Mountains, Canada[END_REF], or the Llanfawr Mudstones (UK; Botting et al., 2011) to a few tens of metres, as in Chengjiang (China; [START_REF] Babcock | Stratigraphy, paleontology and depositional setting of the Chengjiang Lagerstätte (Lower Cambrian), Yunnan, China[END_REF], the Pioche Shale (USA; [START_REF] Moore | Preservation of Early and Middle Cambrian soft-bodied arthropods from the Pioche Shale, Nevada, USA[END_REF] [START_REF] Aldridge | The Soom Shale: a unique Ordovician fossil horizon in South Africa[END_REF]). An exception is the Guanshan Biota (China; Hu et al., 2010), which seems to occur throughout the entire Wulongqing Formation, which can reach up to 60 m in thickness. Environmental conditions maintained over a relatively short timespan may account for the preservation of EPF within a narrow stratigraphic interval [START_REF] Hagadorn | Burgess Shale-type localities: the global picture[END_REF]. Exceptional preservation is often associated with the rapid burial of organisms, which is supposed to promote oxygen restriction and to favour the preservation of soft tissues and lightly sclerotised features (Gaines et al., 2012b). Different mechanisms of entombment have been proposed. For example, a mostly turbiditic origin is often suggested for the mudrock-siltstones of the Burgess Shale [START_REF] Gabbott | Sedimentation of the Phyllopod Bed within the Cambrian Burgess Shale Formation of British Columbia[END_REF], with probably a rhythmic deposition from waxing and waning density currents [START_REF] Gabbott | Sedimentation of the Phyllopod Bed within the Cambrian Burgess Shale Formation[END_REF]. Recent studies of the Burgess Shale communities [START_REF] Caron | Paleoecology of the Greater Phyllopod Bed community, Burgess Shale[END_REF][START_REF] Caron | The Greater Phyllopod Bed community, historical variations and quantitative approaches[END_REF] show that the fauna was mostly preserved in situ and only moderately disturbed by the event of burial. Most organisms were buried in situ by distal mudflow events. In Chengjiang, a typical sedimentary succession is recognised (Hu, 2005). It consists of background layers dominated by hemipelagic mud, which are interrupted by event layers that contain soft-bodied fossils. These event layers are generated by bottom-flowing density currents, which are responsible for the burial of the fauna.

Although storms played an important role in the overall depositional mode of the Fezouata Shale (as indicated by the presence of numerous HCS and tempestites), the storm waves seem to have had a limited influence on the relatively distal deposits that contain EPF. Indeed, storm currents were weak in this area, as attested by the relatively small scale of the HCS found, resulting from a mainly oscillatory movement at the seafloor. The animal communities of the Fezouata Shale were probably buried in situ during storm events, which mobilised more proximally a large amount of coarser sediments. Between two storm events, the background sedimentation resulted from the decantation of mud suspended by wave action. It is not associated with any sedimentological structures, such as HCS and tempestites. This depositional scenario would be comparable to the one invoked for the Chengjiang and other Burgess Shale-type Lagerstätten (Gaines et al., 2012b).

The geochemical context of the Fezouata Shale has not yet been explored and will be investigated on fresh rocks obtained from drill cores (in progress). No detailed information is available on the redox conditions that prevailed at the water-sediment interface and within the sediment. However, the presence of iron oxides in the majority of the weathered EPF suggests that an early mineralisation of pyrite took place on both soft and sclerotised tissues and was induced by microbial activity under anaerobic conditions [START_REF] Gabbott | Preservation of Early Cambrian animals of the Chengjiang biota[END_REF]Vinther et al., 2008;Van Roy et al., 2010). The exceptional preservation of giant anomalocaridids in silica-chlorite concretions is unusual among Burgess Shale-type Lagerstätten and possibly involved the dissolution of volcanic ashes in the sediment (Gaines et al., 2012a). Field observations indicate that bioturbation associated with EPF-bearing intervals was limited to mostly narrow horizontal burrows, very close to the water-sediment interface. This supports a low infaunal activity, possibly related to dysoxic/anoxic conditions within the sediment.

Dating the Fezouata Lagerstätte

Graptolites (A. murrayi and H. copiosus biozones) indicate a late Tremadocian age for the EPF-bearing intervals of the Fezouata Shale. Therefore, the Fezouata Lagerstätte does not extend to the Floian as suggested previously (Van Roy et al., 2010). It cannot be entirely ruled out that H. copiosus occurs lower in the stratigraphy. As the base of the H. copiosus Biozone is defined by the First Appearance Datum of this biozonal marker graptolite (Egenhoff et al., 2004), a larger portion of the interval 2 may belong to the H. copiosus Biozone. However, the resulting changes would be of little significance regarding the age of the Fezouata Biota, which would remain late Tremadocian (Tr3 of Bergström et al., 2009).

The only excavation that was not re-studied in detail (blue asterisk in Fig. 2C) yielded the graptolite Clonograptus along with the trilobite Dikelokephalina brenchleyi. Both would suggest a late Tremadocian age (Fortey, 2009(Fortey, , 2011b)). Graptolites are both abundant and well preserved throughout the Fezouata Shale (Fig. 5). They are reliable markers for dating and correlations, especially in the Ordovician (Webby et al., 2004;[START_REF] Sadler | High-resolution, early Paleozoic (Ordovician-Silurian) time scales[END_REF]. The graptolite assemblages of the Fezouata Shale are dominated by planktonic forms that are assumed to have lived in epipelagic niches (Cooper et al., 1991(Cooper et al., , 2012a)), with a small percentage of rooted benthic dendroids (Fig. 5C andJ). The planktonic ones have a cosmopolitan distribution and have been widely used for global correlations. They occur in Scandinavia (Lindholm, 1991), North America (Williams and Stevens, 1991;Maletz, 1997;Jackson and Lenz, 2003), South America (Maletz and Egenhoff, 2001;Ortega and Albanesi, 2005), China (Feng et al., 2009) and southwestern Europe (Aceñolaza et al., 1996;Pillola et al., 2008).

Microfossils, such as acritarchs, chitinozoans, and conodonts, are also accurate biostratigraphic markers, but their study requires fresh material (e.g. from well drillings or boreholes; Elaouad-Debbaj, 1988) for high-resolution dating. Acritarchs typical of the late Tremadocian to lower Floian messaoudensis-trifidum assemblage occur in the lower part of the Fezouata Shale below and within EPF-bearing interval 1. This particular acritarch assemblage indicates a late Tremadocian age, correlating to the top of the A. murrayi and the base H. copiosus graptolite biozones [START_REF] Molyneux | The messaoudensis-trifidum acritarch assemblage and correlation of the base of Ordovician Stage 2 (Floian)[END_REF]. Cuttings from a drill, which presumably crosses the Fezouata Shale, about 300 km southwest of Zagora, yielded comparable acritarch assemblages equally pointing to a late Tremadocian age (Nowak et al., 2015).

Environmental setting: implications

Sedimentological features indicate that the Fezouata Shale was deposited above storm wave-base, in a relatively shallow, openmarine environment (offshore to lower shoreface, Fig. 6e). Based on comparisons with the depth of storm influence in modern oceans (Immenhauser, 2009), a palaeodepth ranging from 50 to 150 m is proposed for the depositional area of the Fezouata Shale. The Fezouata Biota also delivers interesting indications of water depths. Based on the characteristics of the mitrate (Echinodermata, Stylophora) Peltocystis cornuta from the Fezouata Formation, Lefebvre and Botting (2007) concluded that the environment was shallow and of moderately high energy. In addition, non-calcareous algae are present in the fauna (Van Roy et al., 2010), but they might also have been transported in from shallower waters. Thus, their presence should be considered with caution and is not a reliable depth indicator. In this respect, the visual systems of the articulated benthic trilobites are probably more useful. All trilobites of this stratigraphic interval seem to have eyes (except Ampyx cf. priscus; Vidal, 1998a,b). This would suggest that there must have been at least some visible light, but not necessarily enough for photosynthesis. As a consequence, a maximum depth of 150 m might be overestimated, given the low insolation received in high latitudes [START_REF] Kirk | Light and Photosynthesis in Aquatic Ecosystems[END_REF]. The weak contrasts in lithology and facies throughout the cycles of short-and long-term sea-level changes indicates relatively lowamplitude sea-level fluctuations and thus, a relatively stable sea level close to storm wave-base over a relatively long time span (at least from the A. murrayi to the H. copiosus biochrons that represent a duration of about 2 Myr; Cooper et al., 2012b).

The palaeoenvironmental setting (open shelf close to the limit between the lower shoreface and the offshore) of the Fezouata Biota differs markedly from that of all Ordovician Lagerstätten presently known (Fig. 1), which are associated with particular marine environments. Indeed, the Winneshiek Biota (USA) lived in a possible estuarine embayment (Liu et al., 2006), the Beecher's Trilobite Bed (USA) was deposited under dysoxic conditions in an environment of deposition of distal turbidites [START_REF] Farrell | Paleoredox and pyritization of soft-bodied fossils in the Ordovician Frankfort Shale of New York[END_REF], the Llanfawr mudstones (UK) represented deep-water or abyssal environments (Botting et al., 2011), the Letná Formation (Czech Republic; Fatka et al., 2013), the Airport Cove and William Lake (Canada; Young et al., 2007) were located in shallow peritidal settings and the Soom Shale (South Africa) was close to a retreating ice front (Aldridge et al., 2001).

Therefore, the Fezouata Lagerstätte opens a unique and exceptional window into the marine biodiversity within open-marine environments during the Early Ordovician and makes it possible for the first time to establish accurate comparisons with the older welldocumented open-marine exceptional biotas from the Cambrian. Among them, the Burgess Shale (Canada; [START_REF] Collom | Reinterpretation of "Middle" Cambrian stratigraphy of the rifted western Laurentian margin: Burgess Shale Formation and contiguous units (Sauk II megasequence), Rocky Mountains, Canada[END_REF], Kaili (China; Zhao et al., 2005), Sinsk (Russia; [START_REF] Ivantsov | Palaeoecology of the Early Cambrian Sinsk biota from the Siberian Platform[END_REF], and Sirius Passet (Greenland; [START_REF] Ineson | Geological and depositional setting of the Sirius Passet Lagerstätte (Early Cambrian), North Greenland[END_REF] Lagerstätten are associated with distal offshore environments, whereas the Emu Bay Shale Lagerstätte (Australia; [START_REF] Gehling | The geological context of the Lower Cambrian (Series 2) Emu Bay Shale Lagerstätte and adjacent stratigraphic units, Kangaroo Island, South Australia[END_REF] is characterised by a relative nearshore setting affected by the tectonics of an active margin. In terms of environmental setting, the Fezouata Lagerstätte may best compare with the Guanshan and Chengjiang Lagerstätten (Hu, 2005;Hu et al., 2010;MacKenzie et al., 2015), although the environmental conditions that prevailed at Chengjiang are still debated (see Zhao et al., 2012). A lower shoreface/offshore depositional environment has been inferred for these three localities, in which early diagenetic pyritization represents the dominant mode of soft-tissue preservation [START_REF] Gabbott | Preservation of Early Cambrian animals of the Chengjiang biota[END_REF]Van Roy et al., 2010;Vinther et al., 2008).

Another important characteristic of the Fezouata Lagerstätte is its high latitudinal position (c. 65°S) compared with that of other Ordovician and Cambrian Lagerstätten (Fig. 2B, Cocks and Torsvik, 2004;Torsvik and Cocks, 2011). The Hirnantian Soom Shale Lagerstätte (South Africa) was previously thought to be located at 60°S, but more recent palaeogeographic reconstructions suggest a much lower latitude (i.e. 35°S; [START_REF] Whittle | Taphonomy and palaeoecology of a Late Ordovician caryocaridid from the Soom Shale Lagerstätte, South Africa[END_REF]Torsvik and Cocks, 2013). Unlike the Soom Shale [START_REF] Gabbott | Taphonomy of the Ordovician Soom Shale Lagerstätte: an example of soft tissue preservation in clay minerals[END_REF], no glaciogenic sediments are associated with the Fezouata Biota. This raises the important question of the climatic conditions and seawater temperature that prevailed and may have influenced the diversity and composition of the Fezouata Biota. Nevertheless, the very nature of the global climate state during the Early and Middle Ordovician is under discussion (Trotter et al., 2008;Vandenbroucke et al., 2009;[START_REF] Nardin | Modeling the Early Paleozoic long-term climatic trend[END_REF][START_REF] Turner | Visions of ice sheets in the Early Ordovician greenhouse world: evidence from the Peninsula Formation, Cape Peninsula, South Africa[END_REF].

A Late Tremadocian Lagerstätte : evolutionary perspectives

The Fezouata Biota is a key source of information for understanding the biotic turnover that took place during the Cambrian-Ordovician transition, i.e. after the Cambrian Explosion and before the GOBE. The co-existence of Burgess Shale-type faunal elements (such as marrellomorphs and anomalocaridids) with remarkably derived, typical post-Cambrian organisms (such as crinoids, ostracods and xiphosurids) indicates that the transition between the Cambrian and Palaeozoic Evolutionary Faunas (Sepkoski, 1979) was more complex and extended than hitherto realised. Moreover, the presence of several surprisingly advanced forms in the biota, such as xiphosurid arthropods, machaeridians, or cheloniellids (Van Roy et al., 2010), suggests that at least in some non-biomineralised groups, the GOBE started earlier than previously thought. This supports the idea that, rather than being two distinct events, the Cambrian Explosion and the GOBE might in fact represent different phases of the same dynamic of diversification (Droser and Finnegan, 2003;Van Roy et al., 2010).

Conclusions

Exceptional preservation in the Fezouata Shale is restricted to two relatively narrow intervals that belong to the upper Tremadocian (A. murrayi to basalmost H. copiosus graptolite biozones). The animal communities of the Fezouata biota lived in an open-marine environment, in a relatively shallow setting (offshore to lower shoreface transition) close to the storm wave-base. Organisms were very likely buried in situ during storm events, with a background sedimentation resulting from the decantation of mud suspended by wave action. By offering better chronological and environmental constraints, our results reveal the potential of the Fezouata Biota for understanding the early steps of animal life. Future investigations will concentrate on the identification of the taphonomic and/or abiotic factors controlling the restricted stratigraphic distribution of EPF within the thousand-metre-thick Fezouata Shale.

Introduction

Ordovician graptolites are relatively rare fossils in the Anti-Atlas mountain range of southern Morocco, where identification of Tremadocian and Floian strata was mainly based on typical assemblages of trilobites and other shelly fossils (Choubert et al., 1955;Destombes, 1962aDestombes, , 1967;;Destombes in Destombes et al., 1985). Graptolites were first reported in this geological domain by Destombes and Willefert (1959) and Destombes (1960aDestombes ( , 1960b)). They were discovered earlier in the Ordovician of the High Atlas [START_REF] Roch | Sur un gisement de Silurien dans le Haut-Atlas marocain[END_REF]Termier and Termier, 1947;[START_REF] Du Dresnay | Présence de Skiddawien à graptolites dans le Haut Atlas Oriental (Maroc)[END_REF] and in the western Meseta [START_REF] Yovanovitch | Les boutonnières de Paléozoïque entre Casablanca et Azemmour[END_REF][START_REF] Gigout | Données nouvelles sur le Cambrien et l'Ordovicien entre Casablanca et Mazagan (Maroc occidental)[END_REF][START_REF] Gigout | Sur l'Ordovicien inférieur de Casablanca (Maroc)[END_REF][START_REF] Gigout | Études géologiques sur la Meseta marocaine occidentale (arrière-pays de Casablanca, Mazagan et Safi)[END_REF][START_REF] Gigout | Levés géologiques sur la feuille Sidi-Bennour (Meseta marocaine occidentale)[END_REF][START_REF] Destombes | Observations stratigraphiques nouvelles dans la Meseta côtière marocaine aux environs de Casablanca[END_REF][START_REF] Delarue | Étude géotechnique de la région de Casablanca (Géologie, matières premières minérales, sols)[END_REF], with some specimens figured by Termier and Termier (1950) and [START_REF] Gigout | Études géologiques sur la Meseta marocaine occidentale (arrière-pays de Casablanca, Mazagan et Safi)[END_REF]. Destombes and Willefert (1959) reported the occurrence of early Tremadocian rhabdinoporinids at the base of the Fezouata Shale (= Fezouata Formation) in the central Anti-Atlas, followed by the record of late Tremadocian adelograptids and Bryograptus at a few localities of the same formation from the central and eastern Anti-Atlas (Destombes, 1960a[START_REF] Destombes | Le Cambrien et la base de l'Ordovicien dans la partie orientale et méridionale du Tafilalt (Maroc)[END_REF] and also by some Floian isograptids, tetragraptids and didymograptids, in the western Anti-Atlas (Destombes, 1960b). From these early records, the production of the Geological Map of Morocco at 1:200,000 scale, covering all the Palaeozoic rocks of the Anti-Atlas in ten sheets, led the French geologist Jacques Destombes to discover during the following twenty-four years about one hundred localities or horizons with Tremadocian and Floian graptolites in the Fezouata Formation [START_REF] Destombes | Carte géologique du Maroc au 1/200 000 de l'Anti-Atlas marocain[END_REF](Destombes, , 2006a,b,c,d,e,f,g,h,i,j),b,c,d,e,f,g,h,i,j). These records demonstrate the existence of a regional diachronous and transgressive contact between the base of the Fezouata Formation and the different sandstone and shale units that comprise the middle-upper Cambrian Tabanite Group (Destombes, 1962b andDestombes in Destombes et al., 1985). Also, the southwestern equivalent of the Fezouata Formation unconformably overlies Archean rocks of the West African Craton, for example, the graptolite-bearing shales in the Zemmour region of northern Mauritania (Destombes et al., 1969).

However, none of the Lower Ordovician graptolite records reported in these seminal papers and in the map field-notes from the Anti-Atlas were ever described, although some graptolites from fifteen localities around Zagora were illustrated by Aceñolaza et al. (1996) and Destombes (2006a). In contrast to this, Tremadocian graptolites and trilobites from the southwestern extension of the Anti-Atlas in Mauritania were described and illustrated by [START_REF] Sougy | Les formations paléozoïques du Zemmour noir (Mauritanie septentrionale)[END_REF] Araneograptus identified as a bryozoan-) and Destombes et al. (1969).

The recent discovery of the Fezouata Lagerstätte in the Zagora region renewed the interest in graptolites as the best means of accurate correlation of the soft-bodied metazoanbearing beds. For this purpose, Lefebvre and Botting (2007) 1).

In the present paper a general overview of the graptolite data from the Fezouata Formation is presented, focusing on the review of some important localities of Destombes and presenting new data that allow for a better reappraisal on the Lower Ordovician from a regional and international perspective. The precise position of most of the graptolite localities was indicated in the papers by Destombes and Willefert (1959) and Destombes (1960aDestombes ( , 1960b[START_REF] Destombes | Le Cambrien et la base de l'Ordovicien dans la partie orientale et méridionale du Tafilalt (Maroc)[END_REF][START_REF] Destombes | Carte géologique du Maroc au 1/200 000 de l'Anti-Atlas marocain[END_REF]Destombes ( , 2006b,c,d,e,f,g,h,i,j),c,d,e,f,g,h,i,j). Figured specimens belonging to Jacques

Geological setting

Lower Ordovician rocks of the Moroccan Anti-Atlas are included in the Fezouata and Zini formations. They are overlain by the lower to middle Darriwilian Tachilla Formation in the External (or Outer) Feijas Group (Choubert, 1952;Destombes, 1962aDestombes, , 1971;;Destombes et al., 1985). The Fezouata Formation (=the lower half of the Outer Feijas Shales of Choubert and Termier, 1947) consists of blue-green to yellow-green sandy mudstones and siltstones that coarsen upwards, and they are up to 900 m thick. Due to the regionally transgressive character of the Ordovician sedimentation over Cambrian strata, some thin microconglomeratic and glauconitic horizons are frequently intercalated at or near the base of the unit, and are rare in the middle part of the formation, where a single glauconitic and ferruginous horizon was used traditionally as a boundary-marker to separate the "lower" and "upper" Fezouata "formations" on geological maps (Destombes, 2006a,b,c,d,e,f,g,h,i,j), and later treated as a single formation (Martin et al., in press).

According to Destombes et al. (1985) and Michard et al. (2008), Ordovician sedimentation began during the Tremadocian in an east-west trending sag-basin south of the Saghro-Ougnate high. Afterwards, a gently subsiding shallow-water shelf extended progressively over the entire area, gently deepening northwards and with a shoreline located some distance from the northern rim of the Reguibat Shield (West African Craton). In the absence of significant synsedimentary tectonics, the Tremadocian to lower Floian marine clays and silts unconformably overlie middle to upper Cambrian strata.

The passive margin sedimentation of the Fezouata and Zini formations occurred in a wave-dominated and tide-modulated shallow-marine shelf within a general transgressiveregressive trend (Martin et al., in press). The interpreted palaeoenvironments range from proximal offshore to foreshore (Vaucher and Pittet, 2014;Vaucher et al., 2015aVaucher et al., , 2015b;;Martin et al., in press), although data derived from trilobite biofacies indicate slightly deeper (lower offshore) facies for the middle part of the Fezouata Formation and a progressive shallowing (from upper offshore to shoreface environments) in its upper third (Vidal, 1998).

The record of identifiable Early Ordovician graptolites is so far limited to the Fezouata Formation, where a few graptolite horizons have been recognized in 32 sections along the western, central and eastern Anti-Atlas. Identifiable graptolites have not yet been recorded from the overlying Zini sandstone. Figure 1 shows the examined localities in the Fezouata Formation, mainly discovered by [START_REF] Destombes | Carte géologique du Maroc au 1/200 000 de l'Anti-Atlas marocain[END_REF]Destombes ( , 2006a,b,c,d,e,f,g,h,i,j) ,b,c,d,e,f,g,h,i,j) and Martin et al. (in press).

Graptolites occur generally in fine argillites and siltstones as flattened imprints preserving some organic material, often coated by iron oxides and, in some instances, by pressure-shadow minerals. In the limited interval with soft-bodied and slightly-sclerotised organisms, associated graptolites are usually preserved in partial to full relief, involving an early diagenetic pyritisation and subsequent weathering to reddish iron oxides. These are responsible for the strong contrast shown by many graptolites compared to the surrounding green, blue-grey or yellow matrix. Some internal graptolite casts preserve delicate details such as thecal budding and fusellar rings. None of the specimens examined have traces of stolons or zooidal bodies within the thecae, although an exceptional case of preserved zooids has been claimed to occur in a benthic tuboid colony (Van Roy et al., 2010, fig. S2i). Besides their common preservation in argillites, some graptolites are found in medium-grained sandstones, micaceous siltstones and slightly carbonaceous lenses, where their preservation with some of their original relief may indicate an early diagenetic pyritisation, as well. Atlas at 1:200,000 scale [START_REF] Destombes | Carte géologique du Maroc au 1/200 000 de l'Anti-Atlas marocain[END_REF](Destombes, , 2006a,b,c,d,e,f,g,h,i,j),b,c,d,e,f,g,h,i,j), and are partly reviewed herein.

Schematic maps modified from Destombes (in Horný, 1997, fig. 1). C, Idealised longitudinal section illustrating Lower Ordovician lithostratigraphy of the Moroccan Anti-Atlas. Numbers for sections correspond to the localities indicated on map B. Thicknesses of the Zini and Fezouata formations after Destombes (2006a,b,c,d,e,f,g,h,i,j). a, approximate position of the Tremadocian/Floian boundary; b, location of the stratigraphic contact between the former "lower" and "upper" Fezouata "formations" of Destombes et al. (1985); c, approximate stratigraphic range of the Fezouata Lagerstätte; d, basal unconformity of the Fezouata Formation over different formations of the Tabanite Sandstone Group (middle Cambrian to Furongian). Stratigraphic scheme based on Michard et al. (2008, fig. 3.6). -Key

Graptolite biostratigraphy

Most of the graptolite localities occurring in the Fezouata Shale are isolated exposures, or occur in partial stratigraphic sections with one or a few graptolite horizons, but commonly within a single graptolite biozone. However, some sections include two to five biozones.

These sections are in a large region around the city of Zagora, west of Foum Zguid and east of N'kob (central Anti-Atlas), as well as in the Jbel Tazoult n'Ouzina (eastern Anti-Atlas). The sections near Zagora are located on the flanks of the Bou-Dehir anticline and in the graben of Zagora (sections 14 to 16, Fig. 1).

A composite biostratigraphic scheme based on these sections and from single outcrops is presented for the entire formation (Fig. 2). The graptolite biozones recognized are as follow. et al. (1998) and [START_REF] Cooper | Ecostratigraphy, zonation and global correlation of earliest Ordovician planktic graptolites[END_REF]. The record of Anisograptus in the Anti-Atlas (Destombes et al., 1985) is still pending confirmation, so far limited to a single stipe fragment with well preserved autothecae (not figured) tentatively attributed to this genus in a section located north of Zagora (lat. 30º33'44.8"N,long. 5º49'28.8"W). Previously, Destombes (1960aDestombes ( , 1962Destombes ( , 2006b) ) and Destombes et al. (1985) cited Anisograptus cf. matanensis Ruedemann from an horizon placed 180 m above the base of the Fezouata Formation to the east of Jbel Bou Dehir (Fig. 1, loc. 16). However, it is supposedly associated with much younger graptolites such as "Bryograptus" and "Clonograptus tenellus", which makes that identification highly improbable.

In adition to the recognized subspecies of R. f. flabelliformis, Destombes and Willefert (1959) and [START_REF] Destombes | Carte géologique au 1/200 000 de l'Anti-Atlas marocain[END_REF] mentioned the occurrence of R. f. norvegica (Kjerulf) and R. f. multithecatum (Bulman) from Akka n'Ousdidene (Fig. 1, loc. 20), but this was not confirmed in our review of the locality. The first subspecies was surely mistaken with mineralised remains of R. f. canadensis or even of R. f. flabelliformis, that in some beds show a thick, dark-mineral coating that affected both the stipes and dissepiments, reminiscent of the extensive cortical thickening characteristic of this form. R. f. norvegica was also cited by Destombes (2006b) from the section lying east of the Jbel Bou Dehir (Fig. 1, loc. 16), occurring about 410 m above the base of the formation, at a level placed in the upper Tremadocian. Due to its association with ?paradelograptids and ?kiaerograptids, the specimens were probably confused with some kind of benthic dendroid graptolite.

"Adelograptus" tenellus Biozone

The species "Ad." tenellus was reported in the Anti-Atlas by Destombes (1971Destombes ( , 2006b, 2006e) , 2006e) and Destombes et al. (1985) but, at present, the most convincing specimens closely similar to "Ad." tenellus (Linnarsson) (Fig. 2I) are from a section NW of Ouzina (eastern Anti-Atlas: Fig. 1, loc. 31). However, this occurrence is in a horizon relatively high in the Fezouata Formation that is in an unusually high stratigraphic position relative to the graptolite biozones recorded there. The claimed occurrence of "Ad." tenellus in the Zagora region (Destombes, 2006b) comes from a bed 180 m above the base of the Fezouata Formation in the Jbel Bou Dehir-east section (Fig. 1, loc. 16). This occurrence is also out of place biostratigraphically, and the specimens are probably misidentified.

Aorograptus victoriae Biozone

Among the numerous specimens of "Adelograptus" in the J. Destombes collection, it was impossible to recognize any specimen with the primary robust stipes and the pendent to reclined rhabdosome, with regular dichotomous branching, characteristic of Ao. victoriae (T.S. Hall). In spite of its well-known worldwide distribution, the species is so far apparently absent from Morocco.

Another widespread genus like Bryograptus, commonly recognized elsewhere in the lower part of the Ao. victoriae Biozone [START_REF] Maletz | The Upper Tremadocian (Ordovician) graptolite Bryograptus: taxonomy, biostratigraphy and biogeography[END_REF], or just below it, is rare in Africa.

Its only known occurrences are in Algeria, in the Saharan subsurface [START_REF] Blain | Découverte de la zone à Dictyonema flabelliforme Eichwald dans l'Ordovicien inférieur du Sahara algérien septentrional[END_REF][START_REF] Legrand | Résultats récents sur le problème de la limite Cambrien-Ordovicien au Sahara algérien septentrional[END_REF] and it may also occur in the Ougarta Range (Gomes [START_REF] Silva | Contribution à l'étude du Cambro-Ordovicien des chaînes d'Ougarta[END_REF], but the specimens need examination. Bryograptus has been listed from the Moroccan Anti-Atlas (Destombes, 1971(Destombes, , 2006b, 2006f;, 2006f;Destombes et al., 1985) but the specimens may be misidentified. Such is the case with the reports of "Bryograptus kjerulfi cumbrensis Elles", "B. lapworthi Ruedemann" and "B. gr. divergens Elles and Wood", three forms excluded from Bryograptus (see [START_REF] Zalasiewicz | Graptolites in British stratigraphy[END_REF][START_REF] Maletz | The Upper Tremadocian (Ordovician) graptolite Bryograptus: taxonomy, biostratigraphy and biogeography[END_REF]. However, in the review of one of the localities with Bryograptus at the Jbel Tijekht (eastern Anti-Atlas; Fig. 1, loc. 30), the original specimens correspond not to this genus, but to minute rhabdosomes of Ancoragraptus bulmani (Spjeldnaes) (Fig. 3O-Q). This species has also been recorded from north of Zagora (Fig. 1, loc. 14; Fig. 3N) and is reported here for the first time in Africa.

Ancoragraptus bulmani is a widespread graptolite occurring in the Kiaerograptus kiaeri plus K. stoermeri (=Aorograptus victoriae) biozones of Scandinavia, Canada, Argentina and China [START_REF] Spjeldnaes | Some Upper Tremadocian graptolites from Norway[END_REF]Maletz and Egenhoff, 2001;Jackson and Lenz, 2003;[START_REF] Jackson | Biostratigraphical and ecostratigraphical significance of Tremadoc (Ordovician) graptolite faunas from the Misty Creek Embayment and Selwyn Basin in Yukon and Northwest Territoires[END_REF][START_REF] Zeballo | New late Tremadocian (Early Ordovician) conodont and graptolite records from the southern South American Gondwana margin (Eastern Cordillera, Argentina)[END_REF]Wang et al., 2013a;[START_REF] Toro | Graptolite-trilobite biostratigraphy in the Santa Victoria area, northwestern Argentina. A key for regional and worldwide correlation of the Lower Ordovician (Tremadocian-Floian)[END_REF]. A slightly older species of the same genus is known from the "Ad." tenellus Biozone of Korea [START_REF] Cho | Phylogenetic relationships among Adelograptus, Ancoragraptus and Psigraptus[END_REF].

A second novelty among the Tremadocian graptolite record of the Anti-Atlas is the identification north of Zagora (Fig. 1, loc. 14) of fragmentary specimens of Choristograptus louhai Legrand (Fig. 3J-M). This represents the first occurrence in surface outcrops of a very rare genus, so far only known from a deep borehole in the Algerian Sahara [START_REF] Legrand | Un graptolite remarquable de l'Ordovicien inférieur du Sahara algérien septentrional: Choristograptus louhai nov. gen[END_REF].

Choristograptus louhai was reconized from two horizons 100 m over the base of the Fezouata Formation, directly above the beds yielding Anc. bulmani in the Oued Tarhia n'Imrhaldene section. [START_REF] Legrand | Lower Palaeozoic rocks of Algeria[END_REF] described Ch. louhai in the "γIIb" graptolite assemblage of the Algerian Sahara, which places it in the upper part of the stratigraphic range of "Ad." tenellus.

The close occurrence of Anc. bulmani and Ch. louhai in the lower Fezouata Formation provides an indirect evidence for the Ao. victoriae Biozone in the Anti-Atlas. Although considered barren of graptolites, the overlying 150-200 m of the sections around Zagora should be explored for Ao. victoriae and the many other species that characterize the Aorograptus victoriae Biozone plus the "kiaerograptid interval" in Scandinavia. sensu Lindholm (1991), Clonograptus rigidus (Hall) and "Tetragraptus" bulmani Thomas (=T. longus Lindholm according to Maletz and Egenhoff, 2001), to which we add a recently found 3-stiped tetragraptid possibly related to T. krapperupensis Lindholm.

Araneograptus murrayi

Besides the record of this graptolite biozone in the Zagora area (Fig. 1, loc. 14-16), mass-occurrences of Ar. murrayi were cited by Destombes (2006g) from the northwest of Foum-Zguid (Fig. 1, loc. 11). The first African report of the species was by [START_REF] Sougy | Les formations paléozoïques du Zemmour noir (Mauritanie septentrionale)[END_REF], who described it as "Nissenia zemmourensis" (a primitive bryozoan) found in the Zemmour region of northern Mauritania. The specimens were recognized as graptolites by Destombes et al. (1969), who identified them as "Dictyonema gr. murrayi". The name "Dictyonema murrayi zemmourensis" was used in a report in the Algerian Sahara [START_REF] Legrand | Lower Palaeozoic rocks of Algeria[END_REF], before Gutiérrez-Marco and [START_REF] Gutiérrez-Marco | Araneograptus murrayi (Hall, 1865) (Graptoloidea, Anisograptidae): su identidad con "Dictyonema yaconense" Turner, 1960 y distribución en España y Sudamérica[END_REF] placed this African taxon in synonymy with Ar. murrayi.

Hunnegraptus copiosus Biozone

This biozone was identified based on the occurrence of H. copiosus Lindholm in the section north of Zagora (Martin et al., in press). Only the lower part of the biozone is so far recognized, and the graptolite assemblage includes several taxa that extend upwards from the underlying biozone. These include Paratemnograptus magnificus, P. norvegicus, C. rigidus and "Didymograptus sp. 1". Poorly preserved specimens of "Kiaerograptus" cf. supremus were recorded about 45 m above the FAD of H. copiosus, and probably also range into the biozone. Based on the discovery of a lower occurrence of H. copiosus, the base of the biozone is here placed ca. 5 m lower in the section than that reported by Martin et al. (in press,fig. 4).

Also recently discovered is a possible new species similar to Clonograptus multiplex (Nicholson) but with long first-order stipes.

?Cymatograptus protobalticus Biozone

This biozone was introduced by [START_REF] Egenhoff | Graptolites as indicators of maximum flooding surfaces in monotonous deep-water shelf succession[END_REF] and [START_REF] Maletz | The Lerhamn drill core and its bearing for the graptolite biostratigraphy of the Ordovician Tøyen Shale in Scania, southern Sweden[END_REF] for the upper interval of the T. phyllograptoides Biozone in Scandinavia, being defined by the lowest occurrence of Cymatograptus protobalticus (Monsen). Its possible identification in Morocco is limited to a single section north of Zagora (Fig. 1, loc 14), where a poorly-preserved rhabdosome (Fig. 4J) is very similar to British specimens of Cym.

protobalticus from the Lake District (Rushton in Cooper et al., 2004, fig. 9q). Besides this rare occurrence of Cymatograptus, the strata assigned to this biozone record the incoming of a large clonograptid with highly irregular branching, identified as Clonograptus multiplex (Nicholson) (Fig. 4Q). The species is very abundant throughout the Anti-Atlas and ranges up into the lower beds of the next biozone. It was confused either with "Temnograptus" sp., Schizograptus sp. or Holograptus sp. in earlier papers by Destombes (2006b.

?Baltograptus jacksoni Biozone

This biozone is marked by the occurrence of poorly preserved specimens of Baltograptus jacksoni Rushton (Fig. 4O) in the section of Jbel Bou Legroun (Fig. 1, loc. 28).

It is possibly more widespread and is represented by deflexed didymograptids from a few localities in the central and eastern Anti-Atlas (Fig. multiplex occurs in Morocco towards its lower part, whereas higher beds contain rare specimens of Expansograptus? spp., unknown elsewhere (Fig. 4H,I), and stipe fragments of dichograptoids, which have been identified with problematic forms such as Expansograptus patulus (Hall), E. pennatulus (Hall), E. nitidus (Hall), E. sparsus (Hopkinson), E. extensus (Hall), Acrograptus nicholsoni (Lapworth) or A. nicholsoni planus (Elles and Wood) in previous literature (e.g. Destombes et al., 1985;Destombes et al. 2006bDestombes et al. , 2006c, 2006e, 2006i), 2006e, 2006i). Nevertheless, the review of some of the original samples in the J. Destombes collection reveals that they are misidentifications based on broken stipes, and the specimens could even be multiramous forms.

Sometimes the graptolite assemblage occurring below the B. minutus biozone is composed almost exclusively of tetraramous forms, as occurs for instance in the sections of El Khannfra, Taskala, Timidert, Akka n'Oubsidene and Jbel Zorg (Fig. 1, locs. 1, 3, 18, 20 and 32, respectively). Most of the original material corresponds to horizontal tetragraptids close to

Tetragraptus quadribrachiatus (Hall) preservation", but lacking the "isograptid symmetry" and the proximal arch so characteristic of the genus Isograptus. Tetragraptus approximatus Nicholson has been also listed from the El Khannfra section (Fig. 1, loc 1), but the original specimen corresponds to an obliquelyflattened, slightly reclined Tetragraptus preserved in quadribrachiate mode. The lowest occurrence of true T. approximatus internationally defines the base of the Floian, and it is restricted to lower stratigraphic horizons and deeper-water palaeoenvironments that were represented by the Fezouata Formation.

Baltograptus minutus Biozone

The base of this widespread biozone is established by the FAD of B. minutus (Törnquist). In the Moroccan sucessions, B. minutus (Fig. 4N) occurs at different horizons in an interval 118-167 m below the base of the Zini Formation in the Zagora region (Fig. 1, locs. Baltograptus minutus was cited in the previous Moroccan literature as "Didymograptus cf. -or "gr."-indentus Hall" (Destombes et al., 1985;Destombes, 2006aDestombes, , 2006bDestombes, , 2006cDestombes, , 2006d, 2006e), 2006e). It also occurs in some collections with B. deflexus as recognized in the illustrations by Destombes (2006a, pl. 12, fig. 1) for Jbel Tibasksoutine material (Fig. 1, loc. 17). In a different section from the eastern Anti-Atlas [START_REF] Destombes | Carte géologique au 200 000 de l'Anti-Atlas marocain. Paléozoïque inférieur (Cambrien moyen et supérieur-Ordovicien-base du Silurien[END_REF]: Fig. (Taskala). In the sample from the first locality, B. minutus (labelled either as "D. nanus" or "D. indentus") co-occurs with fragments of a large dichograptoid identified as "Temnograptus multiplex" but with lateral division of "secondary" stipes instead of dichotomous branching.

The same kind of dichograptoid was found in the second sample, labelled as "Schizograptus sp. or Temnograptus sp." Finally, a stipe fragment with a laterally arranged dichotomy was also collected by us at Jbel Tibasksoutine, where it occurs with B. minutus, but the absence of complete rhabdosomes preserving the proximal part prevents their precise identification.

"Azygograptus interval"

The Destombes (2006a, pl. 12, fig. 9) are not adequate for specific identification, and the original material has not been located. However, probably the same horizon with abundant specimens of Azygograptus eivionicus Elles has been recorded at the Jbel Aroudane section in the surroundings of Zagora. This species ranges from the middle Floian to the lower Dapingian (Fortey and Owens, 1987;[START_REF] Beckly | The Ordovician graptolites Azygograptus and Jishougraptus in Scandinavia and Britain[END_REF][START_REF] Zalasiewicz | Graptolites in British stratigraphy[END_REF], but is here correlated with the upper Floian, because of the laterally-coeval strata with B. deflexus and its occurence above the B. minutus Biozone in the central Anti-Atlas. In some Gondwanan shelves of South America showing affinities with the Baltic succession, the B. minutus Biozone is succeeded by a biozone dominated by azygograptids. This is the case in southern Bolivia (Egenhoff et al., 2004) and northern Argentina [START_REF] Monteros | Graptofaunas arenigianas en el borde occidental de la Puna argentina. 12 th[END_REF][START_REF] Martínez | El Ordovícico de la sierra de Quichagua (Puna nororiental argentina): litofacies volcanoclásticas y graptofaunas[END_REF], where the corresponding strata could be referred to the Floian stage.

Of special interest is an unresolved question regarding the precise taxonomic affiliation of graptolites illustrated by Destombes (2006a, pl. 11, fig. 8) as "Acrograptus cf.

nicholsoni" from Jbel Tibasksoutine (Fig. 1, loc. 17). They were collected from a horizon "sandwiched" below the last local occurrence of B. minutus and the first record of Azygograptus, about 73 m below the base of the Zini Formation (Destombes, 2006a,b: sample 431). Although the original material has yet to be found, the figured specimens show a strong resemblance to representatives of the genus Cymatograptus from the lower Floian T.

phyllograptoides Biozone. Thus, many of the illustrations (published without scales) are small declined didymograptids with a long straight sicula and have a morphology very similar to

Cymatograptus rigoletto (Maletz, Rushton and Lindholm). Some specimens of the assemblage also exhibit a variable development of prothecal folds like those of Cym.

undulatus (Törnquist), but not so pronounced as those of Cym. bidextro Toro and Maletz.

Definitively they are biostratigraphically too high to be referred to Cymatograptus, unless this genus persisted in the south-polar Gondwana shelf until the late Floian as is the case for Cym.

bidextro, which in the Gondwana margin of South America occurs as high as the middle Floian Baltograptus jacksoni Biozone (=B. cf. deflexus Biozone of [START_REF] Toro | The proximal development in Cymatograptus (Graptoloidea) from Argentina and its relevance for the early evolution of the Dichograptacea[END_REF]. A poorly preserved specimen in the J. Destombes collection, occurring 344 m above the last record of Rhabdinopora in a section east of Tata (Fig. 1, loc. 9) and identified as A. nicholsoni (Destombes, 2006h, sample 286), also morphologically and dimensionally resembles Cym.

rigoletto, but its proximal part does not show the details required for a precise identification.

Global and regional correlation

The chronostratigraphic terminology based on the historical type areas of the British Ordovician has traditionally been followed in Morocco, where the "Tremadocian" and the "Arenigian" -or "Skiddawian"-(sic) were used as informal stages of a "lower Ordovician"

series. General problems of correlation with the British stratotypes, which yield highly endemic benthic faunas, led [START_REF] Spjeldnaes | The palaeogeography of the Tethyan region during the Ordovician[END_REF] to propose a regional chronostratigraphic scale for the Mediterranean Ordovician. He introduced the series name "Zagorian" -after the Zagora region of the Anti-Atlas-for all strata so far assigned to the Tremadoc-Arenig series in Morocco and southeastern France. Previous to this, [START_REF] Legrand | Contribution à la stratigraphie du Cambro-Ordovicien dans le bassin saharien occidental[END_REF], had already proposed a regional scale for the Ordovician of the Algerian Sahara, after finding the same correlation difficulties with the British scheme. The regional stage "n 1 " (roughly equivalent to the Tremadocian to lower Arenigian interval) was introduced in Algeria and subdivided into substages ("n 1 a, n 1 b, n 1 c") according to the biostratigraphic data provided by the allegedly endemic graptolite faunas [START_REF] Legrand | Lower Palaeozoic rocks of Algeria[END_REF] and Fig. 5).

The conceptual value of the "Zagorian" series was questioned by Destombes (1971), who pointed out the imprecise definition of its boundaries and the better possibilities of correlation with the Bohemian Ordovician, so that this Moroccan-derived regional series was never used again. A more accurate Mediterranean (or south-Gondwanan) chronostratigraphic scheme was later developed in Bohemia [START_REF] Havlíček | Bohemian Ordovician and its international correlation[END_REF] and modified in Iberia [START_REF] Gutiérrez-Marco | Oretanian and Dobrotivian stages vs. "Llanvirn-Llandeilo" Series in the Ordovician of the Iberian Peninsula[END_REF], 2002, 2008, 2015b). The resulting regional scale combined the former Tremadoc and Arenig series -turning them into stages-for the Lower Ordovician, followed by the several Bohemo-Iberian regional stages established for the remaining Middle to Upper Ordovician strata. The validity of the British stages for the Lower and basal Middle Ordovician in the south-Gondwanan regional scheme has been justified thus far by the shared palaeobiogeographical links prior to the complete splitting of Avalonia from southern Gondwana by late Arenigian times (=Dapingian to early Darriwilian): [START_REF] Murphy | Origin of the Rheic Ocean: Rifting along a Neoproterozoic suture?[END_REF].

Regarding the global subdivision of the Ordovician system, all the Ordovician GSSPs for the new series and stages (just like the as-yet-informal "stage slices") are developed in deeper-water facies within continuous and fossiliferous sections (Bergström et al., 2009;[START_REF] Harper | A sixth decade of the Ordovician period: status of the research infrastructure of a geological system[END_REF]Cooper et al., 2012b). In North Africa, as well as in other peri-Gondwanan areas lying in high palaeolatitudes close to the Ordovician South Pole, the general scarcity of graptolites and conodonts for the Lower and Middle Ordovician imposes serious difficulties for correlating the regional successions with the new global chronostratigraphy. Due to this, the sequence of graptolite faunas here described from the Fezouata Formation of Morocco constitutes a first step to advance a more accurate global correlation for the Lower Ordovician (Fig. 5).

The regional chitinozoan zonation [START_REF] Paris | The Ordovician chitinozoan biozones of the Northern Gondwana Domain[END_REF]) is useful for dating Ordovician strata, but its correlation with the global scale is highly speculative, recording continuous changes even by their proposing authors, as was demonstrated for the upper Floian, Dapingian and lower Darriwilian chitinozoan biozones (Gutiérrez-Marco et al., 2014). Similarly, the taxonrange biozones so far considered as indicators of the lower-middle Tremadocian (Lagenochitina destombesi) and lower Floian (Euconochitina symmetrica), and equated by Webby et al. (2004) with their "time slices" 1a-1b and 2a, respectively, occur in part within the range of the upper Tremadocian Araneograptus murrayi graptolite Biozone in peri-Gondwanan, mid-latitude areas of south China (Wang et al. 2013b). Paradoxically, these and other Lower Ordovician chitinozoan biozones of the regional scale have their stratotypes defined by their occurrences in the Moroccan Anti-Atlas or in the Algerian Sahara (Elaouad-Debbaj, 1984, 1988;[START_REF] Paris | The Ordovician chitinozoan biozones of the Northern Gondwana Domain[END_REF], but with little stratigraphic or biochronological control provided by graptolites. Even in the "modern" left-hand columns correlating the main Ordovician succesions from North Africa (e.g. Videt et al., 2010, fig. 2), both chitinozoan and graptolite zonations need a deep review, together with their equivalence with the global stages and "substages" of the Lower Ordovician series (for comparison, see Fig. 5).

The Moroccan graptolite zonal scheme achieved here shows good possibilities for detailed correlation with other Lower Ordovician biozonations around the world (summarized by Loydell, 2012). However, the Moroccan scheme reaches its maximum precision on its correlation potential with the recently reviewed biostratigraphic schemes for Southern Scandinavia, South America and Avalonia (Maletz and Egenhoff, 2001;[START_REF] Maletz | The Lerhamn drill core and its bearing for the graptolite biostratigraphy of the Ordovician Tøyen Shale in Scania, southern Sweden[END_REF] and references therein), summarized in our Fig. 5.

Early Tremadocian

Although the oldest graptolite material from the Fezouata Formation did not usually include complete rhabdosomes of rhabdinoporids, the regularity of the mesh on all specimens is more suggestive of a subspecies close to Rhabdinopora flabelliformis flabelliformis (Eichwald) rather than R. f. parabola (Bulman) or R. f. canadensis (Lapworth), that have less regular mesh. The age of these first graptolites, in the absence of older Staurograptus or good specimens of Anisograptus, is likely to be close of that of "Assemblage 2" of Cooper et al.

(1998, fig. 3), which is early -but not earliest-Tremadocian. Thus, the lower boundary of the Fezouata Formation does not coincide with the base of the Tremadocian, despite earlier studies reporting a Cambrian-Ordovician boundary age (Destombes, 1971).

The Moroccan graptolite assemblages here assigned to the Anisograptus matanensis and Rhabdinopora flabelliformis anglica biozones are correlatable with the R. flabelliformis biozone recorded from the subsurface of the Algerian Sahara [START_REF] Blain | Découverte de la zone à Dictyonema flabelliforme Eichwald dans l'Ordovicien inférieur du Sahara algérien septentrional[END_REF] and from an outcrop in the Ougarta Range [START_REF] Legrand | Précisions biostratigraphiques sur l'Ordovicien inférieur et le Silurien des chaînes d'Ougarta (Sahara algérien)[END_REF]. This was subdivided into four sub-biozones by [START_REF] Legrand | Résultats récents sur le problème de la limite Cambrien-Ordovicien au Sahara algérien septentrional[END_REF] based on the appearance of several endemic subspecies of R. f. socialis that were somewhat allied, in ascending stratigraphical order, with the R. f. socialis, R. f. flabelliformis and R. f. anglica "groups", plus that of "Bryograptus patens Matthew". The latter is probably based on a species of Radiograptus or discoidally preserved specimens of R.

f. anglica (see Erdtmann, 1982b andCooper et al. 1998). The taxonomic status of the Algerian subspecies "Dictyonema sociale nilii, D. s. mehaïgueni (sic), D. sociale praeafricanum, D. s. incommodus", as well as the transitional form "D. s. mehaigueni-D. s.

praeafricanum", is rather obscure. However, some of them show the clear quadriradiate development [START_REF] Legrand | Résultats récents sur le problème de la limite Cambrien-Ordovicien au Sahara algérien septentrional[END_REF][START_REF] Legrand | Development of rhabdosomes with four primary branches in the group Dictyonema flabelliforme (Eichwald)[END_REF] Other than the records mentioned above from North Africa, early Tremadocian (Tr1) graptolites are scarce within the South Polar regions of Gondwana, occurring in scattered localities of Sardinia [START_REF] Pillola | Graptolites du Tremadoc du Sud-ouest de la Sardaigne (Italie): Paléoécologie et contexte tectono-sédimentaire[END_REF]Pillola et al., 2008), southeastern

France [START_REF] Aceñolaza | New Peri-Gondwanan occurrences of Early Tremadoc graptolites from South America and southwestern Europe[END_REF], northern Turkey [START_REF] Yalçinlar | Le massif calédonien de Babadag et ses couvertures anthracolithiques[END_REF]Göncüoglu et al., 2014) and northeastern Iran [START_REF] Jahangir | Biostratigraphy of the Cambrian-Ordovician boundary beds at Kopet-Dah, Iran[END_REF].

Middle Tremadocian

Representatives of the early biradiate graptolite development have rarely been recognized in the "Adelograptus" tenellus and Aorograptus victoriae biozones of the Fezouata Formation.

Previous citations of the distinct pendent triradiate genus Bryograptus have been revised and its presence has been so far disregarded for the Anti-Atlas. An atypical Aorograptus victoriae

Biozone fauna was solely identified on the basis of minute branched rhabdosomes of Ancoragraptus bulmani and fragmentary specimens of Choristograptus louhai occurring in distant localities, under the generalised absence of Ao. victoriae.

Middle Tremadocian graptolites are, however, best represented in the El Gassi Formation of the northwestern and central Algerian Sahara. The diachronic base of the unit closely resembles that of the Fezouata Formation, emphasizing the influence of basement tectonics on Ordovician sedimentation [START_REF] Legrand | Lower Palaeozoic rocks of Algeria[END_REF]. In the northwestern Algerian desert, the R. flabelliformis biozone grades up into the first occurrence datum of Bryograptus [START_REF] Blain | Découverte de la zone à Dictyonema flabelliforme Eichwald dans l'Ordovicien inférieur du Sahara algérien septentrional[END_REF], fig. 3B), a genus also cited in the Ougarta Range (Gomes [START_REF] Silva | Contribution à l'étude du Cambro-Ordovicien des chaînes d'Ougarta[END_REF]. To the southeast, the beginning of Tremadocian sedimentation is slightly younger, and the El Gassi Formation includes successive graptolite assemblages within an Adelograptus Biozone dominated by endemic forms such as Adelograptus bagueli, Ad. messaoudi and Choristograptus louhai [START_REF] Legrand | Un graptolite remarquable de l'Ordovicien inférieur du Sahara algérien septentrional: Choristograptus louhai nov. gen[END_REF][START_REF] Legrand | Deux nouvelles espèces du genre Adelograptus (Graptolites) dans l'Ordovicien inférieur du Sahara algérien[END_REF]. The common record of "Adelograptus" tenellus (see [START_REF] Legrand | Sur la présence du genre Clonograptus au Sahara septentrional[END_REF] in the γII a and γII b graptolite assemblages of [START_REF] Legrand | Lower Palaeozoic rocks of Algeria[END_REF] table 3) supported their correlation with the "Ad." tenellus graptolite Biozone of Morocco and elsewhere (Fig. 5). Of special interest is the occurrence of the endemic African genus

Choristograptus in the γIIb assemblage of the Algerian Sahara, corresponding to the upper part of the stratigraphic range of "Ad." tenellus, as well as from slightly younger beds in Morocco (see above). The Algerian equivalent of the Aorograptus victoriae Biozone may begin within this γIIb assemblage and be essentially represented by the γIII a graptolite assemblage of [START_REF] Legrand | Lower Palaeozoic rocks of Algeria[END_REF], which includes the record of adelograptids, ?clonograptids and thick biramose forms. Chitinozoan biostratigraphy from the El Gassi Formation also indicates that this "Hassi-Messaud adelograptid fauna" might be younger than the early Tremadocian (Paris, 1990, p. 188). [START_REF] Videt | Biostratigraphical calibation of third order Ordovician sequences in the northern Gondwana platform[END_REF] placed it in the Lagenochitina destombesi Biozone, assimilating it to the range of the former "Amphorachitina conifundus

Biozone" of [START_REF] Paris | The Ordovician chitinozoan biozones of the Northern Gondwana Domain[END_REF][START_REF] Paris | Palaeobiodiversification of Ordovician chitinozoans from northern Gondwana[END_REF].

The remaining strata assigned to the middle Tremadocian substage are mostly barren of graptolites in the south-polar peri-Gondwanan areas, as exemplified by the Fezouata Formation, with the exception of the sections of northern Zagora, Jbel Tijekht and NW Ouzina. Two poorly preserved graptolite assemblages occurring below the Ar. murrayi

Biozone were recorded from northern Mauritania (Destombes et al., 1969). They include some "Ad." tenellus-like specimens and remains of branched adelograptids of dubious biostratigraphic assignation, pending more complete and better-preserved material.

Late Tremadocian

In contrast with the few graptolite records from those older Tremadocian strata, late Tremadocian (Tr3) beds of the Fezouata Formation yielded rather abundant graptolites of the Araneograptus murrayi plus Hunnegraptus copiosus biozones, as already stated by Martin et al. (in press). A local diversity peak of more than 10 planktonic species occurs within the Ar.

murrayi Biozone in the central Anti-Atlas, with some species ranging up into the following biozone, defined by the FAD of H. copiosus.

The Ar. murrayi Biozone is widespread in south-polar peri-Gondwanan areas such as southern France (Thoral, 1935), southwestern Spain [START_REF] Robardet | The Ordovician, Silurian and Devonian sedimentary rocks of the Ossa-Morena Zone (SW Iberian Peninsula, Spain)[END_REF], Sardinia (Pillola et al., 2008), Thuringia [START_REF] Heuse | Early Ordovician microfossils (acritarchs, chitinozoans) and graptolites from the Schwarzburg Anticline, Thuringia (Germany)[END_REF], northern Mauritania (Destombes et al., 1969) and the Algerian Sahara [START_REF] Legrand | Lower Palaeozoic rocks of Algeria[END_REF]. In contrast, the H. copiosus Biozone has so far been rarely identified in SW Spain [START_REF] Robardet | The Ordovician, Silurian and Devonian sedimentary rocks of the Ossa-Morena Zone (SW Iberian Peninsula, Spain)[END_REF]Gutiérrez-Marco et al., 2015a) and in the central Taurides, Turkey [START_REF] Sachanski | The graptolite species Hunnegraptus copiosus Lindholm, 1991 from the Ordovician of Central Taurus, Turkey[END_REF].

From the Pontides area of the latter, Göncüoglu et al. ( 2014) add the record of Paradelograptus cf. antiquus (T.S. Hall), which is a characteristic graptolite for the upper Tremadocian [START_REF] Jackson | Some graptolites from the late Tremadoc and early Arenig of Yukon, Canada[END_REF] but may range into the basal Floian (Maletz and Egenhoff, 2001).

Lower and middle Floian

Despite showing a very low diversity when compared with other records in mid to low Ordovician latitudes, the Floian graptolite assemblages from the Fezouata Formation are of some value for biostratigraphical subdivision and correlation, thanks to the probable record of the Cymatograptus protobalticus and Baltograptus jacksoni biozones. Although the range of Cym. protobalticus has been equated with the T. phyllograptoides Biozone in Great Britain (Cooper et al., 2004;[START_REF] Zalasiewicz | Graptolites in British stratigraphy[END_REF], its correlation with Baltica led [START_REF] Egenhoff | Graptolites as indicators of maximum flooding surfaces in monotonous deep-water shelf succession[END_REF] and [START_REF] Maletz | The Lerhamn drill core and its bearing for the graptolite biostratigraphy of the Ordovician Tøyen Shale in Scania, southern Sweden[END_REF] to restrict the FAD of the species to the upper range of T. approximatus [START_REF] Cooper | A precise worldwide correlation of early Ordovician graptolite sequences[END_REF], postdating the LAD of T.

phyllograptoides, and this is followed in the present paper.

A striking biostratigraphic fact regarding the Floian biozones in Morocco is the generalised absence of direct correlation with the Bohemian Lower Ordovician zonation [START_REF] Bouček | Lower Ordovician graptolites of Bohemia[END_REF]Kraft andKraft, 1995, 1999). Work in progress both in Spain and southwestern France also points to the same differences with Bohemia. They may be related with environmental factors favouring the development of endemic faunas in a deeper shelf and more temperate environment. It may also indicate the need for a careful review of some of the Bohemian species, which are closely allied and may eventually be synonymized with certain cosmopolitan or provincial forms.

Late Floian

The upper graptolite assemblages recorded from the Fezouata Formation are also of low diversity by comparison with late Floian occurrences around the world, in particular with the lack of isograptids, pseudophyllograptids, pendent species of Tetragraptus and Didymograptellus, etc. Instead of them, minute pendent, deflexed and declined baltograptids plus fragments of large horizontal dichograptoids predominate in the B. minutus Biozone (see above). The latter is followed by monospecific assemblages of Azygograptus assigned to an "Azygograptus interval", which strongly resemble similar horizons occurring in the same stratigraphic position in other Gondwanan successions of southern Bolivia and northern Argentina (Fig. 5).

Palaeoecological remarks

Sedimentological and palaeobiogeographical evidence indicates that the Lower Ordovician Fezouata Formation of the Moroccan Anti-Atlas was deposited on the Gondwanan shelf at a high palaeolatitudinal context close to the Ordovician South Pole (Martin et al., in press). The corresponding cool-water environments link directly the recorded graptolite assemblages with the "Atlantic Province" faunas, which in Morocco show a minimal diversity compared to many other Ordovician regions around the world.

Graptolite provincialism began in the late Tremadocian with some genera restricted to the shallow-water shelf of high-latitude regions, such as the genera Kiaerograptus [START_REF] Goldman | Graptolite palaeobiogeography[END_REF] and Choristograptus, the latter being endemic to the north African platform.

Another genus biogeographically restricted to high palaeolatitudes is Bryograptus [START_REF] Maletz | The Upper Tremadocian (Ordovician) graptolite Bryograptus: taxonomy, biostratigraphy and biogeography[END_REF], as shown by its Algerian records. By the early and middle Floian, faunal endemism includes T. phyllograptoides (recognised in Spain, but still absent from north Africa) and primitive two-stiped dichograptids such as Cymatograptus and Baltograptus, both related with high-latitude regions [START_REF] Goldman | Graptolite palaeobiogeography[END_REF]. In the late Floian, endemicity in faunas of similar palaeolatitudes is represented by members of the genus Azygograptus, which usually occur in monospecific shallow-water, bedding-plane assemblages [START_REF] Beckly | The Ordovician graptolites Azygograptus and Jishougraptus in Scandinavia and Britain[END_REF].

The horizontal didymograptids Expansograptus nitidus (Hall) and E. pennatulus (Hall), although frequently cited in the old geological literature from North Africa (e.g., Roch, 1939;[START_REF] Gigout | Études géologiques sur la Meseta marocaine occidentale (arrière-pays de Casablanca, Mazagan et Safi)[END_REF][START_REF] Gigout | Levés géologiques sur la feuille Sidi-Bennour (Meseta marocaine occidentale)[END_REF]Destombes, 1960b;[START_REF] Legrand | Contribution à la stratigraphie du Cambro-Ordovicien dans le bassin saharien occidental[END_REF]Destombes and[START_REF] Destombes | Mémoire explicatif de la Carte géotechnique de la Meseta côtière à l'Est de Casablanca au 1/50 000. Régions de Mohammedia, Bouznika et Ben-Slimane[END_REF][START_REF] Beuf | Nouvelles observations sur le « Cambro-Ordovicien » du Bled el Mass (Sahara central)[END_REF], have not been recorded in our present study. Both are warm-water species commonly occurring from the mid Floian Pendeograptus fruticosus Biozone upwards, and restricted to palaeotropical localities [START_REF] Williams | Early Ordovician (Arenig) graptolites of the Cow Head Group, western Newfoundland, Canada[END_REF][START_REF] Goldman | Graptolite palaeobiogeography[END_REF] Apart from the graptolite taxa typical of high-palaeolatitudes, many pandemic taxa are well represented in the Fezouata Formation. The early Tremadocian (Tr1) graptolite assemblages are of low-diversity and include cosmopolitan Rhabdinopora flabelliformis and its ecological subspecies R. f. socialis and R. f. anglica, adapted to outer or mid-shelf environments [START_REF] Cooper | Ecostratigraphy, zonation and global correlation of earliest Ordovician planktic graptolites[END_REF]. The extreme rarity of multiramous horizontal Anisograptus (such as the zonal index An. matanensis) may be related to the deep-water or oceanic preference of this genus, in certain degree similar to Staurograptus, which was restricted to deep biotopes in continental slope deposits (Cooper et al., 1998;[START_REF] Cooper | Ecostratigraphy, zonation and global correlation of earliest Ordovician planktic graptolites[END_REF][START_REF] Goldman | Graptolite palaeobiogeography[END_REF].

Regarding the palaeocological constraints of Ordovician graptolites, Cooper et al. (1991) recognised an inshore graptolite biotope (the "didymograptid biofacies") that contained shelf-restricted species and cosmopolitan epipelagic taxa. The latter are normally found in two other biotopes following a depth-stratification model: a deeper, mesopelagic biotope (the "isograptid biofacies") composed of species restricted to deep-water environments, and a shallow, epipelagic biotope with species found in all biofacies. Although [START_REF] Cooper | Facies preference predicts extinction risk in Ordovician graptolites[END_REF] provided further evidence of depth-stratified biotopes for graptolite assemblages occurring within deep-water sediments, [START_REF] Egenhoff | Graptolites as indicators of maximum flooding surfaces in monotonous deep-water shelf succession[END_REF] noted that the majority of the mesopelagic taxa listed by Cooper et al. (1991) and [START_REF] Cooper | Ecostratigraphy, zonation and global correlation of earliest Ordovician planktic graptolites[END_REF] are instead part of an epipelagic biofacies. The competing model of [START_REF] Finney | New perspectives on graptolite distributions and their use as indicators of platform margin dynamics[END_REF] admitted that depth stratification and lateral differentiation of faunas did occur, but only within the narrow outer-shelf to proximal-slope environment, and that inner-shelf waters were only occasionally inhabited by a few opportunistic species. However, it seems clear that both depth biotope-stratification (mesopelagic/epipelagic) and surface-water temperature (inshore epipelagic endemics-offshore epipelagic pandemics) play an important role in biogeographical differentiation of Ordovician planktonic graptolites (Cooper et al., 2012a;[START_REF] Goldman | Graptolite palaeobiogeography[END_REF].

The graptolite record of the Fezouata Formation strongly contrasts in palaeoecological terms with the very shallow epicontinental sedimentation argued by the sedimentological studies (Vaucher and Pittet, 2014;Vaucher et al., 2015a, 2015b andthis volume;Martin et al., in press). Their interpreted palaeoenvironments range from proximal offshore to foreshore, although palaeoecological data derived from trilobites restricted the occurrence of truly inshore trilobite genera (the Neseuretus biofacies) to the upper third of the formation (Vidal, 1998). This matches the graptolite data, coinciding with the shoreward development indicated by Azygograptus in the highest beds of the formation.

Graptolite occurrences within the "Ad." tenellus, Ar. murrayi and H. copiosus biozones are particularly significant, because these Tremadocian assemblages are largely dominated by genera unambiguously considered elsewhere as representative of the deepwater graptolite biotope, which starts at a depth of 150-200 m (Cooper et al., 1991(Cooper et al., , 2012a)). It is the case of the pandemic species "Adelograptus" tenellus, Hunnegraptus copiosus, Paradelograptus norvegicus, "Tetragraptus" bulmani (=T. longus) and Paratemnograptus magnificus (see [START_REF] Egenhoff | Graptolites as indicators of maximum flooding surfaces in monotonous deep-water shelf succession[END_REF]. [START_REF] Goldman | Graptolite palaeobiogeography[END_REF] suggest that the distribution of "Kiaerograptus" supremus, which they believed to be a two-stiped dichograptid, appears to indicate a deep-water preference for this species as well. Within the Ar. murrayi biozone north of Zagora, up to six mass-accumulation horizons of its naming species were sucessively recorded in the "Bou Izargane" partial section. They range from 10 to 35 cm in individual thickness and the thicker beds may reach several tens of meters of lateral continuity. Also in the same biozone, some bedding planes are covered by monospecific assemblages formed either by large specimens of "T." bulmani found complete and without current orientation in their inferred life orientation (e.g., with the thecal apertures directed downward during settling in the sediment), or by large concentrations of stipes and broken rhabdosomes of P. magnificus, indicating current action and associated fragmentation.

Interestingly, graptolites are often preserved partially pyritised and their dense accumulations are closely associated with the record of the soft-bodied metazoans of the Fezouata Lagerstätte. These are either represented on the same beds, or mesopelagic graptolites occur in close stratigraphic vicinity to the most prolific horizons of the latter.

The mass occurrences of Ar. murrayi may also represent the onset of repeated toxic events in the water column, that rapidly kill en masse hundreds of thousand to millions of colonies in all their astogenetic stages. The resulting "graptolite rain" suddenly entombed complete trilobites and other organisms of the Fezouata Lagerstätte on the sea-floor, and consequently these graptolite beds are being intensively quarried by fossil collectors.

Araneograptus murrayi is a common pandemic species in deeper-shelf to slope environments [START_REF] Egenhoff | Graptolites as indicators of maximum flooding surfaces in monotonous deep-water shelf succession[END_REF]. The propulsion and buoyancy of its large conical rhabdosome (reaching up to 25-30 cm long) were mainly possible by the coordinated movements developed of the zooids. Dissepiments show cortical overgrowths laying obliquely to the dorsal plane of the stipes (Fig. 4F), organizing the water circulation through the rhabdosome for feeding and stabilization purposes (Gutiérrez-Marco and [START_REF] Gutiérrez-Marco | Araneograptus murrayi (Hall, 1865) (Graptoloidea, Anisograptidae): su identidad con "Dictyonema yaconense" Turner, 1960 y distribución en España y Sudamérica[END_REF]Aceñolaza et al., 1996). Rotational movements of the young colonies were also favoured by the development of elaborated nemal structures (Fig. 4A). [START_REF] Egenhoff | Graptolites as indicators of maximum flooding surfaces in monotonous deep-water shelf succession[END_REF] interpreted monospecific mass-occurrences of deep-water graptolites as indicators of maximum flooding surfaces in monotonous shelf successions, normally associated with condensation during sea-level rise and representing single to amalgamated flooding events during a general transgressive phase. But such massoccurrences seem at odds with the sequencial development deduced for the Fezouata Formation (Martin et al., in press; Vaucher et al., this volume), where an especially rapid sedimentation is suggested instead. Fairly similar mass-occurrences of Lower Ordovician graptolites has been explained in a number of ways, either in relation with water-mass specifity of some populations that die after dissipation of its influx when carried by currents into the shallow shelf, or either as graptolites caught in a "dead-trap" by sudden upwelling of anoxic water-masses (Erdtmann, 1982a). The second possibility seems untenable for the extensive epicontinental shelf of very little gradient where the sedimentation of the Fezouata Formation took place.

The Moroccan assemblages recorded from Floian strata also show an unbalanced graptolite record when compared with other sequences around the world. Besides the rare representatives of the genera Cymatograptus, Baltograptus and Azygograptus, classified by Egenhoff and Maletz (2007: supplementary information) as common inshore endemics in Atlantic faunas, large specimens of multiramous dichograptids, usually well-preserved and rather complete, are abundant in many of the studied sections. This is the case of Clonograptus multiplex, a rare graptolite in shallow-water environments, that is strongly dominant in several horizons within the ?Cym. protobalticus and lower ?B. jacksoni biozones, and resembles its common occurrence in deep-water strata elsewhere. Some other pandemic multiramous dichograptids such as Holograptus and Schizograptus have been frequently mentioned for the Fezouata Formation, and are now interpreted as possibly mesopelagic faunal elements [START_REF] Egenhoff | Graptolites as indicators of maximum flooding surfaces in monotonous deep-water shelf succession[END_REF].

The puzzling presence of many deep-water graptolites in the presumed shallow-water sucession of the Fezouata Formation adds to the apparent absence of graptolites clearly indicative of the isograptid biofacies. These were reinterpreted as belonging to the epipelagic biotope because of their flourishing on the medial to distal shelf areas of the Scandinavian platform [START_REF] Egenhoff | Graptolites as indicators of maximum flooding surfaces in monotonous deep-water shelf succession[END_REF], far from a truly deep-marine environment.

Conclusions

The first complete biostratigraphic scheme for the Fezouata Formation is achieved and correlated with previous findings of Lower Ordovician graptolites in northern Mauritania (Destombes et al., 1969), the subsurface of the Algerian Sahara [START_REF] Blain | Découverte de la zone à Dictyonema flabelliforme Eichwald dans l'Ordovicien inférieur du Sahara algérien septentrional[END_REF][START_REF] Legrand | Sur la présence du genre Clonograptus au Sahara septentrional[END_REF][START_REF] Legrand | Un graptolite remarquable de l'Ordovicien inférieur du Sahara algérien septentrional: Choristograptus louhai nov. gen[END_REF][START_REF] Legrand | Deux nouvelles espèces du genre Adelograptus (Graptolites) dans l'Ordovicien inférieur du Sahara algérien[END_REF][START_REF] Legrand | Résultats récents sur le problème de la limite Cambrien-Ordovicien au Sahara algérien septentrional[END_REF][START_REF] Legrand | Development of rhabdosomes with four primary branches in the group Dictyonema flabelliforme (Eichwald)[END_REF] and the Ougarta Range (Gomes [START_REF] Silva | Contribution à l'étude du Cambro-Ordovicien des chaînes d'Ougarta[END_REF][START_REF] Legrand | Précisions biostratigraphiques sur l'Ordovicien inférieur et le Silurien des chaînes d'Ougarta (Sahara algérien)[END_REF].

Many of the identified graptolites, especially those of Tremadocian, have a cosmopolitan or peri-Gondwanan distribution and have been widely used for global correlations. They occur in Scandinavia (Lindholm, 1991;[START_REF] Maletz | The Lerhamn drill core and its bearing for the graptolite biostratigraphy of the Ordovician Tøyen Shale in Scania, southern Sweden[END_REF], Great Britain (Cooper et al., 2004;[START_REF] Zalasiewicz | Graptolites in British stratigraphy[END_REF], North America (Williams andStevens, 1988, 1991;Maletz, 1997;Jackson and Lenz, 2003), South America (Maletz and Egenhoff, 2001;Ortega and Albanesi, 2005;[START_REF] Albanesi | Faunas de conodontes y graptolitos del Palaeozoico inferior en la Cordillera Oriental argentina[END_REF][START_REF] Zeballo | New late Tremadocian (Early Ordovician) conodont and graptolite records from the southern South American Gondwana margin (Eastern Cordillera, Argentina)[END_REF], China (Feng et al., 2009;[START_REF] Li | Floian (Lower Ordovician) graptolites from the Yiyang area, Hunan province[END_REF] and southwestern Europe (Aceñolaza et al., 1996;Pillola et al., 2008).

The Fezouata Lagerstätte-bearing beds were initially estimated as ranging from upper

Tremadocian to upper Floian (Van Roy et al., 2010) and later restricted to the upper Tremadocian (Martin et al., in press). The new graptolite data presented herein extend its record to the ?Cymatograptus protobalticus and ?Baltograptus jacksoni biozones of the lower and middle Floian, respectively. However, the maximum diversity, and abundance of the exceptionally-preserved soft-bodied biota clearly occurs within the late Tremadocian

Araneograptus murrayi and Hunnegraptus copiosus biozones.

According to sedimentological studies (Martin et al., in press;Vaucher et al., this volume), the benthic communities of the Fezouata biota lived in an open-marine environment, in a relatively shallow setting (upper offshore to lower shoreface) above storm wave base.

However, graptolite assemblages recorded in the Konservat-Lagerstätte are dominated by deep-water planktonic forms that are assumed to have lived in mesopelagic niches [START_REF] Egenhoff | Graptolites as indicators of maximum flooding surfaces in monotonous deep-water shelf succession[END_REF][START_REF] Goldman | Graptolite palaeobiogeography[END_REF]. This is the case of the giant conical colonies of Araneograptus murrayi (often reflecting toxic events which generated mass-occurrences), and the large horizontal rhabdosomes of multiramous to pauciramous genera like Paratemnograptus, Paradelograptus, Clonograptus, "Tetragraptus" and Holograptus/Schizograptus. The later group was adapted to living in quiet waters clearly below the storm wave base, where their large colonies could achieve a passive buoyancy, increased by frictional drag in dense, cool water-masses. Maybe some of the monospecific bedding-plane occurrences of such large graptolites were connected with short episodes of onshore transport by currents of mesopelagic graptolites into the shelf, but their repeated occurrence throughout the Fezouata Formation and the taphonomic evidence from its preservation indicates that many other occurrences are linked to the normal record of truly deep-water pandemic graptolite assemblages, as far as presently known from graptolite palaeoecology.

Chapter III The Fezouata Lagerstätte: palaeoeocology and taphonomy

The new environmental and graptolite-based biostratigraphic frameworks defined for the Fezouata Shale (see above) provided the unprecedented opportunity to achieve precise and detailed palaeoecological studies of the Fezouata Biota. To address this question, a large scale excavation project was undertaken during four successive weeks (from the 16 th January to the 8 th February 2014) at the hill of Bou Izargane (N. of Zagora; Fig. 43). This area exposes a 18 meter-thick succession in the lower part of the late Tremadocian stratigraphic interval associated with exceptional preservation in the Fezouata Shale (Martin et al., (Fig. 44). As the leader of this excavation campaign, my first task consisted in listing and buying all the necessary equipment (e.g., pickaxes, gloves, cisels) in Marrakesh with the help of Bertrand Lefebvre, Abel Prieur, and Emmanuel Robert (see fig. 46A). Once in the field, my main task was to lead the scientific exploitation of the excavation as well as the associated every-day organisation during the four weeks (e.g. assigning everyday a precise task to each participant). Three main, large excavations and fifteen smaller, lateral ones were exploited from three distinct, successive stratigraphic intervals (see Fig. 47). In spite of intensive prospection on the flanks of the Bou Izargane hill, it turned out that, in the exposed 18 meter-thick part of the succession, all fossiliferous levels were concentrated in these three intervals, characterized by a slightly coarser lithology (siltstones to fine sands). These three fossiliferous horizons were separated by almost barren intervals consisting of shales and fine silts. A scientific and technical protocol derived from archaeological studies was defined and followed in order to settle a large, proper excavation within each of the three main fossiliferous intervals (Figs. [START_REF] Hofmann | First record of a late Proterozoic faunal assemblage in the North American Cordillera[END_REF] A and B). A similar protocol had been also already used for the late Jurassic Cerin Lagerstätte (Ain, France) during its scientific exploitation from the 1970s to the mid 1990s (see e.g., [START_REF] Bernier | The lithographic limestones of Cerin (southern Jura Mountains, France). A synthetic approach and environmental interpretation[END_REF], and more recently, for the Autunian Lagerstätte of Muse (Saône-et-Loire, France) in the 2000s (see e.g. [START_REF] Gand | Les fouilles paléontologiques de Muse : bilan 2011, projets 2012[END_REF].

The stratigraphically higher excavation was the first to be opened, "randomly", at the top and downslope the western flank of the Bou Izargane hill. The identification of the places where all other excavations were dug resulted from methodic prospections made around the flanks of the Bou Izargane hill, and mostly from the field experience and advices of Mohammed "Ou

Saïd" Ben Moula.

The upper main excavation was initially designed as a large quadrangular area (3 x 4 meters), later extended to cover a 18 m 2 area (3 x 6 meters). This area was subdivided into six rows (labelled A-F) and three columns (1-3), each 1 meter wide. The resulting grid pattern made it possible to subdivide the upper excavation into 18 equal square areas (1 x 1 meter). The aim was to expose a large surface and to exploit it downward, level by level, so as to study the evolution through time and space of palaeocommunities sampled in situ (Figs. 45 C andD).

However, it rapidly turned out that leading a bed by bed excavation was impossible in the Fezouata Shale: all beds are lens-shaped and laterally discontinuous, forming large, metricscale, discrete concretion-like irregular bodies (Figs. 45 E and F). This bed geometry had never been described in the Fezouata Shale, probably because most previous studies had mostly focused on large-scale mapping, on limited surface excavations and/or on fossil material (see e.g. Destombes 2006a; Vidal, 1998a, b). The discontinuous, lens-shaped geometry of most deposits forming the Fezouata Shale probably results from the complex interaction of storms and tides on the muddy sea floor (see Vaucher et al., submitted a, b in appendix). The grid pattern was nevertheless maintained so as to identify the spatial location of samples. However, the stratigraphic position of specimens could not be indicated by level, but instead, was estimated in centimeters from the top of the section.

The stratigraphically intermediate main excavation was the largest one, consisting in a large rectangular area (9 x 4 meters) deeply excavated inside the southwestern flank of the Bou Izargane hill, so as to expose a wide area (36 m 2 ). The stratigraphic position of this fossiliferous level was initially indicated by 'Ou Saïd', and confirmed by preliminary surface scratching yielding exquisitely preserved specimens of arthropods and echinoderms. Several days of hand digging (with hammers and pickaxes) were necessary to remove the about 30 m 3 of barren rocks lying over the fossiliferous interval.

The third and stratigraphically lowermost main excavation had been initially opened (as a small hole) by 'Ou Saïd' and his sons in early January 2014 and had yielded several specimens of soft-bodied taxa. This site is located on the southern edge of the Bou Izargane hill in an almost flat area. During its scientific exploitation, this originally small hole was intensively quarried out and widened to form a large quadrangular area (5 x 5 meters) subdivided into 25 individual, equal square areas.

Main results

The detailed sedimentological study revealed that the 18 excavations corresponded to only three main stratigraphic levels regularly distributed along the 18-m-thick Bou Izargane section (Fig. 47). The graptolite fauna confirmed the stratigraphic position of this section in the lower (late Tremadocian) interval associated with exceptional preservation in the Fezouata Shale (Araneograptus murrayi graptolite zone; Martin et al., in revision below; Vaucher et al., in appendix b). Each fossiliferous interval yielded a wealth of taphonomic, environmental, and palaeoecological data, as well as abundant new soft-bodied specimens (Fig. 48).

The upper fossiliferous level

The highest stratigraphic level probably was probably the one that yielded the less abundant fauna with only 124 specimens listed so far. However, most of the soft-bodied taxa found comes from this interval, especially marrellomorphs arthropods and palaeoscolecid worms (Martin et al., in press b). At least two secondary spines, as well as a cephalic shield, belonging to Aegirocassis benmoulai were also recovered from well-preserved 3-D ripple marks (Figs. 48 F and H). Other components of the fauna mainly include graptolites, a few Symphysurus trilobites (some of which exhibiting the antennae) and Thoralicystis stylophorans (Fig. 48 C). Upper interval yielded marrellomorph arthropods.

The intermediate fossiliferous level

It is probably the best studied level of the excavation campaign at the moment with 626 specimens collected and listed. This abundant fauna is mainly represented by Thoralicystis stylophorans, Bavarilla and Anacheirurus trilobites (Figs. 48 A andB). A rare Chauvelicystis stylophoran was also found (Fig. 48 D). Although only a few marrellomorph athropods were also collected, soft-part preservation is common among trilobites (see below) and stylophorans (Lefebvre et al., 2013(Lefebvre et al., , 2014)).

The lower fossiliferous level

359 specimens have been listed from this level so far, as well as a slab containing a hundred specimens of the trilobite Platypeltoïdes magrebiensis (see section 3.2.1). The fauna mainly comprises the Anatifopsis and Thoralicystis stylophorans, numerous conulariids (Van Iten et al., in press), some bivalves (Polechova, in press), graptolites and a few trilobites (Fig. 48 I).

Numerous burrows were also collected but all of them are relatively shallow and narrow (Fig.

G).

Exceptional preservation in this level yielded an unknown arthropod (Fig. 46 G),

Tremaglaspis remains, and claw appendages (Fig. 48 E).

Palaeoecology of Early Ordovician communities from the Fezouata Shale

Although the detailed study of the Fezouata Shale communities (especially at Bou Izargane) is still ongoing, preliminary results were obtained, based largely on the data obtained from the 2014 excavation, but also from the material collected at other sites visited during the 2012 and 2013 field campaigns.

Palaeoecology of the late Tremadocian Bou Izargane communities

The 18-m-thick section of Bou Izargane is characterized by two main features:

(1) a large part of the succession is almost barren (in fine-grained lithologies: shales and fine siltstones), yielding only extremely rare remains of shelly taxa (e.g. isolated pieces of disarticulated trilobites) ;

(2) both shelly (e.g. echinoderms, trilobites) and soft-bodied taxa (e.g. anomalocaridids, marrellomorphs, palaeoscolecid worms) occur in extremely thin, laterally discontinuous lensshaped levels (in coarser facies: siltstones to fine sands). Moreover, each of the 18 fossiliferous lenses exploited in the three fossiliferous intervals quarried out at Bou Izargane yielded a distinct, low-diversity assemblage. Although abundant specimens were collected in most of these excavations (and, in particular, in the three main, largest ones), each assemblage was quantitatively dominated by one or two taxa, associated to a handful of individuals belonging to other groups (fig. 49).. This pattern is perfectly examplified by the fossiliferous level exploited in the middle main excavation, which yielded the highest number of specimens (534 individuals). This about 1.5 m large, extremely thin (less than 3 cm thick), laterally discontinuous level (lens) was totally extracted in January 2014. Globally, the assemblage collected from this single lens appears to be largely dominated by stylophoran echinoderms (79% of specimens), associated with trilobites (19%), and few other taxa (2%) including brachiopods, marrellomorphs and worms.

Moreover, the stylophoran component of the assemblage consists in an almost monospecific assemblage of the cornute Thoralicystis (97% of stylophoran specimens), associated with two other cornutes: Hanusia (2.8%) and Chauvelicystis (0.2%). Similarly, the trilobite component of the lens is dominated by two taxa: Anacheirurus (58.8% of trilobite individuals) and Bavarilla (38.2%), associated to rare asaphids (3%).

Moreover, the detailed analysis of this level showed that it consists indeed in the superposition of three, distinct, thin (about 1 cm thick each) horizons (Fig. 50): ( 1 for this pattern could be a mechanical size sorting, resulting from the action of storms and/or currents, artificially separating younger (and thus smaller and lighter) individuals from older ones (larger and heavier). However, the careful examination of the trilobites shows that, in spite of their small size, they are not juveniles, but display the typical morphology of adults.

This suggests that the trilobite-dominated horizon is an assemblage of small-sized (dwarfed)

individuals belonging to at least three distinct taxa. The same conclusion cannot be so clearly established for cornute stylophorans, as identifying if they were juveniles or fully-grown adults is more difficult. A possibility would be to check if their gonopore was actually open or not, but the precise location of this orifice is not well-established in this group of primitive echinoderms.

However, a comparable conclusion was reached by Lefebvre and Botting (2007), when they described another late Tremadocian stylophoran-dominated horizon, from a nearby locality in the Ternata plain (at the southern extremity of Jbel Tizagzaouine). In this assemblage, dominated by the mitrate Peltocystis cornuta, they noted that all specimens were unusually small and about half the size of conspecific specimens from the late Tremadocian of Montagne Noire (France). Moreover, although the size of Moroccan specimens was equivalent to that of some of the smallest (juvenile) specimens from Montagne Noire, their morphology was clearly distinct and similar to that of the twice larger (adult) individuals from

France. Indeed, eocrinoid and stylophoran echinoderms occurring in low-diversity, dense assemblages within the 70-m-thick late Tremadocian interval yielding soft-bodied organisms are consistently about twice smaller than those occurring lower or upper in the series. The presence of small-sized (dwarfed) individuals in this part of the succession does not only apply to trilobites (see above) and echinoderms (Lefebvre and Botting, 2007;Lefebvre et al., in press), but also gastropods (see e.g., Ebbestad, in press). Dwarfism is often associated with the occurrence of unhospitable, stressful environmental conditions, and in particular, with oligotrophic waters and/or low oxygen levels (dysoxia; see e.g., [START_REF] Tasch | Causes and paleontological significance of dwarfed fossil marine invertebrates[END_REF][START_REF] Mcroberts | Rhaetian (Late Triassic) Monotis (Bivalvia: Pectinoidea) from the eastern Northern Calcareous Alps (Austria) and the end-Norian crisis in pelagic faunas[END_REF][START_REF] Urlichs | Stunting in some invertebrates from the Cassian Formation (Late Triassic, Carnian) of the Dolomites (Italy)[END_REF][START_REF] Botting | Echinoderm diversity and environmental distribution in the Ordovician of the Builth Inlier[END_REF] Collectively, the faunal diversity observed along the 18-m-thick Bou Izargane section is relatively high, with over 30 distinct taxa, belonging to various groups of both shelly and softbodied organisms (e.g., annelids, arthropods, brachiopods, conulariids, echinoderms, molluscs, sponges). However, the situation is drastically different if each individual horizon is considered: although abundant, remarkably preserved fossil remains were collected in the 18 excavations, they consistently belonged to extremely low-diversity assemblages, typically dominated by one or two taxa.

Comparison with other Early Ordovician localities of the Zagora area

Comparison with other Early Ordovician fossiliferous sites in the Ternata plain shows that:

(1) the same pattern is observed in most other localities belonging to the same, about 70-mthick "lower" interval yielding soft-bodied taxa (A. murrayi to H. copiosus graptolite zones, late Tremadocian);

(2) abundant and extremely diverse assemblages occur in slightly older deposits (?base of the A. murrayi Zone), as well as in a handful of levels located within the "lower" interval with soft-bodied preservation (e.g., Oued Beni Zoli) and mostly, in the totality of the upper part of the Fezouata Shale (Floian), including the levels belonging to the "upper" interval with softbodied taxa;

(3) the base of the Fezouata Shale (0 to 240 m above the contact with the Cambrian) is almost devoid of any remain of macro-organisms (in spite of intensive field work by Destombes in the second half of the 20th c. and of several days of logging and collecting in these levels in 2012-2013). In contrast, the lower part of the Fezouata Shale (early-middle Tremadocian) has yielded extremely abundant and diverse remains of acritarchs, chitinozoans and scolecodonts (e.g., Elaouad-Debbaj, 1988).

In the late Tremadocian "lower" interval yielding soft-bodied taxa, the occurrence of thin, laterally discontinuous mass-occurrences of low-diversity assemblages dominated by one or two taxa is apparently the rule. Most of these dense beds are dominated either by arthropods or echinoderms (see above, intermediate main quarry at Bou Izargane; see also Lefebvre and Botting, 2007;Allaire et al., 2015;Martin et al., 2015;Lefebvre et al., in press). However, conulariids (see e.g., Van Iten et al., in press) and sponges (see e.g., Botting, in press) can also occur in comparable dense, low-diversity assemblages. This pattern of massive, almost monospecific occurrences rather suggests successive brief episodes of colonizations of the sea floor in unstable and unhospitable environmental conditions by "pioneer", generalist "rstrategist" taxa. This interpretation is apparently in good accordance with preliminary geochemical data (see Martin et al. submitted), the repeated presence of dwarfed populations of invertebrates (e.g., arthropods, echinoderms, molluscs), and the extremely low level of bioturbation, all suggesting the probable occurrence of -at least temporary-anoxic to dysoxic sea-bottom conditions. Such environmental conditions were clearly more favorable for the preservation of soft parts and soft-bodied organisms. On the other hand, it can not be fully excluded that the relatively high proportion of "Cambrian-like taxa" in these levels (e.g., cornutes, demosponges, eocrinoids, inarticulate brachiopods, marrellomorphs, palaeoscolecids) may simply result from the adaptation of these organisms to such environmental conditions (oxygen levels were lower in the Cambrian than in the Ordovician), clearly unhospitable for the taxa typical of the PEF (e.g. articulate brachiopods, bivalves, gastropods).

In contrast, the extremely abundant and diverse faunal assemblages occurring in some

Tremadocian levels but mostly in the Floian part of the succession are extremely constant in composition both through time and from one locality to another. They are consistently dominated by the same taxa, typical of "classical" PEF faunas: bivalves, articulate brachiopods, cephalopods, crinoids, gastropods, rhombiferans, mitrate stylophorans, and trilobites. Interestingly, although soft-bodied preservation occurs in some of these levels (e.g., in all levels belonging to the "upper" interval -middle Floian-with exceptional preservation),

very

Comparing Lagerstätte and shelly-fossil site within the Fezouata Shale

A comparison has also been drawn between three different excavations from the Bou Izargane section and a shelly fossil site outside from the Lagersätten intervals (Maëva perroux, undergraduate internship). The three excavations are from the lower fossiliferous interval of Bou Izargane. One excavation (termed BS1 herein) yielded only shelly taxa, while the other two (termed Z-F4(7) and Z-F4(8) herein) yielded shelly and lightly mineralized organisms.148 specimens were recovered from Z-F4( 8), 17 from Z-F4(7), and 44 from BS1.

To compare the communities of each site, we thoroughly listed all the taxa found, and calculate their relative abundances. The shelly fossil site considered for comparisons is termed Z-F26 (Toumiat locality). The sedimentology of this site is the same as previous localities, so our comparison should be based on the same biotope. It is well known from collectors for its abundance and diversity of shelly fossils, typical of Lower Ordovician platform. 314 taxa were listed from this locality.

As there was a great variability in the number of taxa found, we tried to apply a qualitative method to an idea of ecospace utilization, using Bambach diagrams (Bambach et al., 2007).

For this purpose, we tried to determine the main ecological habits of each collected taxon (feeding, motility, and tiering) based on a review of the most recent literature (see databases in appendix). The difference is striking between the difference communities of the Lagerstätte and the one from the shelly fossil site (Fig. 51). 12 niches are occupied in the shelly fossil site, whereas only 6 in the most diverse studied assemblage. This confirms the poor diversity of the communities from the Lagerstätte already highlighted by the relative abundance of taxa. This also suggests that unhospitable environmental conditions were episodic and possibly played a part in the exceptional preservation of the Fezouata Lagerstätte, as no softbodied preservation were found in the diversified shelly fossil site.

Introduction

The Lower Ordovician Fezouata Lagerstätte in the vicinity of Zagora (central Anti-Atlas, Morocco) has yielded rich assemblages of biomineralised and non-biomineralised organisms. These are predominantly composed of arthropods (e.g. anomalocaridids, chelicerates, cheloniellids, marrellomorphs, trilobites), echinoderms (asterozoans, blastozoans, crinoids, edrioasteroids, stylophorans) and graptolites, but also include brachiopods, bryozoans, conulariids, lobopodians, molluscs (e.g. bivalves, cephalopods, gastropods, halwaxiids, hyolithoids), and sponges (Van Roy et al., 2010, 2015a;Lefebvre et al., in this issue-a). The overwhelming majority of these animals lived at or close to the water-sediment interface as vagile or sessile members of epibenthic communities. The water column was also home to large filter-feeding anomalocaridids that probably fed on zooplankton (Van Roy and Briggs, 2011;Van Roy et al., 2015b), including diverse planktonic graptolites (Gutiérrez-Marco and Martin, in this issue). The exceptional preservation of these animal communities may have been facilitated by episodes of oxygen-depletion within the superficial layers of the sea floor, preventing scavenging and slowing down decay (Van Roy et al., 2010, 2015a;Martin et al., in press). This hypothesis of hypoxic/anoxic episodes at the sea floor permitting exceptional preservation questions the view of a permanent habitability of bottom sediment in the Fezouata, as do the huge differences between beds in terms of bioturbation. Indeed, some horizons display abundant subhorizontal or penetrative 3-dimensional burrows (Van Roy et al., 2010;Martin et al., in press), whereas others are barren of trace fossils. Palaeoscolecid worms, which are locally abundant in the Fezouata Shale, might have represented major bioturbators of the sediment according to [START_REF] Huang | The burrow dwelling behavior and locomotion of palaeoscolecidian worms: new fossil evidence from the Cambrian Chengjiang fauna[END_REF]. Although many described palaeoscolecid taxa are known from acid dissolution of limestones [START_REF] Botting | Diverse middle Ordovician palaeoscolecidan worms from the Builth-Llandrindod Inlier of central Wales[END_REF], they also inhabited environments dominated by siliciclastic sedimentation [START_REF] Robison | Annelids from the Middle Cambrian Spence Shale of Utah[END_REF][START_REF] Morris | Middle Cambrian priapulids and other soft-bodied fossils from Utah and Spain[END_REF][START_REF] Kraft | Worm-like fossils (Palaeoscolecida;? Chaetognatha) from the lower Ordovician of Bohemia[END_REF][START_REF] Zhang | Early Cambrian palaeoscolecid cuticles from Shaanxi, China[END_REF][START_REF] Botting | Diverse middle Ordovician palaeoscolecidan worms from the Builth-Llandrindod Inlier of central Wales[END_REF]. This contribution aims to discuss the life habits and feeding ecologies of palaeoscolecids in the light of a recently discovered abundant material from the Lower Ordovician Fezouata Shale in Morocco.

The Palaeoscolecida: previous work

Palaeoscolecids are long, annulated, vermiform organisms of millimetric to decimetric size. Complete specimens from the Chengjiang Lagerstätte show an eversible teeth-bearing pharynx comparable with that of modern priapulid worms [START_REF] Maas | Maotianshan-Shale nemathelminthsmorphology, biology, and the phylogeny of Nemathelminthes[END_REF]. Their most characteristic features are biomineralized microscopic plates covering their trunk. Although known since the nineteenth century [START_REF] Ulrich | Observations on fossil annelids, and descriptions of some forms[END_REF], it was not until the mid-1980s that they were assigned to their own class, the Palaeoscolecida (Conway [START_REF] Morris | Middle Cambrian priapulids and other soft-bodied fossils from Utah and Spain[END_REF]. Palaeoscolecida represent an exclusively fossil group that ranges from the Cambrian Series 2 (Stage 3) to the upper Silurian. Its systematics remains poorly defined and its phylogeny largely unresolved. For instance, palaeoscolecids have been assigned or regarded as closely related to annelids [START_REF] Whittard | Palaeoscolex piscatorum gen. et sp. nov., a worm from the Tremadocian of Shropshire[END_REF][START_REF] Robison | Annelids from the Middle Cambrian Spence Shale of Utah[END_REF][START_REF] Glaessner | Lower Cambrian Crustacea and annelid worms from Kangaroo Island, South Australia[END_REF][START_REF] Kraft | Worm-like fossils (Palaeoscolecida;? Chaetognatha) from the lower Ordovician of Bohemia[END_REF], nematoids (Hou and Bergström, 1994;Budd, 2001), chaetognaths [START_REF] Ahnelt | Chaetognatha[END_REF], or scalidophorans (i.e. priapulids and allies; [START_REF] Wills | Cambrian and recent disparity: the picture from priapulids[END_REF][START_REF] Dong | Fossil embryos from the Middle and Late Cambrian period of Hunan, South China[END_REF][START_REF] Dong | The anatomy, affinity, and phylogenetic significance of Markuelia[END_REF][START_REF] Donoghue | Synchrotron X-ray tomographic microscopy of fossil embryos[END_REF][START_REF] Cobbett | Fossils impact as hard as living taxa in parsimony analyses of morphology[END_REF][START_REF] Maas | Maotianshan-Shale nemathelminthsmorphology, biology, and the phylogeny of Nemathelminthes[END_REF][START_REF] Harvey | Are palaeoscolecids ancestral ecdysozoans?[END_REF]. Dzik and Krumbiegel (l989) even hypothesized that they might have been the ancestors of onychophorans and therefore panarthropods. However, most recent studies concur with the view that palaeoscolecids lie within cycloneuralians, or are at least closely allied to them (Budd, 2001;[START_REF] Maas | Maotianshan-Shale nemathelminthsmorphology, biology, and the phylogeny of Nemathelminthes[END_REF][START_REF] Morris | New palaeoscolecidan worms from the Lower Cambrian: Sirius Passet, Latham Shale and Kinzers Shale[END_REF][START_REF] Zhuravlev | The Palaeoscolecida and the evolution of the Ecdysozoa[END_REF][START_REF] Wills | The disparity of priapulid, archaeopriapulid and palaeoscolecid worms in the light of new data[END_REF]. According to Nielsen (2012), Cycloneuralia is a clade of ecdysozoan animals composed of the Scalidophora (kinorhynchans, loriciferans, priapulids) and the Nematoida (nematodes, nematomorphs). [START_REF] Harvey | Are palaeoscolecids ancestral ecdysozoans?[END_REF] recognized that palaeoscolecids share important features with nematoids and scalidophorans, but not with panarthropods, and accordingly ruled out a position close to the base of the Ecdysozoa in their cladistic analysis.

The plates of palaeoscolecids are usually preserved in calcium phosphate. It is commonly accepted (e.g. [START_REF] Harvey | Are palaeoscolecids ancestral ecdysozoans?[END_REF], although not strongly evidenced, that this phosphatic composition was primary. Three to four different types of palaeoscolecid remains are known [START_REF] Botting | Diverse middle Ordovician palaeoscolecidan worms from the Builth-Llandrindod Inlier of central Wales[END_REF]García-Bellido et al., 2013): 1) compression fossils, with whole or fragmentary body preserved flattened within the rock; 2) Orsten-type fossils, with secondarily phosphatized cuticle fragments; 3) Small Shelly Fossils (SSF), with isolated plates preserved in calcium phosphate; 4) Small Carbonaceous Fossils (SCF) mode, with isolated plates of solely organic composition, most likely due to apatite dissolution during sample processing [START_REF] Butterfield | Small carbonaceous fossils (SCFs): a new measure of early Paleozoic paleobiology[END_REF].

Complete specimens preserved as compression fossils with sufficient cuticular details remain rare, especially in siliciclastic settings [START_REF] Botting | Diverse middle Ordovician palaeoscolecidan worms from the Builth-Llandrindod Inlier of central Wales[END_REF]. Indeed, the conditions permitting the preservation of decay-prone structures, such as the proboscis (inflated anterior part of body lined with scalids) or the fine details of the scleritome (cuticle with mineralized plates) are rarely met [START_REF] Botting | Diverse middle Ordovician palaeoscolecidan worms from the Builth-Llandrindod Inlier of central Wales[END_REF]. Large fragments of trunk scleritomes are more common, although still rare, as documented in the early Cambrian Sirius Passet (Conway [START_REF] Morris | New palaeoscolecidan worms from the Lower Cambrian: Sirius Passet, Latham Shale and Kinzers Shale[END_REF], Sinsk [START_REF] Ivantsov | Articulated palaeoscolecid sclerite arrays from the Lower Cambrian of eastern Siberia[END_REF], or Emu Bay Shale (García-Bellido et al., 2013) biotas, or in the Upper Ordovician of Morocco (Gutiérrez-Marco and García-Bellido, 2015). By contrast, isolated plates are extremely abundant in SSF assemblages and result from the decay of their supporting tissues. For decades, the nature of these isolated elements remained unknown and accordingly, they were described as distinct taxa, which are now regarded as parataxa. Among the most typical examples are the genera Milaculum, Hadimopanella, Kaimenella, and Utahphospha (e.g. [START_REF] Müller | Milaculum n. g., ein phosphatisches Mikrofossil aus dem Altpaläozoikum[END_REF][START_REF] Gedik | Conodont biostratigraphy in the middle Taurus[END_REF][START_REF] Boogaard | Some data on Milaculum Müller[END_REF][START_REF] Hinz | The problematic Hadimopanella, Kaimenella, Milaculum and Utahphospha identified as sclerites of Palaeoscolecida[END_REF]. The fact that they represent isolated sclerites, initially covering the body of palaeoscolecid worms, was first recognized by [START_REF] Boogaard | Isolated tubercles of some Palaeoscolecida[END_REF] and [START_REF] Hinz | The problematic Hadimopanella, Kaimenella, Milaculum and Utahphospha identified as sclerites of Palaeoscolecida[END_REF]. Still, the diversity of preservational types observed in this group significantly complicates the elaboration of sound systematics, as well as the resolution of phylogenetic relationships within the Palaeoscolecida and between them and the other cycloneuralians.

In Chengjiang, mass mortality assemblages comprising dozens of individuals and extreme abundance within particular beds (Zhao et al., 2012a(Zhao et al., , 2012b[START_REF] Zhao | Diversity and species abundance patterns of the early Cambrian (Series 2, Stage 3) Chengjiang Biota from China[END_REF] attest to the fact that palaeoscolecids were ecologically important within early Palaeozoic marine communities [START_REF] Ivantsov | Articulated palaeoscolecid sclerite arrays from the Lower Cambrian of eastern Siberia[END_REF]). However, their exact role within these increasingly complex ecosystems remains unclear. Like priapulid worms [START_REF] Vannier | Gut contents as direct indicators for trophic relationships in the Cambrian marine ecosystem[END_REF], they might have been active bioturbators [START_REF] Vannier | Priapulid worms: pioneer horizontal burrowers at the Precambrian-Cambrian boundary[END_REF] and thus contributed to the Cambrian Substrate Revolution [START_REF] Bottjer | The Cambrian substrate revolution and early evolution of the phyla[END_REF] that gradually led to the replacement of matgrounds by mixgrounds. Alternatively, they might have represented pioneer organisms recolonizing benthic habitats after episodes of oxygen-depletion. The latter hypothesis would explain the fact that they are frequently found in specific horizons, within poorly diverse (if not monospecific) assemblages, and sometimes in great abundance, as illustrated again in the Fezouata Shale.

Geological setting and localities

In the Ternata plain, north of Zagora (Fig. 1A-B), the Guzhangian (Cambrian Series 3) sandstones of the Tabanite Group are uncomformably overlain by the Tremadocian to Floian (Lower Ordovician) deposits of the Lower Fezouata, the Upper Fezouata, and the Zini Formations (Geyer and Landing, 2006), which, together with the Tachilla Formation (Middle Ordovician), compose the Outer Feijas Group (Destombes et al., 1985). The Lower Fezouata and Upper Fezouata Formations constitute a rather monotonous sequence dominated by silty and micaceous argillites, which locally can reach up to 1200 m in thickness. Traditionally, they have been regarded as Tremadocian and Floian in age respectively, and are separated by a glauconitic and ferruginous horizon (Destombes et al., 1985). However, this marker bed can only be observed on the southern flank of Jbel Saghro and in the south of Maïder and of Tafilalt (Destombes et al., 1985). Elsewhere, especially in the Zagora area, where the Fezouata Lagerstätte lies, this horizon is missing (Destombes et al., 1985). Therefore, the mostly homogenous succession can be regarded as a single unit, the Fezouata Shale (= undifferentiated Lower and Upper Fezouata Shale Formations), which is topped by the sandstone-dominated Zini Formation (Destombes, 1962(Destombes, , 1971;;Destombes et al., 1985).

Based on extensive fieldwork, fossil evidence, and facies recognition, Martin et al. (in press) showed that exceptional preservation in the Fezouata Shale essentially occurs within two narrow c. 25 and 15 m thick stratigraphic intervals, both of late Tremadocian age according to graptolites. A more exhaustive analysis of the graptolite assemblages has resulted in a slightly different picture (Gutiérrez-Marco and Martin, in this issue). The two intervals of Martin et al. (in press) are now regarded as parts of a single, thicker (c. 60 m thick) interval with exceptional preservation of late Tremadocian age (red bar Fig. 1C). Additionally, a second, c. 15 m thick interval with exceptional preservation has been identified considerably higher up the stratigraphic succession in the middle Floian? Baltograptus jacksoni biozone (Ortega-Hernández et al. 2016). The Fezouata Biota lived under distal storm influence, in relatively shallow-water environmental conditions in a lower shoreface to upper offshore siliciclastic ramp setting (Martin et al., in press).

All but one of the studied specimens come from a single horizon in the uppermost part of the main stratigraphic interval with exceptional preservation (Fig. 1A-C; late Tremadocian, Araneograptus murrayi biozone). They have been found in an excavation site located on the western flank of Bou Izargane hill, some 20 km North of Zagora. The additional specimen comes from a locality c. 2.5 km to the West of Bou Izargane. It was recovered from a horizon only c. 10 m lower stratigraphically than the palaeoscolecid-bearing horizon at Bou Izargane, still falling within the A. murrayi biozone (late Tremadocian). During the Early Ordovician, Morocco was situated at the West margin of Gondwana (Cocks and Torsvik, 2004;Torsvik and Cocks, 2011), and was located at relatively high palaeolatitudes (c. 65°S) in the southern hemisphere. The entire Lower Ordovician succession was deposited in a rifting context associated with the separation of Avalonia from the Gondwanan supercontinent (Cocks and Torsvik, 2004).

Material and methods

The material consists of 38 flattened specimens, mostly long fragments of trunks (Fig. 2). All were carefully prepared using a needle to remove a crust of clayey matrix and iron oxides and expose the plates (Fig. 3D-E). This mechanical preparation frequently resulted in the abrasion of sculptural features on the plates, but care was taken to describe their morphology from the least abraded ones. The specimens were photographed using a Nikon D3X camera (with a Micro-Nikkor AF 60 mm f/2.8D macrolens) and a Zeiss AxioCam MRc5 digital camera mounted on a Zeiss SteREO Discovery.V8 stereomicroscope. Some fossils were immersed under dilute ethanol to enhance contrast and avoid reflective spots. A FEI Quanta 250 Scanning Electron Microscope (SEM) equipped with an energy dispersive X-ray analyser (EDX) was used at the Centre Technologique des Microstructures, Université Claude Bernard Lyon 1 to image details of the plates and to analyse their chemical composition. Specimens from Bou Izargane (AA prefix) are deposited in the collections of the Cadi Ayyad University, Marrakesh (Faculté des Sciences et Techniques, Guéliz); the specimen recovered from a different locality (YPM IP 525768) is housed in the collections of the Yale Peabody Museum of Natural History. We followed [START_REF] Brock | Shelly fossils from the Early Cambrian (Toyonian) Wirrealpa, Aroona Creek, and Ramsay Limestones of South Australia[END_REF] in using the term 'external surface', to refer to the surface of a plate that was (mostly) uncovered by soft tissues in the living animal, and the term 'visceral surface', to refer to the surface of a plate located deeper within the body envelope, in contact with the soft tissues of the basal cuticle (for similar uses of these terms in other ecdysozoans, see [START_REF] Whittington | Theodenisia and Peracheilus: enigmatic trilobites from Laurentia[END_REF][START_REF] Feist | Coaptative devices, enrollment, and life habits in Paralejurus, a particular case in scutelluid trilobites[END_REF].

Results

Preservation and plate composition

EDX analyses (Fig. 4) performed on well-preserved fragments and their surrounding matrix show that the composition of the plates sharply contrasts with that of the sediment (Fig. 4A-D). Indeed, elemental mapping indicates that the plates are rich in phosphorus and calcium (Fig. 4F-H), suggesting an apatitic composition. The matrix mostly contains oxygen, aluminium, silicon, and potassium (Fig. 4J-L) and therefore may be predominantly composed of aluminosilicates. Once iron oxides are removed, the composition of interplate areas is the same as the sediment. Iron oxides of the crust surrounding the specimens are in the forms of framboids, which indicates that they are derived from pyrite oxidation. In some instances, we could observe that some of these framboids had grown not only at the surface, but also within the plates, thus obscuring their microsculptural details. Carbon is mostly restricted to the plates and apparently in low concentration (Fig. 4M), suggesting that they may contain a small amount of calcium carbonate.

General morphology and identification

All the specimens exhibit the same distribution pattern and unique morphology of the plates, suggesting they all belong to the same species (Fig. 2). The taxonomic treatment of this species, including detailed descriptions of its anatomy and ontogeny, will be published separately. In this section, we only briefly present the main characteristics of this palaeoscolecid worm recovered from the Fezouata Shale (Figs. 2 and5).

The species is characterized by an elongate and slender body, reaching up to three millimetres in width and (at least) five centimetres in length (Fig. 2F). Each annulus bears two transverse rows of c. 20-30 elongate plates (Fig. 2G-H). The plates of a given row are similar in shape and size (c. 90 μm and 45 μm in average length and width, respectively; Fig. 2A-B), and they are orientated with their long axis parallel to the long axis of the body. The distribution, morphology, and size of the plates remain virtually unchanged along the antero-posterior axis of the trunk. No dorso-ventral differentiation is evidenced by the morphology or distribution pattern of the plates. These plates are particularly distinctive, exhibiting a concave external surface with a scalloped margin and a concave visceral surface with circular pits (Fig. 5A-C, and E). The scalloped margins of two adjacent plates were apparently interlocked during the circular contraction of the body (Fig. 5B).

The atypical morphology of the plates and their distribution pattern indicate that the Moroccan specimens belong to Palaeoscolex? tenensis [START_REF] Kraft | Worm-like fossils (Palaeoscolecida;? Chaetognatha) from the lower Ordovician of Bohemia[END_REF]. Until now, this taxon was only known from the external mould of a single small fragment of cuticle (Fig. 5C, andE), which had been found in the Floian (early Arenig, Corymbograptus vsimilis graptolite Zone) Klabava Formation (Bohemia, Czech Republic; [START_REF] Kraft | Worm-like fossils (Palaeoscolecida;? Chaetognatha) from the lower Ordovician of Bohemia[END_REF]. Therefore, the abundant new material from the Fezouata Shale considerably increases our understanding of the morphology of this taxon, even documenting some aspects of its anatomy that are unknown in most palaeoscolecid species.

The posterior region

The posterior region of the body is unfortunately rarely preserved in the Moroccan specimens, being visible in only four out 38 specimens (Fig. 3). Moreover, the preservation of the aboral structures is poor and does not permit fine details of their morphology to be observed. The trunk tapers posteriorly, but only a few millimetres from its posterior end. The latter is associated with smaller and more widely spaced plates, and it bears a pair of thin projections c. 0.65 mm long and c. 0.15 mm wide (at base). These spiny structures are curved and likely represent hooks (Fig. 3). The opening of the anus cannot be observed due to the lateral flattening of the specimens.

The digestive system

One specimen of P.? tenensis preserves a simple, narrow, and straight digestive tract, which occupies a central position within the body cavity and is filled with sediment-like material (Fig. 6A-B). Parts of this infilling exhibit a dark red/purple colour in surface, likely representing a film of iron oxides (gut wall?). Interestingly, the sediment-like infilling and its imprint in the matrix show regularly spaced lateral constrictions (black arrows on Fig. 6B). These are increasingly distinct toward one extremity (on the right on Fig. 6A), where the infilling appears segmented.

Co-occurring fossils

The excavation sites where P.? tenensis was found have also yielded various arthropods (anomalocaridid appendages, marrellomorphs, juvenile Thelxiope, Tremaglaspis sp., trilobites, and small xiphosurids), inarticulate brachiopods, and graptolites. However, the worms have never been found with these taxa on the same slabs and accordingly, whether they really co-occurred with any of them at a given time cannot be ascertained (i.e. they may come from slightly different horizons). Small elongate structures (visible as iron-oxide stained sediment) seldom occur on the worm-bearing slabs, possibly representing burrows. If they do, it seems unlikely that they were made by the palaeoscolecids, for the latter are notably larger. However, it is noteworthy that palaeoscolecid-bearing horizons are intercalated between beds exhibiting abundant, complex, and sometimes deeply penetrative burrows.

On two different samples, the worms are associated with coprolites formed by clusters of pellets called Tomaculum (Fig. 6C-D). These clusters comprise up to 25 rounded and elongate pellets (Fig. 6C), and may be up to 2.4 cm in length and 2 mm in width. Diagenetic (i.e. secondary) phosphatisation is a rather well-known mode of preservation of soft-tissues [START_REF] Schiffbauer | Exceptional fossil conservation through phosphatization[END_REF], which is particularly well-exemplified by the possible metazoan embryos from the Ediacaran Doushantuo Formation (Xiao et al., 1998), or the upper Cambrian Orsten fauna and its variety of tiny arthropods [START_REF] Waloszek | The "Orsten" window: a three-dimensionally preserved Upper Cambrian meiofauna and its contribution to our understanding of the evolution of Arthropoda[END_REF]. Almost 40% of the known species of palaeoscolecids have been described from secondarily phosphatised fragments of cuticle. In such cases, the sclerites and the underlying tissues are mineralized, which allows the observation of exquisite anatomical details. However, a primary phosphatic composition of palaeoscolecid plates has also been repeatedly evocated in the past (e.g. [START_REF] Harvey | Are palaeoscolecids ancestral ecdysozoans?[END_REF], but evidence for this remains scarce. Indeed, palaeoscolecid plates do not usually retain the original composition (García-Bellido et al., 2013), either because of alteration from metamorphism, weathering, or sample processing in Burgess Shale-type and small carbonaceous fossils, or due to secondary phosphatisation in the case of Orsten-type and SSF assemblages. It is also quite common for clay minerals to replicate these sclerites, which lose their phosphatic content during diagenesis [START_REF] Botting | Diverse middle Ordovician palaeoscolecidan worms from the Builth-Llandrindod Inlier of central Wales[END_REF]. Compositional analyses performed on the Moroccan specimens show that their plates are mostly composed of apatite, while the matrix surrounding the specimens (and between the plates) is made of aluminosilicates and does not contain phosphorus and calcium. Since no example of secondary phosphatised tissues has ever been observed in the Fezouata fossils, we interpret the apatitic composition of the plates as primary.

Within Ecdysozoa, a primary and predominantly phosphatic cuticle is also known in a few lower Palaeozoic arthropods: aglaspidids (Lerosey-Aubril et al., 2013), bradoriids [START_REF] Siveter | Cambrian bradoriid and phosphatocopid arthropods of North America[END_REF], phosphatocopines [START_REF] Rode | A new Early Cambrian bradoriid (Arthropoda) from East Antarctica[END_REF], and the enigmatic Phytophilaspis [START_REF] Lin | The cuticle of the enigmatic arthropod Phytophilaspis and biomineralization in Cambrian arthropods[END_REF]. Additionally, the presence of calcium phosphate has been reported in the outer layer of the cuticle of two trilobites [START_REF] Teigler | Microstructure and composition of the trilobite exoskeleton[END_REF][START_REF] Dalingwater | Cuticular ultrastructure of the trilobite Ellipsocephalus polytomus from the Middle Cambrian of Öland, Sweden[END_REF][START_REF] Dalingwater | Cuticular ultrastructure of some Silurian calymenid trilobites from the Welsh Borderland and Gotland[END_REF], but these instances are more likely explained by partial secondary phosphatisation. Sclerites of Cambrian lobopodians have frequently been considered as originally purely organic, sclerotized structures (e.g. [START_REF] Steiner | A new species of Hallucigenia from the Cambrian Stage 4 Wulongqing Formation of Yunnan (South China) and the structure of sclerites in lobopodians[END_REF], but recent compositional analyses indicate otherwise. Indeed, [START_REF] Caron | Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians[END_REF] showed that the sclerites (spines) of Hallucigenia were at least lightly biomineralized and more specifically phosphatized. With their entire trunk largely covered by apatitic plates, palaeoscolecids are definitely unique among cycloneuralians, but the fact that several groups of early Palaeozoic ecdysozoans evolved the capability to secrete calcium phosphate in their cuticle is noteworthy. Thus, while the adaptive benefit of this feature remains unclear, its presence in three different ecdysozoan lineages at that time supports the view that its evolution has been facilitated, if not triggered by a unique chemistry of the oceans in the early Cambrian, the so-called early Cambrian phosphogenic event [START_REF] Wood | Escalation and ecological selectivity of mineralogy in the Cambrian Radiation of skeletons[END_REF].

Ecology of palaeoscolecids

Modes of life

Palaeoscolecid life styles remain poorly known. Burrowing abilities and more generally an endobenthic mode of life have been traditionally inferred based on anatomical characteristics, such as a cylindrical body, a uniform and radial distribution of sclerites at its surface, or the presence of aboral hooks (e.g. Conway [START_REF] Morris | Middle Cambrian priapulids and other soft-bodied fossils from Utah and Spain[END_REF][START_REF] Morris | New palaeoscolecidan worms from the Lower Cambrian: Sirius Passet, Latham Shale and Kinzers Shale[END_REF][START_REF] Hu | A new priapulid assemblage from the early Cambrian Guanshan fossil Lagerstätte of SW China[END_REF]. This assumption may have found some support in the recent claims of in situ preservation of individuals of Cricocosmia jinningensis [START_REF] Hou | Discovery of Chengjiang fauna at Meishucun, Jinning, Yunnan[END_REF] and Maotianshania cylindrica Sun and Hou, 1987 within putative burrows (Zhang et al., 2006;[START_REF] Huang | The burrow dwelling behavior and locomotion of palaeoscolecidian worms: new fossil evidence from the Cambrian Chengjiang fauna[END_REF]. However, these specimens exhibit a great diversity of postures, some being particularly tortuous, and none seem to have their aboral hooks anchored at the wall of the putative burrow. Also, they may instead record the attempts of vagrant epibenthic animals to escape from sediment after rapid burial during an obrution event. Moreover, Cricocosmia and the closely related Tabelliscolex are particularly atypical within the Palaeoscolecida, for they possessed large, paired, and apparently non-biomineralised sclerites [START_REF] Han | Trunk ornament on the palaeoscolecid worms Cricocosmia and Tabelliscolex from the Early Cambrian Chengjiang deposits of China[END_REF][START_REF] Steiner | A new species of Hallucigenia from the Cambrian Stage 4 Wulongqing Formation of Yunnan (South China) and the structure of sclerites in lobopodians[END_REF]. At best, they are regarded as a distinct family within their own class, the Cricocosmidae (Hou et al., 1999;[START_REF] Han | Trunk ornament on the palaeoscolecid worms Cricocosmia and Tabelliscolex from the Early Cambrian Chengjiang deposits of China[END_REF], and therefore can only be used with utter caution when it comes to extrapolating generalized ecological characteristics inferred from them to all palaeoscolecids. Still, most palaeoscolecid species exhibit a general body morphology compatible with an essentially endobenthic life style.

Interestingly, some taxa exhibit hints of a dorso-ventral differentiation of the body, in the form of differences in plate morphology or uneven distributions of plates, aboral hooks, lateral projections or papillae born by the trunk (e.g. [START_REF] Müller | Palaeoscolecid worms from the Middle Cambrian of Australia[END_REF][START_REF] Zhang | Early Cambrian palaeoscolecid cuticles from Shaanxi, China[END_REF][START_REF] Ivantsov | Articulated palaeoscolecid sclerite arrays from the Lower Cambrian of eastern Siberia[END_REF][START_REF] Maas | Maotianshan-Shale nemathelminthsmorphology, biology, and the phylogeny of Nemathelminthes[END_REF]. This differentiation of the external body envelope suggests that the dorsal and ventral sides interacted differently with the surrounding physical environment, which is not the case when animals spend their entire life surrounded by sediment. Also, it has been suggested that these forms with dorso-ventrally differentiated bodies were epibenthic vagrant, at least part of their lives [START_REF] Zhang | Early Cambrian palaeoscolecid cuticles from Shaanxi, China[END_REF][START_REF] Ivantsov | Articulated palaeoscolecid sclerite arrays from the Lower Cambrian of eastern Siberia[END_REF]Muir et al., 2014).

P.? tenensis does not exhibit any hints of dorso-ventral differentiation of the body. Its plates are similar in size, shape and distribution all around the trunk. The regular distribution of the plates does not betray the presence of structures, such as papillae or projections, which might have been lost during fossilization. Only the paired aboral hooks, which are known to curve ventrally in species with dorso-ventrally differentiated bodies, could be used to differentiate a dorsal side and a ventral side. Accordingly, an entirely infaunal mode of life could be reasonably inferred for this species based on its general morphology. However, if a great diversity of burrows are present in the Fezouata Shale (Van Roy et al., 2010;Martin et al., in press), including some compatible in diameters with a palaeoscolecid origin, the horizons yielding the worms are virtually barren in ichnofossils. Elongate features are seldom observed in these beds, which could be regarded as putative burrows, but the palaeoscolecids are too large to be their makers. It could be argued that larger burrows were initially present, but too superficial to survive the sediment reworking accompanying the deposition of the worm-bearing beds. However, sedimentological data suggest that the intervals yielding soft-bodied fossils in the Fezouata Shale were deposited at the limit of impact of storm waves and therefore in a rather quiet environment (Martin et al., in press). The preservation of clusters of faecal pellets on slabs bearing palaeoscolecid remains also speaks to an extremely low-energy environment (see Sections 5.3 and 6.2.2). Accordingly, we believe that pre-burial transport of these delicate remains was minimum, if not totally absent, and that the lack of burrows in palaeoscolecid-bearing beds is not a preservational artefact; instead, it may indicate that these worms were not endobenthic, but epibenthic animals. A similar conclusion was reached by [START_REF] Botting | Diverse middle Ordovician palaeoscolecidan worms from the Builth-Llandrindod Inlier of central Wales[END_REF] and Muir et al. (2014) about palaeoscolecids found in comparable siliciclastic sediments. It is also noteworthy that palaeoscolecids showing no sign of dorso-ventral differentiation of the body are rather abundant in the early Cambrian Emu Bay Shale, while this Lagerstätte is characterized by an almost total absence of bioturbation (García-Bellido et al., 2013;Paterson et al., in press). Similar situations are observed in Guanshan (early Cambrian; [START_REF] Hu | A new priapulid assemblage from the early Cambrian Guanshan fossil Lagerstätte of SW China[END_REF] or in the Weeks Formation (late Cambrian; [START_REF] Lerosey-Aubril | Arthropod appendages from the Weeks Formation Konservat-Lagerstätte: new occurrences of anomalocaridids in the Cambrian of Utah, USA[END_REF]. These observations support the view that at least some palaeoscolecids were not endobenthic organisms.

Lastly, P.? tenensis shows a unique morphological feature: the apparent ability of two adjacent plates of a given row to interlock. This is well illustrated by a pair of plates described by Boogaard (1989, pl. 1 Fig. C). A similar situation was observed a few times during SEM investigations of the Moroccan specimens (Fig. 5B). This interlocking of the plates of a given row apparently only happened when circular muscles contracted, thus reducing the diameter of the body locally. Such a temporary interconnection of the plates may have resulted in a transient and local stiffening of the external envelope of the body. Whether this ability facilitated locomotion or improved defence is unknown.

Feeding habits

Preserved gut remains of palaeoscolecids have been documented in less than ten of the more than sixty species of the group, usually from a single specimen in each species (e.g. Zhang et al., 2006, Fig. 2A;Hu et al., 2012, Fig. 4C;García-Bellido et al., 2013, Fig. 4I-K;[START_REF] Wang | A Tremadocian (Early Ordovician) palaeoscolecidan worm from graptolitic shales in Hunan Province, South China[END_REF], Fig. 2B). However, the morphology of the digestive tract is essentially the same in all these specimens: a straight, rather narrow, cylindrical(?) tube connecting the teeth-bearing pharynx anteriorly to the anus at the posterior body end. There is no evidence of portions of digestive tract differentiated into a crop (anteriorly) or a bursa (i.e. a pouch-like, eversible terminal portion of the gut), whereas such structures are known in some fossil and modern representatives of the closely related priapulids [START_REF] Vannier | Gut contents as direct indicators for trophic relationships in the Cambrian marine ecosystem[END_REF][START_REF] Lan | The feeding behaviour of the Cambrian tubiculous priapulid Selkirkia[END_REF]. Such a simple and conservative organization of the palaeoscolecid gut is further confirmed by the Moroccan specimen described herein (Fig. 6).

To our knowledge, almost all the previously known fossilized palaeoscolecid guts are two-dimensionally preserved as films of carbon, or as a product of diagenetic alteration of organic matter (e.g. pyrite, iron oxides). An exception to this is the unique example of gut phosphatisation in palaeoscolecids that has been recently reported from the Emu Bay Shale by García-Bellido et al. (2013). The phosphatised intestine retains a slight relief in these specimens, but otherwise its morphology is virtually the same as in the other taxa. As documented in arthropods (Butterfield, 2002;[START_REF] Lerosey-Aubril | Controls on gut phosphatisation: the trilobites from the Weeks Formation Lagerstätte (Cambrian; Utah)[END_REF][START_REF] Lerosey-Aubril | Notchia weugi gen. et sp. nov., a new short-headed arthropod from the Weeks Formation Konservat-Lagerstätte (Cambrian; Utah)[END_REF][START_REF] Zacaï | Reconstructing the diet of a 505-millionyear-old arthropod: Sidneyia inexpectans from the Burgess Shale fauna[END_REF], the most exquisite preservation of the gut is via phosphatisation, the other modes of preservation frequently resulting in the loss of structures (e.g. digestive glands; Fatka et al., 2013;Zhu et al., 2014). Accordingly, the simplicity of palaeoscolecid digestive system consistently observed so far is regarded as original.

Another exception in terms of preservational style of the gut is observed in Chalazoscolex from the early Cambrian Sirius Passet. Indeed, some specimens illustrated by Conway [START_REF] Morris | New palaeoscolecidan worms from the Lower Cambrian: Sirius Passet, Latham Shale and Kinzers Shale[END_REF] exhibit parts of their digestive tract filled with sediment-like material. The one re-illustrated herein (Fig. 6E) even shows pellets that merge into a more massive infilling rearwards. The Fezouata specimen in Fig. 6A-B displays a much longer portion of its gut filled with sediment-like material, and the pellet-like structures delimited by the lateral constrictions are in a row. Nevertheless, both examples are strikingly similar to the recently documented pellet-filled guts of the Cambrian priapulid Selkirkia [START_REF] Lan | The feeding behaviour of the Cambrian tubiculous priapulid Selkirkia[END_REF]. In all these specimens, the presence of pellets in the gut leaves little doubt about the fact that the sediment was ingested in vivo in the course of normal feeding. Sediment infilled guts are actually quite common in priapulids from Chengjiang (e.g. Huang et al., 2004;[START_REF] Ma | Phylogeny and evolutionary significance of vermiform animals from the early Cambrian Chengjiang Lagerstätte[END_REF][START_REF] Ma | A new exceptionally preserved Cambrian priapulid from the Chengjiang Lagerstätte[END_REF]), which suggests that deposit feeding was (at least) part of the feeding habits of these worms.

In this regard, the co-occurrence of P.? tenensis and Tomaculum-type coprolites (Fig. 6C-D) on the same slabs is particularly interesting. The faecal pellets composing these clusters are similar to the pellets observed in the guts of the palaeoscolecids from the Fezouata and Sirius Passet, or the priapulid Selkirkia from Chengjiang. The hypothesis of palaeoscolecids being potential makers of Tomaculum-type coprolites is not new; it dates back to Whittard (1953, p. 131), who first suggested it after comparing the size of the gut of Palaeoscolex with that of the pellets composing Tomaculum-type coprolites. [START_REF] Robison | Annelids from the Middle Cambrian Spence Shale of Utah[END_REF] was even more convinced of this relationship between the fossil and the ichnofossil, to the point that he tentatively assigned a Tomaculum-like specimen from the Middle Cambrian Spence Shale of Utah to the cooccurring palaeoscolecid species Wronascolex? ratcliffei (then Palaeoscolex ratcliffei). Tomaculum coprolites also occur in the localities that have yielded the palaeoscolecids of the Guanshan biota (Luo et al., 2008, p. 133, pl. 39, Fig. 4;[START_REF] Hu | A new priapulid assemblage from the early Cambrian Guanshan fossil Lagerstätte of SW China[END_REF][START_REF] Weber | A diverse ichnofauna from the Cambrian Stage 4 Wulongqing Formation near Kunming (Yunnan Province, South China)[END_REF]. Likewise, a few Tomaculum coprolites have been found in the Murero Fm. (Cambrian Series 3, Spain; J.A. Gámez Vintaned, pers. com. 2015), from which rare palaeoscolecid remains have been recovered (see García-Bellido et al., 2013, and references therein). Secondarily phosphatized clusters of comparable faecal pellets were recently described from the middle Cambrian of China [START_REF] Shen | Phosphatized coprolites from the middle Cambrian (Stage 5) Duyun fauna of China[END_REF]. Interestingly, they were found with other coprolites chiefly composed of palaeoscolecid remains (mostly plates), indicating that such worms inhabited this locality at that time. Unlike the similar ichnofossil Alcyonidopsis, which is associated with the filling of a burrow, Tomaculum refers to essentially two-dimensional clusters of faecal pellets found on the surface of a bed, rather than within it. Because of that, Tomaculum coprolites were traditionally regarded as excrementa of epibenthic animals [START_REF] Eiserhardt | Revision of the ichnotaxon Tomaculum Groom[END_REF]. However, it has been recently showed that some modern annelid species can produce the two types of clusters [START_REF] Kulkarni | New insights into Polychaete traces and fecal pellets: another complex ichnotaxon?[END_REF]. In other words, even if a relationship between Tomaculum and palaeoscolecids appears more and more likely, this cannot be used alone to infer an exclusively epibenthic life style for these worms.

Taken together, the co-occurrence of these trace fossils and palaeoscolecid remains in the Fezouata Shale and other localities, and the description of specimens with sediment-filled guts are strong arguments in support of deposit feeding habits in these worms. This does not mean that they never fed on small, dead or living invertebrates at all. Actually, the description of remains of the small arthropod Isoxys in the gut of a specimen of Xystoscolex (Conway [START_REF] Morris | New palaeoscolecidan worms from the Lower Cambrian: Sirius Passet, Latham Shale and Kinzers Shale[END_REF] suggests that this might have happened. But the different lines of evidence discussed above tends to support the view that palaeoscolecids were omnivorous animals, whose generalist feeding strategy involved the ingestion of sediment in a similar way than some modern priapulids do [START_REF] Shirley | Ecology of Priapulus caudatus Lamarck, 1816 (Priapulida) in an Alaskan subarctic ecosystem[END_REF][START_REF] Trott | Gustatory responses of Priapulus caudatus de Lamarck, 1816 (Priapulida, Priapulidae): feeding behavior and chemoreception by a living fossil[END_REF] and review therein). 2016), http://dx.doi.org/10.1016/j.palaeo.2016.04.009 might also occur in the Middle Ordovician (Darriwilian) of Estonia and Sweden, but as isolated plates only [START_REF] Boogaard | Isolated tubercles of some Palaeoscolecida[END_REF]. These different localities belonged to distinct continents in the Early-Middle Ordovician, namely Gondwana, Perunica, and Baltica (Fig. 5G). Virtually nothing is known about the early ontogenetic development of palaeoscolecids. However, considering that extant priapulids do not possess a planktonic larval phase [START_REF] Shirley | Priapulida of the Gulf of Mexico[END_REF], nor do any other living scalidophorans to our knowledge, it seems reasonable to assume that palaeoscolecids had an entirely benthic life cycle and therefore limited dispersal capabilities. The palaeogeographical distribution of P.? tenensis in the Lower Ordovician is thus better interpreted as further evidence of the proximity of Perunica and Gondwana at that time. Indeed, strong faunal similarities between the two terranes are documented by shelly components of the Fezouata biota, such as trilobites and echinoderms (Lefebvre et al., in this issue-b; Martin et al., in this issue). Further support comes from the discovery of marrellomorph and cheloniellid arthropods in the Fezouata Shale, which are strongly reminiscent to taxa asyet exclusively known in the Upper Ordovician of Bohemia (e.g. Duslia, Furca; Van Roy et al., 2010). Similarly, Gutiérrez-Marco and García-Bellido ( 2015) recently described a new palaeoscolecid from the Upper Ordovician of Morocco (Tafilalt region) that is closely related to a Middle Ordovician species from Czech Republic (Bohemia). Thus, if Perunica truly represented a micro-plate independent from Gondwana as early as the late Cambrian [START_REF] Havlíček | Perunica microcontinent in the Ordovician (its position within the Mediterranean Province, series division, benthic and pelagic associations). Sborník geologických věd[END_REF], the persistence of strong similarities between many benthic components of their faunas, including palaeoscolecids, suggests that they probably remained close to one another for most of the Ordovician period.

Conclusions

The abundant material of P.? tenensis offers the opportunity to reconsider some aspects of the ecology of palaeoscolecids. For instance, the description of the frequent occurrence of palaeoscolecid remains in horizons, if not localities barren of burrows casts some doubts on the supposed endobenthic life style of these extinct worms. Instead, even taxa that do not exhibit a dorso-ventral differentiation of the body might have been at least part of their lives epibenthic. Likewise, the presence of a teeth-bearing pharynx in palaeoscolecids should not be regarded as a definitive argument against the possibility that these worms fed on organic matter present in sediment. This argument does not hold for modern priapulids. The presence of sediment-filled guts in some palaeoscolecid specimens and the description of the recurrent association with Tomaculum-type coprolites suggest that it was not the case in palaeoscolecids either. Instead, a generalist feeding strategy (omnivory) consisting of the ingestion of sediment and any organic items (dead or alive) encountered in the process seems entirely possible, especially when considering modern priapulids. These new observations emphasize the need to consider various lines of evidence when attempting to reconstruct the ecology of extinct animals. While the anatomy remains critical in this regard, ichnofossils, faunal associations, and sediment characteristics provide essential information as well.

The Trilobite fauna a) Preservation of soft-parts

Trilobites form a diversified and abundant group in the Fezouata Shale with at least 26 species belonging to 21 genera. The relative abundance of soft-parts preserved in the trilobites from the Fezouata Shale offers interesting opportunities to document the ecology of this group, which remains mostly inferred from the exoskeleton features (Fortey and Owens, 1995), even though the preservation of some digestive systems begins to attract attention (Lerosey-Aubril et al., 2011, 2012;Fatka et al., 2013). Few other Lagerstätten document appendages preserved in trilobites, and most of them belongs to the same family (see database in appendix). An ongoing study with Rudy Lerosey-Aubril aims at describing appendages preserved in several specimens representing two Fezouata species: Anacheirurus adserai (fig. 52) and Bavarilla zemmourensis (fig. 53). As well as giving precious information on the ecology of trilobites, the potential results might also shed a light on the controversial affinities of the genus Bavarilla (Adrain, 2011). The biostratigraphy of the 26 trilobites species from the Fezouata Shale, which display close affinities with Montagne Noire (France), has been thoroughly studied. As a result, a depth control on trilobite assemblages was highlighted, so that three main biofacies could be defined: the pilekiid-bavarillid (fig. 54), Agerina and raphiophorid (fig. 55) biofacies.. An effect of transport resulting in the mixing of communities was detected and discussed.

Finally, geochemical analyses were performed on rock samples from the three fossiliferous levels at the Bou Izargane section to constrain palaeoenvironmental conditions. They suggest temporary more reduced and dysoxic conditions, as already suspected above.

This study is currently under revision and is intended to be part of the special issue entitled "The Fezouata Biota", which will be published a Univ Lyon, Université Claude Bernard Lyon 1, ENS de Lyon, CNRS, UMR 5276 LGL-TPE, F-69622 Villeurbanne, France; emmanuel.martin@univ-lyon1.fr; romain.vaucher@univ-lyon1.fr; bertrand.lefebvre@univ-lyon1.fr

Introduction

The Lower Ordovician of the Anti-Atlas (Morocco; Fig. 1B) is one of the few successions, worldwide, documenting the early stages of the Great Ordovician Biodiversification Event. In the last 50 years, the shales of the Lower and Upper Fezouata formations (early Tremadocian to late Floian; Fig. 1D) have yielded abundant and diverse biomineralized assemblages of marine invertebrates dominated by trilobites, associated with articulate brachiopods, echinoderms, graptolites and molluscs (Destombes et al., 1985) These newly acquired data, along with the material originally collected by Jacques Destombes in the second half of the 20th century, make it possible to describe with unprecedented detail the assemblages of Early Ordovician trilobites from the central Anti-Atlas, and to discuss their variations through time and space in this area. Trilobites are one of the major groups of marine invertebrates in the Fezouata Shale, both in terms of abundance and diversity (Destombes et al., 1985;Vidal, 1996Vidal, , 1998aVidal, , 1998b)). Moreover, trilobites occur both in "classic" fossiliferous levels yielding biomineralized remains throughout the section and in the horizons with exceptional preservation, in which trilobite soft-parts (e.g., gut) and lightly sclerotized portions of their bodies (e.g., appendages) can be preserved (Van Roy et al., 2010).

Although the systematics of trilobites from the Fezouata Shale are still partly underway, their biostratigraphy, palaeoecology and palaeobiogeography can be discussed and compared with previous trilobite-based studies achieved in other areas and/or slightly younger stratigraphic intervals (e.g., Fortey and Cocks, 2003;[START_REF] Turvey | Early Ordovician (Arenig) trilobite palaeoecology and palaeobiogeography of the South China Plate[END_REF][START_REF] Mergl | Tremadocian Trilobites of the Prague Basin, Czech Republic[END_REF][START_REF] Zhou | Trilobite biofacies and palaeogeographic development in the Arenig (Ordovician) of the Yangtze Block, China[END_REF][START_REF] Adrain | A synopsis of Ordovician trilobite distribution and diversity[END_REF]. Consequently, the aims of this paper are: (1) to analyse the variations through time of the successive Early Ordovician trilobite assemblages of the Zagora area, and to propose a trilobite-based biozonation correlated to that established on graptolites;

(2) to discuss the spatial distribution of trilobite assemblages in the Lower Ordovician of the central Anti-Atlas, and to identify trilobite biofacies along a depth-related profile; and (3) to discuss the palaeobiogeographic affinities of the Moroccan trilobite assemblages with other highlatitude, Early Ordovician faunas from peri-Gondwanan areas (e.g., Bohemia, Montagne Noire, Wales).

Historical background

In Morocco, the first Ordovician trilobites were reported successively in the Djebilet hills (Calymene; Barthoux, 1924), in Oulmès area (Trinucleus; Termier, 1927) and in the High Atlas (Placoparia; [START_REF] Roch | Etudes géologiques dans la région 1250 méridionale du Maroc occidental[END_REF]. The presence of Ordovician rocks in the Anti-Atlas, was first mentioned by Neltner (1929), based on the presence of the trilobite Trinucleus in the Tafilalt area. Similarly, the existence of the Ordovician system in the western (Jbel Tachilla, Tiznit area; Bigot and Dubois, 1931) and central Anti-Atlas (Jbel Bani, Foum Zguid area; Bondon in [START_REF] Termier | Etudes géologiques sur le Maroc central et le Moyen-Atlas septentrional[END_REF] was established by the discovery of trilobite remains assigned to Acidaspis buchi and Homalonotus sp., respectively.

In the Anti-Atlas (and in Morocco), the first Early Ordovician trilobites were collected in the 1930s in the Agdz area (Jbel Kissane) by Jacques Bondon and identified as Onchometopus cf.

volborthi by Pierre Pruvost (in Roch, 1939), who suggested a late Tremadocian age. Two specimens of large trilobites (O. cf. volborthi and Ogygia desiderata) from the Lower Ordovician of the Agdz area were figured by Termier and Termier (1950: pl. 193, figs. 1-2).

The Tremadocian age of the large specimens from Agdz was questioned by most authors, who reinterpreted them as more likely 'Arenig' or 'Skiddavian', i.e. Floian [START_REF] Choubert | Aperçu de la géologie marocaine[END_REF][START_REF] Choubert | Note sur la géologie de l'Anti-Atlas[END_REF](Choubert, , 1952;;Choubert and Termier, 1947;Termier andTermier, 1947, 1950;[START_REF] Roch | Histoire stratigraphique du Maroc[END_REF]Choubert et al., 1952) Destombes et al. (1985) and Destombes (2006a-j), who listed 11 and 25 taxa in the lower and upper parts of the Fezouata Shale, respectively (see also Vidal, 1998b).

Systematic descriptions of trilobites have focused mainly on the more diverse assemblages from the upper part of the Fezouata Shale (Floian), with contributions on phacopids (Destombes, 1972), calymenids (Vidal, 1996), asaphids and raphiophorids (Vidal, 1998a), and asaphids (Corbacho and Vela, 2010). In contrast, the description of the trilobite fauna from the lower part of the Fezouata Shale (late Tremadocian) is far less advanced. It was originally undertaken by the late Wolfgang Hammann, who died before it could be completed. So far, only a few late Tremadocian taxa have been described (Rabano, 1990;Fortey, 2009Fortey, , 2011aFortey, , 2011b;;Corbacho and Vela, 2011). No synthetic description of trilobite assemblages from the whole Fezouata Shale has been achieved yet, thus preventing any attempt of detailed comparison with other Early Ordovician trilobite faunas from high-latitude peri-Gondwanan areas, such as Avalonia, England and Wales [START_REF] Fortey | Early Ordovician (Arenig) stratigraphy and fauna of the Carmarthen district, south-west Wales[END_REF], 1987, 1991;[START_REF] Owens | Tremadoc faunas from the Carmarthen district, South Wales[END_REF], Bohemia, Czech Republic (e.g. [START_REF] Mergl | Tremadocian Trilobites of the Prague Basin, Czech Republic[END_REF], or the Montagne Noire, France (e.g. Vizcaïno and Alvaro, 2002). Although the primary aims of this study are to focus on the biostratigraphy, palaeoecology and palaeobiogeography of trilobites from the Lower Ordovician of the central Anti-Atlas, several hundred specimens were identified up to generic, and when possible, specific level. However, their systematic description will be treated elsewhere. In Early Ordovician times, the Anti-Atlas was part of the northern Gondwanan margin, which was located at high latitudes (ca. 65°S; Fig1.C) in the southern hemisphere, relatively close to the South Pole (Cocks and Torsvik, 2004;[START_REF] Torsvik | BugPlates: Linking Biogeography and Palaeogeography[END_REF]Cocks, 2011, 2013). The thick Lower

Geological context

Ordovician succession of the central Anti-Atlas was deposited in a rifting context, related to the opening of the Rheic Ocean and the drift of Avalonia away from the Gondwanan supercontinent (Cocks and Torsvik, 2004).

The Ordovician succession of the Anti-Atlas was originally subdivided into four main lithostratigraphic units by Choubert (1942), which were later formally described as groups by Destombes (1971; Fig. 1D): (1) the External Feijas Group (Tremadocian -early Darriwilian);

(2) the First Bani Group (early Darriwilian -early Sandbian); (3) the Ktaoua Group (early Sandbian -late Katian); and (4) the Second Bani Group (late Katian -Hirnantian). The name of the lower part of the succession ("Feijas") refers, in southern Morocco, to the depressed and elongated areas, which occur between the middle Cambrian sandstones of the Tilemsoun Formation (Tabanite Group, Cambrian Series 3) and the Middle Ordovician sandstones of the First Bani Group (Destombes et al., 1985;Geyer and Landing, 2006). Four main subdivisions were identified by Destombes (1962Destombes ( , 1970Destombes ( , 1971) ) In the eastern Anti-Atlas, the boundary between the Lower and Upper Fezouata Shale formations corresponds to a glauconitic and ferruginous horizon, probably associated with a maximum flooding surface (Destombes et al., 1985). In the central Anti-Atlas, however, this lithological marker bed is absent, and the two lowermost Ordovician formations cannot be distinguished (Destombes et al., 1985;Destombes, 2006a-j). For this reason, the term "Fezouata Shale" was proposed by Martin et al. (in press), for the thick (~ 850 m), monotonous Early Ordovician succession of the Zagora area, consisting of silty and micaceous argillites, and laterally equivalent to both the Lower and Upper Fezouata Shale formations.

The thickness of the Lower Ordovician series varies along the Anti-Atlas, and it reaches its maximum value (~ 900 m) in the Ternata plain (N. of Zagora) in the central part of the Anti-Atlas. In this area, the Fezouata Shale rests unconformably on the middle Cambrian (Guzhangian) Tabanite Group (Fig. 1). The siltstones of the Fezouata Shale were almost exclusively deposited in proximal offshore to lower shoreface environments (Martin et al., in press; Vaucher et al., submitted, this issue). In the central Anti-Atlas, the late Floian deposits of the uppermost part of the Fezouata Shale and of the overlying Zini Sandstone Formation both record a shallowing upward sequence, with a gradual transition from lower shoreface to foreshore environments (Vaucher et al., this issue). Both the Fezouata Shale and the Zini Sandstone exhibit centimeter-to meter-scale oscillatory structures (e.g. wave ripples, hummocky cross stratification), indicating that sedimentation was chiefly controlled by storms and waves (Vaucher et al., this issue), but with the subsidiary influence of tides (Vaucher et al., submitted).

Material and methods

Fieldwork

The whole Lower Ordovician succession exposed in the Zagora area (Central Anti-Atlas, Morocco) was logged, at a decimeter scale, from fourteen different sections (see Martin et al., in press). Data were compiled into a single, synthetic stratigraphic column ~900 meter-thick (Vaucher et al., this issue; Fig. 2). All localities yielding exceptionally preserved fossils (EPF) mentioned in the original contribution of Van Roy et al. (2010) were included, except one located c. 15 km west of the main outcrop area. EPF localities were carefully located and positioned along the stratigraphic column, based on their lithological characteristics (colour, composition, marker beds) and faunal elements (mostly graptolites, trilobites, and to a lesser extent echinoderms). Graptolites were collected throughout the succession, but particular attention was paid to horizons yielding EPF (Gutiérrez-Marco and Martin, this issue).

Changes in lithology and sedimentary structures were observed throughout the succession and were used to reconstruct short-term and long-term relative sea-level changes.

The trilobite material

This study is based on 485 complete and incomplete trilobite specimens, sampled throughout the Fezouata Shale and derived from 19 different stratigraphic levels (Figs. 1A,2). Each sample was identified down to the species level, when possible. All specimens were whitened with ammonium chloride and observed with a binocular microscope. When necessary, camera lucida sketches and photographs were produced, for purposes of comparison with material published in the literature. All trilobite occurrences reported by Jacques Destombes (2006a-j) from other regions of the Anti-Atlas were also plotted on the log, when correlations were possible.

Each identifiable trilobite sclerite was counted according to the MNI protocol used to obtain the number of individuals in each sample [START_REF] Gigout | Estimating Numbers of Whole Individuals from Collections of Body Parts: A Taphonomic Limitation of the Paleontological Record[END_REF]. The state of completeness and the parts preserved were also taken into account.

All studied specimens are deposited in the collections of the Cadi Ayyad University, Marrakesh (Faculté des Sciences et Techniques, Guéliz; sample number prefix AA), and in the palaeontological collections of Lyon 1 University, Villeurbanne (FSL).

Definition of a depth-related zonation

A relative depth zonation for Early Ordovician trilobite assemblages of the Zagora area was defined, based on the sedimentological data obtained from the field (e.g. lithology, grain sizes, sedimentary structures, and biostratinomy). In the Fezouata Shale, two different types of fossiliferous beds are identified (Vaucher et al., this issue). The first type consists of fine to medium grained sands, associated with hummocky cross-stratifications (HCS) of metre-scale wavelength and is considered to represent proximal storm deposits. It commonly yields disarticulated specimens, even for species considered as endobenthic. The second type, which especially contains EPF and/or articulated biomineralized remains, corresponds to a quiet muddy background environment, covered by the most distal storm deposits (several centimetre-thick). The distal storm deposits of the Fezouata Shale consist of fine grained sands diplaying, HCS of decimetre-scale wavelength and were deposited under waxing-andwaning flow activity with potential transport of skeletal material derived from more proximal environments. Three main, partly overlapping zones were defined along a proximal-distal profile: (1) lower shoreface; (2) transition zone; and (3) proximal offshore (Fig. 3). As mentioned above, the typical coastal linear zonation (e.g. McLane, 1995) is modified in the Fezouata Shale, because of the influence of tides. Detailed definitions of the lower shoreface and proximal offshore settings in the Zagora area were given by Vaucher et al. (submitted).

In this peculiar depositional context, lower shoreface environments are characterized by siltstones displaying wave ripples (with a decimetre-scale wavelength), with intercalations of fine, 5 -15 centimetre-thick, sandy storm deposits. The following trilobite localities can be assigned to these settings: 421=425=1677=1678=1680=1681=1683; 1673=1674; Z-F2(0), Z-F3, Z-F4(3), Z-F4(lower excavation), Z-F5 and Z-F6a.

Proximal offshore environments correspond to argillaceous siltstones showing wave ripples (with a centimetre-scale wavelength) and thin (1-5 centimetre-thick) intercalations of coarse silty to fine sandy storm deposits. Trilobite levels associated to such offshore conditions correspond to localities 35, 1685, Z-F7( 2) and Z-F26.

Finally, the sedimentological features observed in the transition zone (defined herein) are a mixture of those typical of both more distal (proximal offshore) and more proximal (lower shoreface) environments. In the Zagora area, several trilobite levels are associated with this intermediate, transitional setting: 1687=Z-F25b, Z-F0, Z-F1, Z-F2(1)a, Z-F2(3a) and Z-F13c.

Geochemical data

Chemical analyses were performed in order to investigate the presence of any significant difference between levels with exceptional preservation and those recording background sedimentation. Because of the intense surface weathering of Ordovician outcrops in Morocco, an alteration of the original chemical signal is likely. Consequently, only a limited number of surface samples (six) was selected for this preliminary geochemical analysis (Trace Element and Organic carbon isotopes, δ 13 C org, analysis). All samples were collected from the same 18 meter-thick section, at Bou Izargane hill (Z-F4), about 20 km N of Zagora. This locality has yielded three fossiliferous levels, all located between 270 and 290 meters above the base of the Ordovician, i.e. within the main, lower interval with exceptional preservation (A. murrayi Zone, late Tremadocian; Figs. 2,5).

The stratigraphically lowest sample (Gc1) was extracted from a small lens (Z-F4 lower excavation, Anatifopsis lens; Fig. 5), where diverse and abundant remains of biomineralized invertebrates were collected: bivalves, brachiopods, conularids, graptolites, mitrate stylophorans (Anatifopsis), and trilobites.

The next three samples were extracted 10 meters above, from a slightly younger stratigraphic interval (Z-F4(3)). One of them (Gc3) was collected within a level yielding fossils with exceptionally preserved soft-tissue: graptolites, marrellomorphs (Furca), cornute stylophorans (Hanusia, Thoralicystis), and trilobites exhibiting antennae and/or appendages (Anacheirurus, Bavarilla). The other samples were extracted from the barren deposits located below (Gc2)

and above (Gc4) the level with exceptional preservation.

The last two samples were collected 5 meters above, from the stratigraphically highest fossiliferous interval (Z-F4, upper excavation). The first one (Gc5) was extracted from a level yielding poorly preserved remains of biomineralized organisms (brachiopods, graptolites and trilobites), whereas the second one (Gc6) was collected within a level with exceptional preservation (appendages of anomalocaridids, graptolites, marrellomorphs, palaeoscolecid worms and trilobites).

All samples were cut, avoiding any fractures or secondary crystallisation and ground in an agate mortar, until having a grain size lower than 145 microns. For trace element analysis, about 100 mg of ground bulk powder was digested with 5% (v/v) acetic acid. Rare earth and other trace element concentrations were measured with an Element 2 ICP-MS at the Pôle Spectrométrie Océan (PSO, Brest). For δ 13 C org , samples were decarbonated in 6 N HCl overnight at room temperature, followed by 2 hours at 80 °C. Residues were washed with distilled water, centrifuged and dried at 50 °C. Around 1 mg of decarbonated powder was then analysed using a EA coupled to a Delta V Plus Thermo Fisher mass spectrometer.

Analitical precision is +-0,2 ‰ (2s). The 485 specimens examined were assigned to 26 different species and 21 genera, belonging to the 15 families listed in the table below (Table 1). Although systematics is not the primary aim of this work, two points have to be stressed.

Results

Distribution of trilobite assemblages through time and space

First, pilekiid trilobites of the Fezouata Shale were assigned to different species of the genus Lehua (junior synonym of Anacheirurus) by Corbacho and Vela (2011). However, a single morphological type of pilekiid was identified in our material and assigned to Anacheirurus adserai (Vela and Corbacho, 2007). Second, all leiostegiid specimens from the Fezouata Shale appear to be conspecific with the Montagne Noire species, originally described as Hoekaspis? quadrata by [START_REF] Dean | The Lower Ordovician stratigraphy and trilobites of the Landeyran valley and neighbouring district of the Montagne Noire, south-western France[END_REF] and here assigned to the genus Agerina.

The distribution of the 26 trilobite taxa identified in the Lower Ordovician of the Zagora area is clearly neither homogeneous nor random within the Fezouata Shale, but rather results from the complex interaction of both macroevolutionary and environmental factors. The putative influence of several of these factors is discussed below.

Trilobite biostratigraphy: evolution through time

The 26 trilobite taxa examined in this study were collected from two main stratigraphic intervals (Fig. 2): a lower (~100 meter-thick) late Tremadocian interval (A. murrayi Zone and base of the overlying H. copiosus Zone), and about 120 meters above it, a larger (230 meterthick) early to middle Floian interval (?C. protobalticus to ?B. jacksoni zones). The two intervals have 10 species in common. The stratigraphically lower one yielded 19 species, whereas 17 taxa were observed in the upper fossiliferous interval.

Trilobites are extremely rare in the lowermost part of the Fezouata Shale (early and middle Tremadocian; Destombes et al., 1985). Consequently, the few available specimens were not included in this study. The stratigraphic intervals located near the Tremadocian -Floian boundary (from about 350 to 470 m above the base of the Ordovician) and in the uppermost Floian part of the Fezouata Shale (from about 700 to 850 m above the base of the Ordovician)

have been less intensively sampled and have thus yielded fewer specimens than the two main intervals investigated. Pending more intensive sampling, the trilobite assemblages from both the earliest Tremadocian and late Floian intervals remain too poorly known to be included in the present analysis.

In the Fezouata Shale, three distinct trilobite assemblages were identified (Fig 2). Assemblage 1 is strictly limited to the A. murrayi Zone. It was observed in the following localities (Fig. 2):

Z-F1, Z-F2(1a), Z-F2(3a), Z-F3, Z-F4 (lower excavation), Z-F4(3), Z-F5, Z-F6a, and Z-F7(2).

It includes eight species: Anacheirurus adserai, Asaphellus stubbsi, Asaphellus tataensis, Bavarilla zemmourensis, Dikelokephalina brenchleyi, Megistaspis (Ekeraspis) cf. filacovi, Platypeltoides magrebiensis and Symphysurus angustatus.

Assemblage 2 was evidenced in various localities ranging from the late Tremadocian (A.

murrayi Zone) to the middle Floian (?B. jacksoni Zone). It occurs in the following localities (Fig. 2): 35, 1687=Z-F25b, Z-F0, Z-F2(0), Z-F2(1a), Z-F2(3a), Z-F3, Z-F4 (lower excavation), Z-F4(3), Z-F5, Z-F6a, Z-F13c and Z-F26. It includes 10 species: Ampyx priscus, Apatokephalus cf. incisus, Asaphellus sp. aff. jujuanus, Colpocoryphe sp., Euloma sp.

Foulonia peregrina, Geragnostus ? sp., Parabathycheilus gallicus, Prionocheilus sp. and Toletanaspis sp. aff. borni.

Finally, assemblage 3 is apparently restricted to the early -middle Floian time interval (?C.

protobalticus to ?B. jacksoni zones; Fig. 2). It was identified in the following localities: 35, 412=425=1677=1678=1680=1681=1683, 1673=1674, 1685, 1687=Z-F25b, Z-F0 and Z-F26.

It comprises seven species: Agerina quadrata, Asaphellus fezouataensis, Basilicus destombesi, Colpocoryphe pradesensis, C. thorali, Neseuretus cf. attenuatus and Pradoella sp.

Trilobite biofacies: a depth control on assemblages

Fortey (1975a, 1975b) was one of the first to highlight a depth control on trilobite communities. He also defined trilobite biofacies patterns by studying trilobite assemblage variation along onshore-offshore transects. Similar studies have been undertaken in the Ordovician, Silurian, and Devonian (for example Fortey andOwens 1978, 1987;[START_REF] Cocks | Lower Palaeozoic facies and fauna around Gondwana[END_REF][START_REF] Chlupáč | Trilobite assemblages in the Devonian of the Barrandian area and their relations to palaeoenvironments[END_REF][START_REF] Chlupáč | Ecostratigraphy of Silurian trilobite assemblages of the Barrandian area, Czechoslovakia[END_REF][START_REF] Zhou | Trilobite biofacies and palaeogeographic development in the Arenig (Ordovician) of the Yangtze Block, China[END_REF][START_REF] Carlucci | Trilobite biofacies and sequence stratigraphy: an example from the Upper Ordovician of Oklahoma[END_REF]. In the Lower Ordovician of Spitzbergen, [START_REF] Fortey | Early Ordovician (Arenig) stratigraphy and fauna of the Carmarthen district, south-west Wales[END_REF] described a basinward succession of illaenid-cheirurid, nileid, and olenid communities close to the palaeoequator, and of Neseuretus, raphiophorid and olenid communities at high palaeolatitudes. Most of the Tremadocian trilobite assemblages in higher latitude and siliciclastic settings of Bohemia might be compared with the illaenid-cheirurid biofacies originally described in lower palaeolatitudes [START_REF] Mergl | Tremadocian Trilobites of the Prague Basin, Czech Republic[END_REF].

In order to discriminate biofacies in the Fezouata Shale, we used the relative abundance of trilobites for each locality to carry out a cluster analysis. The first analysis was based on both articulated and disarticulated remains but did not provide consistent results. However, distinct groups can be eventually recognized when using the relative abundance of articulated remains only. Cluster analysis is a well establishedmethod for defining biofacies [START_REF] Ludvigsen | Dual biostratigraphy: zones and biofacies[END_REF][START_REF] Balseiro | Unusual trilobite biofacies from the Lower Ordovician of the Argentine Cordillera Oriental: new insights into olenid palaeoecology: Unusual Olenid biofacies in Argentina[END_REF], as it groups the different trilobites species according to the shared similarity in distribution (in this case, the pattern of distribution of articulated specimens along a proximal-distal transect). This analysis was performed using the software Past 3.07 (Hammer et al., 2001). The dendrogram was obtained using the square root of the Bray-Curtis similarity index with the Ward grouping method. Because the the metric used is ultrametric with binary data and possibly with abundance data [START_REF] Legendre | Numerical ecology, Third English edition[END_REF], it is suitable for Ward's method [START_REF] Balseiro | Unusual trilobite biofacies from the Lower Ordovician of the Argentine Cordillera Oriental: new insights into olenid palaeoecology: Unusual Olenid biofacies in Argentina[END_REF].

Three different trilobite biofacies were distinguished in the Lower Ordovician of the Zagora area, based on the depth-related zonation defined above (see section 4.3), and the distribution of articulated trilobites observed in the various localities (Fig. 3).

The most proximal trilobite biofacies displays lower shoreface affinities and includes the following nine species: Anacheirurus adserai, Apatokephalus cf. incisus, Asaphellus stubbsi, A. tataensis, Bavarilla zemmourensis, Megistaspis (Ekeraspis) cf. filacovi, Platypeltoides magrebiensis, Prionocheilus sp. and Symphysurus angustatus. The co-occurrence of both pilekiid (Anacheirurus) and bavarillid (Bavarilla) representatives appears to be diagnostic of the biofacies. This is also the case with the proximal association in the Tremadocian of Bohemia [START_REF] Mergl | Tremadocian Trilobites of the Prague Basin, Czech Republic[END_REF]. This proximal assemblage occupies a palaeogeographic position along a proximal-distal transect analogous to that of the illaenid-cheirurid biofacies (Fortey 1975b).

The second biofacies is mainly restricted to intermediate environmental conditions around the storm wave base, and comprises three species: Agerina quadrata, Asaphellus fezouataensis and Euloma sp. Named Agerina biofacies in this study, it is characterized by the eponymous genus, which occurs with representatives of the Eulomidae family, such as Euloma in the Fezouata Shale or Proteuloma in analogous environment from Bohemia [START_REF] Mergl | Tremadocian Trilobites of the Prague Basin, Czech Republic[END_REF]). The onshore-offshore position of the Agerina biofacies is similar to that of the nileid biofacies (as proposed by [START_REF] Mergl | Tremadocian Trilobites of the Prague Basin, Czech Republic[END_REF]. Finally, a third trilobite biofacies occurs in the most distal settings (upper offshore) of the Fezouata Shale and is characterized by two species: Ampyx priscus and Colpocoryphe sp.

Associated with depth-tolerant asaphid species, this assemblage corresponds to the raphiophorid biofacies [START_REF] Fortey | Early Ordovician (Arenig) stratigraphy and fauna of the Carmarthen district, south-west Wales[END_REF] previously described in Morocco (Vidal, 1998b).

Depth-related affinities are unclear for at least three taxa: articulated specimens of Asaphellus sp. aff. jujuanus and Toletanaspis sp. aff. borni are found both in transition zone and upper offshore environments, while articulated individuals of Parabathycheilus gallicus were observed in both offshore and shoreface deposits.

The first proximal assemblage, with pilekiid and bavarillid in the lower shoreface, and the second one, in transitional environment observed in the Fezouata Shale (Agerina biofacies), have many similarities with those described in the same stratigraphic levels from Bohemia [START_REF] Mergl | Tremadocian Trilobites of the Prague Basin, Czech Republic[END_REF], though Platypeltoides appears to have shallower affinities in Morocco than in Bohemia. Representatives of the Olenid biofacies, which are supposed to dominate deeper and oxygen-deficient settings adjacent to palaeoocean, are missing both in the Lower Ordovician of Bohemia and in the Fezouata Shale. To explain this absence, [START_REF] Mergl | Tremadocian Trilobites of the Prague Basin, Czech Republic[END_REF] suggests that the depositional setting of Bohemia during the Ordovician was too shallow as well as too oxygenated, and thus unsuited for olenids. In the Fezouata Shale, geochemical analyses show at least temporary dysoxic conditions on the sea floor (see section 5.2), nevertheless sedimentary analysis (see section 4.3) and trilobite biofacies highlight together that the deposit environments of the Fezouata Shale belong to the proximal part of the shelf, far from the continental slope and deep ocean environments.

Trilobite assemblages: allochthonous vs. autochthonous assemblages

In the five localities of the Zagora area yielding more than 15 specimens of Early Ordovician trilobites palaeoecological aspects of the assemblages were investigated: especially the autochthonous/allochthonous nature of the associations (i.e. biocoenosis vs. thanatocoenosis, Fig. 4).

Although only few trilobite assemblages could be analysed for this purpose, their stratigraphic distribution is relatively wide, with late Tremadocian (Z-F4(3) and Z-F5), early Floian (Z-F26), and middle Floian (Z-F0 and Z-F25b) localities. In these assemblages, the number of taxa varies from five (Z-F25b), to seven (Z-F4(3) and Z-F5) to nine (Z-F26), and to ten (Z-F0).

All five investigated trilobite assemblages display both articulated and disarticulated exoskeletons. Moreover, in a single locality, co-occurring taxa show contrasting environmental affinities with different depth-related habits (Fig. 3). This suggests that these five assemblages all probably result from a combination of both autochthonous and allochthonous elements, transported from nearby settings (Fig. 4). To test this hypothesis, articulated and disarticulated trilobites were plotted separately for each locality. As a result, all articulated forms turned out to have transition and/or offshore affinities, whereas most disarticulated trilobites had shoreface and/or transition affinities (Fig. 4).

In Z-F0, the articulated trilobite assemblage is dominated by taxa with transition setting affinities (Agerina, Euloma, Ampyx, and Asaphellus), which is in good accordance with the sedimentological data. Similarly, in Z-F25b and Z-F26, the analysis of the community structure and field-based sedimentological evidence both suggest a transitional to proximal offshore setting for this locality. The interpretation of the Z-F5 is more problematic. Trilobite assemblage suggests a proximal offshore environment, whereas sedimentological data rather indicate more proximal environmental conditions (lower shoreface for Z-F5). It is thus very likely that Z-F5 and Z-F25b were both located in a quiet, offshore environment, occasionally disturbed by storms events, which could occasionally introduce allochthonous, often disarticulated trilobites living in more proximal environments (Vaucher et al., this issue).

Correlatively, this implies that, if too many storm events were stacked together, the resulting sedimentological signal would then be biased and would suggest deposition in more proximal environments.

Differences in relative depths possibly explain the dramatically distinct compositions of the two contemporaneous trilobite assemblages from Z-F4(3) and Z-F5 (late Tremadocian, A.

murrayi Zone). Z-F4(3) is the oldest and unique locality yielding trilobites with strict shoreface affinities. In contrast, the trilobite community of Z-F5 is typical of deep, offshore settings. It shows striking similarities with the younger (early Floian) Z-F26 assemblage, which was associated with similar, offshore environments. In such relatively deep settings, downslope transported elements are usually rare and not diverse (one to four species) and communities are dominated by autochthonous taxa, such as Ampyx priscus. The numbers of articulated and disarticulated trilobites (N) per locality are indicated. Depth affinities are inferred from the biofacies defined in Fig. 3.

Affinities with other Early Ordovician trilobite faunas

Most genera of trilobites occurring in the Fezouata Shale were widespread in temperate to high palaeolatitudes during the Early Ordovician (e.g. Agerina, Anacheirurus, Apatokephalus, Asaphellus, Dikelokephalina, Geragnostus, Platypeltoides). In contrast, the distribution of calymenacean genera was restricted to (peri)Gondwanan regions (e.g. Bavarilla,

Colpocoryphe, Neseuretus, Parabathycheilus and Pradoella). Those genera are distinctive of proximal to median shelf environments from Avalonia, Bohemia, Armorica and Gondwana continent during the Lower Ordovician (Fortey and Cocks, 2003). Among coeval assemblages of trilobites from these high latitude, peri-Gondwanan terranes, the faunas from the Lower Ordovician of the Zagora area display strong affinities with the Montagne Noire (southern A second major difference concerns the absence of the genus Taihungshania in the Zagora area, whereas it is one of the most common faunal elements in the latest Tremadocian -early Floian time interval in the Montagne Noire (Vizcaïno and Alvaro, 2002). Taihungshania is a typical (peri)Gondwanan trilobite, with a relatively wide palaeobiogeographic distribution: for example, it was reported in China [START_REF] Lu | Ordovician trilobite faunas of central and southwestern China[END_REF][START_REF] Turvey | Early Ordovician (Arenig) trilobite palaeoecology and palaeobiogeography of the South China Plate[END_REF], Iran [START_REF] Ghobadi Pour | An Early Ordovician Trilobite assemblage from the Lashkarak Formation, Damghan area, northern Iran[END_REF], Turkey [START_REF] Dean | The Lower Palaeozoic stratigraphy and faunas of the Taurus Mountains near Beys¸ehir Turkey. II. The trilobites of the Seydisehir Formation (Ordovician)[END_REF], Oman [START_REF] Fortey | Sedimentary facies and trilobite and conodont faunas of the Ordovician Rann Formation, Ras Al Khaimah, United Arab Emirates. GeoArabia[END_REF], Sardinia (Pillola, pers. comm.), and Montagne Noire [START_REF] Courtessole | Etude biostratigraphique et sédimentologique des formations arénacées de 1'Arenigien du Saint Chinianais oriental (Hérault), versant sud de la Montagne Noire (France méridionale)[END_REF]. In all these regions, Taihungshania occurs in shallow shelf environments [START_REF] Turvey | Early Ordovician (Arenig) trilobite palaeoecology and palaeobiogeography of the South China Plate[END_REF][START_REF] Zhou | Trilobite biofacies and palaeogeographic development in the Arenig (Ordovician) of the Yangtze Block, China[END_REF]. Consequently, its absence in the Anti-Atlas can not be simply explained by environmental conditions (Vidal, 1998a), but it more likely suggests that this area was isolated, with restricted communications (and faunal exchanges) with other (peri)Gondwanan regions, at least during the late Tremadocian time interval. This hypothesis is in good accordance with the presence of numerous endemic species in trilobite assemblages of the A. murrayi Zone. In contrast, more intensive faunal exchange in younger (Floian) assemblages indicates the occurrence of a less restricted palaeobiogeographic context for the Zagora area.

Geochemical Data: understanding the environmental context

Geochemical analyses performed on samples extracted from six distinct levels (Fig. 5), all located within the lower interval with exceptional preservation (late Tremadocian, A. murrayi Zone) at Bou Izargane offer the possibility to take into account additional data on environmental conditions, independent of those deduced from the analysis of sedimentology and trilobite assemblages.

The six carbon isotopes on the organic matter show a clear trend towards more negative values from -30.2 to -32.0 ‰, which are consistent with the δ 3 C org compilation presented in [START_REF] Hayes | The abundance of 13 C in marine organic matter and isotopic fractionation in the global biogeochemical cycle of carbon during the past 800 Ma[END_REF] and with the low δ 3 C carb for the same stratigraphic interval in Argentina (Bergström et al., 2009). Trace elements show relatively low differences from one sample to another, with the exception of the two samples collected within the two horizons yielding exceptionally preserved trilobites (Gc6 and Gc3), and both characterized by high values in

Zn. Moreover, one of these two levels (Gc3) is also enriched in Pb and, to a lesser extent, in both U and Cu (when normalized to Al).

These results suggest that the Gc3 fossiliferous level was deposited in a more reduced environment, with the possible influence of chemoautotrophic bacteria participating in the isotopic negative signal. Cu, Zn and Pb are elements highly sensitive to the sulphur cycle, thus suggesting high bacterial sulphate-reduction activity, leading to the rapid isolation of organic matter from oxidation. This "pyrite wall" may have acted as an oxidative sheltering armor. During later diagenesis and exhumation, oxidative weathering led to the replacement of this pyrite coating by iron oxide, liberating the sulphur as sulphate in the dissolved fluid porosity.

The differences observed between the Gc6 and Gc1 fossiliferous levels suggest the presence of a less reduced environment in Gc1, which did not yield any soft-tissue preservation.

Alternatively, these differences could simply result from a better preservation of the original geochemical signal, as samples Gc2, Gc3 and Gc4 were obtained from more deeply excavated rocks (and thus, potentially less weathered) than samples Gc1, Gc5 and Gc6.

In summary, the two levels yielding exceptional preservation (e.g. trilobites with soft-tissue, antennae and/or appendages), Gc3 and Gc6, are both enriched in Zn, which suggests the occurrence of reducing environmental conditions. In contrast, the two levels yielding mineralized fossil remains (e.g. echinoderms, molluscs, trilobites), Gc1 and Gc5, have a geochemical signal very similar to that of barren layers (Gc2 and Gc4).

Moreover, the Co/Th Vs La/Sc diagram makes it possible to identify the origin of the source rocks in the sediment (Fig. 5). On such a diagram, based on a mixing model between a mafic volcanic and a felsic igneous chemistry composition, the six examined samples from the Fezouata Shale all indicate a relatively similar, felsic igneous source rock. Consequently, this preliminary investigation does not support a hypothesis predicting that ions needed for early diagenetic mineralization would come from volcanic ash deposits (Gaines et al., 2012).

Conclusions

Three temporally restricted trilobite assemblages were identified in the Fezouata Shale of the Zagora area, central Anti-Atlas: assemblage 1 is restricted to the late Tremadocian, The identification of three trilobite biofacies in the Fezouata Shale, comprising the newly recognized pilekiid-bavarillid biofacies, Aegerina biofacies and the well known raphiophorid biofacies, is in good accordance with classic models describing the distribution of benthic taxa along an onshore-offshore transect. The Neseuretus biofacies, typical of shallow shelf environments occurs in the Floian part of the Fezouata Shale as in other (peri)Gondwanan regions [START_REF] Fortey | Early Ordovician (Arenig) stratigraphy and fauna of the Carmarthen district, south-west Wales[END_REF][START_REF] Fortey | The Ordovician trilobite Neseuretus from Saudi Arabia and the palaeogeography of the fauna related to Gondwanaland in the earliest Ordovician[END_REF][START_REF] Turvey | Early Ordovician (Arenig) trilobite palaeoecology and palaeobiogeography of the South China Plate[END_REF][START_REF] Zhou | Trilobite biofacies and palaeogeographic development in the Arenig (Ordovician) of the Yangtze Block, China[END_REF].

This research also suggests that the highest diversity is observed in assemblages resulting from the mixing of autochthonous, distal communities with allochthonous elements from more proximal settings, transported downslope by storm waves.

Palaeobiogeographic affinities of Tremadocian Moroccan assemblages with coeval faunas from Bohemia [START_REF] Mergl | Tremadocian Trilobites of the Prague Basin, Czech Republic[END_REF], Avalonia [START_REF] Owens | Tremadoc faunas from the Carmarthen district, South Wales[END_REF]Fortey and Owens, 1991), and Baltica [START_REF] Ebbestad | Trilobites of the Tremadoc Bjørkåsholmen Formation in the Oslo Region, Norway[END_REF] are evident at generic level. In contrast, Floian assemblages of the Zagora area are no longer dominated by endemic species, suggesting the existence of enhanced faunal exchanges and probably more oxygenated bottom waters.

The endemicity of most Tremadocian taxa, along with the absence of taihungshaniids from shallow environments, and of pelagic trilobites in the Lower Ordovician of the Zagora area all suggest that the Fezouata Shales were deposited in a restricted basin. This relative isolation of the present day Anti-Atlas region during the earliest Ordovician is consistent with the occurrence of episodic dysoxic events, which facilitated exceptional preservation in some intervals of the Fezouata Shale. The scarcity of olenid trilobites (generally associated with oxygen-depleted environments), only reported in the lower part of the Fezouata Shale (Destombes et al., 1985) is well explained by the shallow depositional environments of the Fezouata Shale, which, although occasional anoxic, were far removed from the deeper oceanic environments favored by olenids. However, new field investigations are necessary to identify more precisely the stratigraphic position of olenid occurrences in the lowermost part of the Fezouata Shale. Finally, it is possible that the more sustained faunal exchanges documented in the early-middle Floian time interval are tectonically controlled controlled and related to the widening of the basin.

Although the taphonomic context of the Fezouata Shale is the main subject of two contributions based on the sedimentological feature of the Fezouata Shale (Vaucher et al., submitted a, b), some palaeontological studies also brought precious information on the topic.

The taphonomy of stylophoran echinoderms

Many excavations in the Fezouata Shale yielded abundant stylophoran assemblages (see section 2.2.1). The detailed study of such excavations enabled for the first time the recognition of 5 different taphofacies, which were adapted for both cornute and mitrate stylophorans. Together with sedimentological evidence, this newly defined taphonomic scale strongly suggested that at least some stylophoran beds of the Fezouata Shale represented a storm deposit (taphofacies IIC of Brett et al., 1997). In this context, the local accumulation of hundreds of specimens of cornutes very likely results from distal storm scour and redeposition of a para-autochthonous, monospecific community. A similar study was lead on the eocrinoid assemblages (Allaire et al., 2015) This study was detailed in an extended abstract published in 2015 as a book chapter in the proceedings of Progress in Echinoderm Palaeobiology and was the object of an oral presentation. Bertrand Lefebvre and myself collected the specimens of the study that I pictured to illustrate the article. Romain Vaucher made the sedimentological interpretation.

Bertrand Lefebvre conducted the research and helped me in the writing of this chapter. The oral presentation was entirely made by myself (oral and powerpoint slides).

INTRODUCTION

Echinoderms are a major component of benthic communities in the Lower Ordovician of central Anti-Atlas (Noailles et al., 2010;Van Roy et al., 2010). In the Fezouata Shale (Tremadocian-late Floian), stylophorans (cornutes and mitrates) are the most diverse group of echinoderms (24 taxa). Until recently, however, stylophorans were considered as extremely rare fossils in the Lower Ordovician of this region. From 1959 to 1991, a single specimen was collected, despite intensive fieldwork performed by Jacques Destombes. This specimen was found at Destombes' locality 1685 (Fig. 1; latest Tremadocian to earliest Floian) and described by Chauvel (1971) as the holotype of Thoralicystis zagoraensis. This cornute was associated to a diverse benthic assemblage dominated by bivalves, gastropods and trilobites (e.g., Asaphellus fezouataensis, Colpocoryphe thorali), associated with brachiopods (e.g., Paurorthis tadristensis), conularians, crinoids (Ramseyocrinus sp.), glyptocystitid rhombiferans (Macrocystella bohemica), and hyolithids (Chauvel, 1969; Havlicˇek, 1971; Donovan and Savill, 1988; Vidal, 1998).

In the last twelve years, successive field campaigns in the Lower Ordovician of the Zagora area confirmed that stylophorans are a minor component of high-diversity benthic assemblages dominated by arthropods and molluscs. Stylophorans, however, also occur in mass occurrences in densely packed, low-diversity assemblages, within some welldefined, thin horizons (Lefebvre, 2007;Lefebvre and Botting, 2007). For example, Lefebvre (2007) figured a slab containing numerous specimens of the cornutes Cothurnocystis sp. and Galliaecystis sp. (locality Z-F3, late Tremadocian, A. murrayi biozone; Fig. 1). Two distinct stylophoran beds were reported by Lefebvre and Botting (2007) at locality Z-F2(1) (late Tremadocian, A. murrayi biozone; Fig. 1): a Thoralicystis-dominated horizon and, about two meters above it, a Peltocystis-dominated level. Lefebvre (2007) suggested that stylophoran beds observed in the Fezouata Shale of Zagora area were showing taphonomic features characteristic of taphofacies E as defined by Brett et al. (1997) for echinoderms, and were thus associated with relatively deep environmental conditions (outer shelf, slope; see also Van Roy et al., 2010). In contrast, Lefebvre and Botting (2007) suggested that stylophoran beds were possibly associated with much shallower environmental conditions, closer to normal wave base (echinoderm taphofacies C).

The aims of this paper are to describe a new stylophoran-dominated assemblage in the Lower Ordovician of the Zagora area and to discuss its associated palaeoenvironmental context. A taphonomic classification dedicated to stylophorans is proposed here for the first time thus describing the taphonomy of the new assemblage. Finally, all published occurrences of stylophorans in the Lower Ordovician of Zagora area (central Anti-Atlas, Morocco) are replaced in a new, refined biostratigraphic framework (Fig. 1B), based on field campaigns achieved in 2012-2014 (Martin et al., in press).

GEOLOGICAL CONTEXT

The Fezouata Shale (Tremadocian-late Floian) represents the basal member of the Outer Feijas Group (Tremadocian-mid Darriwilian), which corresponds to a shale-dominated interval comprised between the sandstones of the middle Cambrian Tabanite Group and the sandstone-dominated units of the First Bani Group (late Darriwilianearly Sandbian; Destombes et al., 1985;Gutiérrez-Marco et al., 2003). In Zagora area, the Fezouata Shale corresponds to a 1000 m-thick, monotonous succession of fine-grained siltstones. In this area, a detailed logging of the Fezouata Shale and of the overlying Zini Formation (latest Floian) was achieved through a series of field campaigns organized between 2012 and 2014. Sedimentary structures typical of storm deposits (e.g., ripple marks, hummocky crossstratification) were observed repeatedly, at different scales in all parts of the Lower Ordovician succession (Martin et al., in press). Consequently, the general depositional context of the Fezouata Shale more likely corresponds to relatively shallow conditions (lower shoreface), at or above storm-wave base (Lefebvre and Botting, 2007;Martin et al., in press), rather than to a deep, outer shelf setting (Lefebvre, 2007;Van Roy et al., 2010;Gaines et al., 2012).

Moreover, the detailed analysis of both graptolite and acritarch assemblages collected in the last twelve years in Zagora area offered the opportunity to better constrain the biostratigraphic framework of the Fezouata Shale (Martin et al., in press). In this revised biostratigraphic scheme, most deposits formerly interpreted as early to mid Floian in age are now assigned to the Hunnegraptus copiosus biozone (latest Tremadocian). In Zagora area, all described occurrences of stylophoran-dominated beds in the Fezouata Shale are restricted to the A. murrayi biozone (late Tremadocian; Fig. 1B).

Emmanuel Martin, Bertrand Lefebvre and Romain Vaucher 96 (Lefebvre, 2007); Z-F2: Jbel Tizagzaouine (new assemblage); Z-F2(1): locality S of Jbel Tizagzaouine (Lefebvre and Botting, 2007); 1685: type-locality of Thoralicystis zagoraensis (Chauvel, 1971).

The new stylophoran-dominated assemblage reported here was collected from a single, thin, lens-shaped layer of beige siltstones at locality Z-F2, along the very steep slopes forming the eastern flank of Jbel Tizagzaouine. This site is located about 11 km NE of the village of Beni Zoli and 21 km N of Zagora, in the western part of the Ternata plain (W of Jbel Bou Dehir; Fig. 1A). The stylophoran bed was entirely quarried out in 2002-2003. This small lens yielded extremely abundant specimens of cornutes (over 500 individuals), all of them belonging to the same, yet undescribed species. One specimen of this large cornute from Jbel Tizagzaouine was reported as "undescribed Tremadoc bootshaped form" and figured by Ware and Lefebvre (2007, p. 784, fig. 5.1-2). In Z-F2, the monospecific stylophoran assemblage is associated to a low diversity, depauperate fauna comprising fragments of large trilobites (Fig. 2H), along with rare articulate brachiopods, relatively common graptolites (e.g., Paradelograptus norvegicus) and, locally, clusters of tiny gastropods (Fig. 2C). The graptolite assemblage observed in locality Z-F2 suggests a late Tremadocian age for this level (A. murrayi biozone; Fig. 1B).

MATERIAL AND METHODS

The excavation of the Z-F2 stylophoran bed in 2002-2003 yielded 300 specimens (rock samples) containing more than 500 individuals belonging to the same undescribed species of large cornute (Fig. 2). Study material is deposited in the collections of the following public institutions: Cadi-Ayyad University, Marrakesh, 190 specimens (AA.JTZ.OI.1-6, AA.JTZ. , Musée des Confluences, Lyon, 9 specimens (ML20.269238-246), and Muséum d'Histoire naturelle, Nantes, 101 specimens (MHNN.P.045197-297). During fieldwork, sampling was performed as exhaustively as possible: all fossiliferous pieces of rock were collected, even those containing fragmentary, disarticulated material. A sampling bias towards better-preserved, fully articulated specimens is, however, likely. Indeed, minute isolated skeletal elements are less eye-catching, and the possibility that they were under-sampled cannot be excluded.

TAPHONOMY

Five taphonomic groups are defined here for stylophorans, based on the quality of preservation of the specimens. Similar taphonomic classifications have been already proposed for helicoplacoids (Dornbos and Bottjer, 2001) and ophiuroids (Zato ń et al., 2008).

Group 1 includes all cornute specimens with (1) their distal aulacophore preserved, with cover plates articulated (or in close contact) to underlying ossicles; (2) their proximal aulacophore; and (3) a complete to slightly disarticulated theca. Cover plates are extremely delicate skeletal elements protecting the food groove of stylophorans (Ubaghs, 1968;[START_REF] Lefebvre | Functional morphology of stylophoran echinoderms[END_REF]. In cornutes, their preservation requires the rapid burial of live, in situ organisms (Lefebvre, 2007). Unlike cornutes, mitrates had the possibility to flex their distal aulacophore, so as to close their cover plates and thus protect their food groove in case of physical disturbance and/or danger [START_REF] Parsley | Feeding and respiratory strategies in Stylophora[END_REF][START_REF] Lefebvre | Functional morphology of stylophoran echinoderms[END_REF]. In mitrates, the recurved, distress position of the aulacophore greatly enhances the preservation potential of cover plates. Consequently, group 1 only includes mitrates preserved in life position, i.e. with their distal aulacophore held in extended position, over the sea floor (see e.g., Parsley and Gutiérrez-Marco, 2005;Lefebvre and Botting, 2007).

Group 2 corresponds to cornutes with (1) only their brachials (stylocone, ossicles) preserved (cover plates absent); (2) their proximal aulacophore; and (3) a complete to slightly disarticulated theca. In mitrates, group 2 includes all specimens with their distal aulacophore (brachials and cover plates) preserved in flexed, distress position.

Group 3 comprises stylophorans with (1) no distal aulacophore; (2) a complete or more or less dissociated proximal aulacophore; and (3) a complete to partly disarticulated theca.

Group 4 is defined for more or less strongly disarticulated stylophoran thecae, with no aulacophore articulated to them.

Group 5 is proposed for dissociated skeletal elements.

TAPHOFACIES

Three distinct types of preservation (taphofacies) can be identified in the stylophoran bed of locality Z-F2. The first taphofacies is restricted to the lowermost part of the lens, which consists in a thin, irregular layer (0-3 mm in thickness) of coarse siltstones. This level contains abundant isolated skeletal elements of cornutes (taphonomic group 5; Fig. 2G I), sometimes sheltered under large pieces of disarticulated trilobites (Fig. 2H). The second taphofacies is observed in the overlying, thicker, irregular layer (2-5 mm in thickness) of finer siltstones. This horizon corresponds to a dense accumulation of hundreds of cornutes, piled on top of each other (Fig. 2D-F). In this level, individuals do not exhibit any preferential orientation, and about half of the specimens are upside-down. Most cornutes can be assigned to taphonomic group 2, although some individuals belonging to groups 1 and 3 are also present. The third taphofacies occurs in the overlying, thick, upper part of the stylophoran bed (10-30 mm in thickness), which consists in very fine siltstones. This level has yielded scattered, well-preserved specimens of cornutes, frequently associated with graptolites and clusters of tiny gastropods (Fig. 2A-C). In this layer, most individuals belong to taphonomic group 2. Specimens do not show any preferential orientation, and only a few cornutes are upside-down (Fig. 2B).

CONCLUSIONS

In their taphonomic classification of echinoderms, Brett et al. (1997) identified three main groups, based on the resistance of their skeleton to disarticulation. In this scheme, stylophorans belong to type 1 echinoderms (i.e., taxa characterized by loosely articulated skeletal elements; Brett et al., 1997;Lefebvre, 2007). Experimental taphonomic studies on extant type 1 organisms (e.g., ophiuroids) indicate that their skeleton entirely disarticulates within a few days after death [START_REF] Schäfer | Ecology and Palaeoecology of Marine Environments[END_REF]Allison, 1990;Donovan, 1991). Freshly killed specimens of type 1 echinoderms, however, can endure high-energy conditions without severe disarticulation (Kerr and Twitchett, 2004;Gorzelak and Salamon, 2013). Consequently, the preservation of hundreds of exquisitely preserved specimens of cornutes in locality Z-F2 requires exceptional environmental conditions. The available sedimentological evidence (e.g., scoured bioclastic lag at the base of the lens, fining upward lithology) and the geological context of the Fezouata Shale (shallow deposits at or above storm wave base) both strongly suggest that the stylophoran bed of Jbel Tizagzaouine represents a storm deposit (taphofacies IIC of Brett et al., 1997). The local accumulation of hundreds of specimens of cornutes very likely results from distal storm scour and redeposition of a para-autochtonous, monospecific community.

3.2

Trilobite clusters

monospecific accumulations

The presence of large monospecific trilobite accumulations has been reported in the Fezouata Shale (Fortey, 2009(Fortey, , 2011)). During the excavation campaign, a single 25 m 2 slab containing a hundred specimens of the trilobite Platypeltoides magrebiensis was discovered (fig 56 A).

Their exoskeletons were mostly disarticulated and most of them were molds (fig 56 B), in contrast with the other known slabs. However, I measured the direction, length, position (on the back or not) and width of 92 Platypeltoides remains, which I compiled into a database that also indicates their nature (complete, glabella, glabella and thorax, thorax, thorax and pygidium, or pygidium). I also took a picture of each specimen and did a high-resolution panorama of the complete slab. According to this set of data, the studied slabs may reflect a mass mortality event occurring when the oxic/dysoxic limit moved back up just above the sea floor. Indeed, it has been suggested that gaz induced by the decomposition of carcasses would lighten the exoskeleton so that the new equilibrium point would be on its back. But this does not explain why only trilobites of the same species are found. One possible explanation is that the event happened during very specific ecological occasions such as synchronous molting, or mating (hence the same growth stage). In any cases, these explanations may be too speculative for the moment and further specific investigations, taking account all component fauna, are needed before discussing this point.

linear trilobite clusters

Another interesting taphonomic point is the presence of linear clusters in the Fezouata Shale (Chatterton and Fortey, 2008). Some are plurispecific gathering different group from small and often diarticulated agerina trilobites to redonia bivalves, while other are monospecific concerning only the trilobite species Ampyx priscus (see plate 1 in Chatterton and Fortey, 2008). The morphology of this blind species is quite particular with its very long genal spines (sometimes twice the length of the body) and its long glabellar spine (up to the size of the body), so that each specimen seems to imbricate with the following one. A relatively profound groove ran along each of these spines. Additionaly, the presence of calcite infilling inside some of the spines suggest that they were hollow, probably to host sensory organ. An ongoing study with Rudy Lerosey-Aubril, Jean Vannier, Romain Vaucher, and Muriel Vidal aims at characterizing these very peculiar clusters. An unprepared slab, acquired by Jean Vannier, was sewed in order to have a details look at the sedimentological sequence (Fig. 58 A). Preliminary results suggest that the aligned specimens are directly covered by a storm event. A potential explanation comes from the behaviour of modern spiny lobster, which form long procession when the storm season begins (Figs. 58 B andC). Indeed, spiny lobsters are fleeing the decrease in visibility due to saltation of muddy particles and the increasing turbidity. Ampyx, whose morphology is adapted to rest on muddy seafloor, might also have been destabilized by the increasing turbidity due to storm activity. As this species is blind, establishing a contact with its fellow creatures is of prime importance to find its way.

Morever, being aligned breaks the effect of current on the following trilobites. Such linear clusters, implicating trilobites with similar morphology are also reported for the first time from the Lower Ordovician of Montagne Noire in France (fig. 59). Some of these trilobites belong to the species Eoamyxinella villebruni, which lacks a glabellar spine. As a result, linear clusters of this species are much more disorganized (Figs. 59 C andD).

Conclusions 1 The Fezouata Shale: general context and state of the art

Components of the Ordovician shelly faunas from the central Anti-Atlas have been studied since the early 1960's. For example, several contributions were published on bivalves (Babin and Destombes, 1990), brachiopods (Havliček, 1971;Mergl, 1981Mergl, , 1988)), conulariids (Destombes et al., 1985), echinoderms (Ubaghs, 1963;Chauvel, 1966Chauvel, , 1969Chauvel, , 1971aChauvel, , b, 1978;;Chauvel and Régnault, 1986;Donovan and Savill, 1988;Lefebvre and Fatka, 2003), gastropods (Horný, 1997), graptolites (Destombes and Willefert, 1959;Willefert in Destombes et al., 1985;Aceñolaza et al., 1996), hyolithids (Marek, 1983), and trilobites (e.g. Destombes, 1972;[START_REF] Rábano | Trilobites del Museo Geominero. I. Platypeltoides magrebiensis n. sp. (Asaphina, Nileidae), del Ordovícico inferior del Anti-Atlas central (Marruecos)[END_REF]Henry et al., 1992;Vidal, 1998a, b).

However, since the discovery of the Fezouata Shale Lagersätte (Van Roy, 2006;[START_REF] Van Roy | fig. S2c) is closely related to both Cornuta indet. A from the late Cambrian of Korea[END_REF], our knowledge on these communities has increased dramatically. Some studies focused specifically on soft-part preservation and soft-bodied taxa, such as aglaspidids (Ortega-Hernández, 2016), annelids (Vinther et al., 2008), anomalocaridids (Van Roy andBriggs, 2011;Gaines et al., 2012c;Van Roy et al., 2015b), demosponges (Botting, 2007), hyolithids (Marti Mus, in press), marrellomorph arthropods (Van Roy, 2006;[START_REF] Rak | A revision of the Late Ordovician marrellomorph arthropod Furca bohemica from Czech Republic[END_REF][START_REF] Legg | An acercostracan marrellomorph (Euarthropoda) from the Lower Ordovician of Morocco[END_REF], palaeoscolecids (Martin et al., in press b), or problematic arthropods [START_REF] Van Roy | A spinose appendage fragment of a problematic arthropod from the Early Ordovician of Morocco[END_REF]. On the other hand, other contributions pursued the description of shelly taxa: cephalopods (Kröger and Lefebvre, 2012), echinoderms (Lefebvre, 2007;Lefebvre and Botting, 2007;Sumrall and Zamora, 2011), gastropods (Ebbestad and Lefebvre, 2015), hyolithids (Valent et al., 2013;[START_REF] Valent | Pauxillites thaddei a new lower Ordovician hyolith from Morocco[END_REF], and trilobites [START_REF] Vela | Three new species of Lehua from the Lower Ordovician of Dra Valley (Morocco)[END_REF]Vela and Corbacho, 2007;Chatterton and Fortey, 2008;[START_REF] Corbacho | Nuevas especies de Lehua del Ordovícico Inferior del Valle del Dra (Marruecos)[END_REF][START_REF] Corbacho | Kierarges morrisoni: Un nuevo género de Isotelinae (Trilobita) del Floiense de Portugal y Marruecos[END_REF]Fortey, 2009Fortey, , 2011a, b;, b;Corbacho and Vela, 2010, 2011, 2013;[START_REF] Corbacho | A new asaphid trilobite from the Lower Ordovician (Arenig) of Morocco[END_REF][START_REF] Corbacho | Posible dimorfismo sexual en Trilobites del Ordovícico inferior[END_REF].

Then, the contributions on the biostratigraphy of the Fezouata Shale (Martin et al., 2014, in press a;Nowak et al., 2015b, in press;Lehnert et al., in press;Lefebvre et al., in press b;Gutiérrez-Marco and Martin, submitted;Vaucher et al., submitted a, b) opened a new avenue of research. In this context, a special issue, entitled « The Fezouata Biota », is currently in progress and will be published in late 2016-early 2017 in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. It especially comprises detailed studies on bivalves (Polechová, in press), conodonts (Lehnert et al., in press), conulariids (Van Iten et al., in press), echinoderms (Lefebvre et al., in press b), graptolites (Gutiérrez-Marco and Martin, submitted), gastropods (Ebbestad, in press), palynomorphs (Nowak et al., in press), trilobites (Martin et al., submitted), and sponges (Botting et al., in press).

The Fezouata Shale Lagerstätte: main achievements

During the course of my PhD thesis, I have contributed to the acquisition in the field and in the laboratory, of abundant new data, and ultimately, to the advancement of our current knowledge of several key aspects of the Fezouata Shale Lagerstätte:

(1) A sound biostratigraphic and palaeoenvironmental framework was established (Martin et al., 2014, in press a;Lefebvre et al., in press b;Gutiérrez-Marco and Martin, submitted, Vaucher et al., submitted a, b). Before this work, the precise stratigraphic position, range and number of levels yielding soft-bodied taxa were barely known. Exceptional preservation was thought to occur extensively throughout most of the Fezouata Shale, more or less continuously from the late Tremadocian to the late Floian (see e.g. Van Roy et al., 2010). One of the main achievements of my PhD thesis was thus to replace all known levels with exceptional preservation within a single, synthetic stratigraphic column, and to provide a first detailed tentative biostratigraphic framework based on the identification of the successive graptolite assemblages. A major consequence of this refined biostratigraphic framework was to demonstrate that exceptional preservation was neither continuous nor extensive throughout the Fezouata Shale, but restricted to two relatively narrow intervals: (i) a 60-meter-thick late Tremadocian lower interval (A. murrayi to H. copiosus zones) yielding most soft-bodied taxa; and (ii) a 15-meter-thick middle Floian interval (?B. jacksoni Zone), where soft-bodied taxa are apparently less common.

(2) My PhD thesis also paved the way for more comprehensive studies at both community and group levels. Before this work, very little was known about the precise environmental conditions associated to the Fezouata Shale (see e.g., Vidal, 1998b), and constrasted environmental interpretations had been proposed in the literature: e.g. relatively shallow (e.g. Lefebvre and Botting, 2007) vs. deep settings (e.g. Van Roy et al., 2010). Better understanding of the original palaeoenvironmental conditions is essential for the interpretation of fossil communities. My detailed analyses of echinoderm and trilobite communities occurring in the Fezouata Shale were a direct extension of this idea (Allaire et al., 2015;Martin et al., 2015;Martin et al., submitted). They addressed several aspects of their taphonomy, which are essential to identify if the observed assemblages correspond to original in situ communities (biocoenoses) or to time-averaged associations including transported elements (taphocoenoses). Several taphonomic grades were identified for the first time in stylophoran echinoderms, so as to describe the disarticulation process of their skeleton (Martin et al., 2015). Preliminary geochemical analyses of several fossiliferous levels, also confirmed the likely occurrence of -at least temporarily-unhospitable dysoxic environmental conditions associated with exceptional preservation (Martin et al., submitted). Finally, key new data on the morphology, ecology and presumed mode of life of palaeoscolecids were obtained (Martin et al., in press b), thus opening the way to other studies on other soft-bodied, worm-like taxa from other Lagerstätten.

Perspectives and ongoing work

Although reduced both in time (four weeks) and in extent (three main stratigraphic levels, all located within a 15-meter thick interval), the Bou Izargane excavations have yielded an unprecedented wealth of palaeontological, sedimentological, and geochemical data (although a large part of the extracted almost 0.5 ton of material still remains to be exploited). The future extension of such detailed investigations, both at Bou Izargane, but also in other localities and/or in other stratigraphic levels would certainly be crucial for our global understanding of the general context of the Fezouata Biota (e.g. communities, taphonomy, environmental conditions). Some preliminary results have also questioned the interpretation of the Fezouata Biota as a highly diverse community, associated to fully marine, "normal" environmental conditions. Several distinct lines of evidence tend to suggest that, a contrario, most levels with exceptional preservation are indeed associated with temporarily unhospitable (dysoxic) environmental conditions (Botting et al., in press; Lefebvre et al., in press b; Martin et al., submitted). At Bou Izargane, each excavation studied so far has yielded a very abundant but poorly diversified fauna, with sometimes abnormally small (dwarfed?) specimens.

Consequently, an alternative interpretation could be that the massive, spotty, low-diversity occurrences of Cambrian-like taxa correspond indeed to various, successive episodes of colonizations of the sea-floor by opportunistic (k-strategist) taxa. In contrast, most of the other levels, dominated by more classical, Ordovician shelly assemblages (e.g. brachiopods, molluscs and trilobites) could represent more stable communities (R-strategists) adapted to "normal", well-oxygenated environmental conditions. Testing the validity of this alternative scenario (and if it is taphonomically biased -or not-) clearly requires more extensive field data from Morocco, but also from other regions in the world, so as to identify how unique (or not) the Fezouata Biota is. Another avenue for future studies lies in the detailed comparison of the Fezouata Lagerstätte with the better-known Cambrian Lagerstätten in order to unravel the relationships between the Cambrian Explosion and the GOBE.

Other extremely promising perspectives lay in the "side discoveries" made during the excavation campaign. They offer unprecedented opportunities to increase dramatically our knowledge on the detailed anatomy of several groups of marine invertebrates (e.g. trilobite appendages, exceptionally preserved soft parts in echinoderms; see e.g. Lefebvre et al., 2013Lefebvre et al., , 2014)). They also provide new sets of evidence on their putative ethology, allowing comparison with modern counterparts (e.g. linear clusters of Ampyx priscus ), and/or on their taphonomy (the Platypeltoides magrebiensis slab). Additionally, many (most) shelly and softbodied taxa still await formal palaeontological descriptions, and there is little doubt that they will deliver precious ecological and/or phylogenetic information in the future. Finally, a drilling project through the Fezouata Shale was on discussion. It could be a thrilling opportunity to observe fresh rock material and obtain detailed information on the taphonomy of this Lagerstätte. It could also yield precious small carbonaceous fossils (SCF), which remain -so far-unkown in the Fezouata Shale due to its high weathering (Nowak et al., in press). Undeed, SCF provide key-information on smaller animals living in the water column and are essential in the reconstruction of trophic relationships in the lower Palaeozoic.

Interval

Stratigraphy

Thicknesses of fossiliferous intervals

Fauna

Biostratigraphy

Age (± 2 Ma)

Palaeogeography

Palaeoenvi

Cuadernos del Museo Geominero 19, pp. 21-26

INTRODUCTION

Rhopalocystis is a Tremadocian (Early Ordovician) genus of eocrinoids (basal blastozoans), endemic to the Agdz-Zagora area in the central Anti-Atlas, Morocco. Isolated skeletal remains from the Valdemiedes Formation (Cambrian Series 2, Stage 4) of Spain were tentatively assigned to Rhopalocystis [START_REF] Clausen | New Early Cambrian eocrinoids from the Iberian Chains (NE Spain) and their role in nonreefal benthic communities[END_REF]. However, they more likely belong to a yet undescribed gogiid eocrinoid (S. Zamora, pers. comm., 2013). The morphology of Rhopalocystis is characterised by a mixture of plesiomorphic (e.g., epispires) and more advanced characters (e.g., theca organised into well-defined circlets of plates). Its phylogenetic position remains largely unresolved. Rhopalocystis was identified as a gogiid eocrinoid (Sprinkle, 1973;[START_REF] Broadhead | Reappraisal of class Eocrinoidea (Echinodermata)[END_REF], assigned to various groups of more derived blastozoans (e.g., [START_REF] Paul | The phylogeny of the cystoids[END_REF][START_REF] Sumrall | Global lability, regional resolution, and majority-rule consensus bias[END_REF]Nardin, 2007), or interpreted as a possible transitional form between blastozoans and crinoids (e.g., [START_REF] Ausich | Rooting the early crinoid diversification with the eocrinoid Rhopalocystis[END_REF]; but see Ubaghs, 1963;[START_REF] Guensburg | Phylogenetic implications of the Protocrinoida: blastozoans are not ancestral to crinoids[END_REF].

Remains of Rhopalocystis were first collected in 1952 at Tansikht bridge (locality 1; Fig. 1A) during the field excursion of the 19th International Geological Congress (Ubaghs, 1963). However, specimens of Rhopalocystis from Tansikht bridge were not described by Choubert et al. (1953), who only reported on the presence of the associated glyptocystitid Mimocystites (= Macrocystella). Abundant specimens of Rhopalocystis were subsequently collected by Jacques Destombes (from 1959Destombes (from to 1985) ) in several localities located E of the Jbel Bou Dehir, in Zagora area (localities 2, 3 and 5; Fig. 1A), W of Agdz (locality 4; Fig. 1A), and at Tansikht bridge (locality 1). This material was described in a series of papers by Ubaghs (1963), Chauvel (1971Chauvel ( , 1978) ) and Chauvel and Régnault (1986). Ubaghs (1963) created the genus Rhopalocystis based on exquisitely preserved specimens collected at locality 2. All individuals were assigned to the type species, R. destombesi. Four additional species of Rhopalocystis were described by Chauvel (1971), based on new material collected at localities 2 (R. grandis, R. zagoraensis), 3 (R. sp. A), and 4 (R. fraga). Chauvel (1978) redescribed R. zagoraensis and identified three more species, based on new material from localities 1 (R. sp. C), 2 (R. havliceki), and 3 (R. sp. B). Finally, Chauvel and Régnault (1986) discussed the systematics of the different species assigned to Rhopalocystis and suggested the erection of two more taxa, based on new material from localities 2 and 5 (R. dehirensis and R. lehmani).

The aim of this paper is to describe three new Rhopalocystis assemblages (localities 6, 7, and 8) in the Lower Ordovician of the Zagora area and to discuss their taphonomy. Moreover, intensive field work conducted between 2012 and 2014 made it possible to obtain a detailed logging of the Lower Ordovician succession in the Zagora area, and to place all known occurrences of Rhopalocystis in this revised stratigraphic framework (Fig. 1B). The morphological disparity within the genus Rhopalocystis and the validity of its 10 described species will not be investigated here, but will be treated elsewhere.

GEOLOGICAL CONTEXT

In the central Anti-Atlas (Morocco), the Lower Ordovician succession rests unconformably over the middle Cambrian sandstones of the Tabanite Group (Destombes et al., 1985). In the Agdz-Zagora area, the Lower Ordovician sequence corresponds to a thick (1000 m), monotonous series of fine siltstones (Fezouata Shale, Tremadocian to late Floian), capped by the sandstones of the Zini Formation (latest Floian). In the last twelve years, successive field excursions in the Ternata plain (N. of Zagora) resulted in the detailed logging of the whole Lower Ordovician succession and the definition of a precise biostratigraphic framework based on both graptolites and acritarchs (Martin et al., in press). All historical sites yielding remains of Rhopalocystis could be placed in this refined biostratigraphic framework: a late Tremadocian age (A. murrayi biozone) was confirmed for the three localities of the Ternata plain, E of Jbel Bou Dehir (localities 2, 3 and 5; Fig. 1B), and also for the two other sites, in the Agdz area (localities 1 and 4). Moreover, three new sites yielding abundant remains of Rhopalocystis (localities 6, 7, and 8; Fig. 1) were discovered in the Lower Ordovician of the Zagora area.

Locality 6 (Fig. 1) corresponds to a low cliff along the banks of a dry stream bed (oued), about 10 km NE of the small village of Beni Zoli and 21 km N of Zagora, in the western part of the Ternata plain (W of Jbel Bou Dehir). Laminated greenish siltstones have yielded a late Tremadocian assemblage (A. murrayi biozone) consisting in abundant remains of Rhopalocystis spp., associated with an extremely depauperate fauna comprising only rare specimens of the trilobite Bavarilla sp. In 2003, the exploitation of this site yielded 80 specimens deposited in the collections of Cadi-Ayyad University, Marrakesh (AA.TAM.OI.1-80).

Locality 7 (Fig. 1) is located 20 km NE of Zagora, in the eastern part of the Ternata plain (E of Jbel Bou Dehir). Abundant specimens of Rhopalocystis were collected in 2003-2004 by Roland and Véronique Reboul within large, massive lenses of bioclastic sandstones. No other faunal element was observed in these levels. The study material is deposited in the collections of the Musée des Confluences, Lyon (ML20.269379-382) and Lyon 1 University, Villeurbanne (FSL 711650, FSL 711160-163, FSL 712045). Locality 7 is indeed very close to Jacques Destombes' locality 1687 mentioned in Vidal (1998aVidal ( , 1998b)). Destombes' site corresponds to a distinct, concretion-bearing level occurring a few meters below, in the same area. The small alumino-siliceous concretions of locality 1687 have yielded an abundant and diverse assemblage comprising many bivalves (e.g., Redonia michelae), brachiopods, cephalopods (e.g., Bathmoceras australe, Destombesiceras zagorense, Rioceras sp.), conularians, echinoderms (e.g., Anedriophus moroccoensis, Argodiscus espilezorum, Balanocystites primus, Balantiocystis regnelli, Lingulocystis sp.), gastropods (e.g., Carcassonnella courtessolei, Thoralispira cf. laevis), graptolites, hyolithids (e.g., Cavernolites senex), machaeridians, trilobites (e.g., Ampyx sp., Asaphellus fezouataensis, Basilicus sp., cheirurids indet.) and other arthropods (e.g., Eoduslia, Zagoracaris). A mid to late Floian age has been generally assigned to this assemblage (Vidal, 1998a(Vidal, , 1998b;;Van Roy, 2006;Sumrall and Zamora, 2011;Kröger and Lefebvre, 2012). However, the composition of the graptolite assemblage suggests that a latest Tremadocian age (H. copiosus biozone) is more likely (Martin et al., in press). Consequently, a latest Tremadocian (or earliest Floian) age can be inferred for the overlying horizon yielding large Rhopalocystis-bearing concretions.

Locality 8 (Fig. 1) is located at the summit of a small hill, N of Jbel Tizagzaouine, about 2.2 km NE of locality 6 and 23 km N of Zagora, in the western part of the Ternata plain (W of Jbel Bou Dehir). Large fragments of massive lenses of bioclastic sandstones were collected in 2013-2014. They have yielded a late Tremadocian assemblage (A. murrayi biozone) comprising abundant specimens of Rhopalocystis spp., sometimes associated to large fragments of disarticulated trilobites. All available material is deposited in the collections of Cadi-Ayyad University, Marrakesh (AA.TISa.OI. and Lyon 1 University, Villeurbanne (FSL 712044).

TAPHONOMY

Taphonomic features of Rhopalocystis assemblages in the late Tremadocian of the Agdz-Zagora area were thoroughly investigated and discussed by Ubaghs (1963), who identified two main kinds of preservation. The first one corresponds to extremely dense accumulations of individuals in massive, bioclastic sandstones, and the second one, to more scattered individuals preserved in laminated siltstones. The three new Rhopalocystis assemblages confirm the existence of, at least, two main modes of preservation, but they also offer the opportunity to place them in their palaeoenvironmental context.

Both locality 7 and 8 have yielded type 1 assemblages of Rhopalocystis (Fig. 2A-F). They consist in densely packed accumulations of complete, fully articulated thecae in thick, lenticular beds of massive sandstones (Fig. 2C-F). Thecae do not exhibit any preferential orientation. Articulated portions of stems are sometimes preserved, either as isolated stumps, or still in connection with the theca (Fig. 2A,E). No distal extremities of stems have been observed. Brachioles are never preserved. In some other levels, complete thecae are less common and are associated with thousands of disarticulated thecal plates (Fig. 2B) and sometimes, large fragments of trilobites (Fig. 2A). In these levels, isolated skeletal elements consist almost exclusively of basal plates. Type 1 assemblages of Rhopalocystis are interpreted here as storm-generated levels of accumulation, resulting from the downslope transport and rapid burial of shallower communities. A similar interpretation was proposed by Vidal (1998aVidal ( , 1998b) ) for the taphonomy of trilobite assemblages occurring (stratigraphically) a few meters above the Rhopalocystis horizon at locality 7.

The Rhopalocystis assemblage collected at locality 6 illustrates the second type of preservation (Fig. 2G-J) mentioned by Ubaghs (1963). Individuals are not preserved in clusters, but randomly distributed in fine, laminated siltstones (Fig. 2G). Their thecae are large to very large, but never complete, frequently slightly disarticulated and/or collapsed (Fig. 2I-J). Brachioles have not been observed. Fully articulated proximal parts of stems are often preserved in connection with the thecae. The siltstones also contain abundant disarticulated fragments of Rhopalocystis (different types of thecal plates, columnals and numerous smaller elements; Fig. 2H-I). Type 2 assemblages are here interpreted as distal storm-influenced deposits, resulting from the in situ disarticulation and slow burial of Rhopalocystisdominated communities.

CONCLUSIONS

The discovery of three new Rhopalocystis-dominated assemblages confirms the existence of widespread eocrinoid meadows in shallow environmental conditions in the late Tremadocian of the Agdz-Zagora area. The absence or extreme rarity of associated benthic fauna (e.g., molluscs, trilobites) is a characteristic feature also observed in many fossil and modern echinoderm dense beds (Fujita and Ohta, 1989; Lefebvre, 2007; Zaton ´et al., 2008). The abundant material collected at the new localities will offer the opportunity to reevaluate the systematics and morphological disparity within the genus Rhopalocystis.

Introduction

Since their first appearance in the fossil record, in Cambrian Series 2, echinoderms have been a major component of invertebrate marine communities (Paul and Smith, 1984;[START_REF] Guensburg | Rise of echinoderms in the Paleozoic evolutionary fauna: significance of paleoenvironmental controls[END_REF]Zamora et al., 2013a). Their rigid endoskeleton consists of hundreds of skeletal elements, each made of a single crystal of calcite and a variable amount of organic proteins (collagen). The three-dimensional microstructure (stereom) of echinoderm plates is an apomorphy of the phylum (Paul and Smith, 1984;[START_REF] Smith | The pre-radial history of echinoderms[END_REF][START_REF] Bottjer | Paleogenomics of echinoderms[END_REF][START_REF] Zamora | Deciphering the early evolution of echinoderms with Cambrian fossils[END_REF]. Consequently, the identification of echinoderm remains is relatively straightforward, even when they occur in thin sections or as disarticulated skeletal elements.

Experimental taphonomic experiments on modern echinoderms suggest that their endoskeleton more or less fully disarticulates into isolated plates or more resistant modules within a few days or weeks after death [START_REF] Schäfer | Aktuo-Paläontologie[END_REF]Allison, 1990;Kidwell and Baumiller, 1990;Donovan, 1991;[START_REF] Nebelsick | Actuopalaeontological investigations on echinoids: the potential for taphonomic interpretations[END_REF][START_REF] Baumiller | Experimental and biostratinomic disarticulation of crinoids: taphonomic implications[END_REF]Kerr and Twitchett, 2004;Gorzelak and Salamon, 2013). Consequently, the preservation of complete specimens is generally associated with obrution deposits (rapid, in situ burial of live individuals, i.e. by storm or turbiditic flows; Hess, 1999), although a complex suite of diagenetic and/or environmental parameters (e.g., anoxia, cool temperatures) is also very likely involved [START_REF] Bockelie | Functional morphology and evolution of the cystoid Echinosphaerites[END_REF]Donovan, 1991;Brett et al., 1997;[START_REF] Ausich | Echinoderm taphonomy[END_REF]Dornbos and Bottjer, 2001;Lefebvre, 2007). The fossil record of echinoderms is thus strongly biased towards Lagerstätten yielding exceptionally preserved, fully articulated specimens (Smith, 1988;Donovan, 1991;[START_REF] Sprinkle | Radiation of Echinodermata[END_REF]Brett et al., 1997;[START_REF] Lefebvre | This larger amount of available data provides an unprecedented opportunity to better identify if similarities between echinoderm faunas simply reflect comparable palaeoenvironmental conditions and/or if they also support palaeobiogeographic affinities. As a first consequence of this improved knowledge of late Cambrian-Early Ordovician faunas, less than 12% of echinoderms from the Fezouata Shale can be identified as possibly endemic taxa (instead of about 50% in the early 2000s). They include the eocrinoids Rhopalocystis destombesi[END_REF]Zamora et al., 2013a). This is particularly true for the initial stages of their diversification, as the temporal distribution pattern of Lagerstätten is clearly non-random in Lower Palaeozoic rocks [START_REF] Orr | Post-Cambrian closure of the deep-water slopebasin taphonomic window[END_REF]Van Roy et al., 2010, 2015).

In Cambro-Ordovician times, echinoderms underwent a major, stepwise radiation leading to their highest diversity (20 classes, including the five extant ones) for the entire Phanerozoic (Sprinkle and[START_REF] Sprinkle | [END_REF]Lefebvre and Fatka, 2003). In Cambrian Series 2, the high morphological disparity exhibited by the oldest known echinoderms (edrioasteroids, eocrinoids and helicoplacoids) suggests either an extremely rapid initial radiation or a long, unrecorded diversification preceding the acquisition of the biomineralized endoskeleton [START_REF] Ubaghs | Early Paleozoic echinoderms[END_REF]Smith, 1988[START_REF] Smith | The pre-radial history of echinoderms[END_REF][START_REF] Zamora | Deciphering the early evolution of echinoderms with Cambrian fossils[END_REF]. In Cambrian Series 3, echinoderms underwent a first major diversification, possibly driven by the Cambrian substrate revolution and the colonization of new ecological niches by vagile, free-living taxa (Lefebvre and Fatka, 2003;[START_REF] Dornbos | Evolutionary palaeoecology of early epifaunal echinoderms: response to increasing bioturbation levels during the Cambrian radiation[END_REF][START_REF] Alvaro | The role of abiotic factors in the Cambrian substrate revolution: benthic community replacements in West Gondwana[END_REF]Zamora et al., 2013a). Most of these early to middle Cambrian echinoderms (e.g., cinctans, ctenocystoids, eocrinoids, helicoplacoids, solutans, cornute stylophorans) are generally considered as typical members of the Cambrian Evolutionary Fauna (CEF; Sepkoski, 1981;[START_REF] Sprinkle | Radiation of Echinodermata[END_REF][START_REF] Sumrall | Systematics and paleoecology of Late Cambrian echinoderms from the western United States[END_REF].

The transition between the two major bursts in echinoderm diversity in Early Palaeozoic times is poorly documented. Few, lowdiversity echinoderm assemblages (five classes) have been recorded so far from the Furongian-early Tremadocian time interval (Smith, 1988;Smith and[START_REF] Smith | [END_REF][START_REF] Sumrall | Systematics and paleoecology of Late Cambrian echinoderms from the western United States[END_REF][START_REF] Sumrall | Cardiocystella, a new cornute stylophoran from the Upper Cambrian Whipple Cave Formation, eastern Nevada, USA[END_REF][START_REF] Lee | Latest Cambrian cornutes (Echinodermata: Stylophora) from the Taebaeksan Basin, Korea[END_REF]Zamora, 2012;Zamora et al., 2013aZamora et al., , 2013b[START_REF] Zamora | New long-stemmed eocrinoid from the Furongian Point peak shale member of the Wilberns Formation, central Texas[END_REF]Zhu et al., 2014;Botting et al., 2015). They comprise both holdovers of the CEF (eocrinoids, edrioasteroids, solutans and cornute stylophorans) and early representatives of taxa typical of the Palaeozoic Evolutionary Fauna (PEF) (glyptocystitid rhombiferans and mitrate stylophorans). This situation largely results from a strong taphonomic bias (Smith, 1988;Lefebvre and Fatka, 2003). The reduced number of late Cambrian-earliest Ordovician echinoderm Lagerstätten is correlated to the eustatic low stand recorded worldwide for this time interval. In contrast, the two peaks in echinoderm diversity (middle Cambrian and Middle-Late Ordovician) are correlated with both high sea levels and numerous Lagerstätten (Smith, 1988;[START_REF] Peters | A sampling-adjusted macroevolutionary history for Ordovician -Early Silurian crinoids[END_REF][START_REF] Lefebvre | This larger amount of available data provides an unprecedented opportunity to better identify if similarities between echinoderm faunas simply reflect comparable palaeoenvironmental conditions and/or if they also support palaeobiogeographic affinities. As a first consequence of this improved knowledge of late Cambrian-Early Ordovician faunas, less than 12% of echinoderms from the Fezouata Shale can be identified as possibly endemic taxa (instead of about 50% in the early 2000s). They include the eocrinoids Rhopalocystis destombesi[END_REF]Zamora et al., 2013a).

The second major diversification of echinoderms took place in Early-Middle Ordovician times [START_REF] Guensburg | Rise of echinoderms in the Paleozoic evolutionary fauna: significance of paleoenvironmental controls[END_REF]Sprinkle and[START_REF] Sprinkle | [END_REF]Lefebvre and Fatka, 2003;[START_REF] Lefebvre | This larger amount of available data provides an unprecedented opportunity to better identify if similarities between echinoderm faunas simply reflect comparable palaeoenvironmental conditions and/or if they also support palaeobiogeographic affinities. As a first consequence of this improved knowledge of late Cambrian-Early Ordovician faunas, less than 12% of echinoderms from the Fezouata Shale can be identified as possibly endemic taxa (instead of about 50% in the early 2000s). They include the eocrinoids Rhopalocystis destombesi[END_REF]. This time interval is characterized by the onset of the taxa typical of the PEF (e.g., blastoids, crinoids, "cystoids"), but also of the first representatives of the Modern Evolutionary Fauna (MEF: e.g., asteroids, echinoids, holothurians). Extremely diverse echinoderm assemblages dominated by members of the PEF have been described from all palaeocontinents in the Darriwilian-Katian time interval (Sprinkle and Guensburg, 2004;[START_REF] Lefebvre | This larger amount of available data provides an unprecedented opportunity to better identify if similarities between echinoderm faunas simply reflect comparable palaeoenvironmental conditions and/or if they also support palaeobiogeographic affinities. As a first consequence of this improved knowledge of late Cambrian-Early Ordovician faunas, less than 12% of echinoderms from the Fezouata Shale can be identified as possibly endemic taxa (instead of about 50% in the early 2000s). They include the eocrinoids Rhopalocystis destombesi[END_REF]. In contrast, very little is known on the detailed palaeobiogeographic and evolutionary patterns of the initial stages of the Ordovician diversification of echinoderms. In particular, very few abundant and diverse late Tremadocian assemblages have been described so far (Ubaghs, 1969[START_REF] Ubaghs | Les Echinodermes "Carpoïdes" de l'Ordovicien inférieur de la Montagne Noire (France)[END_REF](Ubaghs, , 1983;;[START_REF] Guensburg | Rise of echinoderms in the Paleozoic evolutionary fauna: significance of paleoenvironmental controls[END_REF][START_REF] Sprinkle | Origin of echinoderms in the Paleozoic evolutionary fauna: the role of substrates[END_REF][START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF]Vizcaïno et al., 2001;[START_REF] Mergl | Lower Ordovician cystoids (Rhombifera, Diploporita) from the Prague Basin (Czech Republic)[END_REF][START_REF] Sumrall | Early Ordovician mitrates and a possible solute (Echinodermata) from the western United States[END_REF].

The abundant and diverse, remarkably preserved echinoderm assemblages of the Fezouata Shale (late Tremadocian-Floian), in the central Anti-Atlas of Morocco, provide the opportunity to document both temporal and palaeoecological aspects of the earliest stages of the Ordovician diversification of echinoderms in a high-latitude, intracratonic, Gondwanan context. Although Early Ordovician echinoderms have been known in that area since 60 years (e.g., Choubert et al., 1953;Ubaghs, 1963;Chauvel 1966aChauvel , 1966bChauvel , 1969Chauvel , 1971aChauvel , 1971bChauvel , 1978;;Chauvel and Régnault, 1986;Donovan and Savill, 1988), their diversity and abundance have only recently been revealed by the discovery of numerous levels with exceptional preservation in the early 2000s (Lefebvre and Fatka, 2003;Lefebvre, 2007;Lefebvre and Botting, 2007;[START_REF] Lefebvre | A la poursuite d'une chimère: reconstitution de la morphologie de Plasiacystis mobilis, un solute (Echinodermata, Blastozoa) atypique de l'Ordovicien péri-gondwanien[END_REF]Noailles et al., 2010;Van Roy et al., 2010;Sumrall and Zamora, 2011;Allaire et al., 2015;Martin et al., 2015).

Previous works on Fezouata echinoderms

Although [START_REF] Coquand | Description géologique de la partie septentrionale de l'Empire du Maroc[END_REF] described the first fossils from Morocco nearly 170 years ago (in the Rif area), proper palaeontological investigations did not really begin before the early 1920s in the southern part of the country (Lefebvre et al., in this issue). In Morocco, the first Ordovician fossils were successively reported from the Djebilet hills (Barthoux, 1924) and in Oulmès area (Termier, 1927). The Anti-Atlas range was long considered to consist of a thick succession of Lower Cretaceous (Thomson, 1888;[START_REF] Brives | Voyages au Maroc[END_REF][START_REF] Gentil | La géologie du Maroc et la genèse de ses grandes chaînes[END_REF][START_REF] Gentil | Carte géologique provisoire du Maroc[END_REF] or Devonian (Gentil, 1929) limestones. This interpretation, based solely on lithologies and/or erroneous identifications of fossils, was definitively refuted by [START_REF] Bourcart | Découverte du Cambrien à Archaeocyathus dans l'Anti-Atlas marocain[END_REF], who described the first Cambrian fossils (archaeocyathids) in Tiznit area (western Anti-Atlas). The occurrence of widespread outcrops of Ordovician rocks was then established by Neltner (1929), based on the presence of the trilobite Trinucleus in the Tafilalt. The first remains of Ordovician echinoderms (Calix sedgwicki and cf.? Holocystites sp.) were described by [START_REF] Ségaud | Sur l'Ordovicien du Djebel Tachilla (Maroc Sud-Ouest)[END_REF] in the early Darriwilian (Middle Ordovician) quartzites of Jbel Tachilla (western Anti-Atlas).

In the Anti-Atlas (and also in Morocco), the first remains of Early Ordovician echinoderms (Macrocystella bohemica) were collected in 1952 by G. Choubert and G. Henningsmoen at Tansikht bridge (locality 2082, in between Agdz and Zagora; Fig. 1C) during one of the field excursions organized during the 19th International Geological Congress held in Algiers (Choubert et al., 1953;Ubaghs, 1963;Allaire et al., 2015). The description of this material, originally identified as Mimocystites bohemicus by Choubert et al. (1953), represents the first systematic paper devoted to a fossil from the Fezouata Shale. In the next 40 years, abundant remains of Early Ordovician echinoderms were collected by J. Destombes, who was in charge of the mapping of Cambro-Ordovician rocks on the eight 1:200.000 sheets covering the whole Anti-Atlas range (Destombes, 2006a). During the second half of the 20th century, his material was the exclusive source for the abundant literature devoted to the systematics of echinoderms from the Lower Ordovician of the Anti-Atlas (Ubaghs, 1963;Chauvel, 1966aChauvel, , 1966bChauvel, , 1969Chauvel, , 1971aChauvel, , 1971bChauvel, , 1978;;Destombes et al., 1985;Chauvel and Régnault, 1986;Donovan and Savill, 1988;Smith, 1988). As a result of J. Destombes' activity, 21 species of Early Ordovician echinoderms belonging to 6 distinct classes were reported from the Anti-Atlas in the late 1990s (Table 1; Destombes, 2006a). Ubaghs (1963) described the late Tremadocian eocrinoid Rhopalocystis destombesi, based on abundant and very well preserved material from the Zagora area (locality 809; Fig. 1C). The first remains of mitrate stylophorans (Anatifopsis escandei) were reported by Chauvel (1966a). However, he interpreted them as isolated valves of cirriped crustaceans (locality 983; Fig. 1B). In the western Anti-Atlas, the same region also yielded the eocrinoid Balantiocystis regnelli (locality 983) and the cornute stylophoran Chauvelicystis ubaghsi (locality 985; Fig. 1B), both described by Chauvel (1966b). In the same monograph, Chauvel (1966b) also reported the first occurrence of diploporites (Aristocystites cf. bohemicus sinuosus and Paleosphaeronites? prokopi) in late Tremadocian deposits of the Zagora area (locality 1157 = 809; Fig. 1C). A thorough investigation of Early Ordovician glyptocystitid rhombiferans was published by Chauvel (1969), who confirmed the occurrence of Macrocystella bohemica in the central Anti- 1B). In the Zagora area, the first Early Ordovician cornute from the Fezouata Shale (Thoralicystis zagoraensis) was described by Chauvel (1971a), based on a single specimen (locality 1685; Fig. 1C).

The abundant material of Rhopalocystis collected by J. Destombes in both Agdz (localities 1773 and 2082; Fig. 1B,C) and Zagora areas (localities 809 = 1157 = 1725; 1737 = 1738 = 1750; and 2367; Fig. 1C) was investigated and revised in a series of papers, in which nine additional species were successively described: R. fraga, R. grandis, R. zagoraensis and R. sp. A (Chauvel, 1971b); R. havliceki, R. sp. B, and R. sp. C (Chauvel, 1978); R. dehirensis and R. lehmani (Chauvel and Régnault, 1986). The resulting high intrageneric taxonomic diversity is probably largely overestimated (Smith, 1988;Allaire, 2014). Consequently, only five morphotypes were retained here (Table 1; Appendix 1), pending a detailed systematic revision of the genus Rhopalocystis is performed (Allaire et al., 2015).

Table 1

Increase through time (from 1955 to 2015) in the number of echinoderm taxa reported from the Lower Ordovician of the Anti-Atlas, Morocco. The first two specimens of edrioasteroids from the Lower Ordovician of Zagora area (locality 1687 = Z-F25b; Fig. 1C) were described as Hemicystites? boehmi by Chauvel (1978). In the same monograph, he also reported on the presence of additional material of Balantiocystis from the western Anti-Atlas (locality 1882; Fig. 1B) and provided the first detailed stratigraphic charts showing the ranges of all Ordovician echinoderm species and genera then known from the Anti-Atlas, with the single exception of the mitrate Anatifopsis escandei, still considered to be a crustacean (see also Destombes et al., 1985). Donovan and Savill (1988) described the first specimen of a crinoid (Ramseyocrinus sp.) collected in the Fezouata Shale in the Zagora area (locality 1685; Fig. 1C). Smith (1988) was the first to identify both the lower and upper parts of the Fezouata Shale as echinoderm Lagerstätten. However, his faunal lists were incomplete and partly incorrect. For example, all glyptocystitids (Macrocystella spp.) were reported from the lower part of the Fezouata Shale, although they also occur in the upper part of it. Several taxa were omitted in the faunal lists (e.g., Hemicystites cf. boehmi), whereas other ones were mentioned, although they were not originally described from the Anti-Atlas, but from other regions (e.g., Phyllocystis sp. from Casablanca area, western Moroccan Meseta; [START_REF] Gigout | Sur un hétérostélé de l'Ordovicien marocain[END_REF].

In the early 2000s, numerous localities yielding hundreds of fully articulated remains of Early Ordovician echinoderms were discovered in the Zagora area by Mohamed 'Ou Saïd' Ben Moula, a local fossil collector (see Lefebvre et al., in this issue). It was soon discovered that most of these echinoderm dense beds were restricted to thin, laterally discontinuous horizons, frequently associated with soft-bodied organisms of the Fezouata Biota (e.g., demosponges, marrellomorphs, palaeoscolecid worms, xiphosurans; Noailles et al., 2010;Van Roy et al., 2010). Thousands of echinoderm specimens were collected in situ through successive field campaigns organized from late 2002 to early 2015 (see 'Material and methods' section below; Lefebvre et al., in this issue). This abundant and extremely diverse new material prompted the reevaluation of the Fezouata Shale as one of the few, key echinoderm Lagerstätten documenting the initial stages of the Great Ordovician Biodiversification Event (GOBE) in Early Ordovician times.

In their list of echinoderm genera from the Lower Ordovician of the Anti-Atlas, Lefebvre and Fatka (2003) reported the first taxa collected from Ben Moula's localities in the Zagora area. The new material included the first solutan (Minervaecystis) documented in the Lower Ordovician of Morocco (Lefebvre and Fatka, 2003). This solutan was later identified as Plasiacystis mobilis [START_REF] Lefebvre | A la poursuite d'une chimère: reconstitution de la morphologie de Plasiacystis mobilis, un solute (Echinodermata, Blastozoa) atypique de l'Ordovicien péri-gondwanien[END_REF][START_REF] Rahman | Making sense of carpoids[END_REF][START_REF] Lefebvre | A reinterpretation of the solutan Plasiacystis mobilis (Echinodermata) from the Middle Ordovician of Bohemia[END_REF]. Moreover, the presence of four additional stylophoran taxa, including three cornutes (a new cothurnocystid, a new hanusiid, and Phyllocystis sp.) and one new mitrate was also reported by Lefebvre and Fatka (2003). A slab showing the accumulation of two of these cornutes (Cothurnocystis sp. and Galliaecystis sp.) was figured by Lefebvre (2007: fig. 1A), who also documented the first evidence of somasteroids in the Fezouata Shale (Lefebvre 2007: fig. 1D). The stylophoran dense beds of the Fezouata Shale were interpreted by Lefebvre (2007) as opportunistic, lowdiversity assemblages associated with oligotrophic, relatively deep (below storm-wave base) environmental conditions. Another new cornute from the lower part of the Fezouata Shale was figured by Ware and Lefebvre (2007: fig. 5), to discuss plate homologies in stylophorans. Two additional low-diversity stylophoran dense beds were reported by Lefebvre and Botting (2007) from the southern extremity of Jbel Tizagzaouine (locality Z-F2(1); Fig. 1C): one of them was dominated by the cornute Thoralicystis sp., and the second one, by the mitrate Peltocystis cornuta. In contrast to Lefebvre (2007), Lefebvre and Botting (2007) considered that stylophoran dense beds were occurring in relatively shallow environmental conditions.

The taphonomy of glyptocystitids from the Fezouata Shale and its impact on the systematic identification of the various Moroccan species of Macrocystella were thoroughly discussed by Nardin (2007a), who also figured several new specimens of Rhopalocystis. A representative sample of exceptionally preserved echinoderms from the Fezouata Shale (including a crinoid, an eocrinoid, the glyptocystitid Macrocystella bohemica, a somasteroid, and cornutes) was figured by Van Roy et al. (2010: fig. S2c-g). Two new species of edrioasteroids, Anedriophus moroccoensis and Argodiscus espilezorum were described by Sumrall and Zamora (2011), based on abundant material from Bou Chrebeb (1687 = Z-F25b; Fig. 1C). The two specimens of edrioasteroids originally identified as Hemicystites? boehmi by Chauvel (1978) were reassigned to A. espilezorum by Sumrall and Zamora (2011). The new mitrate mentioned by Lefebvre and Fatka (2003) was interpreted as a basal anomalocystitid by [START_REF] Noailles | A new mitrate stylophoran from the Lower Ordovician of the Central Anti-Atlas (Morocco) and the origin of the anomalocystitids[END_REF]. Several fully articulated echinoderms from the Fezouata Shale were figured by [START_REF] Lefebvre | This larger amount of available data provides an unprecedented opportunity to better identify if similarities between echinoderm faunas simply reflect comparable palaeoenvironmental conditions and/or if they also support palaeobiogeographic affinities. As a first consequence of this improved knowledge of late Cambrian-Early Ordovician faunas, less than 12% of echinoderms from the Fezouata Shale can be identified as possibly endemic taxa (instead of about 50% in the early 2000s). They include the eocrinoids Rhopalocystis destombesi[END_REF]: eocrinoids (Rhopalocystis destombesi), edrioasteroids (Argodiscus espilezorum), glyptocystitids (Macrocystella bohemica), new genera of somasteroids, and cornute stylophorans (Flabellicarpus sp.).

Two preliminary reports briefly mentioned the occurrence of exceptionally preserved, putative soft parts in echinoderms from the Fezouata Shale: in solutans (gut; Lefebvre et al., 2013b), and in cornute stylophorans (gut and water-vascular system; Lefebvre et al., 2014). Finally, taphonomic aspects of selected late Tremadocian echinoderm levels from the Zagora area were investigated: Rhopalocystisdominated assemblages (Allaire et al., 2015), and a single cornute dense bed from Jbel Tizagzaouine (Martin et al., 2015).

In summary, the taxonomic diversity of echinoderms from the Fezouata Shale has increased dramatically since the early 2000s and the discovery of the Fezouata Biota by Ben Moula in the Zagora area (Table 2). Clearly, the list of 18 echinoderm taxa reported in Van Roy et al. (2015) for the whole Fezouata Shale largely underestimates the original diversity of the phylum. A more realistic evaluation of echinoderm taxonomic diversity, based on previous works and the exhaustive examination of specimens deposited in the main European and Moroccan public collections, instead suggests the occurrence of at least 62 species, belonging to at least 45 genera and eight distinct classes (Appendix 1). Consequently, because they are represented by over 60 taxa, echinoderms appear to be one of the two dominant groups of benthic organisms of the Fezouata Biota, along with panarthropods (over 60 taxa; Van Roy et al., 2010, 2015), but far beyond sponges (at least 27 species; Botting, 2007, in this issue) and molluscs (over 20 taxa; Babin and Destombes, 1990;[START_REF] Horny | Ordovician Tergomya and Gastropoda (Mollusca) of the Anti-Atlas (Morocco)[END_REF]Kröger and Lefebvre, 2012;Ebbestad, in this issue;Polechova, in this issue).

Geological setting

The Anti-Atlas is an elongate, Gondwanan, intracratonic basin, which formed during a first phase of extension (aborted rift) in the latest Ediacaran-middle Cambrian time interval (Alvaro et al., 2014a, [START_REF] Clausen | Replacement of benthic communities in two Neoproterozoic-Cambrian subtropical-to-temperate rift basins, High Atlas and Anti-Atlas, Morocco[END_REF]. The middle Cambrian (Cambrian Series 3) shales of the Internal Feijas Group and the sandstones of the Tabanite Group both represent successive intervals of syn-rift progressive infilling of the Anti-Atlas basin (Alvaro et al., 2014b). In latest Cambrian-Early Ordovician times, the opening of the Rheic Ocean and the rifting of Avalonia away from Gondwana prompted the reactivation of major extensive structures in the Anti-Atlas area (Burkhard et al., 2006). In the eastern Anti-Atlas, this second phase of extension was associated with an active volcanism (e.g., interstratified basaltic pillows; [START_REF] Clariond | Sur la présence de roches éruptives acadiennes et postacadiennes au sud du Djebel Ougnat (sud marocain)[END_REF]Choubert et al., 1952;Destombes, 2006c).

In Early Ordovician times, the Anti-Atlas area was located at high palaeolatitudes, close to the South Pole (Cocks and Torsvik, 2002;Torsvik and Cocks, 2013).

In the Zagora area (central Anti-Atlas), the Lower Ordovician series consists of a ~860 m thick, monotonous succession of argillites and siltstones lying unconformably over the middle Cambrian sandstones of the Tabanite Group (Fig. 2; Destombes, 1962Destombes, , 1971;;Destombes et al., 1985;Martin et al., in press). In the Anti-Atlas, the Lower Ordovician series is traditionally subdivided into three successive stratigraphic units: the Lower Fezouata Shale Formation, the Upper Fezouata Shale Formation, and the Zini Sandstone Formation (Destombes, 1962(Destombes, , 1971;;Destombes et al., 1985;Vidal, 1996;Van Roy et al., 2010). In the eastern Anti-Atlas, the boundary between the two lower formations was defined based on the occurrence of an iron-rich, glauconitic marker bed, interpreted as a major flooding surface (Destombes, 1962(Destombes, , 1971;;Destombes et al., 1985). However, this horizon is absent in the central Anti-Atlas (Agdz-Zagora area). In this region, the two lower units are thus frequently considered as forming together the Fezouata Shale Formation (Martin et al., in press). The overlying Zini Sandstone Formation corresponds to a diachronous unit, which is particularly thick in the western Anti-Atlas (300-400 m), but extremely reduced in the Agdz-Zagora area (10-20 m;Destombes, 1962Destombes, , 1971;;Destombes et al., 1985).

In the central Anti-Atlas, the whole Lower Ordovician series corresponds to a single, 2nd order sequence (Fig. 2; Destombes, 1971;Vidal, 1996). In the Zagora area, the Fezouata Shale Formation corresponds to a ~840 m thick relatively monotonous succession (Destombes, 1962(Destombes, , 1971(Destombes, , 2006b;;Destombes et al., 1985;Martin et al., in press). The lower, transgressive part of the Fezouata Shale consists of blue, green and beige argillites and fine siltstones, whereas its upper, regressive part is dominated by beige siltstones containing abundant floated micas, with more and more frequently intercalated sandstone beds towards the top. Storm-related sedimentary structures (e.g., ripple marks, hummocky cross stratifications) occur throughout the whole Fezouata Shale and suggest the persistence of relatively stable and shallow (at or above storm wave base) environmental conditions (Martin et al., 2015, in press;Vaucher et al., in this issue). The transition between the Fezouata Shale and the overlying Zini Sandstone is extremely gradual. The Zini Sandstone Formation corresponds to the ultimate and shallowest part of the Lower Ordovician succession. In the central Anti-Atlas, it is unconformably overlain by the transgressive shales of the Middle Ordovician Tachilla Formation (Destombes, 1962(Destombes, , 1971(Destombes, , 2006b;;Destombes et al., 1985;Vidal, 1996).

In the Zagora area, an early (Tr1) to middle (Tr2) Tremadocian age was proposed for the lowermost 220 m of the Fezouata Shale, based both on graptolites (Destombes and Willefert, 1959;Destombes, 2006b;Gutiérrez-Marco and Martin, in this issue) and palynomorph assemblages (Elaouad-Debbaj, 1988). In the eastern Anti-Atlas, the glauconitic, iron-rich horizon separating the Lower and Upper Fezouata Shale formations was traditionally considered as corresponding to the Tremadocian-Floian boundary (Destombes, 1962(Destombes, , 1971;;Destombes et al., 1985;Vidal, 1996). The absence of this marker bed in the Zagora area prompted the reevaluation of the Lower Ordovician biostratigraphy in that region, based on intensive logging and abundant new collections of graptolites (Martin et al., in press; Gutiérrez-Marco and Martin, in this issue). In the revised biostratigraphic framework, the Tremadocian-Floian boundary occurs at ~450 m above the base of the section and more or less coincides with the maximum flooding interval (Fig. 2). Moreover, the boundary between the two late Tremadocian A. murrayi and H. copiosus graptolite zones (Tr3) was identified at about 330 m above the base of the section (Fig. 2; Martin et al., in press; Gutiérrez-Marco and Martin, in this issue). In the Zagora area, the upper part of the Lower Ordovician succession (uppermost Fezouata Shale and Zini Sandstone) apparently records a relatively complete Floian succession, from the? C. protobalticus graptolite Zone to the Azygograptus interval (Fig. 2; Gutiérrez-Marco and Martin, in this issue). This result is in good accordance with previous investigations based on chitinozoans (Elaouad-Debbaj, 1984). Chitinozoan assemblages suggest a late Floian age (E. brevis Zone; Fl3) for the uppermost part of the Fezouata Shale, and an early Darriwilian age (D. bulla Zone; Dw1) for the lowermost part of the Tachilla Shale, thus implying a stratigraphic gap corresponding to the whole Dapingian time interval, at the top of the Zini Sandstone (Gutiérrez-Marco et al., 2014).

Material and methods

This study relies both on abundant field observations made in the Lower Ordovician of the Zagora area within the last 12 years and an almost exhaustive review of echinoderm specimens from the Fezouata Shale (2960 rock samples containing over 10,000 individuals; Appendix 2) belonging to major European and Moroccan palaeontological collections, with some additional data from some North American collections (e.g., California Academy of Sciences, Yale Peabody Museum of Natural History). This material comes from 39 localities and 63 distinct horizons, providing a relatively comprehensive record of Early Ordovician echinoderm remains from most regions of the Anti-Atlas (Figs. 23; Appendix 3). However, most specimens (89.7%) and localities (79.5%) are from the Agdz-Zagora area, where much of the sampling effort was concentrated in the last 12 years. Other included sites (8) correspond to historical localities originally discovered by J. Destombes in the western and the eastern Anti-Atlas (Chauvel, 1966a(Chauvel, , 1966b(Chauvel, , 1969(Chauvel, , 1978)).

All historical samples described in the literature from Lower Ordovician localities in the Anti-Atlas were reexamined (Appendix 2), with the exception of the original specimens of Macrocystella bohemica described by Choubert et al. (1953), which are probably housed in the palaeontological collections of the Scientific Institute of Rabat, Morocco. Investigated historical material belongs to the palaeontological collections of the Natural History Museum of London, UK (Savill coll.), the Natural History Museum of Oslo, Norway (Spjeldnaes coll.), and Rennes University, France (Destombes coll.). All these specimens were collected in localities originally discovered by J. Destombes. Their stratigraphic position and geographic origin are thus precisely known.

The bulk of the study material (48.5%) belongs to the palaeontological collections of Cadi-Ayyad University of Marrakesh, Morocco (Lefebvre, Reboul, and ANR project "Rise of Animal Life" [RALI] coll.; Appendix 2). It was collected during successive field campaigns organized initially by Dijon and Marrakesh Universities (2003)(2004)(2005)(2006), and then by Lyon 1 andMarrakesh Universities (2007-2014) in the Zagora area. All Marrakesh specimens were collected in situ, either from localities originally discovered by Ben Moula or from various horizons identified along the sections logged for biostratigraphic purposes. The stratigraphic and geographic details of this material are thus well-documented.

The last source of information comes from samples collected in the last 15 years, from various sites originally discovered by Ben Moula in the Zagora area, but also from two other localities from central (Taichoute) and eastern Anti-Atlas (Ouzina; Appendix 2). This material belongs to the palaeontological collections of the Senckenberg Naturmuseum Frankfurt, Germany (Weug coll.), the Natural History Museum of London, UK (Fortey, Jefferies-Savill, and Hunter coll.), the Musée des Confluences de Lyon, France (Reboul coll.), Lyon 1 University, France (Lacombe, Lefebvre, Reboul, Vannier, Van Roy, and Vizcaïno coll.), the Muséum d'Histoire naturelle de Marseille, France (Reboul coll.), the Muséum d'Histoire naturelle de Nantes, France (Botting, Catto, Reboul and Van Roy coll.), the Naturhistoriska Riksmuseet of Stockholm, Sweden (Ouaki coll.), the Muséum d'Histoire naturelle de Toulouse, France (Reboul coll.), and the Museo Paleontológico Universidad Zaragoza, Spain (Zamora coll.). Precise stratigraphic information and locality details were available for most of these specimens (e.g., Botting, Fortey, Lefebvre, Reboul, Vannier, Zamora coll.).

The abundant echinoderm material collected in the Fezouata Shale of the Agdz-Zagora area (2655 specimens) does not display a homogeneous distribution along the stratigraphic column (Table 2). Most localities (21) and specimens (79.1%) are from the late Tremadocian, and in particular from the A. murrayi Zone (Fig. 3). The quantity of available material is significantly more reduced in the middle Tremadocian (1 locality, 0.3% of the specimens), and the early (3 localities, 4.7% of the specimens), middle (5 localities, 15.6% of the specimens) and late Floian parts of the series (2 localities, 0.3% of the specimens; Fig. 2). This distribution pattern questions the existence of a possible sampling bias towards the 70 m-thick stratigraphic interval of the A. murrayi Zone, which yielded most of the soft-bodied organisms of the Fezouata Biota (Fig. 3; Martin et al., in press) and was thus more intensively investigated and quarried by Ben Moula. Clearly, most of the sampling effort focused on this part of the series, where intensive quarrying of echinoderm dense beds at four localities (Z-F2, Z-F3, Z-F4, and Z-F6; Fig. 1C) contributed to 52% of the material collected in the whole Lower Ordovician succession in the Agdz-Zagora area.

However, the same sampling effort was undertaken on all parts of the Lower Ordovician succession during the detailed logging of the series. In spite of intensive investigation, no new echinoderm remains were found in the lowermost part of the Fezouata Shale (below the A. murrayi Zone), thus confirming their extreme rarity in these levels (Destombes and Willefert, 1959;Destombes et al., 1985;Vidal, 1996;Destombes, 2006b). Moreover, the detailed logging of the Fezouata Shale also showed that echinoderms are locally extremely abundant (dense beds) and represent one of the major components of most of the low diversity benthic communities occurring throughout the A. murrayi Zone. The same sampling effort was undertaken in most of the overylying levels (H. copiosus Zone and Floian deposits) during the logging campaigns, and yielded abundant, extremely diverse benthic faunas. However, most of these younger assemblages are dominated by other groups of marine invertebrates (brachiopods, molluscs, trilobites) and yielded comparatively few echinoderm remains. Consequently, the non-homogenous distribution of echinoderm specimens within the Fezouata Shale does not entirely result from a sampling bias, but apparently reflects their large-scale diversity pattern. This conclusion is confirmed by the independent, random sampling performed by J. Destombes, while he was mapping the Agdz-Zagora area: the majority of his echinoderm material (82%) was also collected in the late Tremadocian.

Most echinoderm levels identified in the Fezouata Shale are dominated by one or two taxa, often associated with one or several less frequent species. Producing simple lists of echinoderm taxa collected within a same horizon would not have made it possible to express their relative abundance in the assemblages. Consequently, in all echinoderm levels yielding more than 10 individuals, taxa were identified either as rare (less than 10% of echinoderm specimens), common (10-30%), or abundant (over 30%). In the few horizons yielding less than 10 individuals, taxa were considered as rare or common, depending on the sampling effort and time spent in the corresponding localities. Estimates of relative abundance did not take into account isolated skeletal elements (e.g., thecal plates, columnals). They were established primarily on partly articulated to complete specimens, which represent most of the studied material. The stratigraphic distribution and relative abundance of the 37 echinoderm assemblages collected in situ, from well-identified horizons in the Zagora area (Botting, Destombes, Fortey, Lefebvre, RALI, Reboul pars, Savill, Spjeldnaes, Vannier, and Vizcaïno coll.) were plotted along the detailed stratigraphic columns logged in that region (Figs. 23). The 26 other echinoderm assemblages could not be plotted, either because their stratigraphic position was not precise enough [START_REF] Ortega | Tremadocian graptolite-conodont biostratigraphy of the South American Gondwana margin (Eastern Cordillera, NW Argentina)[END_REF], or because they were identified in geographically distant areas [START_REF] Martin | Palaeoscolecid worms from the Lower Ordovician Fezouata Lagerstätte, Morocco: Palaeoecological and palaeogeographical implication[END_REF]. They are listed in Appendix 3.

Echinoderm assemblages through time

Early-middle Tremadocian

In the Agdz-Zagora area, no echinoderm remains have been collected so far in the lowermost 100 m of the Fezouata Shale (early Tremadocian, A. matanensis to R. flabelliformis anglica graptolite Zones; Fig. 2), in spite of intensive field work conducted by J. Destombes in the second half of the 20th century, and successive logging campaigns (2012)(2013)(2014) during which these levels were thoroughly investigated. In the last 60 years, this part of the series has yielded only a handful of specimens of marine invertebrates (Destombes and Willefert, 1959;Destombes et al., 1985;Elaouad-Debbaj, 1988;Destombes, 2006b). This depauperate, low-diversity assemblage consists mostly of graptolites (Clonograptus sp., Dictyonema sp., Didymograptus sp.), associated with rare trilobites (Beltella sp.).

The overlying 140 m of the succession (middle Tremadocian, A. victoriae graptolite Zone; Fig. 2) have similarly yielded extremely few remains of marine invertebrates, consisting of rare specimens of inarticulate brachiopods (Acrothele sp.), trilobites (Niobe? sp.) and graptolites (Destombes, 2006b). However, abundant, densely packed accumulations of fully articulated thecae of the eocrinoid Rhopalocystis were collected in these levels, from three distinct fossiliferous lenses (localities 1737, 1738and 1750;Figs. 1C, 2), all located in the Zagora area (Chauvel, 1971b;Destombes, 2006b). Brachioles are missing but articulated portions of stems are sometimes preserved, either isolated or in connexion with the theca. These assemblages can be interpreted as storm-generated Konzentrat-Lagerstätten (Vaucher et al., this issue), resulting from the downslope transport and accumulation of allochtonous eocrinoid-dominated communities. They are characterized by relatively small morphotypes, with a robust theca made of few, thick plates (R. destombesi). Such morphologies possibly represent an adaptation to relatively proximal, high energy environmental conditions.

Consequently, with the exception of these downslope transported local accumulations of eocrinoids, the lower part of the Fezouata Shale (0-240 m) is almost entirely devoid of fossil remains of benthic invertebrates. In contrast, extremely abundant and diverse assemblages of acritarchs (over 40 taxa), chitinozoans (4 species) and scolecodonts were described from these levels (Elaouad-Debbaj, 1988). The presence of abundant remains of palynomorphs in the lower part of the Fezouata Formation suggests that the occurrence of a depauperate, low-diversity assemblage of benthic organisms in these levels does not result from oligotrophic (nutrient-depleted) conditions, but more likely from an inhospitable environment on the sea-floor. In the A. victoriae Zone, the Fig. 2. Synthetic, composite stratigraphic column showing the Lower Ordovician succession in the Zagora area, central Anti-Atlas, Morocco. Biostratigraphic data (left columns) are based on graptolite biozones identified by Gutiérrez-Marco and Martin (in this issue). Stratigraphic position and composition of echinoderm assemblages documented in both Destombes localities (white stars) and (new) Floian sites (black stars) are reported along the log. Stratigraphic position and composition of (new) echinoderm assemblages from the late Tremadocian are reported on the detailed log (Fig. 3). The colours on the log correspond to those of the rocks on the outcrop (mostly green, blue and beige). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) occurrence of several allochtonous accumulations of Rhopalocystis suggests the existence of eocrinoid meadows in both shallower and probably more hospitable settings.

Late Tremadocian (A. murrayi Zone)

In the Agdz-Zagora area, the A. murrayi Zone has yielded the most abundant and diverse echinoderm fauna recorded throughout the whole Fezouata Shale (47 taxa belonging to 8 classes; Table 2). In this part of the succession, echinoderms are, along with panarthropods, one of the two dominant groups of marine invertebrates, both in terms of abundance and diversity. However, horizons yielding large numbers of echinoderm remains contain rarely more than five to six taxa, and frequently only one or two (Allaire et al., 2015;Martin et al., 2015). Consequently, the high echinoderm diversity observed in the A. murrayi Zone is cumulative and largely results from the combination of several types of assemblages, with distinct taphonomic signatures and originally associated with contrasting environmental conditions.

Historically, Rhopalocystis-dominated assemblages were the first dense beds reported in the Lower Ordovician of the central Anti-Atlas (Ubaghs, 1963;Chauvel, 1971bChauvel, , 1978;;Chauvel and Régnault, 1986;Allaire et al., 2015). These assemblages have yielded hundreds of remarkably preserved individuals of Rhopalocystis spp. They are known from various localities in the Agdz-Zagora area and have been recorded from several stratigraphic levels in the A. murrayi Zone (Figs. 23). Two main types of preservation were described for these levels (Ubaghs, 1963;Allaire et al., 2015).

The first one was documented from several stratigraphic levels throughout the A. murrayi Zone (e.g., Z-F12c; Figs. 23). It corresponds to abundant, densely packed accumulations of fully articulated thecae, sometimes associated with large trilobite fragments, in massive sandstone lenses (Fig. 4B). These storm-generated, allochtonous assemblages of small, thick-plated thecae of Rhopalocystis (e.g., R. destombesi, R. fraga, R. zagoraensis) indicate the persistence, in shallower environmental conditions, of eocrinoid meadows comparable to those documented in the underlying A. victoriae Zone (see above).

The second type of Rhopalocystis-dominated assemblage is apparently restricted to the lower part of the A. murrayi Zone (from about 240 to 260 m above the base of the Ordovician; Fig. 3). It corresponds to low density accumulations of large portions of more or less disarticulated specimens of Rhopalocystis preserved in siltstones (Fig. 4A,C). Thecal plates are frequently slightly displaced or collapsed. Large portions of articulated stems are often preserved in connexion with the theca. Brachioles are seldom present (Ubaghs, 1963;Allaire et al., 2015). In most localities (e.g., 1157, 1773), such Rhopalocystisdominated beds are associated with a relatively diverse benthic invertebrate fauna including other echinoderms (e.g., diploporites, glyptocystitids) and trilobites (e.g., Dikelokephalina, Platypeltoides; Destombes, 2006b). However, some other localities (e.g., Z-F1) yielded no associated fauna, in spite of intensive sampling. These assemblages can be interpreted as Konservat-Lagerstätten (Vaucher et al., this issue), resulting from the in situ burial of eocrinoid-dominated communities by distal storm events. These Rhopalocystis beds are generally dominated by relatively large morphotypes, with a high, polyplated theca made of numerous, thin plates (e.g., R. grandis, R. havliceki), some of them bearing strong protuberances. Such Calix-like morphologies are possibly adapted to relatively quiet, low energy environmental conditions.

In the Agdz-Zagora area, stylophoran-dominated beds are the major type of echinoderm assemblages documented in the upper part of the A. murrayi Zone (Fig. 5; Lefebvre, 2007;Martin et al., 2015). Their occurrence is restricted to a relatively narrow stratigraphic interval (from about 260 to 330 m above the base of the Ordovician; Fig. 3), corresponding more or less to the levels yielding exceptionally preserved organisms typical of the Fezouata Biota (Van Roy et al., 2015;Martin et al., in press). Stylophoran dense beds are generally dominated by cornutes (e.g. Z-F2(0), Z-F3, Z-F6a, Z-F6b; Fig. 5A-C; Lefebvre, 2007;Martin et al., 2015), and less frequently, by peltocystid mitrates (e.g., Z-F4(7), Z-F9(1); Fig. 5D; Lefebvre and Botting, 2007). These levels are characterized by exceptionally high densities of exquisitely preserved specimens, but also by an extremely low taxonomic diversity (Fig. 5A; Lefebvre, 2007;Martin et al., 2015). Typically, such beds are dominated by one or two species of cornutes, sometimes associated with a limited number of other stylophoran taxa. Very few representatives of other invertebrate groups occur in such levels: some rare other echinoderms (e.g., eocrinoids, somasteroids; Fig. 4D-F), inarticulate brachiopods, graptolites, hyolithids and/or trilobites. In the Zagora area, more than 20 distinct stylophoran dense beds were identified (Fig. 3; Appendix 3). The taphonomy of one of them (Z-F2(0): Jbel Tizagzaouine) was thoroughly investigated by Martin et al. (2015).

Almost all stylophoran dense beds occurring in the upper part of the A. murrayi Zone were collected from thin, lens-shaped horizons of fine-grained siltstones. The lower part of these levels usually consists of relatively coarser siltstones containing abundant disarticulated stylophoran skeletal elements, sometimes associated with large trilobite fragments. This irregular, basal level is typically overlain by a thicker layer of finer material containing extremely numerous, densely packed, fully articulated stylophorans, showing no preferential orientation, frequently upside-down, and sometimes piled on top of each other (Fig. 5A). In most cases, cover plates are preserved open and articulated to the ossicles, on each side of the distal aulacophore (Fig. 5C). Finally, the uppermost part of lenses usually consists of very fine siltstones yielding low density assemblages of remarkably preserved stylophorans. Consequently, these levels can be interpreted as Konservat-Lagerstätten (Vaucher et al., this issue) very likely resulting from the in situ or parautochtonous rapid burial of live stylophorandominated &&communities by distal storm deposits (Lefebvre and Botting, 2007;Martin et al., 2015).

Although the global diversity of stylophorans is extremely high (25 taxa) in the A. murrayi Zone, their distribution within this time interval is apparently restricted to their massive, patchy occurrences in the dense beds. Another intriguing aspect of their record is that the 20 stylophoran dense beds identified in the Zagora area all have different faunal compositions resulting from distinct combinations of the 13 most common taxa (Fig. 3). This pattern suggests the existence of a large pool of available stylophoran species, more or less randomly sampled through massive, though extremely patchy and spatially limited, occurrences. However, this situation in the uppermost part of the A. murrayi Zone is not unique to stylophorans. Most of this 70 m thick portion of the Fezouata Shale is unfossiliferous. In this interval, most fossiliferous levels correspond to discontinuous, lens-shaped siltstone layers yielding extremely low diversity, patchy assemblages dominated by one or two taxa. This observation applies not only to stylophorans, but also to anomalocaridids (Gaines et al., 2012), sponges (Botting, in this issue), marrellomorphs and trilobites (E. Martin, pers. comm.). Consequently, the extremely high diversity of the Fezouata Biota in the A. murrayi Zone appears to be largely cumulative, and to result from the addition of numerous discrete, patchy, low-diversity assemblages dominated by a relatively wide array of arthropods (common) and Chauvelicystis vizcainoi (rare; Fig. 3). Finally, a third distinct type of echinoderm assemblage was identified from a single locality (Z-F5) occurring in the upper part of the A. murrayi Zone (about 275 m above the base of the Ordovician; Figs. 3,6). The green siltstones of Oued Beni Zoli have yielded extremely abundant, exquisitely preserved specimens of the glyptocystitid rhombiferan Macrocystella bohemica. Although they have been crushed nearly flat, most individuals are fully articulated, with intact brachioles, theca and stem in connexion (Fig. 6A,D). This remarkable preservation probably results from the rapid, in situ burial of live communities of rhombiferans by distal storm deposits (Nardin, 2007a). However, the presence of thin levels crowded with amalgamated skeletal elements (e.g., thecal plates of Macrocystella, trilobite fragments) also suggests the occurrence of stronger storm events, implying some possible reworking of in situ to parautochtonous material (Ebbestad and Lefebvre, 2015).

The Oued Beni Zoli echinoderm assemblage is particularly diverse (Fig. 3). It contains very well preserved eocrinoids (Balantiocystis regnelli; Fig. 6C), solutans (Plasiacystis mobilis; Fig. 6B), somasteroids and stylophorans (Anatifopsis trapeziiformis, a new chauvelicystid cornute, Chauvelicystis ubaghsi, Galliaecystis ubaghsi and Prokopicystis sp.). Moreover, this level also strongly departs from all other echinoderm beds from the uppermost part of the A. murrayi Zone by the presence of an abundant and extremely diverse associated invertebrate assemblage comprising numerous trilobites (e.g., Ampyx cf. priscus, Asaphellus nov. sp. aff. jujuanus, Ceraurinella sp., Colpocoryphe sp., Eoharpes? sp., Euloma sp., Parabathycheilus cf. gallicus, Toletanaspis sp.; Martin et al., this issue a), as well as articulate brachiopods, bivalves (Redonia cf. michelae), cephalopods (Polymeres sp.), conulariids, graptolites, hyolithids, ostracods and paragastropods (Kröger and Lefebvre, 2012;Ebbestad and Lefebvre, 2015;Martin et al., in this issue a). This situation is all the more surprising that the Oued Beni Zoli assemblage is stratigraphically bracketed by two typical low diversity stylophoran dense beds (Fig. 3): Z-F4 (about 2 m below) and Z-F3 (about 1 m above).

In many respects, the Z-F5 level shows more similarities with some older localities (e.g., 1773) from the lower part of the A. murrayi Zone (e.g., occurrence of Macrocystella, abundant and diverse associated fauna) than with coeval echinoderm assemblages. These similarities probably reflect relatively comparable environmental conditions and/or taphonomic processes. On the other hand, the composition of the Z-F5 assemblage also appears different and more advanced than underlying Rhopalocystis-dominated faunas, as it records the oldest occurrences of various echinoderm (e.g., Balantiocystis, solutans, somasteroids) and non-echinoderm taxa (e.g., cephalopods, paragastropods) in the Fezouata Shale.

Late Tremadocian (H. copiosus Zone)

Two main types of echinoderm assemblages were observed in the H. copiosus Zone (about 330 to 410 m above the base of the Fezouata Shale). The first one (e.g., locality 2367; Fig. 2) corresponds to allochtonous, extremely dense accumulations of Rhopalocystis in storm-induced, massive sandstone lenses (see above; Allaire et al., 2015). These Konzentrat-Lagerstätten provide the opportunity to document the persistence during the middle-late Tremadocian time interval of Rhopalocystis-dominated eocrinoid meadows in shallower, higher-energy environmental conditions, than those recorded in the more distal deposits of the Zagora area.

A distinct type of assemblage was observed both in Z-F13c, at the base of the H. copiosus Zone (Fig. 3), and much higher in the section, in Z-F24, at the base of the overlying? C. protobalticus Zone (Figs. 2,7). These two localities have yielded abundant isolated, paddle-like dististeles of the solutan Plasiacystis mobilis (Fig. 7B). In some specimens of P. mobilis, the distal part of the proxistele and less frequently, articulated portions of its theca are also sometimes preserved (Fig. 7A). In Z-F13c, solutan remains are associated with an abundant and diverse invertebrate assemblage consisting of bivalves (e.g., Redonia cf. michelae), articulate brachiopods, conulariids and trilobites (e.g., Ampyx cf. priscus, Asaphidae, Geragnostus sp.). The Z-F24 locality corresponds to relatively coarse siltstones, yielding numerous aligned shells of cephalopods and hyolithids, as well as disarticulated pieces of large trilobites (Kröger and Lefebvre, 2012). Various epibiontic organisms, including cemoporid bryozoans and edrioasteroids (e.g., Argodiscus espilezorum), are attached to these large skeletal elements (Kröger and Lefebvre, 2012). Other faunal elements include rare bivalves (e.g., Ekaterodonta courtessolei, Redonia cf. michelae) and gastropods (e.g., Carcassonnella courtessolei). Abundant fragments of disarticulated eocrinoids (columnals, stem portions, isolated thecal plates) suggest the presence of large forms, very likely corresponding to Balantiocystis regnelli. Fully articulated, complete echinoderm specimens are relatively rare and include both crinoids (e.g., Iocrinus n. sp.; Fig. 7D) and kirkocystid mitrates (Balanocystites primus; Fig. 7C).

The coarse lithology of the two Plasiacystis-dominated levels, as well as the alignment of elongate skeletal remains (e.g., cephalopod shells) and the occurrence of numerous, large disarticulated portions of both trilobites and echinoderms (e.g., solutan dististeles) all suggest that these horizons probably correspond to Konzentrat-Lagerstätten (Vaucher et al., in this issue), resulting from the local accumulation of in situ to parautochtonous assemblages by storm-generated currents (Kröger and Lefebvre, 2012). In both Z-F13c and Z-F24, the preferential preservation of the short, massive dististele of P. mobilis, made of tightly sutured plates suggests that this region can be interpreted as a morphological module more resistant to disarticulation than the rest of the body wall (theca, brachiole). Consequently, the solutan Plasiacystis mobilis can be identified as a very likely "type 2" echinoderm (sensu Brett et al., 1997). In Z-F24, the preservation of fully articulated specimens of Argodiscus espilezorum and Balanocystites primus indicates that the abundance of P. mobilis in this locality does not result from a taphonomic bias. Both edrioasteroids and stylophorans are typical "type 1" taxa (sensu Brett et al., 1997), characterized by delicate, polyplated body walls entirely made of numerous, loosely articulated skeletal elements and thus, prone to rapid post-mortem disarticulation. Consequently, the preservation of intact specimens of such type 1 echinoderms suggests that they were probably buried alive by relatively distal storm deposits. In many respects, the Plasiacystis-dominated assemblages of the H. copiosus Zonebase of the? C. protobalticus Zone thus show several similarities with older, Rhopalocystis-and Macrocystelladominated late Tremadocian assemblages (see above).

Early Floian

In the Zagora area, the 400 m thick Floian succession corresponds to a large scale (second order) regressive sequence, comprising the upper part of the Fezouata Shale (~380 m, at the base) and the Zini Sandstone (~20 m, at the top). The most diverse early Floian echinoderm fauna was sampled at Toumiat (Z-F26; Figs. 1C,2), very close to a site originally identified by J. Destombes (locality 35), but from which no echinoderm remains had been reported so far (Vidal, 1996(Vidal, , 1998;;[START_REF] Horny | Ordovician Tergomya and Gastropoda (Mollusca) of the Anti-Atlas (Morocco)[END_REF]Destombes, 2006b). Two additional echinoderm assemblages (Z-F23, Z-F24; Figs. 1C,2) were collected from new localities discovered by Ben Moula in the early 2000s, at the base of the? C. protobalticus Zone. Two main types of echinoderm assemblages could be evidenced in the early Floian succession of Zagora area (about 470 to 510 m above the base of the Ordovician). The first one (Z-F24) corresponds to a Plasiacystis-dominated assemblage associated with an abundant and diverse invertebrate fauna (see above, Fig. 7).

The second type of assemblage was observed in Toumiat (Z-F26) and surrounding sites (e.g., Z-F23). In these localities, beige, relatively fine micaceous siltstones have yielded a diverse, well-preserved echinoderm fauna dominated by Balantiocystis regnelli (Fig. 2). The same new chauvelicystid as in Oued Beni Zoli (Z-F5, A. murrayi Zone; Fig. 3) is also relatively common in Toumiat. Other, less frequent elements of the echinoderm assemblage include crinoids (Ramseyocrinus sp.), edrioasteroids (Argodiscus espilezorum), eocrinoids (Lingulocystis aff. deani), solutans (Plasiacystis mobilis), somasteroids, and mitrate stylophorans (Balanocystites primus). This composition is relatively comparable to that of the stratigraphically older Plasiacystisdominated assemblages (Z-F13c, Z-F24), with in both cases, rare crinoids associated with the same four taxa: Argodiscus espilezorum, Balanocystites primus, Balantiocystis regnelli and Plasiacystis mobilis. Glyptocystitids (e.g., Macrocystella) have not been observed so far in both latest Tremadocian (H. copiosus Zone) and early Floian In many respects, the taphonomy of the Toumiat echinoderm assemblage appears to be very similar to that of earlier, late Tremadocian blastozoan-dominated faunas. In Toumiat, the preservation of extremely delicate echinoderm structures (e.g., large, polyplated eocrinoid thecae, tiny spines articulated to the marginal frame of chauvelicystid cornutes), in anatomical connection and/or only slightly displaced probably results from the rapid burial of live, in situ communities by distal storm deposits. On the other hand, the occurrence of edrioasteroids attached to skeletal fragments, along with the presence of large specimens of Balantiocystis showing various degrees of in situ disarticulation (from complete individuals to articulated modules, and eventually, scattered elements) both indicate relatively quiet environmental conditions, allowing the prolonged exposure of skeletal remains on the sea floor.

This interpretation is supported by the exquisite preservation of the abundant and diverse associated fauna, dominated by bivalves (e.g., Babinka prima, Coxiconcha guiraudi, Ekaterodonta courtessolei, Redonia cf. michelae; Polechova, in this issue), gastropods (e.g., Carcassonnella courtessolei, Lesueurilla prima, Thoralispira laevis; [START_REF] Horny | Ordovician Tergomya and Gastropoda (Mollusca) of the Anti-Atlas (Morocco)[END_REF]Ebbestad, in this issue), and trilobites (e.g., Agerina quadrata, Ampyx cf. priscus, Apatokephalus cf. incisus, Asaphellus aff. jujuanus, Colpocoryphe thorali, Neseuretus aff. attenuatus, Parabathycheilus gallicus, Pradoella tazzarinensis, Toletanaspis aff. borni; Vidal, 1996Vidal, , 1998;;Martin et al., in this issue a). Other faunal elements include articulate brachiopods, cephalopods (e.g., Saloceras sp.; Kröger and Lefebvre, 2012), graptolites (e.g., Didymograptus extensus), hyolithids, ostracods, but no soft-bodied organisms.

Middle Floian

Middle Floian deposits of the Fezouata Shale (?B. jacksoni graptolite Zone, about 570 to 670 m above the base of the Ordovician; Fig. 2) have yielded particularly abundant and diverse echinoderm faunas (Fig. 8). Several levels (e.g., 1685-1687;Figs. 1C, 2) were originally sampled by J. Destombes in the second half of the 20th century (Chauvel, 1969(Chauvel, , 1971a(Chauvel, , 1978;;Donovan and Savill, 1988). Abundant new material of echinoderms from this time interval was collected in the last 12 years both from Destombes' original localities and from new sites discovered by Ben Moula (e.g., Z-F0, Z-F25a; Figs. 1C,2). In the Zagora area, two main types of echinoderm assemblages were observed in middle Floian deposits. The first one (locality 1686; Figs. 1C,2) corresponds to densely packed accumulations of small, fully articulated, robust eocrinoid thecae (Rhopalocystis destombesi) occurring in lenses of massive sandstones (Fig. 8D). The taphonomic features of this Konzentrat-Lagerstätte are identical to those of similar Rhopalocystis-dominated accumulations documented throughout the Tremadocian succession (see above), and suggesting the persistence of eocrinoid-dominated meadows in nearby, shallower, higher energy settings, from the middle Tremadocian (A. victoriae Zone) to the middle Floian (?B. jacksoni Zone).

The second type of echinoderm assemblage was observed in all other fossiliferous middle Floian sites (1685, 1687, Z-F0, Z-F25a; Figs. 1C,2). These localities have yielded numerous and diverse, exquisitely preserved echinoderm remains collected either from beige and pistache siltstones (Z-F0, Z-F25a), or within silico-aluminous concretions (1685,1687). The composition of echinoderm assemblages remains more or less the same through the whole? B. jacksoni Zone (Fig. 2). They are dominated by the stemmed eocrinoid Balantiocystis regnelli (Fig. 8A) associated with locally common cornutes (e.g., same new chauvelicystid as in Z-F5 and Z-F26; Fig. 8C), edrioasteroids Chauvel, 1978, Sumrall andZamora, 2011), glyptocystitids (Macrocystella bohemica;Chauvel, 1969), mitrates (e.g., new anomalocystitid, Balanocystites primus; [START_REF] Noailles | A new mitrate stylophoran from the Lower Ordovician of the Central Anti-Atlas (Morocco) and the origin of the anomalocystitids[END_REF] and/or solutans (Plasiacystis mobilis). Other, less frequent, echinoderm taxa include crinoids, diploporites and somasteroids (Donovan and Savill, 1988). The taphonomy of echinoderms (e.g., large, more or less complete eocrinoid thecae, with slightly collapsed or displaced skeletal elements, stylophorans with fully articulated aulacophore) suggests the rapid burial of both live organisms and in situ disarticulated carcasses by distal storm deposits, in an otherwise relatively quiet environment.

This interpretation is confirmed by the presence, in some localities (e.g., Z-F25b), of dense settlements of edrioasteroids (Anedriophus moroccoensis and Argodiscus espilezorum) attached to many cephalopod and conulariid shells (Sumrall and Zamora, 2011). In the same levels, abundant traces left by other encrusters were also described on most trilobite carapaces (Vidal, 1998). The intensive colonization of skeletal debris by various kinds of epibiontic organisms probably results from their prolonged exposure on the sea floor, and thus of a relatively slow background sedimentation (Vidal, 1998;Lefebvre, 2007). On the other hand, the exquisite preservation of edrioasteroids (Fig. 8B) suggests that they were very likely buried alive and in situ during storm-induced episodes of rapid deposition.

In all localities, Balantiocystis-dominated assemblages are associated with an extremely abundant and diverse invertebrate fauna, dominated by trilobites (e.g., Agerina sp., Ampyx priscus, Asaphellus fezouataensis, A. cf. jujuanus, Basilicus (Basiliella?) destombesi, Bathycheilus gallicus, Euloma sp., Prionocheilus aff. languedocensis, Toletanaspis aff. borni; Destombes et al., 1985;Vidal, 1996Vidal, , 1998;;Martin et al., this issue a). Other members of the fauna include bivalves (e.g., Babinka prima, Ekaterodonta courtessolei, Redonia cf. michelae; Babin and Destombes, 1990;Polechova, in this issue), articulate brachiopods (e.g., Paurorthis tadristensis; [START_REF] Havlíček | Brachiopodes de l'Ordovicien du Maroc[END_REF], cephalopods (e.g., Bathmoceras australe, Destombesiceras zagorense, Polymeres sp., Protocyptendoceras longicameratum, Rioceras sp.; Kröger and Lefebvre, 2012), conulariids, gastropods (e.g., Carcassonnella courtessolei, Lesueurilla prima, Thoralispira cf. laevis; [START_REF] Horny | Ordovician Tergomya and Gastropoda (Mollusca) of the Anti-Atlas (Morocco)[END_REF]Ebbestad, in this issue), graptolites, hyolithids (e.g., Cavernolites senex; Marek, 1983), rostroconchs (Ribeiria sp.; Babin and Destombes, 1990), as well as few soft-bodied organisms such as cheloniellid arthropods (Van Roy et al., 2010) and demosponges (e.g., Pirania auraeum; Botting, 2007, in this issue).

In deep contrast with stylophoran-dominated beds of the underlying uppermost part of the A. murrayi Zone, Balantiocystis-dominated levels are not only associated with high diversity benthic assemblages, but they also occur recurrently in both the? C. protobalticus (Z-F23, Z-F26; see above) and the overlying? B. jacksoni zones. The iterative record of similar echinoderm communities during the early-middle Floian time interval suggests the persistence of relatively stable palaeoenvironmental conditions. In many respects, the palaeoecology and the taphonomy of Balantiocystis-dominated communities both appear much comparable to those documented in older blastozoan-dominated assemblages from the underlying A. murrayi and H. copiosus zones. This is particularly true for the Oued Beni Zoli assemblage (Z-F5, see above; Fig. 6), which shows relatively striking similarities with Balantiocystisdominated communities, in terms of faunal composition, lithology and preservation (Fig. 3). For example, the Z-F5 echinoderm assemblage has more taxa in common with younger faunas from the early or middle Floian (e.g., Balantiocystis regnelli, Macrocystella bohemica, Plasiacystis mobilis, new chauvelicystid cornute) than with coeval stylophorandominated assemblages (e.g., Anatifopsis trapeziiformis, Galliaecystis sp.). Because of its strong similarities with Floian faunas, the Oued Beni Zoli assemblage was sometimes erroneously assigned to a younger stratigraphic position (e.g., Nardin, 2007a;Kröger and Lefebvre, 2012). In recent years, the acquisition of a refined biostratigraphic framework for the Fezouata Shale showed that the Z-F5 levels were located within the A. murrayi Zone, about 55 m below the base of the H. copiosus Zone (Ebbestad and Lefebvre, 2015;Martin et al., in press). Consequently, the strong faunal similarities between the Oued Beni Zoli and younger assemblages are more likely the result of comparable palaeoenvironmental conditions.

Late Floian

Historically, no echinoderm remains were reported from both the uppermost part of the Fezouata Shale (about 670-840 m above the base of the Ordovician) and the Zini Formation (840-860 m), in spite of intensive field work undertaken by J. Destombes in the second half of the 20th century (Destombes et al., 1985;Vidal, 1996;Destombes, 2006b). However, in the last twelve years, several specimens of late Floian echinoderms were collected or observed in situ during field work in the Zagora area. Although this part of the succession still remains largely undersampled, recently acquired preliminary data on late Floian echinoderms of the central Anti-Atlas confirm their rarity and low diversity in these levels.

The largest part of the upper Fezouata Shale (B. minutus graptolite Zone, about 670-790 m above the base of the Ordovician; Fig. 2) is dominated by siltstones, but with more and more frequently intercalated sandstone beds towards the top. This part of the succession corresponds to shallower (lower to upper shoreface) environmental conditions than those (offshore/lower shoreface transition) prevailing from the middle Tremadocian to the middle Floian (Vidal, 1996(Vidal, , 1998;;Destombes, 2006b;Martin et al., in press;Vaucher et al., in this issue). Extremely diverse, though frequently disarticulated benthic invertebrate faunas dominated by bivalves (e.g., Ekaterodonta sp., Hemiprionodonta sp., Redonia sp.; Polechova, in this issue), articulate brachiopods (e.g., Paurorthis tadristensis; [START_REF] Havlíček | Brachiopodes de l'Ordovicien du Maroc[END_REF], gastropods (e.g., Thoralispira laevis, Sinuites sp.), graptolites, hyolithids and trilobites (e.g., Asaphellus sp., Basilicus (Basiliella?) destombesi, Colpocoryphe thorali, Neseuretus aff. attenuatus, Pradoella tazzarinensis; Vidal, 1996) were collected from these levels (e.g., Z-F31, Z-F32; Fig. 1C; see also Destombes, 2006b). In contrast to the situation in the lower portion of the Floian succession (see above), echinoderm remains are extremely rare in the abundant and diverse benthic assemblages occurring in this part of the Fezouata Shale. They consist of few isolated diploporite thecal plates observed in the field (Z-F32). The rarity of echinoderm remains possibly results from a taphonomic bias. The body wall of most Early Ordovician taxa (e.g., edrioasteroids, eocrinoids, somasteroids, stylophorans) was made of hundreds of loosely articulated skeletal elements, and was thus prone to rapid disarticulation in shallow, high energy environmental conditions (Brett et al., 1997;Lefebvre, 2007). On the other hand, the extreme rarity of isolated distinctive skeletal elements (e.g., columnals, thecal plates), along with the absence of taphonomically more resistant modules (e.g., portions of stems, dististeles), all rather point to an original absence or extreme rarity of echinoderms in the diverse late Floian benthic communities associated with shoreface environments in the Zagora area.

The uppermost part of the Fezouata Shale (Azygograptus interval, about 790 to 840 m above the base of the Ordovician; Fig. 2) corresponds to relatively fine siltstones, generally interpreted as deposited under slightly deeper environmental conditions (offshore/lower shoreface transition), comparable to those occurring during most of the succession, from the middle Tremadocian to the middle Floian (Vidal, 1996(Vidal, , 1998;;Destombes, 2006b;Vaucher et al., in this issue). In these levels, echinoderm remains were collected from two localities (Fig. 9). The first one (Z-F30m; Figs. 1C,2) yielded a low-diversity assemblage dominated by a small, nail-shaped, new eocrinoid (Eocrinoidea indet. sp. 1;Figs. 2,9A). A single, fully articulated specimen of mitrocystitid mitrate was also collected from this level. In Z-F30m, the occurrence of abundant, remarkably preserved specimens of "type 1" echinoderms (sensu Brett et al., 1997) suggests that they were probably buried alive by distal storm deposits. The second locality (TO-F2; Figs. 1C,2) corresponds to a concretion-bearing interval at the The youngest late Floian echinoderm assemblage was observed in situ, in two distinct localities (e.g., NDE; Fig. 1C), visited during the logging of the Zini Sandstone Formation (Azygograptus interval, about 840-860 m above the base of the Ordovician). It consists of a single diploporite-dominated bed, occurring within massive quartzites, about 3 m below the contact with the overlying shales of the Tachilla Formation. This bed shows abundant, fully-articulated, threedimensionally preserved, empty moulds of diploporite thecae (Fig. 9B). The sandstones and massive quartzites of the Zini Formation are generally interpreted as high-energy deposits corresponding to shallow, foreshore environmental conditions (Vidal, 1996;Destombes, 2006a;Vaucher et al., in this issue). In the Zagora area, they have yielded few, low-diversity assemblages consisting of brachiopods, hyolithids and disarticulated fragments of trilobites (e.g., Asaphidae indet., Neseuretus sp.; Vidal, 1996;Destombes, 2006b).

Discussion

Palaeoecology

Echinoderms constitute, along with trilobites, one of the two most studied groups of marine invertebrates from the Lower Ordovician of the Zagora area. However, the majority of early contributions dealing with echinoderms has been focusing almost exclusively on their systematics (e.g., Choubert et al., 1953;Chauvel, 1966aChauvel, , 1966bChauvel, , 1971aChauvel, , 1971b;;Chauvel and Régnault, 1986;Donovan and Savill, 1988). With the remarkable exception of Ubaghs (1963), their palaeoecology and taphonomy were not addressed until recently (Lefebvre and Fatka, 2003;Lefebvre, 2007;Lefebvre and Botting, 2007;Noailles et al., 2010;Van Roy et al., 2010;Sumrall and Zamora, 2011;Kröger and Lefebvre, 2012;Allaire et al., 2015;Martin et al., 2015). Ubaghs (1963) was the first to suggest the occurrence of Rhopalocystis-dominated Tremadocian eocrinoid meadows in the central Anti-Atlas (see also Lefebvre and Fatka, 2003;Allaire et al., 2015). Stylophoran dense beds from the A. murrayi Zone were often interpreted as low-diversity, opportunistic assemblages occurring in low-energy, dysoxic, deep environments (outer shelf, slope), well below storm wave base (taphofacies E sensu Brett et al., 1997;Lefebvre, 2007;Noailles et al., 2010;Van Roy et al., 2010). In contrast, the more diverse, early to middle Floian echinoderm assemblages were generally considered as typical of shallower, welloxygenated, relatively quiet environmental conditions, at or slightly below storm-wave base (taphofacies D sensu Brett et al., 1997;Lefebvre, 2007;Noailles et al., 2010). Alternatively, shallower environmental conditions (above storm-wave base) were suggested for stylophoran dense beds (Lefebvre and Botting, 2007;Martin et al., 2015), the Oued Beni Zoli fauna (Nardin, 2007a) and most early to middle Floian echinoderm assemblages (Kröger and Lefebvre, 2012). Finally, the palaeoecology of the two epibiontic edrioasteroids Anedriophus moroccoensis and Argodiscus espilezorum (Fig. 8B) was investigated by Sumrall and Zamora (2011), based on abundant clusters attached to cephalopod shells and conulariids.

Previous investigations on the palaeoecology of echinoderm assemblages from the Lower Ordovician of Zagora area have suggested the influence of several environmental factors on their distribution (e.g., hydrodynamism, dissolved oxygen levels, substrate affinities). However, strongly diverging palaeoenvironmental interpretations were sometimes provided for the same echinoderm levels (e.g., stylophoran dense beds). This confusion was largely the result of the absence of any detailed sedimentologic analysis of the Lower Ordovician succession in the Zagora area. Moreover, the stratigraphic position of the new echinoderm localities discovered by Ben Moula was extremely poorly constrained. These two major difficulties were largely fixed through successive field campaigns in the Zagora area (2012-2015), which provided not only the first precise, graptolitebased, biostratigraphic framework for the whole Fezouata Shale (including all new localities; Figs. 23), but also the first detailed sedimentologic investigation of these levels (Martin et al., in press; Gutiérrez-Marco and Martin, in this issue; Vaucher et al., in this issue). In this context, the palaeoecology of successive echinoderm assemblages identified in the Lower Ordovician of the Zagora area (see above; Fig. 10) and the environmental factors controlling their distribution can be discussed with unprecedented details.

A first, relatively unexpected result obtained from the sedimentological analysis of the Lower Ordovician series in the Zagora area is that most of the Fezouata Shale (from 100 to 670 m and from 790 to 840 m) was deposited under relatively constant environmental conditions at or slightly above storm wave base, in an open marine setting (close to the offshore-lower shoreface transition; Martin et al., in press). In contrast, most of the upper Fezouata Shale (670-790 m) and the overlying Zini Formation (840-860 m) both record a clear regressive trend and the transition towards much shallower, higher energy environmental conditions: upper shoreface and foreshore, respectively (Vidal, 1996;Destombes, 2006b;Martin et al., in press;Vaucher et al., in this issue). Sedimentologic investigations also show that all echinoderm assemblages from the Lower Ordovician of the Zagora area occur in storm-generated deposits. However, contrasted palaeoenvironmental conditions can be deduced from their preservation, which shows almost all intermediate taphonomic grades between Konservat-and Konzentrat-Lagerstätten (Vaucher et al., in this issue). Most echinoderm levels can be interpreted as Konservat-Lagerstätten resulting from the rapid smothering of live in situ communities (biocenoses) by distal storm deposits (e.g., stylophoran dense beds, Z-F0, Z-F5, Z-F26; Figs. 4A,C-F, 5-6, 8A-C). In contrast, some other echinoderm beds of the Fezouata Shale (e.g., 1737, Z-F12c; Figs. 4B,8D) and the diploporite horizon of the Zini Formation (Fig. 9B) more likely correspond to stormgenerated accumulations (thanatocenoses) of transported skeletal debris and/or live, fully articulated individuals (Konzentrat-Lagerstätten). The preservation of complete, very well preserved echinoderms (including type 1 forms, such as Rhopalocystis) in such deposits is not incompatible with their presumed downslope transport by high energy storm currents. Experimental taphonomic analyses on freshly killed individuals of modern type 1 echinoderms (e.g., ophiuroids) have shown that their skeleton could endure several hours of tumbling without any severe disarticulation (Kerr and Twitchett, 2004;Gorzelak and Salamon, 2013).

In the Lower Ordovician of the Zagora area, a coarse bathymetric zonation of echinoderm assemblages into two main categories (proximal vs. distal) can be deduced from their taphonomy and sedimentological data (Fig. 10). The Fezouata Shale yields a relatively continuous record of successive distal echinoderm assemblages from the late Tremadocian to the middle Floian. All these distal faunas are preserved in situ, in their original palaeoenvironmental context, at or slightly above storm wave base (uppermost offshore to lower shoreface). In the middle Tremadocian-middle Floian time interval, the occurrence of shallower echinoderm communities can be also documented, based on the random sampling provided by stormgenerated, downslope accumulations of transported material. In contrast, proximal echinoderm faunas can be observed in their original environmental context, in the Zini Formation (latest Floian).

No data are available on distal echinoderm assemblages in a large part of the upper Fezouata Shale (670-790 m; B. minutus Zone), which corresponds to environmental conditions intermediate between the more distal ones (uppermost offshore to lower shoreface) occurring, stratigraphically, both below (100-670 m) and above (790-840 m), and the shallower ones (foreshore) recorded in the Zini Formation (840-860 m; Vidal, 1996Vidal, , 1998;;Destombes, 2006b;Martin et al., in press;Vaucher et al., in this issue). In the Zagora area, echinoderms are virtually absent from late Floian deposits of the B. minutus Zone, although abundant and diverse benthic faunas were recovered from these levels (see above). This suggests the occurrence, at least during the late Floian time interval, of two distinct echinoderm assemblages (proximal and distal), well separated from each other by an intermediate zone, where they are rare or absent. A similar discontinuous distribution of echinoderms along a bathymetric profile was described in the Lower Ordovician of the Montagne Noire [START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF], and the Middle to Upper Ordovician of the Builth Inlier, Wales [START_REF] Botting | Echinoderm diversity and environmental distribution in the Ordovician of the Builth Inlier[END_REF]. As in the Fezouata Shale, the extreme rarity or absence of echinoderms at intermediate depths coincides in these two regions, with high diversity values for most other groups of benthic invertebrates (e.g., brachiopods, trilobites). Such an antagonistic pattern is apparently the rule for the distribution of most dense assemblages of Palaeozoic echinoderms (Lefebvre, 2007).

In the Lower Ordovician of the Zagora area, two successive echinoderm faunas can be documented in proximal settings (Fig. 10): Rhopalocystis-dominated (middle Tremadocian-middle Floian; Figs. 4B,8D) and diploporite-dominated assemblages (late Floian; Fig. 9B). Diploporites and glyptocystitids are both present, although much less diverse and rare in Rhopalocystis-dominated assemblages. All these proximal, dense populations of blastozoans probably formed extensive meadows in shallow environmental conditions (Ubaghs, 1963;Lefebvre and Fatka, 2003;Allaire et al., 2015). They are characterized by high number of individuals, but an extremely low taxonomic diversity. Similar eocrinoid-dominated meadows were reported in the Lower Ordovician of Montagne Noire, where they are also restricted to the shallowest part of the sequence (Foulon and Cluse de l'Orb formations; [START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF]Nardin, 2007b). Two low-diversity, successive, diploporite-dominated assemblages were also reported from shallow environmental conditions in the late Tremadocian of Bohemia (Třenice and Mílina formations; [START_REF] Mergl | Lower Ordovician cystoids (Rhombifera, Diploporita) from the Prague Basin (Czech Republic)[END_REF]. The low echinoderm diversity observed in all these proximal, high energy assemblages is probably original. This interpretation relies on the exquisite preservation of type 1 echinoderms (sensu Brett et al., 1997), such as eocrinoids and diploporites, in most of these shallow, storm-generated, high energy deposits.

In the Zagora area, the median (240-670 m) and uppermost part (790-840 m) of the Fezouata Shale provide a relatively complex record of successive echinoderm assemblages in distal settings, characterized by several repeated, rapid faunal turnovers involving two main distinct types of echinoderm communities (Fig. 10). The first one corresponds to blastozoan-dominated assemblages, associated with relatively abundant and diverse benthic faunas (e.g., articulate brachiopods, molluscs, trilobites). In the lower part of the A. murrayi Zone (240-260 m), such distal echinoderm communities are dominated by relatively large, polyplated morphotypes of Rhopalocystis (e.g., R. grandis, R. havliceki; Fig. 4A,C), sometimes associated with rare diploporites and glyptocystitids. Higher up in the section, the Oued Beni Zoli assemblage (275 m) corresponds to a Macrocystella-dominated community (Fig. 6A,D), associated with various other echinoderms (e.g., Balantiocystis, Plasiacystis, somasteroids and stylophorans; Fig. 6B,C). In the H. copiosus Zone and at the base of the overlying? C. protobalticus Zone (340-470 m), distal echinoderm communities are dominated by Plasiacystis (Fig. 7A-B), sometimes associated with crinoids (Fig. 7D), edrioasteroids (Argodiscus), eocrinoids (Balantiocystis) and mitrates (Balanocystites; Fig. 7C). In the earlymiddle Floian time interval (470-670 m), distal assemblages are dominated by the eocrinoid Balantiocystis regnelli (Fig. 8A), associated with several, locally abundant, other echinoderms (crinoids, edrioasteroids, glyptocystitids, Plasiacystis, somasteroids and stylophorans; Fig. 8B,C). Finally, a single late Floian locality (Z-F30m) has yielded a low diversity distal echinoderm assemblage dominated by a small, nail-shaped eocrinoid (Fig. 9A) associated with mitrocystitid mitrates (Azygograptus interval, 790-840 m).

However, in the upper part of the A. murrayi Zone (260-330 m), blastozoan-dominated faunas are repeatedly replaced by massive occurrences of low-diversity, stylophoran assemblages (Fig. 5). Stylophoran dense beds are typically dominated by one or two cornute taxa, and the associated fauna is generally rare and poorly diverse (e.g., few inarticulate brachiopods, rare trilobites and graptolites). The successive replacements of one community by the other one cannot be simply explained by variations of the relative depth, as initially suggested (Lefebvre, 2007;Noailles et al., 2010). Sedimentological data indicate that both types of echinoderm assemblages were associated with similar depth ranges at or slightly above storm wave base. Moreover, this pattern of repeated faunal shifts in the lower part of the Fezouata Shale is not only observed for echinoderms, but indeed for all invertebrate groups. The lower part of the A. murrayi Zone (240-260 m), the Oued Beni Zoli assemblage (275 m), the H. copiosus Zone to middle Floian time interval (330-670 m) and the uppermost part of the Fezouata Shale (790-840 m) have all yielded diverse, abundant benthic invertebrate faunas dominated by blastozoans, molluscs and trilobites, associated with articulate brachiopods, conulariids, hyolithids, but with few or no soft-bodied organisms typical of the Fezouata Biota. However, the occurrence of scarce, exceptionally preserved specimens of softbodied taxa (e.g., cheloniellid arthropods, demosponges) in some of these levels (e.g., Z-F0, Z-F25b) suggests that their rarity (or their absence) cannot be simply explained by a taphonomic bias. The diverse benthic communities of the median and upper parts of the Fezouata Shale are comparable to other fully marine, Early Ordovician faunas reported from similar high-latitude peri-Gondwanan areas, such as the Eastern Cordillera, Argentina (e.g., Waisfeld andSánchez, 1996, Aceñolaza andGutiérrez-Marco, 2002), the Montagne Noire, France (e.g., Thoral, 1935;Vizcaïno et al., 2001) or Shropshire, UK (e.g., [START_REF] Paul | Macrocystella Callaway, the earliest glyptocystitid cystoid[END_REF]Fortey and Owens, 1991).

In contrast, the upper part of the A. murrayi Zone (260-330 m) is largely unfossiliferous. With the exception of the Oued Beni Zoli assemblage (275 m; Fig. 6), all fossil-bearing levels correspond to patchy, laterally discontinuous horizons, each yielding a distinct, low diversity assemblage, generally dominated by one or two taxa represented by high numbers of exquisitely preserved individuals. In the lower part of the Fezouata Shale, these massive, low-diversity occurrences are generally dominated either by graptolites (e.g., Araneograptus murrayi; Martin et al., in press), various types of Cambrian-like panarthropods (e.g., anomalocaridids, marrellomorphs; Van Roy et al., 2010, 2015;Gaines et al., 2012), sponges (Botting, 2007, in this issue), or cornute stylophorans (Fig. 5A-C; Lefebvre, 2007;Lefebvre and Botting, 2007;Martin et al., 2015). Other relatively frequent faunal elements include inarticulate brachiopods, hyolithids, palaeoscolecid worms and trilobites (Van Roy et al., 2010, 2015;Martin et al., in press). Clearly, this pattern of patchy, dissimilar, lowdiversity assemblages in an otherwise almost azoic sequence instead suggests the record of successive, local colonizations of the sea floor by opportunistic organisms (r-strategy), than the frequently advocated occurrence of fully marine, "normal" communities (e.g., Van Roy et al., 2010, 2015). This interpretation is in good accordance with the presence of numerous stylophoran dense beds, which are generally interpreted as opportunistic colonizations of the sea floor by these echinoderms, during brief periods of environmental instability [START_REF] Alvaro | The Middle-Upper Cambrian of the southern Montagne Noire[END_REF]Lefebvre, 2007). Conversely, the diverse benthic communities occurring in the lower part of the A. murrayi Zone, at Oued Beni Zoli, in the H. copiosus Zone and in the Floian more likely represent 'normal' assemblages, adapted to long-term, relatively stable environmental conditions (K-strategy).

In the lower part of the Fezouata Shale, the transition from one type of community to the other one is relatively sharp, thus suggesting extremely rapid environmental changes. For example, the Oued Beni Zoli locality (275 m) represents a brief episode of colonization of the sea floor by diverse and abundant invertebrate benthic communities, stratigraphically bracketed by two low-diversity stylophoran dense beds: Z-F4 (about 2 m below) and Z-F3 (about 1 m above). These rapid faunal turnovers cannot be explained by sudden changes in the nature of the substrates. The lower part of the Fezouata Shale (100-670 m) is characterized by relatively homogenous lithologies dominated by argillites and fine sandstones. In this part of the succession, almost all echinoderm levels are occurring in fine sandstones, possibly as a result of their burial by distal storm deposits. One exception is the Z-F9( 2) assemblage (about 280 m above the base of the Ordovician), which is preserved in argillites and exhibits a slightly higher taxonomic diversity than most other stylophoran dense beds (Fig. 3). Moreover, the transition from the almost entirely azoic 240 first metres of the Fezouata Shale (no benthic fauna) and the overlying first fossiliferous levels (e.g., Z-F1, Z-F12c; Fig. 3) is not matched by any lithological change. In the Zagora area, the occurrence of a siliclasticdominated sedimentation during the whole Early Ordovician time interval probably explains the extreme rarity of taxa adapted to attachment on hard substrates (e.g. crinoids, edrioasteroids).

Dense populations of ophiuroids were identified by Lefebvre (2007) as a possible analogue of stylophoran dense beds. Since the advent of duraphagous predators in the late Jurassic, the distribution of widespread, low-diversity ophiuroid communities is restricted to welloxygenated, oligotrophic environments with a low predation pressure [START_REF] Fujita | Dense beds of ophiuroids from the Paleozoic to the Recent: the significance of bathyal populations[END_REF][START_REF] Aronson | Evolutionary paleoecology of dense ophiuroid populations[END_REF][START_REF] Emson | Making the best of it: the ecology of deep sea echinoderms[END_REF]. Although it remains extremely difficult to estimate it in the Fezouata Shale, the occurrence of palaeoscolecid worms in the same levels as stylophorans (e.g., in Z-F9(2); Martin et al., in this issue b) and/or of large, soft-bodied, putatively carnivorous, swimming arthropods (e.g., anomalocaridids; Van Roy and Briggs, 2011) in the upper part of the A. murrayi Zone suggests that predatory pressure was neither absent, nor the chief factor controlling the distribution of echinoderm dense beds. Consequently, the opportunistic colonization of the sea floor by widespread, shortlived echinoderm communities was interpreted by Lefebvre (2007) as resulting primarily from oligotropohic environmental conditions, unsuitable for most other groups of marine invertebrates (e.g., articulate brachiopods, molluscs). However, this interpretation is not confirmed by the occurrence of extremely abundant and diverse palynomorph assemblages (acritarchs, chitinozoans) in the whole Fezouata Shale, even in the 240 lower-most metres yielding almost no benthic fauna (Elaouad-Debbaj, 1988;Nowak et al., in this issue). The persistence of a relatively abundant microphytoplancton in the water column during the whole Tremadocian time interval suggests that the rapid, successive faunal turnovers observed for benthic communities of the Zagora area more probably result from variations in environmental conditions on the sea floor.

A relatively striking feature of most stylophoran dense beds occurring in the lower part of the Fezouata Shale is the extremely small size of all included echinoderms (e.g., cornutes, eocrinoids, mitrates), which rarely exceeds 5 to 10 mm. This observation is particularly true for most of the stratigraphically lower levels (e.g., Z-F3, Z-F4, Z-F7 and Z-F9(2); Fig. 3), which display some of the smallest echinoderm specimens observed in the whole Fezouata Shale. For example, the thecal width of the same Phyllocystis-like cornute (Cornuta n. gen. 1, n. sp.; Fig. 5B) does not exceed 8 to 10 mm in specimens from Z-F7 (A. murrayi Zone), whereas it can reach more than 16 mm in Z-F0 (?B. jacksoni Zone). In contrast, echinoderms occurring in the diverse assemblages of the lower part of the A. murrayi Zone (240-260 m), at Oued Beni Zoli (275 m), the H. copiosus,? C. protobalticus and? B. jacksoni zones (330-670 m) can display extremely large sizes. For example, in locality Z-F1 (250 m), the thecal length of most specimens of Rhopalocystis havliceki exceeds 120 mm (Fig. 4A,C; Allaire, 2014). Similarly, most blastozoans occurring at Oued Beni Zoli (Fig. 6) or in younger, high-diversity assemblages (e.g., Balantiocystis regnelli, Plasiacystis mobilis; Figs. 7A,B,8A) are characterized by large body sizes, with thecal lengths frequently exceeding 50 mm.

A first possible explanation for these dramatic differences in size between specimens from low-and high-diversity assemblages is that communities occurring in dense beds could correspond to short-lived populations, consisting mostly of juveniles. Conversely, the larger sizes observed in the high diversity assemblages would then simply result from the presence of older (adult?) individuals, which could reach their optimum size because of the persistence of more stable environmental conditions. However, Lefebvre and Botting (2007) showed that the small-sized specimens of the mitrate Peltocystis cornuta from localities Z-F2(1) and Z-F4 (A. murrayi Zone; Fig. 3) were not juveniles, because their theca showed the typical broad morphology of adult individuals, clearly distinct from the more elongate one of juveniles. Moreover, the occurrence of small-sized specimens in low-diversity assemblages of the upper part of the A. murrayi Zone is not unique to echinoderms, but is indeed observed for most other benthic organisms (e.g., molluscs, trilobites). For example, local dense accumulations of the gastropod Carcassonnella sp. in Z-F2 consist of hundreds of tiny specimens (less than 2 mm wide), whereas individuals of the same species are much larger (4 to 5 mm) in both early (e.g., Z-F26) and middle Floian (e.g., Z-F0) levels (Ebbestad, in this issue). As for the mitrate Peltocystis cornuta, small-sized trilobites from these levels (e.g., Anacheirurus and Bavarilla in Z-F4) do not display juvenile morphologies, but correspond to small-sized adults (E. Martin, pers. comm., 2015).

The occurrence of "dwarfed" echinoderm faunas is not unique to the lower part of the Fezouata Shale. Similar small-sized assemblages occur in the Landeyran Formation (late Floian) of Montagne Noire [START_REF] Ubaghs | Deux Stylophora (Homalozoa, Echinodermata) nouveaux pour l'Ordovicien inférieur de la Montagne Noire (France méridionale)[END_REF] and the Llanfawr Mudstones Formation (latest Darriwilianearly Sandbian) of Wales [START_REF] Botting | Echinoderm diversity and environmental distribution in the Ordovician of the Builth Inlier[END_REF]. In both modern and fossil communities, the occurrence of dwarfed populations is usually associated with unhospitable, stressful environmental conditions, such as oligotrophic waters, low oxygen levels, and/or poisoning with sulfur compound [START_REF] Tasch | Causes and paleontological significance of dwarfed fossil marine invertebrates[END_REF][START_REF] Mcroberts | Rhaetian (Late Triassic) Monotis (Bivalvia: Pectinoidea) from the eastern Northern Calcareous Alps (Austria) and the end-Norian crisis in pelagic faunas[END_REF][START_REF] Urlichs | Stunting in some invertebrates from the Cassian Formation (Late Triassic, Carnian) of the Dolomites (Italy)[END_REF][START_REF] Botting | Echinoderm diversity and environmental distribution in the Ordovician of the Builth Inlier[END_REF]. As argued above, there is no evidence supporting any decrease in microphytoplancton productivity during the A. murrayi Zone in the Zagora area (Elaouad-Debbaj, 1988;Nowak et al., in this issue). In the lower part of the Fezouata Shale, most levels yielding low-diversity, dwarfed assemblages are characterized by an extremely reduced bioturbation, consisting of few, shallow, narrow burrows, more or less parallel to the bedding (Van Roy et al., 2010). This low intensity, non-penetrative bioturbation strongly suggests the occurrence of unhospitable, very likely dysoxic conditions at the water-sediment interface. This interpretation is in good accordance with preliminary geochemical data (Martin et al., in this issue a) and the occurrence of the mitrate Lagynocystis pyramidalis in several stylophoran dense beds in the lower part of the Fezouata Shale (e.g., Z-F3, Z-F4; Fig. 3,Appendix 3). In the Ordovician, the distribution of this mitrate was restricted to dysoxic environments [START_REF] Henry | Stratification thermique probable des eaux marines sur la marge nord-gondwanienne (Massif armoricain) pendant l'Ordovicien (Llanvirn): implications paléogéographiques[END_REF][START_REF] Parsley | Morphological and palaeoecological analysis of the Ordovician ankyroid Lagynocystis (Stylophora: Echinodermata)[END_REF]Lefebvre, 2007;Lefebvre et al., 2010). Finally, the high pyrite content of the sediment in the upper part of the A. murrayi Zone could also question the possibility that the reduced size of invertebrates results from their poisoning with sulfur compound. However, this seems improbable, as the origin of the pyrite is secondary (early diagenesis) and results from the decay of the organic matter originally contained in the sediment.

At a larger scale, the Lower Ordovician succession in Central Anti-Atlas apparently records the progressive transition from mostly anoxic sea-bottoms (0-240 m, early to middle Tremadocian) to more dysoxic conditions, including some well-oxygenated intervals (240-330 m, A. murrayi Zone), and finally, the definitive occurrence of more hospitable, well-oxygenated sea-floors (330-880 m, H. copiosus Zone and Floian). This long-term evolution of environmental conditions at the water-sediment interface probably results from the progressive, structurally controlled, widening of the Anti-Atlas intracratonic basin and the accompanying transition from a stratified to a mixed water column.

In summary, the spatial distribution of Early Ordovician echinoderm assemblages from the Zagora area and the main environmental factors controlling it can be identified. In proximal, high energy settings, low-diversity blastozoan meadows were dominated successively by eocrinoids (Rhopalocystis) and diploporites. In more distal environments, two distinct assemblages can be identified. The first one corresponds to diverse echinoderm communities dominated successively by Rhopalocystis, Macrocystella, Plasiacystis, Balantiocystis and small, nail-shaped eocrinoids. They were associated with well-oxygenated environmental conditions and diverse, abundant, benthic invertebrates typical of Early Ordovician, high-latitude, peri-Gondwanan faunas. The second assemblage corresponds to low-diversity, dwarfed stylophoran-dominated communities. They were restricted to opportunistic, short-lived, massive occurrences in dysoxic environmental conditions. They were associated with other low-diversity benthic communities dominated by Cambrian-like taxa (e.g., anomalocaridids, demosponges, marrellomorphs). This suggests that the occurrence of Burgess Shale-like taxa in the lower part of the Fezouata Shale is not typical of "normal" environments (Van Roy et al., 2010, 2015), but rather restricted to stressed, dysoxic conditions, unhospitable for most taxa typical of the GOBE, such as articulate brachiopods, bivalves, crinoids, and gastropods.

Palaeobiogeographic affinities

Since the first description of echinoderm faunas in the Lower Ordovician of the Anti-Atlas, their palaeobiogeographic affinities have been regularly discussed (Choubert et al., 1953;Chauvel, 1966aChauvel, , 1966bChauvel, , 1971aChauvel, , 1978;;Donovan and Savill, 1988;[START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF]Lefebvre and Fatka, 2003;Lefebvre, 2007;Lefebvre and Botting, 2007;Sumrall and Zamora, 2011;[START_REF] Lefebvre | This larger amount of available data provides an unprecedented opportunity to better identify if similarities between echinoderm faunas simply reflect comparable palaeoenvironmental conditions and/or if they also support palaeobiogeographic affinities. As a first consequence of this improved knowledge of late Cambrian-Early Ordovician faunas, less than 12% of echinoderms from the Fezouata Shale can be identified as possibly endemic taxa (instead of about 50% in the early 2000s). They include the eocrinoids Rhopalocystis destombesi[END_REF]. Broad similarities with other high-latitude peri-Gondwanan regions were noted, in particular with the diverse and well-known Ordovician faunas from Bohemia (Czech Republic; [START_REF] Barrande | Système Silurien du Centre de la Bohême: Volume VII. Classe des Echinodermes[END_REF] and Montagne Noire (France; Thoral, 1935). However, faunal similarities of Fezouata echinoderm assemblages with other peri-Gondwanan faunas more often reflect comparable environmental conditions, rather than any close palaeogeographic distribution. For example, the occurrence of relatively comparable diploporite-dominated assemblages in the Zini Formation (late Floian, Anti-Atlas) and both the Třenice and Mílina formations (late Tremadocian, Bohemia; [START_REF] Mergl | Lower Ordovician cystoids (Rhombifera, Diploporita) from the Prague Basin (Czech Republic)[END_REF] is clearly related to the existence of similar proximal, high energy conditions in these two regions. This interpretation is confirmed by the strong similarities between Early Ordovician faunas from the Anti-Atlas (e.g., Argodiscus, Balanocystites, Hanusia, Lagynocystis, Prokopicystis) and their younger counterparts from the Šárka and Dobrotivá formations (Darriwilian, Bohemia), occurring in relatively similar dysoxic settings [START_REF] Henry | Stratification thermique probable des eaux marines sur la marge nord-gondwanienne (Massif armoricain) pendant l'Ordovicien (Llanvirn): implications paléogéographiques[END_REF][START_REF] Parsley | Morphological and palaeoecological analysis of the Ordovician ankyroid Lagynocystis (Stylophora: Echinodermata)[END_REF]. A very similar stylophoran assemblage was also reported from the dysoxic black shales of the Pontyfenni Formation (Dapingian, Wales; [START_REF] Jefferies | The chordatesa preliminary note[END_REF]Lefebvre, 2007). The striking faunal similarities (over 66% of common genera, and several common species) between echinoderm faunas from the Fezouata Shale (Anti-Atlas) and the Saint-Chinian Formation (Montagne Noire) probably result both from their similar, late Tremadocian age and their occurrences in relatively comparable, relatively shallow environmental conditions at or slightly above storm-wave base [START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF]. Similarly, the dwarfed echinoderm faunas from the Landeyran Formation (late Floian, Montagne Noire) have over 70% of common genera with the slightly older assemblages from the median part of the Fezouata Shale (late Tremadocian to middle Floian).

The frequently advocated palaeobiogeographic affinities of Early Ordovician echinoderm assemblages from the Anti-Atlas with other high-latitude peri-Gondwanan areas (e.g., Bohemia, Montagne Noire) are not only the result of relatively similar environmental conditions (e.g., siliciclastic substrates), but they are also largely the consequence of a strong sampling bias [START_REF] Lefebvre | This larger amount of available data provides an unprecedented opportunity to better identify if similarities between echinoderm faunas simply reflect comparable palaeoenvironmental conditions and/or if they also support palaeobiogeographic affinities. As a first consequence of this improved knowledge of late Cambrian-Early Ordovician faunas, less than 12% of echinoderms from the Fezouata Shale can be identified as possibly endemic taxa (instead of about 50% in the early 2000s). They include the eocrinoids Rhopalocystis destombesi[END_REF]. Until the 1990s, abundant and diverse Early Ordovician echinoderm faunas were only known from the Montagne Noire (Thoral, 1935;Spencer, 1951;[START_REF] Ubaghs | Le genre Lingulocystis Thoral (Echinodermata, Eocrinoidea) avec des remarques critiques sur la position systématique du genre Rhipidocystis Jaekel[END_REF]Ubaghs, , 1969[START_REF] Ubaghs | Les Echinodermes "Carpoïdes" de l'Ordovicien inférieur de la Montagne Noire (France)[END_REF][START_REF] Ubaghs | Le genre Balantiocystis Chauvel (Echinodermata, Eocrinoidea) dans l'Ordovicien inférieur de la Montagne Noire (France)[END_REF]Ubaghs, , 1983[START_REF] Ubaghs | Deux Stylophora (Homalozoa, Echinodermata) nouveaux pour l'Ordovicien inférieur de la Montagne Noire (France méridionale)[END_REF][START_REF] Ubaghs | Echinodermes nouveaux (Stylophora, Eocrinoidea) de l'Ordovicien inférieur de la Montagne Noire (France)[END_REF]Fell, 1963; Vizcaïno cosmopolitan, low-diversity communities including representatives of five classes (Edrioasteroidea, Eocrinoidea, Rhombifera, Soluta, Stylophora).

The fossil record indicates that in both Laurentia and peri-Gondwanan areas, the transition from late Cambrian-like echinoderm faunas to the more diverse, Ordovician-like assemblages occurred almost synchronously, in middle to late Tremadocian times. This sharp transition can be documented in Montagne Noire, which is one of the few regions showing a relatively continuous sedimentation from the early Cambrian to the end of the Lower Ordovician (Thoral, 1935;[START_REF] Capera | Biostratigraphie de l'Ordovicien inférieur de la Montagne Noire (France méridionale). Trémadocien inférieur[END_REF][START_REF] Ubaghs | Echinodermes nouveaux du Cambrien supérieur de la Montagne Noire (France méridionale)[END_REF][START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF][START_REF] Alvaro | The Middle-Upper Cambrian of the southern Montagne Noire[END_REF]. In Montagne Noire, the low diversity echinoderm assemblages of the Val d'Homs (Furongian: edrioasteroids, glyptocystitids and stylophorans) and la Dentelle formations (middle Tremadocian: glyptocystitids) are abruptly replaced by the diverse faunas of the Saint-Chinian Formation (late Tremadocian: crinoids, edrioasteroids, eocrinoids, glyptocystitids, ophiuroids, solutans, somasteroids, stylophorans). Such a transition cannot be documented in the central Anti-Atlas, because of the absence of late Cambrian deposits in the Zagora area and the occurrence of unhospitable environmental conditions (anoxic sea-bottoms) during the early Tremadocian. Consequently, the Fezouata Shale only records the initial stages of the late Tremadocian diversification.

However, contrary to the situation in the late Tremadocian of the Montagne Noire, the repeated ocurrence of unhospitable, probably dysoxic environmental conditions in the lower part of the Fezouata Shale provides a relatively complex pattern of successive echinoderm assemblages in the Zagora area. In the Anti-Atlas, the diverse benthic assemblages of invertebrates, typical of the late Tremadocian diversification are thus regularly replaced by low-diversity, opportunistic, cosmopolitan communities of Cambrian-like organisms during most of the A. murrayi Zone. In the Zagora area, the resulting evolutionary pattern is thus slightly delayed, compared to the situation in the Montagne Noire, with the persistence of a relatively high proportion of Cambrian-like taxa during most of the A. murrayi Zone. In the central Anti-Atlas area, typical diverse benthic communities dominated by post-Cambrian taxa are definitively established in the latest Tremadocian-early Floian time interval.

Although all Early Ordovician echinoderm faunas diversified from more or less the same ancestral, cosmopolitan stock (including late Cambrian taxa, and possibly asterozoans and crinoids as well), they rapidily gave rise to distinct, endemic assemblages in both Laurentia and peri-Gondwanan areas (Lefebvre and Fatka, 2003;Sprinkle and Guensburg, 2004;[START_REF] Lefebvre | This larger amount of available data provides an unprecedented opportunity to better identify if similarities between echinoderm faunas simply reflect comparable palaeoenvironmental conditions and/or if they also support palaeobiogeographic affinities. As a first consequence of this improved knowledge of late Cambrian-Early Ordovician faunas, less than 12% of echinoderms from the Fezouata Shale can be identified as possibly endemic taxa (instead of about 50% in the early 2000s). They include the eocrinoids Rhopalocystis destombesi[END_REF]. Environmental conditions, and in particular the nature of the substrates, very likely played a key role in this large scale evolutionary pattern [START_REF] Guensburg | Rise of echinoderms in the Paleozoic evolutionary fauna: significance of paleoenvironmental controls[END_REF][START_REF] Sprinkle | Origin of echinoderms in the Paleozoic evolutionary fauna: the role of substrates[END_REF]. In the shallow, tropical seas of Laurentia, both crinoids and other attached echinoderms (e.g., edrioasteroids, paracrinoids) underwent a major diversification during the Floian. This radiation was largely driven by a high carbonate productivity and the occurrence of widespread hard and firmground surfaces (e.g., sponge-algal mounds), suitable for their attachment. No comparable diversification of crinoids is observed in the Floian of high-latitude, peri-Gondwanan regions (e.g., Anti-Atlas, Montagne Noire). This very likely results from the absence of carbonate productivity and suitable, hard substrates in these areas during most of the Ordovician (Lefebvre and Fatka, 2003;[START_REF] Lefebvre | This larger amount of available data provides an unprecedented opportunity to better identify if similarities between echinoderm faunas simply reflect comparable palaeoenvironmental conditions and/or if they also support palaeobiogeographic affinities. As a first consequence of this improved knowledge of late Cambrian-Early Ordovician faunas, less than 12% of echinoderms from the Fezouata Shale can be identified as possibly endemic taxa (instead of about 50% in the early 2000s). They include the eocrinoids Rhopalocystis destombesi[END_REF]. Echinoderm faunas from the Anti-Atlas and the Montagne Noire both document the initial diversification of endemic peri-Gondwanan Ordovician assemblages (diploporites, eocrinoids, solutans, somasteroids and mitrate stylophorans) adapted to siliciclastic, soft substrates. The Early Ordovician echinoderm faunas of the Anti-Atlas also provide the rare opportunity to document the initial diversification of aristocystitid diploporites. The occurrence of diploporite beds in the late Floian of the Zagora area echoes the dominance of this group in younger Ordovician peri-Gondwanan echinoderm faunas. Préservation exceptionnelle de parties molles chez des échinodermes stylophores de l'Ordovicien inférieur de la région de Zagora (Anti-Atlas central, Maroc)

Bertrand LEFEBVRE Les stylophores (Cambrien moyen -Carbonifère supérieur) sont des fossiles énigmatiques, de taille centimétrique, constitués d'un appendice articulé, à symétrie bilatérale (l'aulacophore) et d'un corps aplati et fondamentalement asymétrique (la thèque). Bien que les éléments squelettiques constitutifs de ces deux régions soient calcitiques et présentent la microstructure caractéristique des échinodermes (le stéréome), la position phylogénétique des stylophores au sein des deutérostomes est fortement débattue depuis plus d'un siècle. La véritable nature de l'aulacophore est au coeur de la controverse et à l'origine des différentes interprétations proposées pour les stylophores : (1) cystoïdes aplatis pourvus d'une tige ; (2) chordés primitifs (calcichordés) possédant une queue pourvue d'une notochorde ; (3) échinodermes dérivés munis d'un bras nourricier ; et enfin (4) membres du groupe-souche des échinodermes ayant conservé un pédoncule comparable à celui des hémichordés. La campagne de fouilles paléontologiques menée en janvier et février 2014 dans l'Ordovicien inférieur de la région de Zagora (Anti-Atlas central, Maroc) a permis d'exploiter et de dégager sur de grandes surfaces plusieurs horizons fossilifères qui ont livré des organismes à préservation exceptionnelle (par exemple : aglaspidides, anomalocaridides, cirripèdes, marrellomorphes, trilobites pourvus de leurs appendices, vers palaeoscolécides). Ces niveaux ont également fourni de très nombreux restes de stylophores : principalement des cornutes (Hanusia, Thoralicystis) et des mitrates (Anatifopsis). Des parties molles ont été préservées chez plusieurs individus appartenant au genre Hanusia, dans les régions distale (pieds ambulacraires) et proximale (oesophage) de l'aulacophore. Ces structures, mises en évidence pour la première fois chez des stylophores, apportent des arguments décisifs qui permettent de valider leur interprétation en tant qu'échinodermes dérivés pourvus d'un bras nourricier.

The Cambrian-Ordovician time interval (ca. 541-443 Ma) represents a pivotal period in the history of the biosphere, with the relatively sudden appearance of unprecedentedly complex anatomies and functionalities associated with dramatic and irreversible changes in marine ecosystems, such as trophic escalation, the Cambrian agronomic revolution, and the Ordovician planktonic revolution. The initial stages of the early Palaeozoic diversification of metazoans (the so-called ''Cambrian explosion'') are particularly well documented based on a large number of Konservat-Lagerstätten; for example, the Burgess Shale (Canada; Briggs et al. 1994), Chengjiang and Kaili (China; Hou et al. 2004;[START_REF] Lin | Bioturbation in Burgess Shale-type Lagerstätten-Case study of trace fossil-body fossil association from the Kaili Biota (Cambrian Series 3), Guizhou, China[END_REF], the Emu Bay Shale (Australia; Paterson et al. 2008), and Sirius Passet (Greenland; Conway [START_REF] Morris | A Burgess Shalelike fauna from the Lower Cambrian of North Greenland[END_REF]). These localities have yielded not only well-preserved elements typical of the Cambrian evolutionary fauna (e.g., eocrinoids, hyolithids, inarticulate brachiopods, and trilobites), but also abundant remains of nonbiomineralized organisms, unusually preserved in the fossil record (e.g., a huge diversity of arthropods, various kinds of worm, and primitive chordates).

The Great Ordovician Biodiversification Event (GOBE) represents the second major burst in marine biodiversity during early Palaeozoic times. It was characterized by a sustained, almost exponential diversification rate during most of the Ordovician Period and the replacement of the Cambrian evolutionary fauna by the Palaeozoic evolutionary fauna (e.g., articulate brachiopods, crinoids, graptolites, and nautiloid cephalopods), which dominated marine ecosystems until the end-Permian mass extinction (Harper 2006;Servais et al. 2008Servais et al. , 2010)). However, in contrast to the Cambrian explosion, the GOBE is almost exclusively documented based on the shelly fossil record. Few Ordovician Konservat-Lagerstätten with softtissue preservation have been described thus far, and most of them have yielded low-diversity assemblages, associated with restricted marine environments; for example, Beecher's Trilobite Bed and Winneshiek (USA; Liu et al. 2006;Farrell et al. 2009), Llanfawr (UK; Botting et al. 2011), Manitoba (Canada;Young et al. 2007), and the Soom Shale (South Africa; Aldridge et al. 2007).

A notable exception is the Fezouata Biota from the Outer Feijas Group of the central Anti-Atlas, southeastern Morocco (Van Roy et al. 2010). In the Ternata Plain, north of Zagora, the Lower Ordovician succession lies unconformably over the middle Cambrian sandstones of the Tabanite Group, and consists of an extremely thick series of more or less silty and micaceous argillites (ca. 700 m; Lower and Upper Fezouata formations) overlain by the sandstones of the Zini Formation (Destombes 1962(Destombes , 1971;;Destombes et al. 1985). Since the early 2000s, several fossiliferous horizons within the Lower Fezouata Formation (Tremadocian) and the conformably overlying Upper Fezouata Formation (Floian) have yielded extremely abundant and diverse remains of fully marine assemblages. The Fezouata assemblages comprise numerous exquisitely preserved remains of shelly taxa typical of post-Cambrian faunas, including articulate brachiopods, asterozoans, bivalves, cornute and mitrate stylophorans, crinoids, edrioasteroids, gastropods, glyptocystitid rhombiferans, graptolites, hyolithids, machaeridians, nautiloid cephalopods, ostracods, and trilobites (Destombes et al. 1985;Vidal 1998;Vinther et al. 2008;Noailles et al. 2010). However, the Fezouata Biota also contains abundant and diverse remains of nonbiomineralized organisms. Exceptionally well preserved soft-bodied taxa comprise several representatives of the Palaeozoic evolutionary fauna (most of them previously known only from younger deposits), for example, aglaspidid and cheloniellid arthropods, cirripedian crustaceans, and xiphosurans (Van Roy 2006;Van Roy et al. 2010). More strikingly, the Fezouata Biota also contains extremely numerous and diverse nonbiomineralized taxa typical of early to middle Cambrian Burgess-Shale-type assemblages; for example, anomalocaridids, demosponges, halkieriids, marrellomorphs, naraoids, palaeoscolecids, and wapkiids (Van Roy 2006;Botting 2007;Van Roy et al. 2010;Van Roy and Briggs 2011). Consequently, the Fezouata Biota offers a unique opportunity to compare similarly preserved early to middle Cambrian assemblages with Early Ordovician ones. The co-occurrence of Burgess-Shale-like taxa along with typical post-Cambrian organisms is suggestive of a relatively smooth transition between the Cambrian and Palaeozoic evolutionary faunas.

However, most recent studies focusing on the Fezouata Biota relied mainly on materials acquired from local collectors, without precise knowledge of their exact stratigraphic positions. In the last two years, under the auspices of the ANR project RALI (2012RALI ( -2015) ) and the French-Moroccan CNRS-CNRST (SDU 05/09 and SDU 02/13) project VALORIZ (2013VALORIZ ( -2015)), two field campaigns logged in detail the whole Lower Fezouata Formation, as well as the lower half of the Upper Fezouata Formation, and identified the precise stratigraphic positions of all the main fossiliferous levels yielding soft-bodied remains. The occurrence of mm-to cm-thick silty or sandy layers displaying storm-wave influence (wave ripples) points to relatively shallow offshore conditions for the deposition of the Fezouata argillites. The new data obtained in the field provide essential information on the evolution of successive assemblages through time, as well as on their associated palaeoenvironmental conditions.

Introduction

The facies model for wave-and storm-dominated depositional environment is well established in the literature for both the modern and the fossil records (e.g., [START_REF] Davies | A morphogenic approach to world shorelines[END_REF]Bluck, 1967;Goldring & Bridges, 1973;Harms, 1979;Heward, 1981;Brenchley & Newall, 1982;Dott & Bourgeois, 1982;Davis Jr & Hayes, 1984;[START_REF] Davis | Beach and nearshore zone[END_REF][START_REF] Short | Rip-current type, spacing and persistence, Narrabeen Beach, Australia[END_REF][START_REF] Brenchley | Proximal and distal hummocky cross-stratified facies on a wide Ordovician shelf in Iberia. Shelf Sands and Sandstones[END_REF]Boyd et al., 1992;Dumas & Arnott, 2006;Plint, 2010), providing a predictable zonation of the sedimentary structures created by the collapse of wave orbitals on the sea-floor (Allen, 1997). However, wave-and storm-dominated environment represents one end members of the three processes (river, tides, waves) that influence coastal environments. Mixed coastal depositional system is generated when the dominant controlling process is overlapped by one of the two others (Boyd et al., 1992;[START_REF] Dalrymple | Tidal depositional systems[END_REF]. Recent studies have discussed the sedimentary organization of modern coastal systems under the influence of waves and tides (e.g. [START_REF] Li | Open-coast intertidal deposits and the preservation potential of individual laminae: a case study from east-central China[END_REF][START_REF] Yang | A seasonal model of surface sedimentation on the Baeksu open-coast intertidal flat, southwestern coast of Korea[END_REF][START_REF] Dalrymple | Sedimentology and stratigraphy of a tide-dominated, foreland-basin delta (Fly River, Papua New Guinea)[END_REF][START_REF] Yang | Sedimentation on a wave-dominated, opencoast tidal flat, south-western Korea: summer tidal flat -winter shoreface[END_REF]Yang et al., , 2006;;[START_REF] Dashtgard | Tidally modulated shorefaces[END_REF][START_REF] Dashtgard | Tidal effects on the shoreface: Towards a conceptual framework[END_REF]. Similar systems certainly existed in the past but only few of them were described so far in ancient sedimentary successions (e.g. [START_REF] Plink-Björklund | Wave-to-tide process change in a Campanian Shoreline Complex, Chimney Rock Tongue, Wyoming/Utah[END_REF]Billeaud et al., 2009;[START_REF] Basilici | A depositional model for a wavedominated open-coast tidal flat, based on analyses of the Cambrian-Ordovician Lagarto and Palmares formations, north-eastern Brazil[END_REF][START_REF] Basilici | Hummocky cross-stratification-like structures and combined-flow ripples in the Punta Negra Formation (Lower-Middle Devonian, Argentine Precordillera): A turbiditic deep-water or storm-dominated prodelta inner-shelf system?[END_REF][START_REF] Vakarelov | Recognition of wavedominated, tide-influenced shoreline systems in the rock record: Variations from a microtidal shoreline model[END_REF]Rossi & Steel, 2015). The studies on modern and fossil coastal environments focused on the shallowest parts of these mixed systems, namely on the intertidal and shallow subtidal zones. The continuation of these systems toward the offshore is not constrained, and very few models exist that integrate such sedimentary context from the coastline to the offshore have been proposed.

In sedimentary systems dominated by waves and storms, tidal influence is generally obliterated, and tide-related sedimentary structures have a much lower preservation potential than wave-related structures [START_REF] Li | Open-coast intertidal deposits and the preservation potential of individual laminae: a case study from east-central China[END_REF]. Consequently, evidencing the influence of tides in a wave-dominated sedimentary environment by the identification of unambiguous tidal structures can be difficult or unfeasible. This difficulty probably explains why these types of environments have not been often recognized in ancient sedimentary successions [START_REF] Fan | Influences of storm erosion and deposition on rhythmites of the upper Wenchang Formation (Upper Ordovician) around Tonglu, Zhejiang Province, China[END_REF][START_REF] Yang | Sedimentation on a wave-dominated, opencoast tidal flat, south-western Korea: summer tidal flat -winter shoreface[END_REF]. However, one study led by [START_REF] Vakarelov | Recognition of wavedominated, tide-influenced shoreline systems in the rock record: Variations from a microtidal shoreline model[END_REF] identified one example of a hybrid system in the Campanian recording both wave and tide influence on the sedimentation. This identification was based on the observation of sedimentary features generated by both waves and tides [START_REF] Vakarelov | Recognition of wavedominated, tide-influenced shoreline systems in the rock record: Variations from a microtidal shoreline model[END_REF]. An alternative approach for recognizing tide influence (when no tide generated structures are present) is to understand how changes in sea level during tide cycles can modify both the sedimentary structures formed by waves and storms and the general sedimentary architecture of these peculiar wavedominated systems (e.g. [START_REF] Dashtgard | Tidal effects on the shoreface: Towards a conceptual framework[END_REF]. of the Cadi Ayyad University, Marrakech), probably belonging to the genus Furca, typical of the middle Cambrian, but discovered in the Lower Ordovician excavation site (Martin et al., in press).

The Lower Ordovician succession of Morocco (Fezouata and Zini formations; central Anti-Atlas; Zagora area; Fig. 1A) was deposited in a wave-dominated sedimentary system (Martin et al., in press) that however exhibits uncommon sedimentary features and geometrical organization when compared to the well-established facies model for wave-dominated coastal environments. The present study shows that tide modulation strongly impacts the organization of wave-dominated deposits and explains the uncommon sedimentary features and architecture observed in the Lower Ordovician succession of the central Anti-Atlas. A facies model for this type of mixed system is proposed, which helps to improve the facies model researches on this topic by giving new keys of recognitions.

The Lower Ordovician of the Anti-Atlas is also of prime importance for the understanding of the rise of animal life (e.g. Van Roy et al., 2010, 2015;Martin et al., 2014, in press) as the Fezouata Formation contains one of the few Lagerstätten (Fig. 1B) recording the initial stages of the Great Ordovician Biodiversification Event (e.g., Bambach et al., 2004;Servais et al., 2008Servais et al., , 2010)). A better constrain of the sedimentary dynamics involved in the exceptional preservation of soft tissues is also briefly discussed in the present work.

Geological setting

At the end of the Precambrian, the Pan-African tectonic event formed the super-continent Gondwana. This super-continent extended from the Equator to the South Pole (Fig. 2A).

During the Cambrian Period the mean sea level rose related to the oceanic expansion and resulted in the development of shallow epicontinental seas (Destombes et al., 1985;Fabre, 1988;Boote et al., 1998;Carr 2002;Coward & Ries, 2003). At beginning of the Ordovician, a long-term transgression took place (e.g. Meyers & Peters, 2011). In Early Ordovician, a rifting phase took place resulting in the separation of Avalonia from Gondwana (Fig. 2A).

The entire Lower Ordovician sedimentary succession of Morocco (Anti-Atlas; Fig. 2B) was deposited on a passive margin (Cocks & Torsvik, 2004).

Our study focuses on the Fezouata and Zini formations in the central-eastern part of the Anti-Atlas, in the area of Zagora, southeastern Morocco (Fig. 1A). In this area, the Lower Ordovician (Tremadocian and Floian) deposits are laying unconformably on the Guzhangian sandstones (Cambrian series 3; middle Cambrian; Fig. 2C) of the Tabanite Group (Geyer & Landing, 2006). The two studied formations (Fezouata and Zini) belong to the Outer Feijas Group (Fig. 2C) (Choubert, 1942), which is composed of the Lower Ordovician Fezouata and Zini formations, and of the early Middle Ordovician Tachilla Formation. [START_REF] Veevers | Edge tectonics (trench rollback, terrane export) of Gondwanaland-Pangea synchronized by supercontinental heat[END_REF]. Note that the study area (red star) was located in an epicontinental sea.

(B) Simplified geological map of the Anti-Atlas, after Hollard & Choubert (1985). (C) Lithostratigraphic column of the Cambro-Ordovician succession in the Zagora area, modified after Destombes et al. (1985); Ordovician sequences of [START_REF] Gasquet | Carte géologique du Maroc au 1/50 000. Feuille De Tamazrar[END_REF] are also shown; the studied interval (in pink) is bounded by two major 2 nd -order discontinuities. Destombes (1962) was the first to describe the stratigraphic units of the Ordovician series in the central Anti-Atlas. The lower part of the Fezouata Formation (Tremadocian in age; Fig. 2C and3) consists of quartz-rich siltstones. Glauconite is an important component of the sediment. The upper part of the Fezouata Formation (Floian in age) is locally based by a horizon rich in ferruginous ooids or glauconite, and is composed of siltstones that gradually alternate with more frequent sandstones in the uppermost part of the Fezouata Formation below the Zini Formation (Fig. 2C and3). The Fezouata Formation is locally overlain by massive black sandy beds characteristic of the Zini Formation (late Floian). In some localities, a layer containing ferruginous ooids is present in the uppermost sandstone bed constituting the Zini Formation (Destombes, 1962). In the Zagora area, the thickness of the Zini Formation does not exceed 10 m. Both the Fezouata and the Zini formations were deposited in an epicontinental sea (Fig. 2A) at a high palaeolatitude (ca 60°S) at the periphery of the Gondwana [START_REF] Smith | Estimates of the Earth's spin (geographic) axis relative to Gondwana from glacial sediments and paleomagnetism[END_REF]Cocks & Torsvik, 2004;[START_REF] Veevers | Edge tectonics (trench rollback, terrane export) of Gondwanaland-Pangea synchronized by supercontinental heat[END_REF]Torsvik & Cocks, 2013).

Sediments are mainly composed of quartz and muscovite. Pyrite and iron oxides are also present. U-Pb analyses of detrital zircons from the Cambro-Ordovician successions of Algeria and Israel [START_REF] Williams | Large-scale sediment dispersal associated with the late Neoproterozoic assembly of Gondwana[END_REF][START_REF] Avigad | Origin of northern Gondwana Cambrian sandstone revealed by detrital zircon SHRIMP dating[END_REF] indicate a Neoproterozoic age for the source of the sediments, thus pointing to pan-african terranes.

Materials and methods

All the accessible and well-outcropping Lower Ordovician sections in the surroundings of Zagora were analyzed. Ten stratigraphic sections (red crosses, Fig. 1A), ca 50 to 200 m thick, of the upper part of the Fezouata and the Zini formations were logged at decimeter scale and analyzed for their facies. This part of the Lower Ordovician succession exhibits the highest facies variability, from facies typical of the Fezouata Formation (argillites and siltstones) to sandstone facies, more typical of the Zini Formation. One longer (ca 900 m thick) stratigraphic log, less detailed, was built from composite sections representing the complete Lower Ordovician sedimentary succession (Fig. 3). The combined analysis of lithology, grain size, sedimentary structures (and their wavelength; Fig. 4) and depositional geometries made it possible to describe 14 different facies (Table 1 ;Figs. 5,7 and 8). excavation site is shown. The colors used on the logged section approximate the colors of the rocks observed in the field.

These facies were then grouped in four facies zones corresponding to distinct depositional environments. A facies model for the deposition of the Lower Ordovician sedimentary succession is then proposed that takes into account the facies succession in the studied sections in application of the Walther's law (1894), and the geographical distribution of facies in the ca 1500 km 2 -wide study area (Fig. 1A).

Sedimentary facies characteristics and facies zones

In the Zagora area, the Lower Ordovician sedimentary succession is upward coarsening (Figs.

2C, 3 and 4A) and shows no or limited lateral continuity of the sandstone deposits in both the Fezouata (Fig. 4B) and the Zini formations (Fig. 4C). Symmetrical (oscillation) and slightly asymmetrical (combined flow-oscillation) ripples, with 2D and 3D morphologies, and hummocky cross-stratification (HCS; scour-and-drape and accretionnary) (Figs. 4D, E andF), are the main sedimentary features observed in the silty to fine-sandy parts of the sections (upper part of the Fezouata Formation). Horizontal lineation on bed surfaces (Fig. 4G), tabular cross-beds (Fig. 4H), meter-to tens of meter-scale wide erosional surfaces (Fig. 4I), tangential cross-stratification (Fig. 4J), plane-parallel stratification (Fig. 4K), and large amalgamated oscillation (symmetrical megaripple) structures (Fig. 4L), are commonly observed in the sandstones of the Zini Formation. Based on lithology, grain size, sedimentary structures and their size, as well as the stratinomy, 11 facies (F1 to F11) are defined (Fig. 5; Table 1). Three distinct, additional facies (SL 1 to SL 3) associated with sand-to-silt lenses, were also observed in the sedimentary succession and are described here (Figs. 6,7 and 8).

All facies were grouped into 4 Facies Zones (FZ). Facies description is synthesized Table 1.

Sand-to-silt lenses

Descriptions

Sand-to-silt lenses (SL; see Table 1) are meter-scale sedimentary features composed of several beds (2 to 8) of few meters long (idealized SL Fig. 6). SL can be easily identified in the field, based on both their color (yellowish/orange) and their cementation (Fig. 8). In section, the center of each bed consists of fine-grained sandstones (locally coarse bioclastic discontinuous layers occur), which gradually pass laterally into siltstones (Figs. 6 and7). The first bed at the bottom of SL commonly displays an erosional base (ca dm-deep). Usually, the sandstone parts of the SL are commonly thicker (ca dm-thick) compared to the siltstone parts (ca cm-thick). The transition between beds is given by a normal or erosive contact. Since SL are composed of several beds thicker at the center than in lateral with usually a erosional base, the overall morphologies are elliptical with a concave base (channel-like) and a mound shape top (lobe). Bioclasts have a size ranging from 0.5 to 5 cm, but are not graded. These bioclastic lags occur in different positions within the SL, at their base, in their middle part, and rarely at their top.

Their lateral extension corresponds to that of the more cemented part of the SL. SL2 (Figs.

8C) is defined as containing thin (cm-thick), commonly discontinuous bioclastic lags with surface bed of a sand-t-silt lens shows that they are elongated along a proximal-distal axis; here, a N20° direction was measured in agreement with the orientation of the sedimentary system (Fig. 1). (E)

Typical channel-lobe morphology of one sand-to-silt lens (here observed in SL2) displaying oblique cross-stratifications (magnified in (F) and (G)) interpreted as structures of lateral accretion due to the morphological compensation of the lobe.

millimeter to centimeter bioclasts. SL3 (Fig. 8D) corresponds to SL composed almost entirely of a fine sandy fraction with a minor contribution of bioclasts, which are deposited as thin millimeter lags or are isolated in the sandstone part.

Interpretation

Sand-to-silt lenses are almost only constituted by scour-and-drape HCS. However, HCS as well as swaley cross-stratifications (SCS) may display common characteristics with sedimentary structures formed under supercritical flows, as antidunes or chutes-and-pools (e.g. [START_REF] Alexander | Bedforms and associated sedimentary structures formed under supercritical water flows over aggrading sand beds[END_REF][START_REF] Lang | Lateral and vertical facies relationships of bedforms deposited by aggrading supercritical flows: From cyclic steps to humpback dunes[END_REF][START_REF] Cartigny | Morphodynamics and sedimentary structures of bedforms under supercritical-flow conditions: New insights from flume experiments[END_REF]. The difficulty in the distinction between HCS/SCS vs antidune/chutes-and-pools was already pointed out as a source of errors for paleoenvironmental reconstructions (e.g. [START_REF] Prave | Small-scale hummocky cross-stratification in turbidites: a form of antidune stratification?[END_REF][START_REF] Rust | Three-dimensional antidunes as HCS mimics in a fluvial sandstone: the Pennsylvanian South Bar Formation near Sydney, Nova Scotia[END_REF][START_REF] Cotter | Coastal plain sedimentation in the late Devonian of Southern Ireland; hummocky cross-stratification in fluvial deposits? Sediment[END_REF]. [START_REF] Alexander | Bedforms and associated sedimentary structures formed under supercritical water flows over aggrading sand beds[END_REF] surhorizontal laminae in the vertical section normal to the flow. Cartiny et al. (2014) mentioned in their study that chutes-and-pools may mimics HCS, but noted that the preservation of the entire chute-and-pool structures is promoted by a high aggradation rates.

SL exhibit several types of stratifications as subhorizontal laminae, trough and hummocky stratifications. SL were observed in all possible cross-sections (360°) and similar patterns were observed. By observing all cross section, at least once, the orientation should be parallel to the paleoflow. However no dominance of upstream dipping laminae were observed as well as high-angle downstream dipping. Similarly, in section normal to the flow, no dominance of subhorizontal laminae were observed, always subhorizontal laminae associated with trough and hummocky stratifications. Therefore, the stratifications observed in SL are considered as HCS (scour-and-drape). According to most previous studies (e.g. Harms, 1979;Dott & Bourgeois, 1982;[START_REF] Yang | Sedimentation on a wave-dominated, opencoast tidal flat, south-western Korea: summer tidal flat -winter shoreface[END_REF]Yang et al., , 2006)), HCS are considered as storm depositional structures preferably generated and preserved between the storm-weather wave base (SWWB)

and the fair-weather wave base (FWWB;McLane, 1995). Since SL mainly consist of HCS, the storm processes hypothesis is preferred for the formation of SL instead supercritical flows.

Difference of oscillatory structure wavelengths between the center of SL (dm-to multi-dmscale) and the lateral (cm-to dm-scale) can be explained by the grain-size difference [START_REF] Cummings | Fine-grained versus coarsegrained wave ripples generated experimentally under large-scale oscillatory flow[END_REF]. The cross-bedded part (Figs. 7E, F and G) sometimes observed in the upper part of SL is interpreted as lateral accretion due to morphological compensation of the lobe (e.g. Nichols, 2009). The SL consist of different units (i.e. couplets large to small HCS constituting one bed; Figs. 7A andC) suggesting either that the sand-to-silt lenses are formed (1) by numerous amalgamated storm events or (2) that the record of storm intensity at the seafloor was modulated by another process than the storm. Distribution of SL on outcrops (Figs. 8A andE) and on bed surfaces (Fig. 7C) show that deposition is very localized. It is therefore highly improbable that successive (two to eight) storm events have deposited sands on exactly the same small area of few square meters. It appears more probable that SL have resulted from a single storm event rather than numerous. The absence of both muddy deposits and bioturbation between two successive units also suggests a fast, continuous deposition, and that one SL corresponds to one single storm event as suggested by the step-by-step diagram (Fig. 9).

Facies Zone 1 (FZ1): Proximal Offshore

This facies zone (FZ1) consists of three main facies (F1 -F3 -F4; see Table 1). F1 forms the host sedimentation dominated by siltstones (Fig. 5A) and rare very fine-sandy layers displaying cm-scale wave-ripples. Thin, mm-scale, argillaceous intercalations devoid of structures frequently occur. F3 consists of coarse siltstones to very fine sandstones that are up to 8 m-thick (several dm-thick in the thinnest case) and few hundreds m-wide (Fig. 5C). From base to top of F3, a thickening upward of the beds is observed as well as a decrease in the thickness and abundance of muddy intercalations between the beds (Fig. 5D). Meter-scaled erosional surfaces are observed within F3 (Fig. 5E). Plane-parallel laminations and few current ripples (Fig. 5F) occur in F3, oscillatory structures are absent. Facies F3 is intercalated within thick (up to 10 m) siltstones or argillaceous siltstones of F1. Facies F4 consists of very fine sandstones displaying cm-to dm-scale wave ripples (Fig. 5G); the laminations are underlain by thin layers of coarser quartz grains (Fig. 5H). In FZ1, few SL3 (Fig. 8F) occasionally occur.

FZ1 is interpreted as being deposited in the proximal offshore, close to the limit of the SWWB. Host sediments express by F1 would represent the background sedimentation deriving from terrestrial input in the basin. The argillaceous fraction in F1 would result from gravitational settling. Here, the slightly coarser sediments (F4) deposited during storm events suggest a strongly attenuated influence of storm current and oscillation, and are interstratified with the background muddy to silty sediments of F1. Locally, F3 is interstratified with the background fine-grained sediments (F1). The lack of unequivocal oscillation-ripples in F3

suggests that this facies was deposited below the SWWB, deeper than F4. The overall morphology of F3 with consequent thickness variations associated with current ripples, plan parallel stratifications, and large erosional surfaces suggest a lobe deposition. The meterscaled erosional surface exhibited within F3 suggests multiple amalgamated lobes and/or channels feedings lobes (e.g. [START_REF] Fildani | Late Quaternary evolution of channel and lobe complexes of Monterey Fan[END_REF]. Since F3 corresponds to lobe deposition, the process responsible of its formation is not clearly identifiable and these distal lobes, either formed by turbiditic currents, storm back-currents, storm-derived turbidity currents or Froud supercritical flows.

Facies zone 2 (FZ2): Lower shoreface

The facies zone 2 (FZ2) is formed by five main facies (F2 -F4 -F5 -F6 -F7; see Table 1).

F2 corresponds to the host sediments constituted by coarse siltstones to very fine sandstones displaying dm-scale wave ripples (Fig. 5B). F4 to F7 are all fine-to medium-grained sandstones interstratified in F2. Facies F5 displays dm-scale wave ripples (Fig. 5I) and combined lateral and vertical accretionary wave-ripples. F6 exhibits dm-scale of wavelength scour-and-drape HCS (Fig. 5J), and few combined lateral and vertical accretionary waveripples. Facies F7 is characterized by m-scale wavelength scour-and-drape and rare accretionary HCS (Fig. 5K).

FZ2 is interpreted as the lower shoreface, here defined as being comprised between the SWWB and the FWWB. F2 would correspond to the continuous terrestrial input of sediment in the basin. From F4 to F7, a gradual increase in the size of the oscillatory structures (HCS) is associated to a coarsening and thickening upward of the storm layers (Figs. 4G, I, J and K), thus suggesting that F4 to F7 were respectively deposited in distal to proximal setting within the lower shoreface (Heward, 1981;Davis Jr & Hayes, 1984;Boyd et al., 1992;Yang et al., 2006). This facies zone FZ2 can therefore be subdivided in two subzones: FZ2-a for the distal lower shoreface with F4 and F5 as characteristic facies and FZ2-b for the proximal lower shoreface indentified by F6 and F7. Commonly F2 is several m-thick and laterally continuous.

However, a peculiar organization between F1 and F2 can be observed close to the SWWB. A repeated, regular alternation of F1 and F2 at dm-scale is recorded, thus suggesting a cyclic deposition.

The three types of sand-to-silt lenses (SL 1 -SL 2 -SL 3; see Table 1) commonly occur in FZ2. SL are therefore typical of the lower shoreface. SL3 is commonly found with F4, SL2

with F5 and SL1 with F6-F7. Thus, SL1 (Fig. 8B) belongs to the proximal, SL2 (Fig. 8C) to the intermediate, and SL3 (Fig. 8D) to the distal lower shoreface. Only few SL3 were observed in the proximal offshore (FZ1). In SL1 and SL2, the edge of the lenses generally corresponds to the termination of the bioclastic lags. The cementation of the deposits during diagenesis seems to have been driven by the occurrence of these bioclastic lags. Thus, diagenesis enhanced the primary channel-lobe shaped organization of sandstone deposits and gave them their typical lens shape. Differential compaction is only locally observed between the cemented sandstone bodies and the underlying sediments.

4.4. Facies Zone 3 (FZ3): Upper shoreface FZ3 consists of three facies (F2 -F8 -F9; see Table 1). F2 corresponds to the host sedimentation. Fine-to medium-grained sandstones displaying amalgamated HCS (F8; Fig. 5L) and rare sandstones displaying cross-cutting sets of gently dipping, concave-up laminations, interpreted as SCS with rare local overdigging interpreted as gutter (F9; Fig. 5M) are intercalated with F2. These sandstone beds commonly have an erosional base.

Amalgamated HCS and SCS are formed above the FWWB. They are the results of storm and wave activity in proximal environment, where the waves constantly rework the sediment during both fair-and storm-weather conditions, thus vertically stacking HCS to form amalgamated HCS or truncating the hummocks of HSC to create SCS. This facies zone testifies for constant high-energy conditions in the upper shoreface (e.g. [START_REF] Hampson | Geomorphological and sequence stratigraphic variability in wave-dominated, shoreface-shelf parasequences[END_REF].

Facies Zone 4 (FZ4): Foreshore

This facies zone (FZ4) is composed of two facies (F10 and F11; see Table 1). Facies 10 (Fig. 5N) is composed of fine-to medium-grained sandstones and displays cross-bedded tabular stratifications (Fig. 4H), rare tens of m-wide erosional surfaces (Fig. 4I), very rare tangential cross-beds (Fig. 4J), plane parallel stratifications (Fig. 4K), tens of dm-to m-scale oscillatory structures (Fig. 4L). Few sandstone levels display aligned pores (mm-scale of diameter on outcrops) tentatively interpreted as keystone vugs. Facies F11 (Fig. 5N), composed of fine to medium sand is characterized by decimeter to multi-decimeter oscillation structures and frequent bioturbations (white arrows in Fig. 5O), probably resulting from the activity of annelids, trilobites or other arthropods on the sediment surface. F11 forms fine-grained sandstone layers discontinuous at m-scale, always intercalated in F10 (Fig. 5N).

Low angle parallel stratifications with levels of probable keystone vugs, large erosional surfaces interpreted as beach cusps due to the storm swash (e.g. Inman and Guza, 1982), megaripples, and tabular cross-beds corresponding to berms, are only observed in the sandstones of the Zini Formation (FZ4). These observations strongly suggest that FZ4 records the most proximal deposits (foreshore). F11 with decimeter scale wave ripples, vertical and horizontal burrows only present and intercalated between F10 is interpreted as an intertidal

zone in an open-coast environment, whereas F10 would represent foreshore environments or surf-bars developing between the high tide beach and the low tide beach [START_REF] Masselink | The effect of tide range on beach morphodynamics and morphology: a conceptual beach model[END_REF]Davis & Dalrymple, 2012). Furthermore, the tangential cross-stratified sandstones can also point out a tidal influence as to be bottom sets of tidal-bundles.

Development of the depositional model

In application of the Walther's law (1894), facies succession and facies zone succession in the studied sections can provide informatieon on how the facies and facies zones were spatially disposed along a proximal to distal transect, thus reinforcing their interpretation in terms of palaeoenvironmental significance. An example is shown in Figure 10 that makes evident that the succession FZ1-FZ2-FZ3-FZ4 (and the facies they contain) corresponds to palaeonvironmental zones that can be placed in a model of deposition from the proximal offshore to the foreshore (Fig. 11). The outcrop analysis, shown a dominance of FZ3 and FZ4

in the SW of Zagora (Fig. 1A) while in the NW of Zagora ( formations (Fig. 10). They are here defined as elementary, small-, medium-and large-scale parasequences [START_REF] Van Wagoner | Overview of sequence stratigraphy of foreland basin deposits: Terminology, summary of papers, and glossary of sequence stratigraphy. In: Sequence Stratigraphy of Foreland Basin Deposits[END_REF].

Depositional model: a potential tide modulation?

The main sedimentary features of the Fezouata and the Zini formations are oscillatory structures pointing fair-weather waves and storm dominance on the sedimentation. The interpretation of a wave-storm dominance in this studied interval is supported by (1) a landward coarsening in grain size; (2) the presence of planar bedding and tabular cross-beds (berms) in the foreshore;

(3) tens of m-wide erosional surfaces interpreted as reactivation surfaces in the foreshore formed during storms (beach cusps) and; (4) oscillatory structures in most facies, from small-to large-scale wave ripples, HCS (mainly scour-and-drape, few accretionary) and SCS. Many authors have already studied wave-dominated systems, and the facies model for these coastal environments is now well constrained (e.g., [START_REF] Davies | A morphogenic approach to world shorelines[END_REF]Bluck, 1967;Goldring & Bridges, 1973;Harms, 1979;Heward, 1981;Brenchley & Newall, 1982;Dott & Bourgeois, 1982;Davis Jr & Hayes, 1984;[START_REF] Davis | Beach and nearshore zone[END_REF][START_REF] Short | Rip-current type, spacing and persistence, Narrabeen Beach, Australia[END_REF][START_REF] Brenchley | Proximal and distal hummocky cross-stratified facies on a wide Ordovician shelf in Iberia. Shelf Sands and Sandstones[END_REF]Boyd et al., 1992;Dumas & Arnott, 2006;Plint, 2010). However, all sedimentary structures and all bedding architectures cannot be explained only by wave and storm action.

According to standard definitions, foreshore to shoreface deposits accumulates to form rather homogenous sandstone bodies without well-defined bedding planes due to continuous regime of accumulation at the coast (Nichols, 2009). Only the reactivation surfaces (often beach cusps) formed during storms interrupt sediment accumulation. In the Lower Ordovician successions of the central Anti-Atlas, the foreshore environments are uncommon and characterized by the alternation of bioturbated intervals with wave-ripples (F11) and more massive sandstone beds with parallel or low angle tabular stratifications (F10). Furthermore, peculiar features, unlike in a purely wave-storm dominated system, are observed in the studied successions: (a) cyclical changes in both grain and structure size within a single storm event (here SL; Fig. 12A), cyclical organization at the transition between F1 and F2 (Figs.

12B and C), as well as in fair-weather conditions with alternation of smaller and larger wavelength of wave ripples (Fig. 12D); (b) local overdigging in SCS interpreted as gutter (laminae in SCS rarely dip more than 10°, [START_REF] Leckie | Storm-and tide-dominated shorelines in Cretaceous Moosebar-Lower Gates interval -outcrop equivalents of Deep Basin gas trap in western Canada[END_REF]) (Fig. 5M); (c) deep internal erosion in the sand-to-silt lenses, i.e. within the storm deposits themselves (Fig. 13A and B); (d) discontinuous sandstone layers (Fig. 4B, C and 8) in all environments that can locally form lobes or channel-lobes (Figs. 6, 7A and E), and thus lead to the formation of sand-to-silt lenses (Fig. 8). Moreover, in distal environments the deposition is discontinuous with the formation of silty to fine-sandy lobes (F3; Fig. 5C); and (e) simultaneous aggradation-progradation of the wave-ripples alternating with purely aggrading wave-ripples (Fig. 13E). The latter observation suggests that, during one storm event, periods with oscillation dominantly controlling sediment deposition (aggradation; Fig. 13F) alternated with periods during which combined flow (progradation; Fig. 13F) controlled sediment accumulation. As observed in Figure 13 F, the change from one hydrodynamic regime to the other seems to be gradual. We propose that the studied interval was storm-wave dominated but indirectly tide-modulated [START_REF] Dashtgard | Tidal effects on the shoreface: Towards a conceptual framework[END_REF].

Changes in sea level during tidal cycles would change the size of wave orbitals interacting with the sediment surface (Fig. 12E). At low tide, water depth is reduced; oscillation at the sea floor is expected to be of higher amplitude than at high tide, if tidal range is important.

This implies that, during a storm event, larger oscillatory structures would be generated at low tide than at high tide (Figs. 12E andF). The observed changes in the amplitude of oscillation structures (or changes in grain size in the sand-to-silt lenses, with coarse bioclastic layers alternating with fine sands) would characterize storm deposits in this study (Fig. 12A).

Furthermore, SL are formed by two to eight cycles (Fig. 7) and for diurnal tide cycles, this would correspond to one to four days of storm. This duration is comparable to that of modern storms [START_REF] Dolan | Mid-Atlantic coastal storms[END_REF]. Similarly, velocity of storm currents at the sea floor is expected to be higher during low tide than during high tide, implying that erosion would be more frequently recorded during falling tide than during rising tide, thus possibly explaining the decimeter scale gutters commonly observed at the bottom or at the center of SL1 (Figs. 13A andB) which correspond to one storm event. Moreover, changes in sea level related to tide cycles would continuously change the storm equilibrium profile during the storms. Thus, erosion is not only recorded as guttercasts, but could also create an irregular topography along the sea-floor, with zones of erosion and of deposition (Fig. 9), explaining that the Lower Ordovician sediments are generally deposited as lobes, or channel-lobes (Figs. 6,7A,7E and 9), and that storm deposits most commonly form discontinuous layers (Figs. 4B andC).

In a wave-dominated coastal system modulated by tidal cycles, the wave breaking zone moves on the intertidal zone, from the high tide beach to the low tide beach during falling tide and conversely, from low tide beach to high tide beach during rising tide, thus forming surf bars (when submerged) or wave breaking bars (when located at the sea level). A modern equivalent of this type of coastal system can be observed in northern France (Fig. 14A).

Breaking or surf bars are both characteristic of the intertidal zone, where plane or low-angle parallel stratifications are formed (Fig. 14B). In topographical lows, large and small-scale oscillation structures are generated when the intertidal zone is partly or fully submerged (Fig. 14B). This could explain the atypical coastal facies (F10 -F11), where oscillation structures are sandwiched between parallel stratifications (Fig. 4K). The morphology of the intertidal zone with disconnected surface bars and topographical lows at their back are only formed in macro-tidal systems [START_REF] Short | Macro-meso tidal beach morphodynamics: An overview[END_REF][START_REF] Masselink | The effect of tide range on beach morphodynamics and morphology: a conceptual beach model[END_REF] and it can imply that both tide currents and storm back-currents are channelized (i.e. rip currents in channels or baines), thus focusing transport of the sediments seaward (Fig. 14A) [START_REF] Shepard | Rip currents a process of geological importance[END_REF][START_REF] Short | Rip-current type, spacing and persistence, Narrabeen Beach, Australia[END_REF]; weakened, implying that current may be better expressed in the sedimentary structures, thus creating combined oscillation-flow structures. (E) and (F) At low tide, oscillation dominates over currents, and mainly aggrading oscillation structures form. This results in aggrading -prograding wave ripples (red lines) stacked between wave ripples of larger wavelength (green lines). [START_REF] Aagaard | Mean currents and sediment transport in a rip channel[END_REF][START_REF] Macmahan | Rip current review[END_REF]. This mechanism is likely contributing to deposition as lobes as commonly recorded in the Lower Ordovician of the central Anti-Atlas.

Tide ranges were classified in three categories by [START_REF] Davies | A morphogenic approach to world shorelines[END_REF]: micro-(<2m), meso-(2-4m), macro-(>4m), and mega-tidal (>8m). Macro-tidal beaches could be separated in two distinctive groups, and a third intermediate case [START_REF] Short | Macro-meso tidal beach morphodynamics: An overview[END_REF][START_REF] Masselink | The effect of tide range on beach morphodynamics and morphology: a conceptual beach model[END_REF]. End members are "higher wave, planar, uniform slope" and "moderate waves, multi bars". Intermediate one is "low wave beach and tidal flat". The studied Lower Ordovician deposits probably belong to a system, where the coastal environment corresponds to the "moderate waves, multi bars" macro-tidal beaches. Macro-tidal beaches showing "moderate waves and multi bars" as shown in Fig. 14, consist of small-and large-scale oscillatory structures alternating with large surf bars characterized by low angle parallel stratifications.

Evolution of this kind of coastal environment by migration of the surf bars would sandwich small-and large-scale oscillatory structures between low angle parallel stratifications as observed in the Zini Formation. Current ripples, tidal channels, tidal bundles, as well as flaser, wavy or lenticular beddings are missing in both the Fezouata and the Zini formations. The only sedimentary structures possibly tide-generated could be the tangential cross-bedded sandstones observed in one place in the Zini Formation (Fig. 4J), which could be eventually interpreted as the bottom part of tidal bundles. SL frequently display internal erosion surfaces (see Figs. 13A andB) thus each tidal cycle could not be preserved through SL. Furthermore the general sedimentation in the Lower Ordovician succession is mainly constituted of lobe-shaped deposits which imply that tidal cycles would not be recorded everywhere. However, the hypothesis that the studied deposits belong to a macro-tidal beach system is in agreement with a dominance of wave-related structures and the scarcity of typical tidal sedimentary structures and geometries in such context [START_REF] Dashtgard | Tidally modulated shorefaces[END_REF][START_REF] Dashtgard | Tidal effects on the shoreface: Towards a conceptual framework[END_REF]. Through this study, the prolongation of the "moderate waves, multi-bars" model [START_REF] Short | Macro-meso tidal beach morphodynamics: An overview[END_REF][START_REF] Masselink | The effect of tide range on beach morphodynamics and morphology: a conceptual beach model[END_REF] is then proposed up to proximal offshore environments. Others authors have already observed hybrid sedimentary systems up to offshore environment in the rock records (e.g. [START_REF] Basilici | A depositional model for a wavedominated open-coast tidal flat, based on analyses of the Cambrian-Ordovician Lagarto and Palmares formations, north-eastern Brazil[END_REF][START_REF] Basilici | Hummocky cross-stratification-like structures and combined-flow ripples in the Punta Negra Formation (Lower-Middle Devonian, Argentine Precordillera): A turbiditic deep-water or storm-dominated prodelta inner-shelf system?[END_REF][START_REF] Vakarelov | Recognition of wavedominated, tide-influenced shoreline systems in the rock record: Variations from a microtidal shoreline model[END_REF]Rossi & Steel, 2015). However, the Lower Ordovician of the Anti-Atlas is the first sedimentary system exhibiting storm and wave structures and geometries, which indirectly record a tide modulation. The other described examples of hybrid systems are characterized by a clear expression of both processes (waves and tides) with distinctly expressed wave and tide sedimentary structures.

Early Palaeozoic times were characterized by tides of higher amplitudes than today [START_REF] Scrutton | Long-term changes in the rotation rate of the Earth[END_REF], due to the shorter Earth-Moon distance and hence faster Earth rotation [START_REF] Scrutton | Periodic Growth Features in Fossil Organisms and the Length of the Day and Month[END_REF][START_REF] Lambeck | The Earth's Palaeorotation[END_REF][START_REF] Lambeck | The Earth's variable rotation: geophysical causes and consequences[END_REF]. In the Anti-Atlas, the Lower Ordovician succession filled the intracratonic basin created by the rifting phase between Avalonia continent and the Gondwanaland (Fig. 2) (Cocks & Torsvik, 2004). The proximity of Avalonia and Gondwana, separated by a wide shallow epicontinental sea could be responsible for a significant effect of resonance (Davis & Dalrymple, 2012) that increased tidal range [START_REF] Cram | The influence of continental shelf width on tidal range: paleoceanographic implications[END_REF]. Other sedimentary succession from Early Paleozoic report mixed wave-and tide-processes e.g. the Cambro-Ordovician successions of Brazil [START_REF] Basilici | A depositional model for a wavedominated open-coast tidal flat, based on analyses of the Cambrian-Ordovician Lagarto and Palmares formations, north-eastern Brazil[END_REF] and of Algeria [START_REF] Ghienne | The Cambrian-Ordovician succession in the Ougarta Range (western Algeria, North Africa) and interference of the Late Ordovician glaciation on the development of the Lower Palaeozoic transgression on northern Gondwana[END_REF], the Upper Ordovician of China [START_REF] Fan | Influences of storm erosion and deposition on rhythmites of the upper Wenchang Formation (Upper Ordovician) around Tonglu, Zhejiang Province, China[END_REF], or the Upper Ordovician of Morocco [START_REF] Loi | The Late Ordovician glacio-eustatic record from a high-latitude storm-dominated shelf succession: The Bou Ingarf section (Anti-Atlas, Southern Morocco)[END_REF]. Brenchley & Newall (1982) also described SL similar to those described in this study in the Middle Ordovician of England (Shropshire) and interpreted these sedimentary bodies as deposited under ebb-forced, storm back-currents, localized by reef barriers. Moreover, in the central Anti-Atlas, the middle Cambrian Tabanite Group (below the Fezouata Formation; Fig. 1), and the Middle Ordovician First Bani Group (above the Zini Formation; Fig. 2) both exhibit typical combined wave and tide sedimentary structures (field observations; Marante, 2008). Furthermore, a study of the Lower Ordovician succession of Algeria [START_REF] Ghienne | The Cambrian-Ordovician succession in the Ougarta Range (western Algeria, North Africa) and interference of the Late Ordovician glaciation on the development of the Lower Palaeozoic transgression on northern Gondwana[END_REF] have recognized the influence of wave and tide trough the interval. Therefore, it is likely that, in the Zagora area, the Lower Ordovician succession was also influenced by both waves and tides. the intertidal zone implies that small-and large-scale oscillatory structures will be sandwiched between low angle parallel stratifications as observed in the facies (F10 and F11; FZ4) of the Zini Formation.

Palaeoenvironment of the Lower Ordovician Fezouata Biota

Processes responsible for exceptional preservation of soft-body tissues in the fossil record are still under investigation. However, the fossils of the Moroccan Fezouata Lagerstätte could exhibit, according to Gaines et al. (2012), characteristic features of a Burgess Shale-type preservation. Processes of organic matter degradation (decay) were stopped early, and authigenic minerals replaced soft tissues [START_REF] Gaines | Cambrian Burgess Shale-type deposits share a common mode of fossilization[END_REF]. Rapid burial is one key factor for organic matter preservation in order to escape oxidation by oxygen, sulphate reduction, or other processes of organic matter oxidation (e.g. Froelich et al., 1979;Allison, 1988;[START_REF] Briggs | Also important is the Chengjiang Lagerstätte, which documents the Cambrian Explosion. In the first decade (1984-1994) of excavation, although numerous taxa in both diploblasts and protostomes were recovered, none of the deuterostomes was known in the Chengjiang fauna (Zhang and Hou[END_REF]Allison & Brett, 1995;Butterfield, 1995;Gaines et al., 2005). Exceptionally preserved soft-tissues are found in thin layers of muddy sediment that are covered by cmthick coarser, sandy deposits (Vaucher et al., submitted). This implies that the fossil rich layers correspond to peculiar stratigraphic levels and cannot be found everywhere through the entire Lower Ordovician succession (Martin et al., in press). The fact that these layers are only found in F1 when associated to F4 (distal tempestites) suggests that burial by storm deposits is required, but that energy was not high enough to rework fossil assemblages that can thus be preserved in situ (Martin et al., in press; Vaucher et al., submitted). As previously mentioned, the dominant depositional geometry in most of the environment is lobe, which implies that thick amount of sediment can be deposited rapidly. The fast deposition of sediment appears to be a key factor to initiate the exceptional preservation. These conditions are fulfilled in the most distal storm influenced zone (distal lower shoreface; Fig. 11).

Van Roy et al. (2010) described the environment of the Fezouata Biota as deep water and of low energy, whereas coarser sediments were interpreted as distal tempestites. Our interpretation suggests a shallower setting in agreement with [START_REF] Lefebvre | Peltocystis cornuta is also known from the late Tremadocian of Montagne Noire (Ubaghs, 1970; Vizcaïno and Lefebvre[END_REF] who analysed assemblage composition of stylophoran echinoderms from the Fezouata Formation.

This shallower setting is proposed here to be close/above to the limit of the storm weather wave base. In term of water depth, 30 -70 m are suggested (Immenhauser, 2009).

Conclusions

The Fezouata and the Zini formations were subdivided in 14 facies, all grouped in four facies zones, namely the proximal offshore, the lower shoreface, the open coast subtidal-upper shoreface and the intertidal-foreshore (respectively FZ1 to FZ4). The Lower Ordovician succession records offshore to foreshore environments dominated by oscillatory structures.

Tidal effect can only be recognized by the modulation of oscillatory structures during sea level fluctuations induced by tidal cycles, and no diagnostic tidal sedimentary structures are present.

A new model of deposition for a wave-dominated, tidally-modulated siliciclastic system is proposed. The two main characteristics of such a depositional environment are: (1) the deposits are fundamentally discontinuous, at different scales, and for different water depths (from the intertidal-foreshore to the offshore), and; (2) the deposits indirectly record tidallycontrolled cyclical changes in sea level, and their expression depends on the position along the siliciclastic ramp.

This study revealed one of the very few wave-dominated and tide-modulated shallow epicontinental sea recording foreshore to offshore environments. The few examples described in the geological record focus only on proximal environments (e.g. [START_REF] Basilici | A depositional model for a wavedominated open-coast tidal flat, based on analyses of the Cambrian-Ordovician Lagarto and Palmares formations, north-eastern Brazil[END_REF][START_REF] Basilici | Hummocky cross-stratification-like structures and combined-flow ripples in the Punta Negra Formation (Lower-Middle Devonian, Argentine Precordillera): A turbiditic deep-water or storm-dominated prodelta inner-shelf system?[END_REF] but could exist from modern studies (e.g. [START_REF] Yang | Sedimentation on a wave-dominated, opencoast tidal flat, south-western Korea: summer tidal flat -winter shoreface[END_REF][START_REF] Dashtgard | Tidally modulated shorefaces[END_REF][START_REF] Dashtgard | Tidal effects on the shoreface: Towards a conceptual framework[END_REF].

Only [START_REF] Vakarelov | Recognition of wavedominated, tide-influenced shoreline systems in the rock record: Variations from a microtidal shoreline model[END_REF] described a wave-dominated tide-influenced system from proximal offshore to foreshore environment by the recognition of both wave-and tidegenerated sedimentary structures.

This study provides key data on the sedimentological context associated with exceptional preservation in the Early Ordovician Fezouata Biota. Rapid burial during storm events in a relatively distal environment seems to be a pre-requisite for the in situ preservation of organisms by inhibiting their early decay.

Introduction

The Great Ordovician Biodiversification Event (GOBE) is one of the most important radiations of the Phanerozoic Eon. The GOBE began some 35 million years after the Cambrian Explosion, during the Early Ordovician (ca 485.4 -470 Ma;Bambach et al., 2004;Servais et al., 2008Servais et al., , 2010) ) and is characterized by an exponential diversification of classes within phyla that appeared during the Cambrian (e.g. Harper, 2006;Servais et al., 2008Servais et al., , 2010)). Paleontologists have recognized a major gap in the record of soft-bodied faunas spanning the late Cambrian (Furongian) to the Middle Ordovician (Darriwilian). Middle to Late Ordovician Konservat Lagerstätten are rare and post-date the initial stages of the GOBE (e.g., Briggs et al., 1991;Liu et al., 2006;Aldridge et al., 2007;Young et al., 2007;Botting et al., 2011). The recent discovery of a late Tremadocian Konservat Lagerstätte in Morocco (Zagora region; Van Roy et al., 2010, 2015a, 2015b;Martin et al., 2014, in press) in the lower part of the Fezouata Shale (Martin et al., in press) (Fig. 1A) is thus of prime importance for understanding the rise of animal life (Van Roy et al., 2010). According to Van Roy et al.

(2015a), these fossils may display characteristics of Burgess Shale-type (BST) preservation, as also suggested by more recent findings (Marti Mus, this issue). If so, their preservation resulted from a combination of favorable sedimentary conditions: rapid burial in fine-grained sediment, early inhibition of microbial activity, low concentrations in both sulfate (in the global ocean) and oxygen (in the bottom waters) (Gaines et al., 2012). However, a second type of Lagerstätte also occurs in the Fezouata Shale, represented by massive bioclastic accumulations in dm-thick layers (Konzentrat Lagerstätte). The aim of this paper is to present the sedimentological field analyses performed in the Lower Ordovician deposits of the Zagora area in order to illustrate that the sedimentological processes involved in the formation of these two types of Lagerstätten are part of the same depositional system. Both are associated with different water depths in a storm-dominated sedimentary environment.

Geological setting

By the end of the Precambrian, the Pan-African Orogeny had resulted in the formation of Gondwana. This supercontinent was located in the southern hemisphere and extended from the South Pole to the Equator. Oceanic expansion during the Cambrian led to the rise of mean sea level, thus creating shallow epicontinental seas on the Gondwanan margins (Destombes et al., 1985;Fabre, 1988;Boote et al., 1998;Carr, 2002;Coward and Ries, 2003) and a longterm transgression took place at the beginning of the Ordovician (Meyers and Peters, 2011).

At the end of the Cambrian, the initiation of a rifting phase in the western part of Gondwana resulted in the northward drift of Avalonia and the opening of the Rheic Ocean (Cocks and Torsvik, 2004). The entire Lower Ordovician succession of Morocco was deposited on the rifted margin of Gondwana (Cocks and Torsvik, 2004), and is now well-exposed in the central part of the Anti-Atlas Mountains of Morocco (Fig. 1B). Lower Ordovician deposits exhibit a wide geographic distribution of ca. 800 km from Tan-Tan to Erfoud (Destombes et al., 1985).

In the Zagora region, the Fezouata Shale (Tremadocian to Floian in age) consists of ca. 900 m (Fig. 2) of quartz-rich siltstones locally overlain by massive black sandstones characteristic of the Zini Formation (late Floian) (Destombes, 1962;Martin et al., in press). Recently, the Lower Ordovician succession was interpreted to have been deposited in a storm-wave dominated sedimentary environment (Martin et al.,in press;Fig. 3), but modulated by tides (see Vaucher and Pittet, 2014;Vaucher et al., 2015;submitted). Tidal action is indirectly recorded in both the Fezouata Shale and Zini Formation by modification of storm-wave generated structures during their geneses, however no strictly tidally generated structures are present.

Material and methods

In January 2014, a large palaeontological excavation campaign was organized in the late Tremadocian series (Araneograptus murrayi graptolite Zone, Martin et al., in press;

Gutiérrez-Marco and Martin, this issue; messaoudensis-trifidum acritarchs assemblages, Nowak et al., this issue) of the Zagora area. The aim of this campaign was to collect in situ assemblages of exceptionally preserved fossils typical of the Lower Ordovician Fezouata Biota, which occur abundantly within several discrete horizons (Van Roy et al., 2010, 2015a, 2015b;Martin et al., in press). The excavation site is located on a hillside (Bou Izargane) covering an area of ca. 2000 m 2 (Fig. 4A). Twenty-two excavations were conducted, and all known levels yielding EPF were incorporated into a detailed stratigraphic log. Elsewhere in the Zagora area, a second type of fossiliferous horizons, which occur only in the upper part of the Fezouata Shale, were also investigated: 77 sandstone lenses containing mm-to dm-thick bioclastic lags were analysed from four distinct Lower Ordovician (Floian) sections (Figs. [1][2]. Sandstone lenses were classified according to their bioclastic content (amount, clast size and thickness of bioclastic layers). The two types of fossil-rich intervals (with and without exceptional preservation of soft tissues) encountered in the Fezouata Shale (Figs. [1][2] were subjected to sedimentological analyses in the field, and their lithology, grain size, sedimentary structures and bed geometries were described.

Results and interpretations

Levels with soft-tissue preservation

Description

Bou Izargane, a 20-m high hill (Fig. 4A) yielding exceptionally well-preserved fossils (EPF;

Van Roy et al., 2010Roy et al., , 2015aRoy et al., , 2015b;;Martin et al., in press) claystones with sparse intercalations of siltstones (mm-thick). Argillaceous siltstones and siltstones display symmetrical cross-laminations of cm-scale wavelength (Fig. 3C) and claystones are devoid of sedimentary structures. The second facies association (FA2) consists of very fine sands that display hummocky cross-stratification (HCS) with dm-scale wavelength (Fig. 4C). The sandy layers (cm-thick) are commonly made of three parts (Fig. 4D): (1) a poorly sorted base displaying upward fining from coarse to fine sands (in Fig. 4D orange dots are iron-stained ooids); (2) an intermediate part with mostly plane parallel lamination, with a base comprised of fine-grained quartz sand; and (3) an upper part displaying small-scale HCS of dm-scale wavelength consisting of fine-grained sandstones to siltstones. The sandy deposits of FA2 are discontinuous at the meter-to decimeter-scale, with an overall flattened lobe shape. The 23 excavations containing EPF were incorporated onto a detailed stratigraphic log (Fig. 4B). In the Bou Izargane section, EPF consistently occur in the three intervals (from bottom to top, ca 1.5, 3 and 3.5 m thick respectively), which show a more important quantity and thicker sandy layers (cm-scale thick) of FA2 (Fig. 4B). Fossils are commonly disarticulated without exceptional preservation (Fig. 4E), but sometimes preserved as EPF (Fig. 4F). EPF are almost always found at the top of the finest grained thin layers (claystones or argillaceous siltstones) of FA1 (Fig. 4G) directly overlain by coarsergrained sandy layers of FA2.

Interpretation

The succession of sedimentary structures in the sandy layers in FA2 from Part 1 (bottom) to Part 3 (top) likely corresponds to a distal storm sequence deposited under a wax and wane flow close to the storm wave base (SWB) (e.g. Harms, 1979;Brenchley and Newall, 1982;Dott and Bourgeois, 1982;[START_REF] Aigner | Storm Depositional Systems[END_REF]Duke, 1985;Duke et al., 1991). Part 1 corresponds to the basal part of a storm sequence deposited during the waxing phase of the storm flow. derived from terrestrial input in the basin, likely resulted from gravitational settling (Davies, 1964;Bluck, 1967;Goldring and Bridges, 1973;Dumas and Arnott, 2006;Plint, 2010). FA1 display symmetrical cross-laminations of small wavelength (cm-scale) (Fig. 3C). This indicates oscillation processes (e.g. Nichols, 2009) at the sea-floor during low-intensity storm conditions due to the collapse of storm orbits on the sea-floor (Allen, 1997) during and/or after deposition. The alternations of argillaceous siltstones dominated intervals (FA1) and sandstones (FA2), with rare claystones, suggest a depositional environment that lays below the fair weather wave base (FWWB) and above SWB, and corresponds to lower shoreface environments (Fig. 3A) (McLane, 1995).

Shell beds

Description

Shell accumulations primarily occur in the upper part of the Fezouata Shale (Fig. 2).

Bioclastic material occurs as layers (from mm-to dm-thick) inside sandstone lenses (SL) (Fig. 5A). SL have a lobate or a channel-lobe morphology (concave erosive base and lobate top), ranging in width from 55 cm to 9 m (average of 3 m) and in thickness from 15 cm to 1 m (average 50 cm). Bioclastic layers uniformly occur within the thickest intervals of the SL, which is also the most indurated zone. SL are interstratified in coarse siltstones displaying symmetrical cross-laminations (dm-scale wavelength) (Fig. 3B-D) as well as HCS (Fig. 3E).

SL are a stack of units (8 maximum), each consisting of three distinct parts (Fig. 5B): ( 1 and isolated bioclasts or mm-thick lags (SL3).

Interpretation

On the basis of the sedimentary structures (and their dimensions) and associated grain-size, each complete unit (consisting of three identified parts) of the SL likely corresponds to the typical sequence of a storm event in a lower shoreface environment (e.g. [START_REF] Allen | Physical processes of sedimentation[END_REF][START_REF] Komar | The initiation of oscillatory ripple marks and the development of plane-bed at high shear stresses under waves[END_REF]Harms, 1979;[START_REF] Kreisa | Storm-generated sedimentary structures in subtidal marine facies with examples from the Middle and Upper Ordovician of southwestern Virginia[END_REF]Brenchley and Newall, 1982;Dott and Bourgeois, 1982;[START_REF] Kreisa | The Role of Storm Processes in Generating Shell Beds in Paleozoic Shelf Environments[END_REF][START_REF] Aigner | Storm Depositional Systems[END_REF]Duke et al., 1991). Their basal bioclastic part was likely deposited during storm peak, which corresponds to the waxing flow [START_REF] Kreisa | The Role of Storm Processes in Generating Shell Beds in Paleozoic Shelf Environments[END_REF][START_REF] Aigner | Storm Depositional Systems[END_REF]. The transition to finer sediments in Part 2 suggests decreasing, but still rapid flow of the storm back current (start of the waning phase during the storm). Part 3 corresponds to the end of the storm event, when oscillatory flow overcomes back-current flow to generate oscillatory structures. SL formed by up to 8 units therefore correspond to the stack of successive storm events, or alternatively, as hypothesized by Vaucher et al. (submitted) to the modulation of storm intensity by tides, each unit corresponding to one tide cycle and SL to single storm events. SL are interstratified in a host sediment which consists of coarse siltstones displaying symmetrical cross-laminations and HCS pointing out oscillatory processes (waves and storms) (e.g. Nichols, 2009). SL1 is associated to wave ripples of dm-scale of wavelength (Fig. 3D) and HCS of m-scale wavelength (Fig. 3E); SL2 is found with wave ripples of dm-scale of wavelength and HCS of multi dm-scale wavelength; and SL3 is observed with wave ripples of cm-to dm-scale wavelength with HCS of dm-scale wavelength. Environmental conditions necessary to produce the SL were clearly shallower than those associated with the formation of EPF levels.

The wavelengths of both HCS in storm sequences and wave ripples in the host sediment are longer than in EPF layers, due to the stronger interaction of the wave orbits on the seafloor (Heward, 1981;[START_REF] Davis | What is a wave-dominated coast?[END_REF]Boyd et al., 1992;Yang et al., 2006;Plint, 2010). The decrease in bioclastic content within the SL as well as the decrease in wavelength of oscillatory structures in both the host sediment and the SL suggest a decreasing impact of the storms related to the increase of the water depth (e.g. [START_REF] Aigner | Storm Depositional Systems[END_REF]. This implies that SL1 formed higher in the lower shoreface as SL2, while SL3 occurred in the distal part of the lower shoreface (Fig. 3A).

Discussion

Originally, the term "Lagerstätte" (plural Lagerstätten) comes from mining terminology used to describe a rock of economic interest [START_REF] Seilacher | Taphonomy of Fossil-Lagerstätten: overview[END_REF]. Applied to palaeontology, this term describes stratigraphic units containing exceptionally well-preserved fossils or remarkably abundant fossils (Seilacher, 1970;Allison, 1988a). Commonly, two types of Lagerstätten are distinguished: one related to the exceptional preservation of the fossils, and one to their concentration (e.g. [START_REF] Seilacher | Sedimentological, Ecological and Temporal Patterns of Fossil Lagerstätten [and Discussion[END_REF][START_REF] Seilacher | Taphonomy of Fossil-Lagerstätten: overview[END_REF][START_REF] Speyer | Taphofacies controls: Background and episodic processes in fossil assemblage preservation[END_REF][START_REF] Briggs | Exceptionally Preserved Fossils[END_REF]. Sedimentary rocks yielding soft-bodied fossils are conservation deposits, so called Konservat-Lagerstätten (sensu Seilacher, 1970). In this case, not only the hard parts but also the soft tissues are preserved. This unusual preservation of soft tissues is related to a combination of favorable depositional, environmental and/or early diagenetic conditions for the preservation or mineral replication of soft tissue (e.g. Froelich et al., 1979;Allison, 1988b;Briggs and Kear, 1994;Allison and Brett, 1995;Butterfield, 1995;Gaines et al., 2005). On the other hand, sediments with a high fossil abundance are concentration deposits, which are referred as Konzentrat-Lagerstätte (sensu Seilacher, 1970). These two types of Lagerstätten are considered as end-members of a spectrum of preservation (Allison andBriggs, 1991a, 1991b). This study highlights two different cases of fossil occurrences in the Lower Ordovician Fezouata Shale of Morocco: EPF and shell beds. According to the previously mentioned definitions, EPF horizons correspond to a Konservat-Lagerstätte (Ksl) in which the quality of the fossil preservation is abnormally good, and shell beds to a Konzentrat-Lagerstätte (Kzl) that are characterized by a large quantity of articulated and/or fragmented skeletal remains.

The two types of fossil-rich intervals in the Fezouata Shale (Fig. 2) are linked to storm deposition . Claystones and argillaceous siltstones layers containing EPF were almost always immediately buried under coarser sediments deposited during the waxing phase of storms. Similarly, the shelly layers of SL were formed during the waxing phase of storms. Shell accumulation in SL corresponds to the basal part of the storm sequence, when the velocity is maximum and leads to the deposition of the unsorted coarser material of the sequence (e.g. [START_REF] Kreisa | The Role of Storm Processes in Generating Shell Beds in Paleozoic Shelf Environments[END_REF][START_REF] Aigner | Storm Depositional Systems[END_REF]. Based on sedimentological evidences, Ksl and Kzl were both assigned to a lower shoreface environment, but with Ksl in a more distal position than Kzl (Fig. 6). In terms of water depth, 30 -70 m are suggested for the depositional environment of Ksl and 10 -30 m for Kzl (Immenhauser, 2009). Both types of fossil-rich occurrences in the Fezouata Shale are related to rapid burial or obrution events. This implies that a sufficiently thick amount of sediment was deposited very rapidly during a storm event [START_REF] Martin | Taphonomy: a process approach[END_REF]. Obrution deposits appear to be the most common type of KsL [START_REF] Brett | Obrution deposits[END_REF][START_REF] Brett | Sequence Stratigraphy, Biostratigraphy, and Taphonomy in Shallow Marine Environments[END_REF] and preservation potential of soft tissues within them is especially high when rapid accumulation occurs in a quiet background sedimentary context (Brett et al., 1997). In shallower settings, wave activity continuously reworks previously deposited sediments. Commonly, organisms buried by obrution deposits experience little to no transport [START_REF] Brett | Mid-Paleozoic trilobite Lagerstätten: models of diagenetically enhanced obrution deposits[END_REF]. The preservation of soft tissues in EPF may suggest that the transport of organisms was almost null to promote the conservation of soft tissues (Allison and Briggs, 1991a). The abnormally good preservation observed in the KsL is in agreement with lower velocity of the storm flows compared to proximal settings. The fine-grained sandy layers (several cm-thick) deposited in the distal lower shoreface during storm events have enough velocity to rapidly bury the organisms but not enough to transport them seaward (e.g. [START_REF] Aigner | Storm Depositional Systems[END_REF]. Nevertheless, some experimental studies [START_REF] Allison | Soft-bodied animals in the fossil record: The role of decay in fragmentation during transport[END_REF]Kidwell and Baumiller, 1990;Kerr and Twitchett, 2004) show that soft-bodied and lightly skeletonized organisms can resist disarticulation for several hours in a tumbler and that fragmentation potential is largely controlled by the extent of decay prior to transport. This means that the assemblages found in the Ksl probably correspond to autochthonous to para-autochthonous communities, although an allochthonous origin cannot be fully excluded. Storm deposits in the EPF were likely responsible for transferring (by fast burial) the fossils (dead or alive) from oxic or dysoxic conditions at the seafloor to anoxic conditions in the sediment. Fossils were always directly buried by sandy deposits even if in rare case fossils were first covered by a thin (mm-thick) muddy layers. The absence of mud in most of the cases can also result from its removing during storms or its non-deposition. However, for the Fezouata Ksl, the precise role of mud in exceptional preservation is not clear. The burial by mud alone appears insufficient, while no EPF were found in the muddy intervals in absence of sandy storm deposits. After Martin et al. (in press), the more similar Ksl in terms of environmental setting compared to the Fezouata Ksl are the Guanshan and Chengjiang Ksl (e.g. Hu, 2005;Hu et al., 2010;MacKenzie et al., 2015).

In proximal storm deposits, Kzl consist of shelly fossils, with skeletal remains occurring in the shell beds being either complete (e.g. echinoderms, see Allaire et al., 2015;Martin et al., 2015) or commonly fragmented. Kzl are also obrution deposits (here storm deposits). In shell beds in general, the component of allochthonous fauna is generally high in proximal deposits and decreases seaward [START_REF] Reineck | Sedimentologie, Faunenzonierung und Faziesabfolge vor der Ostküste der inneren Deutschen Bucht[END_REF]. The high abundance of shelly fragments is likely explained by interludes of reduced sedimentation and/or repeated winnowing before being sealed during a storm event (e.g. [START_REF] Brett | Depositional dynamics recorded in mixed siliciclastic-carbonate marine successions: Insights from the Upper Ordovician Kope Formation of Ohio and Kentucky, USA[END_REF]Datillo et al., 2008). It can be assumed that fully articulated organisms were buried close to their life habitat, whereas disarticulated to fragmented ones were probably transported seaward, or were already dead and subject to prior decay (Martin et al., this issue). Other examples of Kzl exist in the geological record and are also formed by current activity as in the Middle Jurassic (ammonite rich intervals; [START_REF] Zatoń | Konzentrat-Lagerstätte-type carbonate concretions from the uppermost Bajocian (Middle Jurassic) of the Częstochowa area, South-Cental Poland[END_REF], in the Late Jurassic (bivalve rich intervals; [START_REF] Kin | Palaeoenvironment and taphonomy of a Late Jurassic (Late Tithonian) Lagerstätte from central Poland[END_REF] and in the Late Cretaceous of Poland (crinoid rich intervals; [START_REF] Salamon | A crinoid concentration Lagerstätte in the Turonian (Late Cretaceous) Conulus Bed (Miechów-Wolbrom area, Poland)[END_REF].

In the Lower Ordovician of the Zagora area, the whole succession shows that sedimentary structures linked to storm and wave oscillation (Fig. 3) largely dominate over those formed by currents (e.g. Martin et al., in press, Vaucher et al., submitted). The collapse of wave orbits on the seafloor generated oscillatory currents (e.g. [START_REF] Bagnold | Motion of waves in shallow water. Interaction between waves and sand bottoms[END_REF][START_REF] Perillo | A unified model for bedform development and equilibrium under unidirectional, oscillatory and combined-flows[END_REF], which were not able to transport material over large distance; it only reworked the sediment locally, in contrast to unidirectional currents that can transport material for large distances. Kzl exhibit evidence of current, and they can be considered as thanatocenoses. The rare current structures and the small wave ripples seen in the Ksl are suggestive of a very reduced transport: the fauna can therefore be considered as living or having lived in situ or close to their burial sites, and the Ksl thus likely correspond to biocoenoses.

Sedimentogical study of the Fezouata Lagerstätte provides evidence that rapid burial (here storm deposits) is a necessary pre-requisite condition for exceptional fossil preservation in the Fezouata Shale. Indeed, the known levels containing EPF are always related to storm deposits whereas the finest intervals devoid of storm deposits did not yield EPF. However, rapid burial alone cannot account for the preservation of soft tissues. If similar intervals did not yielded EPF, its means that environmental conditions were not present at this time for promoting the exceptional preservation of the fossils, or that there were no organisms in this place at this time or that fossils are present but not founded. In most BST Lagerstätten, in addition to fast burial, oxygen deficient conditions are also required for exceptional preservation of organic remains as well as by low sulphate concentrations (e.g. [START_REF] Jorgensen | Mineralization of organic matter in the sea bed-the role of sulphate reduction[END_REF]Allison, 1988b;Butterfield, 1995;Gaines et al., 2005Gaines et al., , 2012)). Such environmental conditions very likely occurred in the Fezouata Lagerstätte. Low oxygenation can be inferred by both the presence of "dwarfed" benthic faunas (Lefebvre et al., this issue) and the absence of infaunal bivalves in the Araneograptus murrayi biozone (Polechová et al., this issue). However, the conditions of exceptional preservation in the Fezouata Lagerstätte require further study to determine the specific taphonomic conditions surrounding EPF in the Fezouata.

Conclusion

Fig. 1 .

 1 Fig. 1. Where Do We Come From? What Are We? Where Are We Going? (Museum of Fine Arts;

Fig. 2 .

 2 Fig. 2. The phylogenetic position of animals in the Tree of Life. A, the Tree of Life, according to

Fig. 3 .

 3 Fig. 3. A possible phylogeny of the Apoikozoa (Choanoflagellata+Metazoa), adapted from Budd and

Fig. 4 .

 4 Fig. 4. The main biological, environmental, and geological events leading to the emergence and

Fig. 5 .

 5 Fig. 5. Main steps in the history of life during the Archean and Proterozoïc eons (Ediacaran

Fig. 6 .

 6 Fig. 6. The Ediacaran fauna. A, sea diorama on display in the Smithsonian Museum

Fig. 7 .

 7 Fig. 7. Global distribution of sites where Ediacara-type fossils have been reported (red stars).

Fig. 8 .

 8 Fig. 8. The Avalon assemblage, adapted from Narbonne (2005). A, current-aligned fronds of

Fig. 9 .

 9 Fig. 9. The White Sea assemblage, adapted from Narbonne (2005). Images A-C courtesy of

Fig. 10 .

 10 Fig. 10. The Nama assemblage, adapted from Narbonne (2005). A, mold of the holotype of

Fig. 13 .

 13 Fig. 13. Molecular phylogeny of the animals showing possible positions of selected Ediacaran taxa

Fig. 14 .

 14 Fig. 14. The first lightly skeletonized taxa. A, Cloudina carinata from the terminal Ediacaran

Fig. 15 .

 15 Fig. 15. Published estimates of the protostome-deuterostome divergence time, adapted from Wray

Fig. 16 .

 16 Fig. 16. Analogy between savannah and Ediacaran envionments, modified from Budd and Jensen

Fig. 17 .

 17 Fig. 17. Chronogram of the diversification of major animal groups based on first appearances of taxa

Fig. 18 .

 18 Fig. 18. The historical background of the Cambrian Explosion. A, Buckland (1836). B, Darwin

Fig. 19 .

 19 Fig. 19. The record of ichnofossils during the Cambrian Explosion. A, Schematic reconstructions of

Fig. 20 .

 20 Fig. 20. Typical Early Cambrian assemblage of Small Shelly Fossils (adapted from Bengtson, 2004).

Fig. 21 .

 21 Fig. 21. Distribution of the main Cambrian Lagerstätten (from Conway-Morris, 1990; Hagadorn,

Fig. 22 .

 22 Fig. 22. Exceptional preservation of soft-bodied or lightly sclerotized marine animals from Cambrian

  . A, C, E, G-J are from the Middle Cambrian Burgess Shale (British Columbia, Canada) biota ; B, D, F are from the Early Cambrian Chengjiang biota (Yunnan, China). All photographs from J. Vannier except e (courtesy J.-B. Caron). Scale bars : 1 cm in A, C, D, F-H; 5 mm in B, E, J; 1 mm in I. Though the pattern seen from the skeletal and trace fossil record is mirrored by soft-bodied fossils (Erwinn, 2011), we can only expect to see a relatively complete picture of the early faunas through extraordinarily preserved soft-bodied fossil (Figs. 22 and 23 A; Chen and

Fig. 23 .

 23 Fig. 23. The Cambrian communities. A, an artist view of the Cambrian communities made by Masato

Fig. 24 .

 24 Fig. 24. Timeline of environmental, molecular and ecological events that were proposed as triggers of

Fig. 25 .

 25 Fig. 25. Evaluation of the power of individual factors hypothesized to be relevant to an understanding

Fig. 26 .

 26 Fig. 26. The classical "Sepkoski" diversity curves (black line) of marine invertebrate families through

Fig. 27 .

 27 Fig. 27. The different tempos of diversity increases according to major fossil groups during the

Fig. 28 .

 28 Fig. 28. The colonization of Land during the Ordovician Radiation. A, Stem segment of the lycopod

Fig. 29 .

 29 Fig. 29. The ecological shift of the Ordovician Radiation. A, Reconstruction of a marine ecosystem

Fig. 31 .

 31 Fig. 31. The main triggers commonly invoked to explain the Ordovician Radiation (from Zhang et al.,

Furongian.

  The highest diversities were reached globally in the uppermost Cambrian Stage 10. They precede a substantial phase of acritarch species extinction below and at the Cambrian/Ordovician boundary. Nearly all the genera present in Stage 10 survived into the Ordovician and became the foundation for the rapid radiation of acritarchs during the GOBE.

Fig. 32 .

 32 Fig. 32. The Ordovician context: continental dispersion, high sea level and tropical shelf area

Figure 33 .

 33 Figure 33. Diagram summarizing the factors affecting diversity of Builth Inlier faunas (modified

Fig. 34 .

 34 Fig. 34. The main Cambro-Ordovician Lagerstätten, which yielded soft-bodied macroorganisms

Fig. 35 .

 35 Fig. 35. Recently found soft-bodied taxa in the Early Ordovician showing close affinities to Cambrian

Fig . 36 .

 36 Fig . 36. A, From left to right, Lahcen Ben Moula, Bertrand Lefebvre, Ben Moula's son, and

Fig. 37 .

 37 Fig. 37. Exceptionally preserved soft-bodied organisms from the Fezouata Shale. A, cheloniellid

Fig. 38 .

 38 Fig. 38. Geographic positions of the main localities yielding soft-bodied organisms around Zagora

The

  Rif and Tell domain is located at the far North of Morocco. It corresponds to the remains of the West Mediterranean part of the Alpine orogeny. It comprises allochthonous units, including Palaeozoic sediments, which have already been affected by the previous Hercynian event. The Meseta domain is divided by the middle Atlas into two distinct parts: the western and the eastern Meseta. Three massive Palaeozoic hills are found in the western part: the Rehamma, the Central Massif, and the Jebilet. All are covered by intact Mesozoic to Cenozoic sediments. The Palaeozoic massifs of the Eastern Meseta are far less developped. The Atlas domain includes the High and Middle Atlas. It is a 2500 km long mountain range extending from Morocco to Tunisia (Tunisian Atlas), and resulting from the convergence between Africa and Europe during the Cenozoic (Frizon de Lamotte et al., 2000; Bracène and Frizon de Lamotte, 2002). Its base is Hercynian and covered by folded Mesozoic to Cenozoic sediments. The global direction of this domain is East-West, so that it constitutes a natural barrier separating the northern Meseta and the southern Anti-Atlas domains. The 700-km-long Anti-Atlas domain is extending Northeast-Southwest. It is bounded by the South-Atlas fault to the North and the Reguibat ridge to the South. Its base is Precambrian and has been affected by the Panafrican orogeny. Late Proterozoic and lightly folded Palaeozoic sediments were deposited on this base. On top of that, intact Mesozoic to Cenozoic sediments can be found. It comprised the thickest Palaeozic sequence of Morocco. Finally, the Reguibat Arch, which is mainly located in Algeria and Mauritania, used to belong to the West-African Precambrian shield. It has been strongly affected by the Panafrican orogeny before being covered by Palaeozoic to Cenozoic sediments in the Tindouf bassin.

Fig. 39 .

 39 Fig. 39. The structural Geology of Morocco (adapted from Michard et al., 2008). A, Tectonic map of

Fig. 40 .

 40 Fig. 40. The oldest occurrence of the term Anti-Atlas on a map (from Hooker and Ball, 1878). Arrow

  : western, central, and eastern parts. The western part runs from the Atlantic Ocean through the Souss plain along a NW-SE axis, which is imprinted by the Meseta. This part includes the thickest Palaeozoic deposits of the Anti-Atlas, but also the most affected by the Hercynian orogeny. The central Anti-Atlas, in which the Fezouata Lagerstätte (Van Roy et al., 2010) is located, follows the direction of the Atlas, being primarily oriented NE-SW from the Souss plain, then E-W to the Draa. Valley. The eastern part is dominated by the direction of the Ougarta (NW-SE), starting from the Draa Valley to the Gui Hamada at the East. No precise borders have been defined

Fig. 41 .

 41 Fig. 41. Simplified geological map of the Moroccan Anti-Atlas (courtesy of Romain Vaucher, after

Fig. 42 .

 42 Fig. 42. Lithostratigraphic framework of the Ordovician in the Anti-Atlas (Courtesy of Romain

  of the ANR projectRALI (2012RALI ( -2015) ) and the French-Moroccan CNRS-CNRST (SDU 05/09 & SDU 02/13) projectVALORIZ (2013VALORIZ ( -2015)), two field campaigns logged in detail most of the Fezouata Shale Formation, in order to identify the precise stratigraphic positions of all main fossiliferous levels yielding soft-bodied preservation. Based on extensive fieldwork, fossil evidence, and facies recognition, my colleagues and myself could show that exceptional preservation in the Fezouata Shale essentially occurs within two narrow c. 25 and 15 m thick stratigraphic intervals, both of late Tremadocian age according to graptolites (Araneograptus murrayi and Hunnegraptus copiosus graptolite zones). Moreover, sedimentological features indicate that communities of the Fezouata Biota lived under distal storm influence, at relatively shallow-water environmental conditions in a lower shoreface to upper offshore siliciclastic ramp setting. This study was accepted by the journal Gondwana Research (IF: 8,235) and is available online since the 5 th May 2015. I conducted this research and participated at the logging of the Fezouata Shale Formation and definition of the sea-level variation, together with Romain Vaucher and Bernard Pittet, who did most of the sedimentological interpretation of the Fezouata Shale. I also collected numerous fossils and did a first attempt of the taxonomic recognition of the graptolite fauna, which was later refined under the expertise of Juan Carlos Gutiérrez-Marco. I also wrote the first draft of this paper, which was improved in collaboration with Juan-Carlos Gutiérrez-Marco, Bertrand Lefebvre, Rudy Lerosey-Aubril, Bernard Pittet, Jean Vannier. Hendrik Nowak, Thomas Servais and Thijs Vandenbroucke brought information on the age suggested by palynomorphs. Peter Van Roy gave critical information on the positions of the localities and their faunistic content. Khadija El Hariri and Moussa Masrour preciously helped us on the field, as well as most of co-authors mentioned above.

Fig. 1 .

 1 Fig. 1. Stratigraphic distribution of the main diverse exceptionally preserved faunas from the Cambrian and the Ordovician. Numerical ages in Ma.

Fig. 2 .

 2 Fig. 2. A, Lower Ordovician outcrops of northern Morocco and location of Zagora; B, Palaeogeography of Africa (blue star represents Zagora) in the Early Ordovician (modified from Cocksand Torsvik, 2004; Torsvik and Cocks, 2011); and C, Lower Ordovician outcrops in the Zagora area. Small black crosses indicate logged sections and red crosses the three key sections used to build the composite stratigraphic column (Fig.4). Small grey crosses and blue asterisk indicate the inspected and non-inspected localities that yielded EPF respectively(Van Roy et al., 2010; modified). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3 .

 3 Fig. 3. Examples of exceptionally preserved fossils from the Fezouata Biota (latest Tremadocian; Zagora area, Morocco). A, undetermined palaeoscolecid worm, AA-BGF2-OI-1; B, Pirania auraeum Botting, 2007, demosponge, AA-JBZ-OI-115; C-E, arthropods with preserved appendages (arrows); C, Furca sp. (Marrellomorpha), AA-BIZ31-OI-39; D, Xiphosuran (Chelicerata) with fully segmented opisthosoma, FSL 712 411; E, Bavarilla sp. (Trilobita), AA-BIZ15-OI-16. All specimens were photographed under alcohol. Scale bars: A-C, 5 mm; D-E, 2 mm.

Fig. 4 .

 4 Fig. 4. Partial composite section of the Fezouata Shale (Lower Ordovician, Zagora area) showing the distribution of Exceptionally Preserved Fossils (EPF; red stars) and the vertical range of diagnostic graptolites. Right columns modified from (1) Loydell (2012) and (2) Bergström et al. (2009). The colours in the logged section roughly correspond to the colours of outcropping rocks. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5 .

 5 Fig. 5. Graptolites of the Fezouata Biota (late Tremadocian; Zagora area, Morocco). A, Araneograptus murrayi (Hall, 1865), AA-BIZ3-OI-35 showing nematularium (upper left) and distal end on another young colony (centre-right); B, Paradelograptus tenuis Lindholm, 1991, AA-OFTa-OI-23; C, Koremagraptus sp., AA-FETi-OI-1; D, Clonograptus rigidus (Hall, 1858), AA-JTZb-OI-19; E, Paradelograptus norvegicus (Monsen, 1937), AA-JTZ-OI-10; F, "Tetragraptus" bulmani Thomas, 1973, AA-BIZ13-OI-9; G, "Didymograptus sp. 1" (cf. Lindholm, 1991), AA-BIZ31-OI-40; H-I, Paratemnograptus magnificus (Pritchard, 1892), fragment of a large rhabdosome (F, AA-BZG-OI-13) and detail of a stipe showing plaited thecal structure (triad budding without bithecae), AA-BIZ13-OI-10; J, Dictyonema? sp., AA-BZG-OI-14; K-M, "Kiaerograptus" supremus Lindholm, 1991, K = AA-BGF2-OI-7; L = AA-BGF2-OI-6, with detail of the proximal end (M) showing sicular bitheca (arrow); N-O, Hunnegraptus copiosus Lindholm, 1991, N = AA-JBZ-OI-117; O = AA-JBZ-OI-116; and P, Paradelograptus sp., AA-JTZ-OI-9. All specimens, except A, H, I and M, were photographed under alcohol. Scale bars: A, D, and H, 20 mm; E, 10 mm; C, F, K, L, and J, 5 mm; B, G, N, and O, 3 mm; I, and P, 2 mm; and M, 0.5 mm.

Fig. 6 .

 6 Fig. 6. Typical sedimentary structures used to characterise the palaeoenvironment of the Fezouata Shale. A-B, 2D-wave ripples in a distal tempestite, in section and slab surface, respectively. C, small, centimetre-scale hummocky cross stratifications (HCS) from the top of the section. D, thin, possibly wave-induced cross-laminated structures in a distal silty tempestite. E, general palaeoenvironmental reconstruction for the Fezouata Shale deposition. Terminology of depth zones follows the definition of McLane (1995). Scale bars: A-B, 1 cm; and C-D, 3 cm.

  fig.S2h-i), figured specimens of ?Clonograptus (=P. norvegicus) and putative tuboids,

  Destombes' collection (prefixed JD) are housed in the Ministry of Energy, Mines, Water and Environment (Rabat). Aditional material collected by the authors was deposited in the Faculty of Sciences and Technics of the Cadi-Ayyad University of Marrakesh (prefixed AA, material from Martin et al., in press), and in the Department of Palaeontology of the Complutense University of Madrid (temporary deposit, prefixed DPM).

Fig. 1 .

 1 Fig. 1. A-B, Location sketch-map of all known occurrences of Ordovician graptolites in the Lower

for localities: 1 ,

 1 Jbel El Khannfra; 2, Kheneg M'Karz (Aouin-Deliouine); 3, Taskala; 4, W Torkoz (Aouinet-Torkoz); 5, Timoulaye (SE Bou-Izakarn); 6, Tisgui Cheikh (NW Foum-el-Hassane); 7, Foum-Akka n'Tamia (W Akka); 8, Kheneg/Hassi Brahim (W Tata); 9, Adrar n'Tassefat (E Tata); 10, Assif n'Tafraout n'Bani (W Foum-Zguid); 11, Oued Lamhamid/Hdeb Mhdi (NW Foum-Zguid); 12, Zaouit/Sidi Blal (SE Tazenakht); 13, SW Ksar Igdaoun (= "Oulad Slimane" or "Oulad-Atmane" locality of Fortey, 2009 and Moroccan trilobite dealers); 14, diverse sections 15-25 km N Zagora ("Bou Izargane", Oued Tarhia n'Imrhaldene, Oued Ouaoufraou and others); 15, Jbel Amergou (W Zagora) and Jbel Adafane (E Zagora); 16, E Jbel Bou Dehir (NE Zagora); 17, Jbel Tibasksoutine (E Zagora); 18, N Timiderte (N Jbel Kissane); 19, N Jbel Taddrist; 20, Akka n'Ousdidene (E N'kob/Nekob); 21, Bou Ljir (NE Mlal/Mellal); 22, Isk n'Tafsast (N Tazzarine); 23, Isk Tamotjane (Amerdoul n'Oufsad); 24, Ikhf n'Ourarh (Arhenbou n'Dali); 25, Afrou n'Tarhoucht (S Tinejdad); 26, Tourza; 27, Tizi n'Taourakt (N Istlhou); 28, Jbel Bou Legroun (NW Rissani); 29, Jbel Signit (E M'cissi/Mecissi); 30, Jbel Tijekht; 31, E Jbel Tazoult-n'Ouzina (NW Ouzina); 32, N Jbel Zorg (SSE Tawz/Taouz).

Fig. 2 . 6 ,

 26 Fig. 2. Stratigraphic distribution of selected graptolite taxa from the Fezouata Formation in the

  BiozoneAbove a large stratigraphic interval (up to 250 m thick) with rare fragments of unidentifiable graptolites, the Araneograptus murrayi Biozone begins with mass-occurrences of the nominal form, first forming monospecific assemblages and gradually incorporating other biramous to multiramous taxa. Higher in the biozone Ar. murrayi is less common, occurring as single rhabdosomes and small fragments on bedding planes that are dominated by long stipes of big forms such as Paratemnograptus and Clonograptus. The identification and vertical range of graptolite species recognized within this biozone has been illustrated in a partial composite section presented byMartin et al. (in press, fig. 4). It resulted from bed-bybed exploration of the Fezouata Lagerstätte through more than 80 m of strata. The zonal assemblage is composed of Ar. murrayi(Hall) (Fig. 4A-F), "Kiaerograptus" supremus Lindholm, Paratemnograptus magnificus (Pritchard) (Fig. 4G), Paradelograptus norvegicus (Monsen) (Fig. 3R-U), P. tenuis Lindholm, Paradelograptus sp., "Didymograptus sp. 1"

  [START_REF] Ortega | Tremadocian graptolite-conodont biostratigraphy of the South American Gondwana margin (Eastern Cordillera, NW Argentina)[END_REF][15], and is associated with B. deflexus (Elles and Wood) (Fig.4K-L) at the horizon of 112 m below the Zini Formation. In Scandinavian sucessions, B. deflexus is restricted to the uppermost part of the B. minutus Biozone (Maletz and Ahlberg, 2011). Therefore, the cooccurrence of both species in the lower part of the Moroccan B. minutus Biozone in Zagora may indicate that a major part of this biozone essentially corresponds to the upper part of the B. minutus Biozone on peri-Gondwana and Baltica.

  highest graptolites recorded in the Fezouata Formation, from beds 5-63 m below the top of the formation in the eastern Anti-Atlas (Destombes 2006e; Fig. 1, locs. 26 and 29), are forms similar to B. deflexus and the specimens of "A. nicholsoni" reported from the western Anti-Atlas (Destombes 2006h; Fig. 1, loc. 9). However, in two different sections of the central Anti-Atlas, monospecific assemblages of Azygograptus have been discovered 15-49 m below the base of the Zini Formation (Fig. 1, locs. 15 and 17). Drawings of nice rhabdosomes of Azygograptus sp. provided by

Fig. 3 .

 3 Fig. 3. Tremadocian graptolites from the Fezouata Formation, southern Morocco.

Fig. 4 .

 4 Fig. 4. Tremadocian (A-G) and Floian (H-Q) graptolites from the Fezouata Formation, southern

Fig. 5 .

 5 Fig. 5. Correlation of Lower Ordovician graptolite zones (for details see text), after revising the

  characteristic of the genus Rhabdinopora, and can be synonymized with other currently accepted subspecies of R. flabelliformis. According toCooper et al. (1998),[START_REF] Cooper | Ecostratigraphy, zonation and global correlation of earliest Ordovician planktic graptolites[END_REF], and[START_REF] Goldman | Graptolite palaeobiogeography[END_REF], lower and middle Tremadocian graptolite biozones can be used on a worldwide scale because of the absence of provincialism. The existence of these Saharan endemics has not been confirmed in the adjacent Moroccan Anti-Atlas, although some Gondwanan endemics seem to occur for example in northern Argentina, where R. f. acenolazai constitutes a mimic of the old subspecies R. f. parabola[START_REF] Gutiérrez-Marco | Graptolitos del Tremadociense (Ordovícico Inferior) de la Formación Volcancito, Sistema de Famatina (La Rioja, Argentina)[END_REF] being recorded as high as the An. matanensis biozone.

  in press a,Lefebvre et al., in press b, Gutiérrez-Marco and Martin, submitted).During the logging of the 950 m-thick Fezouata Shale in 2012 and 2013, several potential excavation sites were identified in the 60 meter-thick, lower stratigraphic interval (A. murrayi to H. copiosus zones, late Tremadocian), yielding the most abundant and diverse soft-bodied assemblages. All candidate sites were visited again in 2013, and the Bou Izargane hill was definitively retained as the final choice (Fig.43A). The second criterion was geomorphologic: as the dip of the Lower Ordovician succession is very gentle (about 1°) in the area N. of Zagora, the presence of slopes/hill was a pre-requisite for opening large excavations in different stratigraphic levels. For this reason, the fossiliferous site of Bou Glef was discarded, as it was located on a mostly flat area in the Ternata plain. Conversely, the extremely wellexposed succession of Jbel Tizagzaouine (logged in 2013) was also discarded, because the flanks of this hill were extremely steep, thus making it extremely difficult to dig out large surfaces. The gentle slopes of the Bou Izargane hill were offering a good compromise, with the possibility to exploit large surfaces and distinct stratigraphic levels. The third and last criterion was to be both easily accessible by van and/or large cars from Zagora and to be far enough from the villages. The hill of Bou Izargane was also meeting this last condition, as it could be easily accessed using a regular track from the small village of Beni Zoli, located about 7.5 km SW of the excavation site (Figs.43 B and C).

Fig. 43 . 2 Methodology

 432 Fig. 43. The excavation site at Bou Izargane Hill. A, panoramic view of the excavation site.

Fig. 44 .

 44 Fig. 44. The excavation team. A: Rudy Lerosey-Aubril. B: Mohammed 'Ou Saïd' Ben Moula

Fig. 45 .

 45 Fig. 45. The excavation settlement at Bou Izargane hill. A, levelling of a former excavation

Fig. 46 .

 46 Fig. 46. The hard life of an excavation leader. A, our hardware store in Marrakesh. B, myself

Fig. 47 .

 47 Fig. 47. Detailed log of the excavation campaign (modified from Vaucher et al., in appendix

Fig. 48 .

 48 Fig. 48. Examples of shelly and soft-bodied fossils found at Bou Izargane hill. A-B,

Fig. 49 .

 49 Fig. 49. The poorly diversified communities of the middle fossiliferous interval at Bou

  ) the lower (and spatially smallest one) consists of an almost monospecific dense assemblage of marrellomorphs (Furca); (2) the intermediate one is largely dominated by trilobites; and (3)the upper (and most extensive one) is a stylophoran-dominated assemblage. This suggests that the low global diversity observed for the whole lens (9 taxa) is indeed overestimated, and results from the combination of three distinct assemblages, each characterized by an even lower diversity and dominated by distinct taxa, with from bottom to top: a Furca-dominated

Fig. 50 .

 50 Fig. 50. Detailed sedimentological and palaeontological study of the middle main excavation

  few soft-bodied organisms have been recorded so far (e.g., a handful of individuals of the demosponge Pirania, few specimens of cheloniellids), in spite of 15 years of intensive quarrying activity (e.g. at Bou Chrebeb, Jbel Bou Zeroual or Toumiat). The presence of softbodied structures (e.g., trilobite gut; seeVan Roy et al., 2010) and organisms in these levels suggests that the rarity of unmineralized and lightly sclerotized taxa in these levels does not simply result from a taphonomic bias. Although the effect of a putative sampling bias can not be entirely ruled out (was the sampling effort similar in late Tremadocian and Floian intervals with exceptional preservation?), it seems plausible that the extreme rarity of "Cambrian-like" taxa in the largest part of the Fezouata Shale more likely results from the occurrence of more hospitable, well-oxygenated environmental conditions, more favorable for the colonization of the sea-floor by stable communities dominated by K-strategist taxa belonging to the PEF.At a more global scale, the Lower Ordovician succession of the Zagora apparently illustrates the progressive opening of an intracratonic basin, in a general distensive context affecting the high-latitude southern margin of Gondwana and leading to the rifting of Avalonia away from it. In the 240 basal meters of the Fezouata Shale (early-middle Tremadocian), the occurrence of a high microphytoplankton productivity (abundant and diverse assemblages of acritarchs and chitinozoans) contrasts with the extreme rarity (and in most cases, absence) of benthic fauna. This situation probably results from the occurrence of a stratified water column, with a well-oxygenated upper portion and an anoxic, lower part. The late Tremadocian part of the succession is characterized by the succession of brief intervals yielding highly diverse benthic assemblages, separated by the levels typical of the "lower" interval with exceptional preservation, characterized by massive occurrences of low-diversity assemblages. In this context, the late Tremadocian part of the sequence can be interpreted as transitional, with brief episodes of well-oxygenated sea-floors (mixed water column) interrupted by intervals of more unhospitable, dysoxic conditions, favorable for the establishment of opportunistic communities dominated by "Cambrian-like". Finally, the occurrence of diverse, abundant and extremely stable benthic communities dominated by PEF taxa in the upper part of the Fezouata Shale suggests the definitive onset of a mixed water column and well-oxygenated sea-bottom conditions.

Fig. 51 .

 51 Fig. 51. Comparison of ecospace utilization between a shelly fossil site and Bou Izargane

2 E

 2 .L.O. Martin et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx-xxx Please cite this article as: Martin, E.L.O., et al., Palaeoscolecid worms from the Lower Ordovician Fezouata Lagerstätte, Morocco: Palaeoecological and palaeogeographical implication, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.04.009

Fig. 1 .

 1 Fig. 1. Geographic and stratigraphic locations of excavation sites yielding Palaeoscolex? tenensis Kraft and Mergl, 1989 in the Fezouata Shale. A, location of Zagora in Morocco. B, location of the excavation sites yielding palaeoscolecids in the vicinity of Zagora. C, composite stratigraphic column showing the locations of the palaeoscolecid-bearing horizons (red stars) within the Fezouata Shale (modified after Martin et al., in press). Background images in A and B from Google Earth. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2 .

 2 Fig. 2. Tremadocian Palaeoscolex? tenensis Kraft and Mergl, 1989 from the Fezouata Shale (Zagora area, Morocco). A, B, trunk fragment (AA-BIZ30-OI-62-(1)a-2). C, cluster of trunk fragments (AA-BIZ30-OI-33-(1)). D, Interpretative drawing of C, trunk fragments are highlighted in grey. E, cluster of trunk fragments (AA-BIZ30-OI-63-(2)). F, almost complete and partially coiled (posterior region?) specimen (AA-BIZ30-OI-62-(1)b-1). G, trunk fragment (AA-BIZ30-OI-23-(5)). H, trunk fragment (AA-BIZ30-OI-23-(2)-1). A, C, E and H, specimens immersed in dilute ethanol. B, SEM picture. Scale bars: 2 mm for C-F, 1 mm for G, 500 μm for A, B and H.

Fig. 3 .

 3 Fig. 3. Posterior region of the body of Palaeoscolex? tenensis Kraft and Mergl, 1989 from the Fezouata Shale. A-E, specimens immersed in dilute ethanol, showing a pair of aboral hooks. A, specimen AA-BIZ30-OI-62; note that the hooks are adpressed on the body in this specimen. B, specimen AA-BIZ30-OI-63-(2). C, D, specimen AA-BIZ30-OI-23-(1)a-1, before (C) and after (D) preparation. E, specimen AA-BIZ30-OI-63-(3)b-1. Scale bars: 500 μm for C-E, 200 μm for A-C.

Fig. 4 .

 4 Fig. 4. Compositional analyses of a specimen of Palaeoscolex? tenensis Kraft and Mergl, 1989 (AA-BIZ30-OI-63-(1)-2). A, picture of the specimen immersed in dilute ethanol. B, SEM picture of the specimen. C, D, EDX spectra of the surrounding matrix (C) and a plate (D). E, SEM picture and results of elemental mapping of a portion of the specimen. Scale bars: 500 μm for A and B, 200 μm for E.

Fig. 5 .

 5 Fig. 5. Plate morphology and palaeobiogeography of Palaeoscolex? tenensis Kraft and Mergl, 1989. A-C, E, late Tremadocian specimens from the Fezouata Shale (Morocco), SEM pictures. A, three incomplete rows of plates (AA-BIZ30-OI-63-(1)-2), exhibiting external surfaces with scalloped margins. B, incomplete row of plates, including two interlocked ones (AA-BIZ30-OI-63-(1)-2). C, visceral surface of a single plate showing circular pits (AA-BIZ30-OI-63-(2)-1). E, incomplete row of plates (AA-BIZ30-OI-63-(2)-1) exhibiting external surfaces with scalloped margins. D, F, Floian specimen (GS-MM 199) from the Klabava Formation (Czech Republic), SEM pictures of latex cast; courtesy of M. Mergl. D, rows of plates exhibiting external surfaces with scalloped margins. F, visceral surface of a single plate showing two rows of circular pits. G, palaeogeographic distribution of Palaeoscolex? tenensis in the Early-Middle Ordovician: Fezouata Shale in Morocco (1), Klabava Formation in Czech Republic (2), Dubowiki Formation in Estonia (3), and Komstad Limestone and 'Orthoceras Limestone' in Sweden (4); palaeogeographical map modified from Torsvik and Cocks, 2013. Scale bars: 150 μm for D and E, 40 μm for A, 20 μm for B, C and F.

Fig. 6 .

 6 Fig. 6. Digestive system of palaeoscolecids and Tomaculum-type coprolites. A, B, specimen (YPM IP 525768) of Palaeoscolex? tenensis Kraft and Mergl, 1989 from the Fezouata Shale with sediment-filled gut. A, general view. B, detailed view showing lateral constriction of the gut infilling (black arrows). C-D, Tomaculum-type coprolites co-occurring (same slabs) with specimens of Palaeoscolex? tenensis Kraft and Mergl, 1989 from the Fezouata Shale. C, specimen (AA-BIZ30-OI-23-(1)b-4) immersed in dilute ethanol. D, specimen (AA-BIZ30-OI-23-(1)b-3) immersed in dilute ethanol. E, sediment-infilled gut in a specimen (MGUH 29149) of Chalazoscolex pharkus from the lower Cambrian Sirius Passet, Greenland; courtesy of J. Peel. Scale bars: 5 mm for E, 2 mm for A, B and D, 1 mm for C.

  6.3. Palaeogeographical implications P.? tenensis occurs in the Lower Ordovician of Morocco (late Tremadocian) and Czech Republic (Floian;[START_REF] Kraft | Worm-like fossils (Palaeoscolecida;? Chaetognatha) from the lower Ordovician of Bohemia[END_REF]; it 9 E.L.O. Martin et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx-xxx Please cite this article as: Martin, E.L.O., et al., Palaeoscolecid worms from the Lower Ordovician Fezouata Lagerstätte, Morocco: Palaeoecological and palaeogeographical implication, Palaeogeogr. Palaeoclimatol. Palaeoecol. (

Fig. 52 .

 52 Fig. 52. Soft-part preservation in the species Anacheirurus adserai. A-C, preservation of

Fig. 53 .

 53 Fig. 53. Soft-part preservation in the species Bavarilla zemmourensis. A-B part and

  in the journal Palaeogeography, Palaeoclimatology, Palaeoecology (IF: 2,339). Together with Muriel Vidal, I reviewed the whole trilobite collections at Lyon and Brest, which were mostly collected by Jacques Destombes, Bertrand Lefebvre, and Daniel Vizcaïno from the Fezouata Shale. Romain Vaucher defined a precise depth-related zonation for each excavation yielding trilobite remains. I compiled the data gathered from 485 specimens in a database and ran the statistical and palaeoecological studies of the trilobites communities. Muriel Vidal reviewed the literature to compare the communities found with other well-known ones from Lower Ordovician localities. Bertrand Lefebvre and myself collected rock samples for geochemical analyses that were performed and interpreted by Pierre Sans Jofre. With the precious help of Muriel Vidal, I wrote a first draft of the manuscript, which was later improved in collaboration with all co-authors.

Fig. 54 .

 54 Fig. 54. The pilekiid-bavariid biofacies. A, Anacheirurus adserai. B, Symphysurus

Fig. 55 .

 55 Fig. 55. The Agerina and raphiophorid biofacies. A-C, the Agerina biofacies. A, Agerina

Fig. 1 .

 1 Fig. 1. (A) Satellite image (Google Earth) of the studied area, locations and names of the excavations

Fig. 2 .

 2 Fig. 2. Biostratigraphy of the trilobite fauna from the Lower Ordovician Fezouata Shale. The

Fig. 3 .

 3 Fig. 3. Definition of trilobite biofacies along an onshore-offshore transect. Depth zonations follow

Fig. 4 .

 4 Fig. 4. Relative abundance of articulated (left column) against disarticulated (right column) specimens

  France), as indicated by the shared occurrence of several species: Agerina quadrata, Ampyx priscus, Colpocoryphe pradesensis, C. thorali, Foulonia peregrina and Parabathycheilus gallicus. Two of them, Ampyx priscus and Parabathycheilus gallicus, are characterized by a long stratigraphic range in the Anti-Atlas, extending from the late Tremadocian (A. murrayi Zone) to the middle Floian (?B. jacksoni). Similarly, in the Montagne Noire, these two taxa show relatively large stratigraphic extensions(Vizcaïno et al., 2001). In that region, A. priscus is known from the Saint-Chinian (late Tremadocian) to the Landeyran Formation, and P. gallicus is recorded in both the Foulon (middle Floian) and Landeyran formations (late Floian). Consequently, Ampyx has a slightly longer range in the Montagne Noire, whereas that of Parabathycheilus is shorter. The late Floian trilobite faunas from the Landeyran Formation show strong affinities with the slightly older Floian assemblages of the Zagora area, with for example the occurrence of three common species: Agerina quadrata, Foulonia peregrina and Colpocoryphe thorali. Both in Morocco and in the Montagne Noire, the stratigraphic occurrence of C. thorali is preceded by that of a closely related species: C. pradesensis. In the Montagne Noire, Colpocoryphe pradesensis (originally assigned to Platycalymene by[START_REF] Courtessole | Calymenina, Echinodermata et Hyolitha de l'Ordovicien inférieur de la Montagne Noire (France méridionale)[END_REF] occurs in the la Maurerie Formation (early Floian), and in Morocco, it was documented in Destombes locality 35 (early Floian). The observation of the same succession of closely related species in the two regions supports the existence of common evolutionary trends and strong faunal exchanges between them.However, significant differences also exist between the trilobite faunas from Morocco and those of southern France. First, faunal elements adapted to deep platform settings (i.e. Bohemilla (Fenniops), Cyclopyge, Degamella, Pricyclopyge) are present in the Montagne Noire(Vizcaïno et al., 2001), whereas they are absent in the central Anti-Atlas. Their absence in Morocco is in good agreement with sedimentological data, all suggesting that the depositional environment of the Fezouata Shale was relatively shallow, from lower shoreface to upper offshore and probably never reached the most distal part of the shelf (see above, section 4.3; Vaucher et al., this issue). Consequently, the absence of deeper water trilobites in the Anti-Atlas probably results from the original absence of corresponding outer shelf environments in this region and/or the absence of exchanges with an oceanic basin.

Fig. 5 .

 5 Fig. 5. Geochemical analysis performed on the first Late Tremadocian interval of the Fezouata Shale

assemblage 2

 2 extends from the late Tremadocian to the middle Floian, and assemblage 3 is observed only the early-middle Floian. Consequently, though less precise than graptolites, trilobite assemblages can be used as a complementary stratigraphic tool in the Lower Ordovician of Morocco. A dramatic faunal turnover takes place at the Tremadocian/Floian boundary, with the disappearance of several typical late Tremadocian endemic taxa (e.g. Anacheirurus, Bavarilla, Dikelokephalina), and their replacement by Floian (peri)Gondwanan assemblages dominated by Calymenidae (in particular, Reedocalymeninae and Colpocoryphinae). Strong affinities between trilobite faunas from the Floian of Morocco and Montagne Noire suggest the existence of faunal exchanges between these two areas.

Figure 1 .

 1 Figure 1. Location and stratigraphic positions of all stylophoran occurrences described in the Lower Ordovician of Zagora area. A. Simplified geological map of central Anti-Atlas, Morocco. Ordovician outcrops are indicated in green. B. Partial log of the Lower Ordovician succession in Zagora area (interval comprised between 170 and 405 m above the unconformity between the Tabanite Group and the Fezouata Shale) showing the stratigraphic positions of stylophoran beds (in red) and stylophorans occurring within diverse benthic assemblages (in blue). Z-F3: locality NE of Beni Zoli village(Lefebvre, 2007); Z-F2: Jbel Tizagzaouine (new assemblage); Z-F2(1): locality S of Jbel Tizagzaouine(Lefebvre and Botting, 2007); 1685: type-locality of Thoralicystis zagoraensis(Chauvel, 1971).

  -97 TAPHONOMY OF A STYLOPHORAN-DOMINATED ASSEMBLAGE IN THE LOWER ORDOVICIAN OF ZAGORA AREA (CENTRAL ANTI-ATLAS, MOROCCO)

Figure 2 .

 2 Figure 2.

Figure 2 .

 2 Figure 2. Different types of preservation in the stylophoran bed of locality Z-F2 (Jbel Tizagzaouine, Zagora area), Lower Ordovician (late Tremadocian, A. murrayi biozone). All photographs show original specimens. A-C. Isolated cornutes in uppermost part of fossiliferous layer. A-B. MHNN.P.045281. A. Large slab showing four complete individuals and a few isolated skeletal elements. B. Cluster of three specimens: two of them are in lower aspect, one is in upper aspect. C. Large slab showing six complete individuals (all in lower aspect), associated with portions of other specimens, isolated plates, and numerous poorly preserved tiny gastropods, MHNN.P.045296. D-F. Accumulation layer full of complete cornutes. D. AA.JTZ.OI.5. E. AA-JTZ-OI-50. F. AA-JTZ-OI-14. G-I. Bioclastic lag at base of stylophoran bed. G. AA-JTZ-OI-50. H. MHNN.P.045295. I. AA-JTZ-OI-34. Repositories: Cadi-Ayyad University, Marrakesh (AA), Muséum d'Histoire naturelle, Nantes (MHNN).

Fig. 56 .

 56 Fig. 56. Mass occurrence of the trilobite Platypeltoides magrebiensis. A, high resolution panorama of the 9 m 2 slab. B, close-up of the slab showing some of the most complete

Fig. 57 .

 57 Fig. 57. Analyses of the massive monospecefic occurence of Platypeltoides magrebiensis. A,

Fig. 58 .

 58 Fig. 58. Linear cluster of Ampyx priscus from the Fezouata Shale and equivalent formations

Fig. 59 .

 59 Fig. 59. Linear clusters of Ampyx priscus and Eoampyxinella villebruni from the Lower

Flanc

  à trilobites avec appendices Flanc Sud de Bou Izargane, Z-F4(8), site à trilobites avec appendices, 30°29'52,28''N, 05°50'51,72''W, province de ZAGORA, Anti-Atlas central, MAROC Ordovicien inférieur, Trémadocien supérieur, Formation inférieure des Fezouata, Zone à A. murrayi Trémadocien supérieur, Formation inférieure des Fezouata, Zone à A. murrayi Bou Izargane, Chantier 3, fouille du bas, niveau à euloma, bavarilla, lehua, furca, thoralicystis et eldonioide, 30°29'59,5''N, 05°50'58,00''W, province de ZAGORA, Anti-Atlas central, MAROC Flanc Ouest de Bou Izargane, Chantier 3, fouille du bas, niveau à euloma, bavarilla, lehua, furca, thoralicystis et eldonioide, 30°29'59,5''N, 05°50'58,00''W, province de ZAGORA, Anti-Atlas centralOrdovicien inférieur, Trémadocien supérieur, Formation inférieure des Fezouata, Zone à A. murrayi Ordovicien inférieur, Trémadocien supérieur, Formation inférieure des Fezouata, Zone à A. murrayi supérieur, Formation inférieure des Fezouata, Zone à A. murrayi Flanc Ouest de Bou Izargane, Chantier 3, fouille du bas, niveau à euloma, bavarilla, lehua, furca, thoralicystis et eldonioide, 30°29'59,5''N, 05°50'58,00''W, province de ZAGORA, Anti-Atlas centraldu premier Flanc Ouest de Bou Izargane, BS4(b), équivalent gauche N6a premier chantier, 30°30'00,87''N, 05°50'57,60''W, province de ZAGORA, Anti-Atlas central, MAROC Ordovicien inférieur, Trémadocien supérieur, Formation inférieure des Fezouata, Zone à A. murrayi BS4(a) extension latérale droite équivalente au niveau 6a du premier chantier Flanc Ouest de Bou Izargane, BS4(a), équivalent droite N6a premier chantier, 30°30'00,56''N, 05°50'57,16''W, province de ZAGORA, Anti-Atlas central, MAROC Ordovicien inférieur, Trémadocien supérieur, Formation inférieure des Fezouata, Zone à A. murrayi Niveau du bas 1.3 Database of the trilobites having appendages preserved so far 1.4 Supplementary data of Martin et al., submitted. Database including the listing, counting, and determinations of each trilobite species per locality. Presence/absence of trilobite species in each locality are also given, as well as the depth settings for each locality. ., Rabano, I. (Eds.), Progress in Echinoderm Palaeobiology.

Figure 1 .

 1 Figure 1. Location and stratigraphic position of Rhopalocystis assemblages. A. Simplified geological map of the central Anti-Atlas, Morocco. Ordovician outcrops are indicated in green. B. Partial log of the Lower Ordovician succession in the Zagora area (interval between 170 and 405 m above the unconformity between the Tabanite Group and the Fezouata Shale) showing the stratigraphic position of Rhopalocystis beds. Locality 1: Tansikht bridge (= Destombes locality 2082); locality 2: Jbel Bou Dehir (= Destombes localities 1157 and 1725); locality 3: Jbel Bou Dehir (= Destombes localities 1737, 1738 and 1750); locality 4: W. of Agdz (= Destombes locality 1773); locality 5: Jbel Bou Dehir (= Destombes locality 2367); locality 6: Tameraout, Z-F1; locality 7: Bou Chrebeb, Z-F25; locality 8: small hill, N of Jbel Tizagzaouine, Z-F12c.

Figure 2 .

 2 Figure 2.

2 B.

 2 Lefebvre et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx-xxx Please cite this article as: Lefebvre, B., et al., Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02.039 Atlas (locality 1685; Fig. 1C). Chauvel (1969) also reported the presence of two other taxa: M. cf. mariae, from various sites in the western, central and eastern Anti-Atlas (localities 302, 1540, 1769 and 1773; Fig. 1B), and a new species, M. tasseftensis, restricted to the western Anti-Atlas (localities 289 and 303; Fig.

Fig. 1 .

 1 Fig. 1. Locality maps of main Lower Ordovician echinoderm sites from the Anti-Atlas, Morocco. (A) Location of the Anti-Atlas range in northwestern Africa. (B) Map of the Anti-Atlas showing the distribution of Ordovician outcrops, the location of the Zagora area and of fossiliferous localities mentioned in the text and/or listed in Appendix 3. (C) Landsat view of the Zagora area showing the location of all main Early Ordovician echinoderm localities mentioned in the text and/or positioned on the logs (Figs. 2-3); photograph courtesy of the U.S. Geological Survey.

  et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx-xxx Please cite this article as: Lefebvre, B., et al., Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02.039

5

  B. Lefebvre et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx-xxx Please cite this article as: Lefebvre, B., et al., Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02.039

7

  B. Lefebvre et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx-xxx Please cite this article as: Lefebvre, B., et al., Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02.039 Please cite this article as: Lefebvre, B., et al., Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02.039

Fig. 3 .

 3 Fig.3. Detailed, stratigraphic columns showing the late Tremadocian interval (A. murrayi Zone and part of the H. copiosus Zone) containing most echinoderm localities and levels with exceptional preservation (Fezouata Biota). Stratigraphic columns obtained from the logging of three sections: Oued Ouaoufraou (Z-F13; Fig.1C), left column; Oued Tarhia n'Imrhaldene (Z-F12; Fig.1C) and SE of Jbel Tizagzaouine (Z-F2(3); Fig.1C), right column. The boundary between the A. murrayi and H. copiosus zones is located at the top of the Oued Tarhia n'Imrhaldene log. Stratigraphic position and composition of all (new) echinoderm assemblages from the late Tremadocian are reported along the log. The colours on the log correspond to those of the rocks on the outcrop (mostly green, blue and beige).

9 B.

 9 Lefebvre et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx-xxx Please cite this article as: Lefebvre, B., et al., Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02.039

Fig. 5 .

 5 Fig. 5. Stylophorans from the A. murrayi Zone (late Tremadocian), Fezouata Shale, Zagora area, central Anti-Atlas, Morocco. All scale bars: 10 mm. (A) Cornute indet. n. gen., n. sp. 2 (Cornuta), Ouaoufrout (Z-F6), cluster of 12 individuals with well-preserved thecae and attached aulacophores (FSL 711637). (B) Cornute indet. n. gen., n. sp. 1 (Cornuta), SE of Jbel Tizagzaouine (Z-F2(3)), fully articulated specimen (MHNM.15690.182). (C) Chauvelicystis vizcainoi (Cornuta), SE of Jbel Tizagzaouine (Z-F2(3)), fully articulated specimen with the two series of cover plates preserved in open position (MHNM.15690.172). (D) Anatifopsis trapeziiformis (Mitrata), Bou Izargane (Z-F4), cluster of several individuals with their aulacophore in recurved position (ML20-269,192).

Fig. 4 .

 4 Fig. 4. Echinoderms from the A. murrayi Zone (late Tremadocian), Fezouata Shale, Zagora area, central Anti-Atlas, Morocco. All scale bars: 10 mm. (A) Rhopalocystis havliceki (Eocrinoidea), Tameraoute (Z-F1), large partly disarticulated theca (AA.TAM.OI.13). (B) Rhopalocystis zagoraensis (Eocrinoidea), hill N of Jbel Tizagzaouine (Z-F12c), accumulation of fully articulated thecae in a large concretion (FSL 712044). (C) Rhopalocystis havliceki (Eocrinoidea), Tameraoute (Z-F1), numerous disarticulated elements of eocrinoids and a large partly disarticulated theca with strong basal knobs (AA.TAM.OI.21). (D) cf. Pareocrinus (Eocrinoidea), Bou Izargane (Z-F4), apical portion of the theca and array of at least 16 brachioles (ML20-269,158). (E) cf. Pareocrinus (Eocrinoidea; right) stem and lower part of theca associated with two fully articulated aulacophores of cornute stylophorans (left), SE of Jbel Tizagzaouine (Z-F2(3); MHNM.15690.162). (F) Somasteroid indet. sp. 1 (Somasteroidea), locality unknown, fully articulated specimen (FSL 424961).

Fig. 6 .

 6 Fig. 6. Echinoderms of the Z-F5 (Oued Beni Zoli) assemblage, A. murrayi Zone (late Tremadocian), Fezouata Shale, Zagora area, central Anti-Atlas, Morocco. All scale bars: 10 mm. (A) Macrocystella bohemica (Glyptocystitida), cluster of 12 complete individuals with enlarged proximal and thin distal stems (FSL 711664). (B) Plasiacystis mobilis (Soluta), large, fully articulated specimen with enlarged, paddle-shaped dististele (MHNM.15690.200). (C) Balantiocystis regnelli (Eocrinoidea), complete theca with brachioles (MHNM.15690.197). (D) Macrocystella bohemica (Glyptocystitida), cluster of three individuals with long stems and brachioles (MHNM.15690.196).

Fig. 7 .

 7 Fig. 7. Echinoderms of the Z-F24 (hill N of Toumiat) assemblage,? C. protobalticus Zone (early Floian), Fezouata Shale, Zagora area, central Anti-Atlas, Morocco. All scale bars: 10 mm. (A) Plasiacystis mobilis (Soluta), proximal part of the theca and complete homoiostele of a large individual (MHNM.15690.212). (B) Plasiacystis mobilis (Soluta), cluster of isolated, enlarged dististeles (MHNM.15690.222). (C) Balanocystites primus (Stylophora), complete specimen in upper aspect (MHNM.15690.210). (D) Iocrinus sp. (Crinoidea), fully articulated specimen with long arms and stem (MHNM.15690.209).

Fig. 8 .

 8 Fig. 8. Echinoderms from the? B. jacksoni Zone (middle Floian), Fezouata Shale, Zagora area, central Anti-Atlas, Morocco. All scale bars: 10 mm. (A) Balantiocystis regnelli (Eocrinoidea), Bou Chrebeb (locality 1687), articulated aboral portion of the theca of a large indivudual (left) and scattered skeletal elements (AA.BCBb.OI.134). (B) Argodiscus espilezorum (Edrioasteroidea), Bou Chrebeb (locality 1687), holotype theca with imbricate plates and short ambulacra (AA.BCBb.OI.19). (C) Thoralicystis zagoraensis (Stylophora), Bou Chrebeb (locality 1685), holotype theca with distinctive triangular shape (IGR.1685z). (D) Rhopalocystis destombesi (Eocrinoidea), Bou Chrebeb (locality 1686), latex cast of an accumulation of thecae without stems or brachioles (ML20-269,382).

14 B.

 14 Lefebvre et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx-xxx Please cite this article as: Lefebvre, B., et al., Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02.039 top of the Fezouata Shale. This level yielded two specimens of the mitrate Chinianocarpos sp. associated with bivalves, articulate brachiopods and trilobites.

Fig. 9 .

 9 Fig. 9. Echinoderms from the Azygograptus interval (late Floian), Zagora area, central Anti-Atlas, Morocco. (A) Nail-shaped, small eocrinoid indet. (Eocrinoidea indet. sp. 1), Jbel Zagora (Z-F30m), Fezouata Shale, complete theca and proximal part of the stem (AA.JZGm.OI.2); scale bar: 5 mm. (B) Moulds of thecae in diploporite-bearing horizon, North Douirat East (NDE), Zini Sandstone Formation.

Fig. 10 .

 10 Fig. 10. Successive echinoderm assemblages documented in both proximal and distal settings, in the Lower Ordovician of the Zagora area, central Anti-Atlas, Morocco.

  B. Lefebvre et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx-xxx Please cite this article as: Lefebvre, B., et al., Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02.039

  21 B. Lefebvre et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2016) xxx-xxx Please cite this article as: Lefebvre, B., et al., Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02

Fig. 1 .

 1 Fig. 1. (A) Satellite image from Google Earth showing the location of the study area in the central part

Fig. 2 .

 2 Fig. 2. Geological context. (A) Palaeogeography of Gondwana during the Early to Middle Ordovician

Fig. 3 .

 3 Fig. 3. Sections showing the lithostratigraphic succession of the Fezouata and Zini formations in the

  Figures

Fig. 4 .Fig. 4 .

 44 Fig. 4.

Fig. 5 .Fig. 5 .

 55 Fig. 5.

Fig. 6 .

 6 Fig. 6. Idealized sand-to-silt lenses. Simplified sketch of the two observed morphologies of SL:

Fig. 7 .

 7 Fig. 7. Sand-to-silt lenses description. (A) Sand-to-silt lens (here SL1) in outcrops, which displays

  led an experimental study in order to give keys of recognition for the sedimentary structures generated by supercritical flows. Patterns only observed in supercritical flow deposits were: (1) high-angle stratifications dipping downstream corresponding to rapid migration of the bedforms and (2) a dominance of

Fig. 8 .

 8 Fig. 8. (A) Sand-to-silt lenses (here SL2) in outcrop showing that deposits are very localized even if a

Fig. 9 .

 9 Fig. 9.Step-by-step diagram explaining the genesis of the sand-to-silt lenses, their sedimentary

Fig. 9 .

 9 Fig. 9.

  Fig. A) FZ1 and FZ2 are dominant. This imply a SE (proximal environment) -NW (distal environment) axis of progradation of the system and giving a SW -NE shoreline orientation. A return to a facies zone belonging to deeper environment on the top of facies of shallowing upward bedset implies a rupture in the characteristic facies succession as predicted by the Walther's law (1894) that can be defined as a transgressive surface bounding a parasequence. Consequently, parasequences related to relative sea-level changes can be identified throughout the sections, and parasequences of different scales are recognized in the uppermost Fezouata and Zini

Fig. 10 .

 10 Fig. 10. Detailed section of the uppermost Fezouata and Zini formations (location: North Douirat East;

Fig. 11 .

 11 Fig. 11. Facies model for a wave-dominated, tide-modulated epicontinental system based on the

Fig. 12 .

 12 Fig. 12. (A) Cyclical changes in grain size in a sand-to-silt lens. The finest sediments are interpreted

Fig. 13 .

 13 Fig. 13. (A) and (B) Bioclastic infills of gutters in SL1. (C) Gutters are interpreted as generating

Fig. 14 .

 14 Fig. 14. (A) Satellite view in Google Earth, north of the Baie de Somme (Northern France), showing a

Fig. 1 .

 1 Fig. 1. (A) Satellite image (Google Earth) of the study area. (B) Geological map of the Zagora area

Fig. 2 .

 2 Fig. 2. Composite stratigraphic section of

Fig. 3 .

 3 Fig. 3. (A) Simplified palaeoenvironmental reconstitution of the Fezouata Shale and the Zini

Fig. 4 .

 4 Fig. 4. (A) Hillock of the excavation site (location red star Fig. 1). (B) Detailed section of the

  The transition to Part 2 suggests a decreasing flow velocity (waning flow). However, plane parallel laminae indicate that flow velocity was still high. The presence of HCS indicates that the deposition of Part 3 resulted from a mechanism mainly controlled by oscillation and thus suggests a decrease of the current component during waning flow (plane parallel bedding evolving into oscillatory structures). The laterally discontinuous aspect of storm deposits and their general flattened lobe-shaped morphology indicate very localized deposition of the sandy layers. Host sedimentation (FA1) is dominated by argillaceous siltstones to siltstones

  ) a basal part formed of a bioclastic lag without obvious sedimentary structures;(2) an intermediate part consisting of fine-to medium-grained sandstones with plane parallel laminae; and (3) an upper part consisting of fine-to medium-grained sandstones showing accretionary or scour-and-drape HCS of decimeters-to meter-scale wavelength. Each unit is separated from the other by an erosion surface (Fig.5B). Since SL are composed by several units, the bioclastic part may occur at different positions throughout the SL. Commonly, the bioclastic part is present at the base or in the center of the SL, whereas it is rarely observed in the upper unit of the SL. The bioclasts are unsorted and no soft tissues are preserved (Figs.5C-D). Gastropods, brachiopods, bivalves and fragments of trilobites form the main component of the fauna in the bioclastic parts. SL have been classified according to the relative amount of bioclasts and to the thickness of the bioclastic layers. Three types of SL are distinguished: dm-thick bioclastic layer (SL1); cm-to pluri-cm thick bioclastic layers (SL2);

Fig. 5 .

 5 Fig. 5. (A) Proximal storm deposits containing Konzentrat Lagerstätte; hots sediments: coarse

3

  Carcassonnella sp (gastropod). (D) Surface of Shell bed; 1: Pradoella sp (trilobite); 2: undetermined gastropod; 3: Cephalic doublure of a large asaphid trilobite.

Fig. 6 .*

 6 Fig. 6. Distribution of storm deposits (from SL1 to distal tempestites) in the lower shoreface (between

  Sedimentological analysis of the Fezouata Shale (Tremadocian -Floian) in the Zagora area reveals the existence of two types of fossil-rich intervals: Ksl and Kzl horizons. Ksl are characterized by exceptionally well-preserved fossils, and Kzl, by shell beds. Ksl intervals occur in argillaceous siltstone beds covered by storm deposits (fine-grained sandstones) in the distal lower shoreface environment close to SWB. Kzl are part of waxing storm deposits in the proximal lower shoreface environment. The Kzl contain only hard skeletal fossils, mainly gastropods, bivalves, brachiopods, and fragments of trilobites. Both soft and skeletal remains of organisms are preserved in Ksl. During storm events, sediment was transported from coastal environments and deposited seaward, then burying organisms in Ksl, or concentrating living or dead organisms by the storm current, and deposited as Kzl. Fossil rich-intervals are present in different environments (proximal and distal lower shoreface), but the same process (storms) was responsible for their formation. Specific environmental conditions, such as low concentrations in both oxygen and sulphate may have served to promote the exceptional preservation of fossil soft-tissues.

  Terreneuvian or Cambrian Stage 2 (commonly refered to as Tommotian; 530 to 520 Ma), the microscopic fossils, many of which are only hundreds of microns in size, became much more

numerous (see review by Kouchinsky 2001). While many represent individual animals, others represent individual components of the armor of much larger animals (Conway

1.2 The Fezouata Lagerstätte: a new window on Early Ordovician marine communities 1.2.1 Historical background of the discovery

	According to Lefebvre et al. (in press a), the first soft-bodied specimen of the Fezouata Biota
	was discovered in late 1999 or early 2000 by Mohamed 'Ou Said' Ben Moula, a local
	collector from Taichoute, during his prospection in the Lower Ordovician of the Zagora area
	(Fig. 36A). For two years, the sole exceptionally preserved fossil known from the Zagora
	area was a single Tremaglaspis specimen. In June 2002, Tahiri provided Peter Van Roy (Fig.
	36B), who was then a graduate student working on the Upper Ordovician of Morocco, with
	three specimens of a cheloniellid arthropod recently collected by Ben Moula. The exquisite
	preservation of the material, convinced Van Roy of the potential of the newly discovered
	sites. Accordingly, in November 2002, he visited these localities. Half a day of collecting
	yielded a rich shelly assemblage, carapaces of non-biomineralised bivalved arthropods and a
	few soft-bodied vermiform fossils (likely annelids; Van Roy, 2006: p. 40 -43, fig. 3.6). A
	year later, a 10-day field campaign was conducted, and more exceptionally preserved fossils,
	including Burgess Shale-type demosponges were recovered (Van Roy, 2006; Botting, 2007).
	Independently, in early 2002, Roland and Véronique Reboul, a couple of French amateur
	palaeontologists, visited most excavations produced by Ben Moula in the Fezouata Shale, and
	signalled the occurrence of several echinoderm Lagerstätten (sensu Smith, 1988) in the
	Zagora area to Bertrand Lefebvre, who was a researcher studying Early Palaeozoic
	echinoderms. As a result, a collaboration between the universities of Burgundy (Dijon) and
	Cadi-Ayyad (Marrakesh) was initiated by B. Lefebvre and Khadija El Hariri the same year
	(Figs 36A and

  sensu lato, in part confused with proximal parts of ?multiramous forms. But others are true reclined tetragraptids (Fig.4M) among which the

long-ranging species Tetragraptus amii (Elles and Wood) and T. reclinatus Elles and Wood ssp. indet. have been identified. It is likely that "Isograptus caduceus var. gracilis Rued., I. caduceus var. armatus Rued." and "I. furcula Rued." from locality Taskala

(Willefert in Destombes, 1960b: Fig. 1, loc. 3

) are mistaken identifications based on deformed specimens of one or more species of Tetragraptus, showing sicula and highly reclined stipes in "1-2

  and his sons. C: Artur Sa, Julio Martin and Saturnino,Lorenzo. D: Elise Nardin. E: Peter Van Roy. F: Ahmid Hafid and Moroccan students. G: the driver Omar, Khadija El Hariri and Moroccan students. H: Juan Carlos Gutiérrez-Marco. I: Muriel Vidal, Abdel Azizi, Daniel

	Vizcaïno, Bernard Pittet, Moussa Masrour, Clément Jauvion, Romain Vaucher, Khaoula
	Kouraïss, Emmanuel Martin, Manon Martin, Jean Vannier and Abel Prieur. J: Khadija El
	Hariri, Emmanuel Martin, Khaoula Kouraïss, Abdel Azizi,Emmanuel Robert, Abel Prieur,
	Moussa Masrour, Bertrand Lefebvre, and Ahmid Hafid.

  . The putative occurrence of oligotrophic conditions is not supported by the presence of abundant and diverse palynomorph assemblages throughout the whole Fezouata Shale (e.g.Elaouad-Debbaj, 1988; Nowak et al., in press), as well as of large planktotrophic swimming anomalocaridids in the late Tremadocian time interval(Van Roy et al., 2015b). Conversely, in the lower interval yielding assemblages result from the limited transport and local accumulation of parautochtonous live (or freshly killed) organisms by distal storms, piled on top of each other in small depressions on the sea floor. The preservation of soft-bodied organisms (e.g., Furca, worms), as well as of soft parts in both trilobites (e.g., antennae) and echinoderms (e.g., podia, gut) indicates the occurrence of relatively comparable taphonomic processes in all horizons of the lens. The rapid burial of organisms was a necessary, but not sufficient condition to explain the exceptional preservation in this lens. It is very likely that other parameters played a key role (e.g. low oxygen levels; see above).

soft-bodied organisms, the extremely reduced bioturbation intensity, with few shallow, vertical burrows is more compatible with the occurrence of -at least temporarilyunhospitable, dysoxic settings.

All three discrete horizons of the fossiliferous level exploited in the main intermediate quarry share comparable taphonomic attributes, with complete, fully-articulated, densely packed (sometimes on top of each other) individuals, showing no preferential orientation. It is likely that such massive low-diversity

  . In the early 2000s, the discovery of exceptionally preserved remains of soft-bodied organisms in the Lower Ordovician of the Zagora area (central Anti-Atlas) dramatically altered the views on the tempo and patterns of the major diversification of metazoans in Early Palaeozoic times(Van Roy et al., 2010). The Burgess Shale type Fezouata Biota includes a mixture of archaic,

	this issue). As a consequence of the discovery of the Fezouata Biota, successive field
	campaigns were organized from 2003 to 2015, and abundant, new material was collected in
	the Lower Ordovician of the Zagora area. This material was recently replaced within both its
	biostratigraphic and environmental contexts (Gutiérrez-Marco and Martin, this issue; Lehnert

Cambrian-like taxa, typical of early to middle Cambrian Lagerstätten (e.g., anomalocaridids, marrellomorphs) and the earliest occurrences of typical post-Cambrian organisms, previously known only from more recent Siluro-Devonian Lagerstätten (e.g., cirriped crustaceans, eurypterids, highly derived xiphosurids;

Van Roy et al., 2015)

.

In the Zagora area, levels with exceptional preservation are restricted to two narrow stratigraphic intervals within the Fezouata Shale, which corresponds to a monotonous series of argillites about 850 meters thick (Fig.

2

;

Martin et al., in press)

. Recent advances in graptolite biostratigraphy in the central Anti-Atlas indicate that the main, lower level with exceptional preservation corresponds to a ~70 m thick interval at the top of the A. murrayi Zone (late Tremadocian), whereas the second, upper level is only ~15 m thick and occurs about 200 m above, in the ?B. jacksoni Zone (middle Floian; Gutiérrez-Marco and

Martin, et al., this issue; Nowak et al., this issue; Vaucher et al., this issue)

.

  identified byDestombes (1960) in the Agdz-Zagora area. A more detailed account of trilobite diversity in the Lower Ordovician of the central Anti-Atlas was provided byDestombes (1967), who mentioned the occurrence of Asaphellus, Bavarilla, Beltella,

	Dikelokephalina, Orometopidae, Pharostomina, Pilekia, Platypeltoides?, Raphiophoridae and
	Symphysurus in the Tremadocian, and of Ampyx, Apatokephalus, Asaphus? and undetermined
	asaphids, Bathycheilus, Bavarilla, Ceraurinella, Colpocoryphe, Euloma, Neseuretus,
	Plesiomegalaspis, Prionocheilus, Pterygometopus and Symphysurus, in the Floian. In the
	"Zemmour area" (Northern Mauritania), Destombes et al. (1969) described a small trilobite

. However, a new fossil site discovered East of Agdz (Tansikht bridge), yielded through successive field campaigns from 1952 to 1954, a diverse trilobite assemblage definitively interpreted as late Tremadocian by Pierre Hupé (in Choubert et al., 1955) and consisting of Asaphellus cf homfrayi, Asaphopsis sp., Parapilekia cf. olesnaensis and Platypeltoides croftii.

From the late 1950s to the late 1990s, Jacques Destombes collected thousands of Early Palaeozoic fossils during the mapping of Cambro-Ordovician rocks for the series of 1:200.000 geological maps of the Anti-Atlas. Several late Tremadocian levels yielding large trilobites were fauna including Bavarilla zemmourensis, Megistaspis (Ekeraspis) sp., Nileus deynouxi, Parapilekia sougyi, Platypeltoides sp., and Prionocheilus sp. A relatively complete overview of trilobite diversity in the Lower Ordovician of the Anti-Atlas was later provided by

Table 1 .

 1 List of the 26 trilobite species identified in the Lower Ordovician Fezouata Shale. Family of each species and references of their systematic description are also provided.

	Family	Genus	Species	References
	Agnostidae	Geragnostus	sp.
		Asaphellus	fezouataensis	Vidal, 1998a
			sp. aff.
		Asaphellus		Harrington, 1937
			jujuanus
		Asaphellus	stubbsi	Fortey, 2009
	Asaphidae
		Asaphellus	tataensis	Vidal, 1998a
		Basilicus	destombesi	Vidal, 1998a
				(Munier-Chalmas, Bergeron,
		Megistapis (Ekeraspis)	cf. filacovi
				1888)
	Bathycheilidae	Parabathycheilus	gallicus	(Dean, 1965)
	Bavarillidae	Bavarilla	zemmourensis	Destombes, 1969
				(Courtessole, Pillet, Vizcaïno,
		Colpocoryphe	pradesensis
				1983)
		Colpocoryphe	thorali	Dean, 1966
	Calymenidae
		Colpocoryphe	sp.
		Neseuretus	cf. attenuatus	(Gigout, 1951)
		Pradoella	sp.

identified as Euloma sp., Pradoella sp. and Toletanaspis sp. belong to new species, yet

Table 2

 2 Change in echinoderm taxonomic diversity (number of taxa) through time in the Lower Ordovician of the Zagora area, central Anti-Atlas, Morocco, with: (Tr2) middle

	Tremadocian; (Tr3) late Tremadocian; (Fl1) early Floian; (Fl2) middle Floian; and (Fl3)
	late Floian. The diversity of late Floian diploporites (1) is indicated with an asterisk,
	because it still has to be determined. Diploporite-bearing beds of locality North Douirat
	East (NDE, Fig. 1C) contain at least one species.
		Tr2	Tr3	Fl1	Fl2	Fl3
	Crinoidea	0	1	2	1	0
	Diploporita	0	2	0	1	1*
	Edrioasteroidea	0	1	1	2	0
	Eocrinoidea	1	12	3	6	1
	Rhombifera	0	1	0	1	0
	Soluta	0	3	1	1	0
	Somasteroidea	0	2	1	3	0
	Stylophora	0	25	4	6	2
	TOTAL species	1	47	12	21	4

Please cite this article as: Lefebvre, B., et al., Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02.039 2014b;

  1,* , Khadija EL HARIRI 2 , Khaoula KOURAISS 2 , Emmanuel MARTIN 1 & Fleur NOAILLES 3

	1 UMR CNRS 5276, LGLTPE, bâtiment Géode, campus de la Doua, Université Lyon 1, 2 rue Dubois,
	69622 Villeurbanne cedex, FRANCE
	e-mails : bertrand.lefebvre@univ-lyon1.fr et emmanuel.martin@univ-lyon1.fr
	2 Département des Sciences de la Terre, Faculté des Sciences et Techniques, Université Cadi Ayyad,
	avenue Abdelkrim Khattabi, BP 549, 40000 Marrakech, MAROC
	e-mail : elhariri@fstg-marrakech.ac.ma
	3 Department of Earth Sciences, University of Bristol, Wills Memorial building, Queen's road, Bristol
	BS8 1RJ, ROYAUME-UNI
	e-mail : fleur.noailles@bristol.ac.uk

  , consists of deposits of late Tremadocian age (Fig.4B; Araneograptus murrayi biozone,Martin et al., in press, Gutiérrez- Marco and Martin, this issue; messaoudensis-trifidum acritarchs assemblages,Nowak et al., this issue). The sedimentary succession (Figs.2-4A) is characterized by two alternating facies associations. The first one (FA1) is dominated by argillaceous siltstones and thin (mm-thick)

Please cite this article as: Martin, E.L.O., et al., Palaeoscolecid worms from the Lower Ordovician Fezouata Lagerstätte, Morocco: Palaeoecological and palaeogeographical implication, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.04.009

S. Zamora & I. Rábano (eds.), Progress in Echinoderm Palaeobiology. Cuadernos del Museo Geominero, 19. Instituto Geológico y Minero de España, Madrid. ISBN: 978-84-7840-961-7 © Instituto Geológico y Minero de España 2015

EmmanuelMartin, Bertrand Lefebvre and Romain Vaucher

Cuadernos del Museo Geominero, 19. Instituto Geológico y Minero de España, Madrid. ISBN: 978-84-7840-961-7 © Instituto Geológico y Minero de España 2015

NinonAllaire, Bertrand Lefebvre, Emmanuel Martin, Elise Nardin and Romain Vaucher

Please cite this article as: Lefebvre, B., et al., Palaeoecological aspects of the diversification of echinoderms in the Lower Ordovician of central Anti-Atlas, Morocco, Palaeogeogr. Palaeoclimatol. Palaeoecol. (2016), http://dx.doi.org/10.1016/j.palaeo.2016.02.039

The Fezouata Biota (Central Anti-Atlas, Morocco)

bertrand.lefebvre@univ-lyon1.fr

Acknowledgments

This work is part of the ANR (Agence Nationale de la Recherche) research project entitled "The Rise of Animal Life (Cambrian-Ordovician): organisation and tempo" (grant number RALI 197) and of the CNRS-CNRST cooperation project VALORIZ (grant number 52943). J. C. G-M thanks MINECO (project CGL2012-39471) and P. V. R. the National Science Foundation (grant EAR 1053247) for financial support. The authors are particularly grateful to C. E. Brett (Cincinnati), M. Williams (Leicester), A. Rushton (London), and I. D. Somerville (Dublin) for reviewing the manuscript and making many helpful remarks. We also thank Abdel Azizi, Ali Bachnou, Ahmid Hafid, Khaoula Kouraïss, Mohamed "Ou Saïd" Ben Moula, Elise Nardin, Fleur Noailles, Abel Prieur, Roland and Véronique Reboul, Emmanuel Robert, Muriel Vidal, Daniel Vizcaïno and the numerous French and Moroccan student volunteers for assistance in the field and Carlos Alonso (Madrid) for photography.

Acknowledgements

The authors are indebted to Julio Martín (Madrid, Spain), Artur A. Sá (Vila Real, Portugal) and Saturnino Lorenzo (Almadén, Spain) for the help during fieldwork on the Moroccan outcrops, Jacques Destombes (Pessac, France) and Philippe Legrand (Gradignan, France) for providing support information on the Saharan localities and graptolites. The authors cordially thank Jörg Maletz (Berlin) for his help with some graptolite identifications, as well as Jan Zalasiewicz (Leicester, UK) and Petr Kraft (Prague, Czech Republic) for their very helpful comments and suggestions, Diego García-Bellido (Adelaide, Australia) and Stan Finney (California, USA) for their linguistic work on the manuscript, and Ismail Said (Rabat, Morocco) for facilitating the search of graptolite samples in the J. Destombes collection, housed in the Ministry of Energy, Mines, Water and Environment (Rabat). Carlos Alonso (Universidad Complutense de Madrid, Spain) photographed the graptolite specimens. This paper was financed by the project CGL2012-39471 of the Spanish Ministry of Economic Affairs and Competitivity, and is also a contribution to the projects RALI 197 of the French ANR and IGCP 591 (IUGS-UNESCO). The present study is dedicated to the French geologists J. Destombes, Ph. Legrand and Solange Willefert, pioneers in the modern research on Ordovician graptolites in the African continent.

Acknowledgments

This work is a contribution to the ANR (Agence Nationale de la Recherche) research project 'The Rise of Animal Life (Cambrian-Ordovician): organisation and tempo' (grant number RALI 197) and the CNRS-CNRST cooperation project VALORIZ (grant number 52943). We thank A. Azizi, A. Bachnou, K. El Hariri, Khaoula Kouraïss, B. Lefebvre, M. Ben Moula, E. Nardin, F. Noailles, B. Pittet, A. Prieur, R. and V. Reboul, E. Robert, P. Van Roy, R. Vaucher, M. Vidal, D. Vizcaïno and the numerous French and Moroccan students who assisted us in the field. We are also grateful to John Peel and Michal Mergl, who kindly provided us with the pictures of C. pharkus (Fig. 6G) and P.? tenensis (Fig. 3C, and E), respectively. Lastly, the two referees, T. Harvey and W. Wang, and P. Van Roy are thanked for their constructive comments.

Acknowledgments

The authors would like to thank the Agence National de la Recherche -RALI project (E. M., grant number grant number ANR-11-BS56-0025; "Rise of Animal Life (Cambrian-Ordovician): organization and tempo"), and the CNRS-CNRST cooperation project VALORIZ (grant number 52943) for the financial support. This paper is a contribution to the IGCP project 591'The Early to Middle Palaeozoic Revolution', and of the team "Vie primitive, biosignatures, milieux extrêmes", UMR CNRS 5276 LGLTPE. Many important data on the stratigraphy and localities were obtained from M. Ben Moula (Taichoute), J.P. Botting and L. Muir (Llandrindod Wells), J.C. Gutiérrez-Marco (Madrid), R. and V. Reboul (Saint-Chinian), B. Pittet (Villeurbanne), E. Nardin (Toulouse), and P. Van Roy (Yale). Critical field assistance was provided by A. Azizi, A. Bachnou, M. Bajedoub, K. Kouraïss, A. Hafid, K. El Hariri (Marrakesh), C. Jauvion and J. Vannier (Villeurbanne), M. Masrour (Agadir), H. Nowak and T. Vandenbroucke (Villeneuve d'Ascq). The authors are particularly thankful to Rudy Lerosey Aubril, Richard Fortey and Carlton Brett for reviewing the manuscript and making many helpful remarks.

Acknowledgements

This paper is a contribution of the ANR (Agence Nationale de la Recherche) research project entitled "The Rise of Animal Life (Cambrian-Ordovician): organisation and tempo" (RALI, coord. Jean Vannier). The authors are particularly grateful to Myriam Lefebvre, Roland and Véronique Reboul, and Daniel and Framboise Vizcaïno for their assistance in the field, to Abel Prieur, for his help with photographs, and also to James Nebelsick and Samuel Zamora, for their constructive comments and corrections.

Acknowledgements

This paper is a contribution of the ANR (Agence Nationale de la Recherche) research project entitled "The Rise of Animal Life (Cambrian-Ordovician): organisation and tempo" (RALI, coord. Jean Vannier). The authors are particularly grateful to Khadija El Hariri, Moussa Masrour, Fleur Noailles, Bernard Pittet, Roland and Véronique Reboul, and Daniel Vizcaïno for their assistance in the field, and also to Abel Prieur, for his valuable help with photographs. Samuel Zamora and James Sprinkle reviewed this manuscript and made many helpful remarks.

Acknowledgements

This study was financed as part of the RALI (Rise of Animal Life) project, which is funded by the ANR (Agence Nationale de la Recherche, France; grant number ANR-11-BS56-0025), and as part of the CNRS-CNRST cooperation project VALORIZ (grant number 52943). S.Z. was funded by a Ramón y Cajal Grant (RYC-2012-10576 and projects CGL2012-39471 and CGL2013-48877) from the Spanish Ministry of Economy and Competitiveness. This paper is a contribution to the IGCP project 591 'The Early to Middle Palaeozoic Revolution', and of the team "Vie primitive, biosignatures, milieux extrêmes", UMR CNRS 5276 LGLTPE. The authors are particularly thankful to J. Sprinkle (Austin) and M. Reich (Munich) for reviewing the manuscript and making many helpful remarks. The authors are also grateful to R. Roch (Porrentruy) for the delicate preparation of specimens preserved in concretions, to A. Prieur and E. Robert (Villeurbanne) for taking photographs, assistance in the field and the access to important specimens housed in the collections of Lyon 1 University. Many important data on the stratigraphy and localities were obtained from M. Ben Moula (Taichoute), J.P. Botting and L. Muir (Llandrindod Wells), J. Destombes (Pessac), J.C. Gutiérrez-Marco (Madrid), R. and V. Reboul (Saint-Chinian), P. Van Roy (Yale), R. Vaucher (Villeurbanne), M. Vidal (Brest) and D. Vizcaïno (Carcasssonne). Precious field assistance was provided by A. Azizi, A. Bachnou, M. Bajedoub, A. Hafid, and K. El Hariri (Marrakesh), C. Jauvion, M. Martin and J. Vannier (Villeurbanne), M. Lefebvre (Grézieu-la-Varenne), M. Masrour (Agadir), H. Nowak and T. Vandenbroucke (Villeneuve d'Ascq). Access to important specimens housed in private collections was kindly provided by P. Catto (Bainde-Bretagne), C. Guillou (Guichen), L. Lacombe (Ouveillan) and C. Milne (Berkeley). The curators or colleagues of the following institutions are also acknowledged for access to the material and their kind help: D. Berthet (Musée des Confluences, Lyon), P. Dalous and Yves Laurent (MHN, Toulouse), T. Ewin (NHM, London), C. Franzen-Bengtson (Naturhistoriska Riksmuseet, Stockholm), R. Lerosey-Aubril (Senckenberg Museum, Frankfurt-am-Main), F.J. Lindemann and H.A. Nakrem (NHM, Oslo), A. Médard and S. Pichard (MHN, Marseille), J. Plaine (Géosciences, Rennes), and S. Régnault (MHN, Nantes). Access to important specimens housed in several European collections was provided to B.L. by the European Synthesys projects GB-TAF-4565 (The Natural History Museum, London) and SE-TAF-4765 (Naturhistoriska Riksmuseet, Stockholm).

Acknowledgments

The authors would like to thank the Agence National de la Recherche (E. M., grant number RALI 197; "The Rise of Animal Life (Cambrian-Ordovician): organisation and tempo") and CNRS-CNRST cooperation project VALORIZ for financial support. We are grateful to Ninon Allaire, Ali Bachnou, Khadija El Hariri, Juan-Carlos Gutiérrez-Marco, Ahmid Hafid, Rudy Lerosey-Aubril, Moussa Masrour, Elise Nardin, Abel Prieur, Emmanuel Robert, Thomas Servais, Thijs Vandenbroucke, Jean Vannier, Peter Van Roy, Muriel Vidal and Daniel Vizcaïno for their precious help in the field and/or for fruitful discussions. We are also thankful to the Associate Editor Mariano Marzo as well as Shahin Dashtgard, Piret Plink-Björklund, Valentina Rossi and two anonymous reviewers for their sensible remarks that helped us to improve the quality of the manuscript.

Acknowledgments

This work was supported by the ANR (Agence National de la Recherche) project 197 RALI "Rise of Animal Life (Cambrian-Ordovician): organisation and tempo" and by the CNRS-CNRST cooperation project VALORIZ. Ninon Allaire, Khadija El Hariri, Bertrand Lefebvre, Rudy Lerosey-Aubril, Elise Nardin, Abel Prieur, Emmanuel Robert, Thomas Servais, Thijs Vandenbroucke, Jean Vannier, Muriel Vidal, Daniel Vizcaïno, Guillaume Suan and Corentin Gibert are thanked for their precious help on the field and fruitful scientific discussions. We are also thankful to the managing guest-editor of this special issue Bertrand Lefebvre (Villeurbanne), to the guest editor Peter Van Roy (Yale University), and to the three reviewers Carlton E. Brett (University of Cincinnati), Robert R. Gaines (Pomona College) and an anonymous reviewer for their precious comments that helped us improving the overall quality of the manuscript.

guttenbergensis, Enoploura popei; [START_REF] Kolata | Anomalocystitid mitrates (Stylophora-Echinodermata) from the Champlainian (Middle Ordovician) Guttenberg Formation of the Upper Mississipi Valley Region[END_REF][START_REF] Parsley | Review of selected North American mitrate stylophorans (Homalozoa: Echinodermata)[END_REF].

In the shallow, tropical seas of Laurentia, anomalocystitids underwent a major diversification event in late Ordovician times and ultimately, became a significant component of Siluro-Devonian benthic, vagile echinoderm communities (Lefebvre, 2007). The kirkocystid mitrate Balanocystites primus (Fig. 7C), which is recorded from several early to middle Floian localities in the Zagora area (Fig. 2), is another example of stylophoran echinoderm typical of late Early to Middle Ordovician high-latitude peri-Gondwanan regions. For example, B. primus is known from the Floian of Montagne Noire (la Maurerie and Landeyran formations; [START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF]Vizcaïno et al., 2001), the Dapingian of Wales (Pontyfenni Formation; [START_REF] Jefferies | The chordatesa preliminary note[END_REF], and the late Darriwilian of Brittany (Traveusot Formation;[START_REF] Chauvel | Etude critique de quelques échinodermes stylophores du Massif armoricain[END_REF]Lefebvre, 2000b) and Bohemia (Šárka and Dobrotivá formations; [START_REF] Barrande | Système Silurien du Centre de la Bohême: Supplément au volume I. Trilobites, Crustacés Divers et Poissons[END_REF][START_REF] Barrande | Système Silurien du Centre de la Bohême: Volume VII. Classe des Echinodermes[END_REF][START_REF] Lefebvre | Some critical comments on "ankyroids[END_REF]). In the central Anti-Atlas, the solutan Plasiacystis mobilis first occurs in the Oued Beni Zoli assemblage (Z-F5, A. murrayi Zone; Figs. 3,6B). In this region, it represents a major component of younger (H. copiosus Zone to early Floian) echinoderm communities (e.g., Z-F0, Z-F13c, Z-F24, Taichoute; Figs. 3, 7A-B; Appendix 3). With its short and wide, paddle-like dististele, Plasiacystis was well adapted to the life on soft, siliclastic substrates. This solutan was also recorded from the late Tremadocian of Montagne Noire (Saint-Chinian Formation; B. Lefebvre, pers. obs.) and the Darriwilian of Bohemia (Šárka Formation; [START_REF] Prokop | Plasiacystis mobilis, gen. et sp. n., a strange "carpoid" (Echinodermata, ?Homoiostelea: Soluta) in the Bohemian Ordovician (Czech Republic)[END_REF][START_REF] Lefebvre | A reinterpretation of the solutan Plasiacystis mobilis (Echinodermata) from the Middle Ordovician of Bohemia[END_REF] and Shropshire [START_REF] Lefebvre | A reinterpretation of the solutan Plasiacystis mobilis (Echinodermata) from the Middle Ordovician of Bohemia[END_REF].

The eocrinoid genera Balantiocystis and Lingulocystis are two other examples of typical Ordovician taxa restricted to high-latitude, peri-Gondwanan regions. In the Anti-Atlas area, the oldest known occurrences of Balantiocystis are located in the A. murrayi Zone (e.g., Z-F5; Figs. 3,6C). However, in stratigraphically higher levels (early-middle Floian), this eocrinoid becomes particularly abundant and it dominates most echinoderm assemblages (e.g., Z-F0, Z-F25a, Z-F25b, Z-F26; Figs. 2,8A). The morphology of Balantiocystis was well adapted to life on soft, siliciclastic substrates, on which it could anchor with the grapnel-like distal extremity of its stem (kite-strategy; [START_REF] Ubaghs | Le genre Balantiocystis Chauvel (Echinodermata, Eocrinoidea) dans l'Ordovicien inférieur de la Montagne Noire (France)[END_REF]. Outside of the Anti-Atlas, Balantiocystis was also reported from the Early Ordovician (late Tremadocian to late Floian) of Montagne Noire (Saint-Chinian, la Maurerie and Landeyran formations; [START_REF] Ubaghs | Le genre Balantiocystis Chauvel (Echinodermata, Eocrinoidea) dans l'Ordovicien inférieur de la Montagne Noire (France)[END_REF]Ubaghs, , 1983;;[START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF]Vizcaïno et al., 2001). In the Anti-Atlas, Lingulocystis is known from a single occurrence in the A. murrayi Zone (Z-F9(2); Fig. 3). However, it is a regular component of early to middle Floian echinoderm assemblages (e.g., Z-F25a, Z-F25b, Z-F26; Fig. 2). Lingulocystis represents a typical example of Early Ordovician kite-strategist eocrinoid adapted to life on soft, siliciclastic substrates in high-latitude peri-Gondwanan areas [START_REF] Ubaghs | Le genre Lingulocystis Thoral (Echinodermata, Eocrinoidea) avec des remarques critiques sur la position systématique du genre Rhipidocystis Jaekel[END_REF]Nardin, 2007b). This genus was originally described in the Early Ordovician of Montagne Noire (Thoral, 1935;[START_REF] Ubaghs | Le genre Lingulocystis Thoral (Echinodermata, Eocrinoidea) avec des remarques critiques sur la position systématique du genre Rhipidocystis Jaekel[END_REF][START_REF] Ubaghs | Echinodermes nouveaux (Stylophora, Eocrinoidea) de l'Ordovicien inférieur de la Montagne Noire (France)[END_REF]Nardin, 2007b), where it occurs massively (as eocrinoid meadows) in shallow, high energy environments (le Foulon and Cluse de l'Orb formations), but forms a minor component of more distal, low energy communities (Saint-Chinian, la Maurerie and Landeyran formations). Lingulocystis was also reported from the Floian of Argentina [START_REF] Aceñolaza | Ordovician echinoderms of Argentina[END_REF] and Bolivia (Sprinkle, 1973b).

The small nail-shaped eocrinoid (Eocrinoidea indet. sp. 1) dominating distal late Floian assemblages (see above; Fig. 2) is another example of typical high-latitude, peri-Gondwanan post-Cambrian echinoderm. Initially documented by Chauvel and Régnault (1986: fig. 4) in the H. copiosus Zone of the Zagora area (locality 2367; Fig. 1C), this eocrinoid occurs regularly throughout the Fezouata Shale, from the A. murrayi Zone (e.g., Z-F3, Z-F9(2); Fig. 3) to the Azygograptus interval (Z-F30m; Figs. 2,9A). This undescribed eocrinoid is also locally abundant in the late Floian of Montagne Noire (Landeyran Formation; D. Vizcaïno, pers. comm. 2011) and it is morphologically very similar to? Ascocystites cuneiformis from the early Darriwilian of the Armorican Massif (Moitiers d'Allonne Formation; [START_REF] Chauvel | Recherche sur les cystoïdes et les carpoïdes armoricains[END_REF][START_REF] Régnault | Morphologie et ontogenèse des Ascocystitidae (Eocrinoidea): données fournies par l'étude de faunes de l'Ordovicien du Massif armoricain[END_REF].

The two diploporites described so far from the Fezouata Shale (Aristocystitidae indet. and Palaeosphaeronites prokopi; Chauvel, 1966b) are typical examples of post-Cambrian echinoderm faunas. Diploporites are minor components of distal echinoderm communities in the Fezouata Shale (Figs. 23), but they dominate the shallower, high energy assemblages observed in the Zini Formation (late Floian; Figs. 9B,10). The single specimen of aristocystitid described by Chauvel (1966b) from the upper part of the A. murrayi Zone (locality 1157; Fig. 2) represents the oldest known record of this major group of diploporites. In Ordovician times, their distribution was strictly restricted to peri-Gondwanan regions (Chauvel, 1966b;Lefebvre and Fatka, 2003;[START_REF] Lefebvre | This larger amount of available data provides an unprecedented opportunity to better identify if similarities between echinoderm faunas simply reflect comparable palaeoenvironmental conditions and/or if they also support palaeobiogeographic affinities. As a first consequence of this improved knowledge of late Cambrian-Early Ordovician faunas, less than 12% of echinoderms from the Fezouata Shale can be identified as possibly endemic taxa (instead of about 50% in the early 2000s). They include the eocrinoids Rhopalocystis destombesi[END_REF]. In late Early to early Middle Ordovician times, most of the diversification of aristocystitids apparently took place at intermediate palaeolatitudes, in Iran (e.g., Sinocystis; [START_REF] Lefebvre | Ordovician echinoderms from the Tabas and Damghan regions, Iran: palaeobiogeographical implications[END_REF], Sibumasu (e.g., Pseudoaristocystis, [START_REF] Bather | The Lower Palaeozoic Fossils from the Northern Shan States, Burma[END_REF] and South China (e.g., Aristocystites, Sinocystis; [START_REF] Sun | On the occurrence of Aristocystis fauna in China[END_REF][START_REF] Sun | The early occurrence of some Ordovician and Silurian cystoids from western Yunnan and its significance[END_REF]. However, in Middle and Late Ordovician times, aristocystitids were the dominant group in most high latitude, peri-Gondwanan echinoderm assemblages (e.g., Armorican Massif, Bohemia, Morocco, Spain; [START_REF] Barrande | Système Silurien du Centre de la Bohême: Volume VII. Classe des Echinodermes[END_REF][START_REF] Chauvel | Recherche sur les cystoïdes et les carpoïdes armoricains[END_REF]Chauvel, , 1966b[START_REF] Chauvel | Données nouvelles sur quelques cystoïdes diploporites (échinodermes) du Paléozoïque armoricain[END_REF][START_REF] Gutiérrez-Marco | Los equinodermos (Cystoidea, Homalozoa, Stelleroidea, Crinoidea) del Paleozoico inferior de los Montes de Toledo y Sierra Morena (España)[END_REF], 1996;[START_REF] Parsley | Aristocystites, a recumbent diploporid (Echinodermata) from the Middle and Late Ordovician of Bohemia[END_REF]. The second diploporite (Paleosphaeronites prokopi) was also originally reported by Chauvel (1966b) from the same locality (1157) in the upper part of the A. murrayi Zone. Its range is now extended to the? B. jacksoni Zone (locality Z-F0; Fig. 2). Outside of the Anti-Atlas, the genus Paleosphaeronites was also described from the Třenice and Mílina formations (late Tremadocian) of Bohemia (P. crateriformis, P. grossularia; [START_REF] Mergl | Lower Ordovician cystoids (Rhombifera, Diploporita) from the Prague Basin (Czech Republic)[END_REF]. The two late Tremadocian occurrences of Paleosphaeronites in Morocco and Bohemia both represent the oldest known record of sphaeronitid diploporites. In late Early to early Middle Ordovician times, this family underwent a major diversification in peri-Gondwanan areas (e.g., Armorican Massif, Bohemia, Iran, Morocco, South China, Spain; [START_REF] Barrande | Système Silurien du Centre de la Bohême: Volume VII. Classe des Echinodermes[END_REF][START_REF] Chauvel | Recherche sur les cystoïdes et les carpoïdes armoricains[END_REF]Chauvel, , 1966b[START_REF] Chauvel | Données nouvelles sur quelques cystoïdes diploporites (échinodermes) du Paléozoïque armoricain[END_REF][START_REF] Sun | The early occurrence of some Ordovician and Silurian cystoids from western Yunnan and its significance[END_REF][START_REF] Gutiérrez-Marco | Los equinodermos (Cystoidea, Homalozoa, Stelleroidea, Crinoidea) del Paleozoico inferior de los Montes de Toledo y Sierra Morena (España)[END_REF][START_REF] Lefebvre | Ordovician echinoderms from the Tabas and Damghan regions, Iran: palaeobiogeographical implications[END_REF] and in Baltica (e.g., Estonia, Norway, Russia, Sweden; [START_REF] Paul | Evolution and functional morphology of the cystoid Sphaeronites in Britain and Scandinavia[END_REF][START_REF] Bockelie | The Diploporita of the Oslo region[END_REF].

The edrioasteroid genus Argodiscus (Fig. 8B) represents a possible additional example of typical, peri-Gondwanan, Ordovician echinoderm (Sumrall and Zamora, 2011). In the Zagora area, it is known from several Floian localities (Fig. 2), both in the? C. protobalticus (Z-F24, Z-F26) and the? B. jacksoni zones (Z-F25a, Z-F25b). This genus was originally reported from the Darriwilian Šárka Formation (A. rarus; [START_REF] Plas | ?Argodiscus rarus sp. n. (Edrioasteroidea) from the Šárka Formation (Llanvirn) of Bohemia[END_REF] and the late Sandbian Letná Formation (A. hornyi; [START_REF] Prokop | Argodiscus hornyi gen. n. et sp. n. (Edrioasteroidea) z Českého středního ordoviku[END_REF] of Bohemia. It may also be present in the Lower Ordovician of Montagne Noire (Thoral, 1935;[START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF].

Finally, both crinoids and somasteroids represent definitive members of post-Cambrian echinoderm faunas in the Fezouata Shale. Although crinoids are particularly rare in the Lower Ordovician of the Zagora area, at least three distinct taxa are present: Aethocrinus cf. moorei in the upper part of the A. murrayi Zone (Appendix 3), Iocrinus sp. in the early Floian (Z-F23; Fig. 2), and Ramseyocrinus sp. in both the? C. protobalticus (Z-F26; Fig. 2) and the? B. jacksoni zones (locality 1685; Donovan and Savill, 1988). Aethocrinus and Ramseyocrinus are two typical Early Ordovician crinoids from high-latitude, peri-Gondwanan areas. The first one was reported from the late Tremadocian of Montagne Noire (A. moorei; Ubaghs, 1969;[START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF] and the Floian of Shropshire (A. murchisoni; [START_REF] Donovan | Pelmatozoan columnals from the Ordovician of the British Isles. Part 1[END_REF]. Ramseyocrinus was described in the early Floian of Wales (R. cambriensis; [START_REF] Donovan | Ramseyocrinus and Ristnacrinus from the Ordovician of Britain[END_REF], the late Floian of Montagne Noire (R. vizcainoi; Ubaghs, 1983;[START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF] and Argentina (R. argentinus; [START_REF] Guensburg | South America's earliest (Ordovician, Floian) crinoids[END_REF], and the Darriwilian of Bohemia (R. primus; Waagen and Jahn, 1899). The stratigraphic distribution of these two crinoids is remarkably comparable in both the Anti-Atlas and the Montagne Noire. In these two regions, Aethocrinus occurs in the late Tremadocian, and Ramseyocrinus in the Floian. The occurrence of a Iocrinus-like crinoid (Fig. 7D) in the early Floian of the Zagora area is less informative from a palaeobiogeographic point of view, as iocrinids were cosmopolitan Ordovician crinoids.

The Fezouata Shale has yielded a relatively diverse assemblage of somasteroids, consisting of at least three distinct, yet undescribed taxa (Figs. 2, 3, 4F; Appendix 3). In the Zagora area, somasteroids occur in several stratigraphic levels of the Fezouata Shale: in the A. murrayi (e.g., Z-F5, Z-F9(1); Fig. 4F), the? C. protobalticus (e.g., Z-F23, Z-F26), and the? B. jacksoni zones (e.g., Z-F25b). Consequently, Moroccan somasteroids represent, along with those described from the middle Tremadocian of Tasmania (Maydena roadsidensis is here considered as a somasteroid; Jell, 2014), the late Tremadocian of Montagne Noire (Ampullaster ubaghsi, Chinianaster levyi, Thoralaster spiculiformis, Villebrunaster thorali; Thoral, 1935;Spencer, 1951;Fell, 1963;[START_REF] Dean | Evolutionary diversification of asteroids and ophiuroids[END_REF]Shackleton, 2005;[START_REF] Blake | The class Somasteroidea (Echinodermata, Asterozoa): morphology and occurrence[END_REF] and the Floian of Nevada (Ophioxenikos langenheimi; [START_REF] Blake | New Lower and Middle Ordovician stelleroids (Echinodermata) and their bearing on the origins and early history of the stelleroid echinoderms[END_REF], one of the oldest and most diverse occurrence of these early asterozoans. Surprisingly, no ophiuroids have been collected so far in the Fezouata Shale, although this class has so far only been documented in the late Tremadocian of the Montagne Noire (Pradesura jacobi; Thoral, 1935;[START_REF] Vizcaïno | Les échinodermes du Paléozoïque inférieur de Montagne Noire: biostratigraphie et paléodiversité[END_REF], with all other ophiuroid occurrences globally being mid to late Floian. There is thus good evidence that the earliest asterozoan communities were somasteroid-dominated, with the Fezouata fauna being no exception.

In summary, the oldest echinoderm assemblages documented in the Early Ordovician of the Anti-Atlas (A. victoriae and A. murrayi zones) are largely dominated by cosmopolitan Cambrian-like taxa, associated with endemic forms. In contrast, younger communities (H. copiosus Zone and Floian) are largely composed of post-Cambrian taxa, typical of highlatitude, peri-Gondwanan areas. Striking similarities between Early Ordovician faunas from the Anti-Atlas and Montagne Noire probably result from their occurrence in comparable environmental conditions and palaeogeographic context. France ;bertrand.lefebvre@univ-lyon1.fr

Abstract

Hybrid depositional systems are created by the interaction of two or more hydrodynamic processes that control facies distribution and their characteristics in terms of sedimentary structures and depositional geometry. The interaction of wave and tide both in the geological sedimentary record and modern environments has been rarely described in the literature. Univ Lyon, Université Claude Bernard Lyon 1, ENS de Lyon, CNRS, UMR 5276 LGL-TPE, F-69622 Villeurbanne, France ; romain.vaucher@univ-lyon1.fr, emmanuel.martin@univ-lyon1.fr, helene.hormiere@ens-lyon.fr, bernard.pittet@univ-lyon1.fr,

Abstract

In the central Anti-Atlas (Morocco; Zagora region), the Fezouata Shale has yielded an extraordinary fossil Lagerstätte documenting the initial stages of the Great Ordovician Biodiversification Event. This Lagerstätte contains abundant and exceptionally wellpreserved fossils (EPF) exhibiting soft-bodied preservation as well as "hard", mineralized parts of extinct organisms. While soft-bodied preservation in the Fezouata Shale is confined to a few stratigraphic intervals, other fossiliferous intervals contain only shelly fossils in sandstone lenses (SL).

Placed in the context of a previously established depositional model, intervals with EPF are interpreted as buried in situ by storm deposits, close to storm wave-base. There, the current component of storms is reduced compared to proximal settings and weak oscillation is dominant resulting in the record of oscillation ripples. In contrast, bioclastic accumulations were deposited upslope in shallower, more proximal lower shoreface environments. There, the current component of storms was significant, as suggested by the disarticulation of skeletal remains. Bioclastic materials are localized in SL, which are lenses a few tens of centimeters in thickness and a few meters in width and length. These lenses commonly display erosive bases and convex tops, resulting in a lobate or a channel-lobe morphology. A large amount of sediment was thus rapidly deposited, resulting in the rapid burial of allochthonous material across a strongly localized surface of the sea floor.

Integrating both sedimentological and paleontological evidence, intervals yielding EPF are considered as Konservat Lagerstätten (Ksl), in contrast with bioclastic accumulations defined as Konzentrat Lagerstätten (Kzl). Thus, Ksl assemblages are interpreted to approximate biocenoses, in which fossils experienced rapid burial by distal tempestites, while Kzl assemblages more likely correspond to thanatocenoses, locally deposited by storm backcurrents. Here, a genetic link between Ksl and Kzl is suggested within the Fezouata Shale.