Aaron W Hunter
email: aaron.hunter@u-bourgogne.fr

Bertrand Lefebvre

Serge Régnault

Philippe Roussel

Roland Claverie

A mixed ophiuroid-stylophoran assemblage (Echinodermata) from the Middle Ordovician (Llandeilian) of western Brittany, France

de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

of infaunal or epibenthic ophiuroids forming an actual living 'carpet' in or on the sea floor [START_REF] Fujita | Spatial structure within a dense bed of the brittle star Ophiura sarsii (Ophiuroidea: Echinodermata) in the bathyal zone off Otsuchi, northeastern Japan[END_REF][START_REF] Young | Ordovician sedimentary facies and faunas of southwest Europe: palaeogeographic and tectonic implications[END_REF][START_REF] Aronson | Evolutionary paleoecology of dense ophiuroid populations[END_REF]. For example, the sea floor off the northern coast of Japan is nearly totally blanketed by dense beds of the modern ophiuroid Ophiura sarsii, between depths of 200 and 600 m [START_REF] Fujita | Size structure of dense populations of the brittle-star Ophiura sarsii (Ophiuroidea: Echinodermata) in the bathyal zone around Japan[END_REF]. Such assemblages are usually monospecific or largely dominated by a single species [START_REF] Brun | Aggregation of Ophiotrix fragilis (ABILDGAARD) (Echinodermata: Ophiuroidea)[END_REF][START_REF] Fujita | Size structure of dense populations of the brittle-star Ophiura sarsii (Ophiuroidea: Echinodermata) in the bathyal zone around Japan[END_REF][START_REF] Hily | Is the activity of benthic suspension feeders a factor controlling water quality in the Bay of Brest?[END_REF]. They are also typically characterized by a rare and poorly diversified associated fauna [START_REF] Fujita | Spatial structure within a dense bed of the brittle star Ophiura sarsii (Ophiuroidea: Echinodermata) in the bathyal zone off Otsuchi, northeastern Japan[END_REF][START_REF] Young | Ordovician sedimentary facies and faunas of southwest Europe: palaeogeographic and tectonic implications[END_REF][START_REF] Ball | The rocky subtidal assemblages of Kinsale Harbour (south coast of Ireland)[END_REF]. Several examples of comparable dense beds of ophiuroids have been also documented in the fossil record [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]. The oldest indisputable occurrences of such ophiuroid meadows were reported from the Zahor hany and Bohdalec formations (Caradoc, Upper Ordovician) of Bohemia [START_REF] Petr | Revision of morphology and ecology of Bohemura jahni J AEKEL, 1903 (Ophiuroidea, Protasteridae) from Bohemian Middle Ordovician[END_REF]Mikulás et al. 1995). In the Middle Ordovician of Spain (Montes de Toledo), [START_REF] Hammann | Depositional environment and systematics of a new ophiuroid, Taeniaster ibericus n. sp., from the Middle Ordovician of Spain[END_REF] also reported the occurrence of several specimens of the ophiuroid Large concentrations of non-colonial organisms such as, for example, ophiuroids and crinoids are good sources of taphonomic, palaeoecological and taxonomic information (see [START_REF] Donovan | The taphonomy of echinoderms: calcareous multi element skeletons in the marine environment[END_REF]. Observations of modern communities from submersibles demonstrate that such accumulations could reach a great extent, not only in tropical waters but also on shallow temperate shelves, abyssal plains and even around the fringes of Antarctica [START_REF] Aronson | Evolutionary paleoecology of dense ophiuroid populations[END_REF][START_REF] Juterzenka | Echinoderm assemblages in the deep Greenland Sea: patterns in space[END_REF]). Understanding such assemblages allows us to reconstruct ancient palaeocommunities and isolate the factors that control biofacies and taphofacies [START_REF] Hunter | Field sampling bias, museum collections and completeness of the fossil record in echinoderms from the Campanian-Maastrichtian of Northern Europe[END_REF]. Recent work on modern environments has allowed us to understand mass accumulations and their relevance to fossil bioaccumulations as far back as the Palaeozoic (e.g. [START_REF] Brett | Echinoderm taphonomy, taphofacies, and Lagerstätten[END_REF].

In modern seas, ophiuroid meadows probably represent one of the most significant and widely distributed examples of such accumulations of echinoderms [START_REF] Aronson | Evolutionary paleoecology of dense ophiuroid populations[END_REF]. They correspond to extensive, dense populations (several tens to several hundreds of individuals per m 2 ) 1 Taeniaster ibericus in a single horizon. However, this aggregation more probably results from the fortuitous local accumulation of several individuals transported by currents over a short distance, rather than an actual dense assemblage [START_REF] Hammann | Depositional environment and systematics of a new ophiuroid, Taeniaster ibericus n. sp., from the Middle Ordovician of Spain[END_REF].

In this paper, we report on the discovery of an autochthonous assemblage consisting of a dozen, well-preserved, articulated individuals of the ophiuroid Taeniaster in the Upper Middle Ordovician (Llandeilian) of Gourin, Brittany (western France). This assemblage thus represents the oldest indisputable evidence of a gregarious mode of life for ophiuroids.

Interestingly, in the Middle Ordovician of Gourin, abundant remains of the mitrate stylophoran Mitrocystella were also sampled in the same horizon as well as specimens of ophiuroids. Stylophorans are an extinct class of free-living echinoderms (Middle Cambrian-Upper Carboniferous), characterized by a flattened theca and the possession of a single feeding arm (aulacophore: [START_REF] Ubaghs | Stylophora[END_REF][START_REF] Chauvel | Etude critique de quelques échinodermes stylophores du Massif armoricain[END_REF][START_REF] Parsley | Feeding and respiratory strategies in Stylophora[END_REF]Lefebvre 2003a). Their mode of life is supposed to be comparable to that of ophiuroids (Lefebvre 2003a[START_REF] Lefebvre | Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms[END_REF]. The occurrence of a dozen, fully articulated, autochthonous individuals of the mitrate Mitrocystella in a single horizon is indicative of a gregarious mode of life for these echinoderms. This interpretation is in good accordance with several previous records suggesting a gregarious mode of life for stylophorans: such stylophoran-dominated meadows are documented from the Middle Cambrian [START_REF] Ubaghs | Homalozoan echinoderms of the Wheeler Formation (Middle Cambrian) of Western Utah[END_REF] to at least the Devonian [START_REF] Ruta | A redescription of the anomalocystitid mitrate Rhenocystis latipedunculata from the Lower Devonian of Germany[END_REF]. However, the co-occurrence of abundant ophiuroids and mitrates in the same horizon in the Middle Ordovician of Gourin is an extremely significant observation from a palaeoecological point of view. Several examples of such mixed ophiuroid-stylophoran meadows have been reported in younger deposits of the Upper Ordovician (e.g. [START_REF] Lefebvre | Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms[END_REF]) to Lower Devonian (e.g. [START_REF] Caster | A new carpoid from the Parana Devonian[END_REF]). Consequently, this paper reports on the oldest definitive example of a mixed ophiuroid-stylophoran assemblage documented so far, and it demonstrates that the co-occurrence of these two echinoderm groups is a not uncommon occurrence in the fossil record. This study incorporates data from recent research on brittlestar preservation, palaeoecology and observations of modern communities from submersibles, along with numerous recent works on the probable palaeoecology of stylophorans (see Lefebvre 2003a) to track the early evolution of such bioaccumulations.

Finally, this paper describes the oldest remains of ophiuroids reported so far in the Armorican Massif and the third species of this Class ever described in the Palaeozoic of this region. The two previously described brittlestars are both early Devonian in age, and have been reported from Daoulas, Finistère (Protaster daoulasensis: Davy 1886), and from Saulges, Mayenne (Ophiurina armoricana: [START_REF] Morzadec | Ophiurina armoricana n. sp., ophiuroïde nouveau du Dévonien inférieur de la Bretagne[END_REF]. The comparable Middle Ordovician successions and faunas observed in Spain, Portugal and the Armorican Massif are considered by most authors as evidence for a direct geographic continuity between these three regions (e.g. [START_REF] Paris | Paléogéographie et relations ibéro-armoricaines au Paléozoïque anté-Carbonifère[END_REF][START_REF] Romano | The trilobite genus Eoharpes from the Ordovician of Brittany and Portugal[END_REF][START_REF] Young | Ordovician sedimentary facies and faunas of southwest Europe: palaeogeographic and tectonic implications[END_REF][START_REF] Gutiérrez-Marco | Faunal dynamics between Iberia and Bohemia during the Oretanian and Dobrotivian (late Middle-earliest Upper Ordovician), and biogeographic relations with Avalonia and Baltica[END_REF]. Consequently, the rarity of ophiuroids in the Middle Ordovician of Brittany is intriguing, as at least three different forms have been reported in coeval levels from elsewhere (Llandeilian) in Spain [START_REF] Chauvel | Les échinodermes (cystoïdes, astérozoaires, homalozoaires) de l'Ordovicien moyen des Monts de Tolède (Espagne)[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] Hammann | Depositional environment and systematics of a new ophiuroid, Taeniaster ibericus n. sp., from the Middle Ordovician of Spain[END_REF]. This paper also presents the first report of the mitrate Mitrocystella in the Chateaulin Basin (western Brittany). In the Armorican Massif, this stylophoran was previously only documented from several localities in eastern Brittany (e.g. Guichen), where most specimens were collected in siliceous concretions of the Traveusot Formation (Llandeilian: [START_REF] Chauvel | Recherches sur les cystoïdes et les carpoïdes armoricains[END_REF][START_REF] Chauvel | Etude critique de quelques échinodermes stylophores du Massif armoricain[END_REF]Lefebvre 2000a). In eastern Brittany, this type of fossil occurrence severely limits palaeoecological interpretations (especially as far as a putative gregarious mode of life is concerned). Apart from the Armorican Massif, the mitrate Mitrocystella was also reported from the Middle Ordovician (Llandeilian) of Bohemia [START_REF] Barrande | Système Silurien du centre de la Bohême[END_REF][START_REF] Jefferies | Some fossil chordates with echinoderm affinities[END_REF][START_REF] Jefferies | Locomotion, shape, ornament, and external ontogeny in some mitrate calcichordates[END_REF]Parsley 1994), Spain andPortugal (Gutiérrez-Marco &[START_REF] Gutiérrez-Marco | Nuevos hallazgos de Estilóforos (Homalozoos) en los materiales ordovícicos de la zona Centroibérica[END_REF][START_REF] Couto | Alguns fósseis raros ou pouco conhecidos da Formação de Valongo (Ordovícico Médio, Norte de Portugal)[END_REF].

Repositories

The material referred to in this paper is deposited in the following public institutions: MHNN (Muséum d'Histoire naturelle de Nantes, France) and NMP (Narodni Muzeum Prague, Czech Republic).

Geological setting and associated fauna

Both the ophiuroids (consisting entirely of the genus Taeniaster) and the mitrate stylophoran (Mitrocystella) were sampled from a single unit within the Upper Member of the Schistes de Postolonnec Formation (Llandeilian) in the abandoned slate quarry of Guernanic, near Gourin, Morbihan (grid ref. (GR) 157210688), in the central 'synclinorium' system of Brittany (western France). The occurrence of Middle Ordovician fossils in the Gourin area was first documented by [START_REF] Tromelin | Essai d'un catalogue raisonné des fossiles siluriens des départements de Maine-et-Loire, de la Loireinférieure et du Morbihan, avec des observations sur les terrains paléozoïques de l'Ouest de la France[END_REF], who mentioned the presence of bivalves (Redonia deshayesiana, R. duvaliana) and trilobites (Calymene aragoi, C. salteri, C. tristani, Dalmanites macrophtalma, Illaenus beaumonti, I. giganteus) in the 'schiste ardoisier'. Later on, [START_REF] Barrois | Le granite de Rostrenen, ses apophyses et ses contacts[END_REF][START_REF] Barrois | Légende de la feuille de Châteaulin[END_REF] reported a comparable, although more diverse, fossil assemblage in the 'schistes ardoisiers d'Angers' (Middle Ordovician) of Roch-Arvran (north of Gourin), including bivalves (Redonia deshayesiana, R. duvaliana), brachiopods (Orthis berthoisi), graptolites and trilobites (Calymene aragoi, C. tristani, Dalmanites macrophtalma, Illaenus giganteus). The presence of Middle Ordovician echinoderm remains in the area of Gourin was quoted by Chauvel (1941, p. 65), who mentioned the probable occurrence of diploporite cystoids (Aristocystites cf. bohemicus). However, contrary to the situation in other regions of the Armorican Massif (e.g. Crozon peninsula, synclinals south of Rennes), where Middle Ordovician successions and faunas have been intensively studied (e.g. [START_REF] Kerforne | Etude de la région silurique occidentale de la presqu'île de Crozon (Finistère)[END_REF][START_REF] Chauvel | Recherches sur les cystoïdes et les carpoïdes armoricains[END_REF][START_REF] Chauvel | Données nouvelles sur quelques cystoïdes diploporites (Echinodermes) du Paléozoïque armoricain[END_REF][START_REF] Chauvel | Etude critique de quelques échinodermes stylophores du Massif armoricain[END_REF][START_REF] Babin | Mollusques bivalves et céphalopodes du Paléozoïque armoricain[END_REF][START_REF] Henry | Trilobites ordoviciens du Massif armoricain[END_REF][START_REF] Henry | Paléoenvironnements et dynamique de faunes de trilobites dans l'Ordovicien (Llanvirn supérieur-Caradoc basal) du Massif armoricain (France)[END_REF][START_REF] Henry | Apparition soudaine de Protolloydolithus (Trilobita, Trinucleidae) dans l'Ordovicien (Llandeilo) du Massif armoricain: un essai d'interprétation[END_REF], those occurring in the Gourin area remained very poorly investigated (Pruvost & Le Maître 1949;[START_REF] Le Gall | Notice explicative, carte géologique de la France (1/50 000), feuille Gourin[END_REF]. Consequently, this study represents the first systematic description of Middle Ordovician fossils collected in the eastern part of the Chateaulin Basin.

In the Gourin area, the Schistes de Postolonnec Formation (Abereiddian-Llandeilian) corresponds to a thick succession (at least 450 m thick) of shales and slates lying conformably above the sandstones of the Grès Armoricain Formation (Arenig). The uncertain presence of the Middle Member of the Schistes de Postolonnec Formation (Grès de Kerarvail)

has not yet been definitively established in the Gourin area [START_REF] Le Gall | Notice explicative, carte géologique de la France (1/50 000), feuille Gourin[END_REF]. In more western outcrops (Crozon peninsula), the Kerarvail Member corresponds to a relatively thick sandstone intercalation (up to 30 m thick) between the shales of the Lower Member (Abereiddian) and those of the Upper Member (Llandeilian) of the Schistes de Postolonnec Formation (see [START_REF] Barrois | Observations sur la constitution géologique de l'Ouest de la Bretagne (3ème article)[END_REF][START_REF] Kerforne | Etude de la région silurique occidentale de la presqu'île de Crozon (Finistère)[END_REF][START_REF] Babin | Lexique des Formations de l'Ordovicien armoricain[END_REF][START_REF] Henry | Paléoenvironnements et dynamique de faunes de trilobites dans l'Ordovicien (Llanvirn supérieur-Caradoc basal) du Massif armoricain (France)[END_REF]. In spite of a difficult geological context (numerous faults, absence of continuous sections), intensive field studies lead since the mid 1990s by two of the authors (P. Roussel and R. Claverie) has made it possible to recognize at least four distinct fossiliferous intervals (typically 10-50 cm thick) within the Schistes de Postolonnec Formation (Fig. 1). These four intervals are characterized by their different faunal content and can be correlated between outcrops. They are designated here, from bottom to top, as the 'Pont Saint-Hervé', 'Monaco', 'Guernanic' and 'Pontvern Nord' units. Fossils collected in these four intervals of the Schistes de Postolonnec Formation are flattened and some of them are preserved in pyrite.

The Pont Saint-Hervé unit occurs about 40 m above the contact between the Grès Armoricain and Schistes de Postolonnec formations. It has yielded a rich Abereiddian assemblage, with abundant remains of cystoid echinoderms (Calix), graptolites and trilobites (e.g. Neseuretus, Plaesiacomia). The second fossiliferous interval (Monaco) is located about 350 m above the pont Saint-Hervé unit, and it corresponds to a Llandeilian assemblage, dominated by trilobites (e.g. Placoparia, Prionocheilus). The third fossiliferous unit (Guernanic) occurs about 40 m above the Monaco unit. It is characterized by the presence of numerous micas (absent in the underlying Monaco unit). The Guernanic unit has yielded a rich and diverse fossil assemblage, dominated by articulated trilobites (e.g. Salterocoryphe, illaenid indet., Zeliskella), associated with echinoderms (e.g. crinoids, diploporite cystoids, mitrate stylophorans), brachiopods, bivalve and cephalopod molluscs. The presence of tarphicerid cephalopods is indicative of a possible late Llandeilian age. In one locality (abandoned slate quarry of Guernanic), a well-defined echinoderm-rich horizon occurring within this fossiliferous interval has yielded a dozen specimens of ophiuroids (Taeniaster) associated with a comparable proportion of mitrate stylophorans (Mitrocystella). It is this assemblage that will be described herein. It is apparent that this echinoderm horizon contains very little associated fauna. Finally, the fourth fossiliferous unit (Pontvern Nord) is located about 10 m above the Guernanic unit. It consists in a well-defined, mica-rich interval. The Pontvern Nord unit has yielded a late Llandeilian assemblage dominated by bivalves and trilobites (e.g. Phacopidina micheli).

The composition of the four fossiliferous assemblages recognized so far in the Middle Ordovician of Gourin shows clear affinities with coeval faunas reported from elsewhere in Brittany (e.g. western part of the Chateaulin Basin, synclinals south of Rennes: [START_REF] Chauvel | Recherches sur les cystoïdes et les carpoïdes armoricains[END_REF][START_REF] Chauvel | Données nouvelles sur quelques cystoïdes diploporites (Echinodermes) du Paléozoïque armoricain[END_REF][START_REF] Chauvel | Etude critique de quelques échinodermes stylophores du Massif armoricain[END_REF][START_REF] Henry | Trilobites ordoviciens du Massif armoricain[END_REF][START_REF] Henry | Paléoenvironnements et dynamique de faunes de trilobites dans l'Ordovicien (Llanvirn supérieur-Caradoc basal) du Massif armoricain (France)[END_REF] but also in Portugal and Spain (see [START_REF] Chauvel | Les échinodermes (cystoïdes, astérozoaires, homalozoaires) de l'Ordovicien moyen des Monts de Tolède (Espagne)[END_REF][START_REF] Henry | Trilobites ordoviciens du Massif 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][START_REF] Gutiérrez-Marco | Nuevos hallazgos de Estilóforos (Homalozoos) en los materiales ordovícicos de la zona Centroibérica[END_REF]. However, in the absence of a complete survey of the Gourin fauna (and especially of the trilobites), it is beyond the scope of this paper to elaborate further on its biostratigraphic and/or palaeobiogeographic significance.

Taphonomy

Both ophiuroids and stylophorans are regarded universally as having echinoderm skeletons (e.g. [START_REF] Jefferies | Some fossil chordates with echinoderm affinities[END_REF][START_REF] Ubaghs | Stylophora[END_REF][START_REF] Chauvel | Etude critique de quelques échinodermes stylophores du Massif armoricain[END_REF]). In the taphonomic classification proposed by [START_REF] Brett | Echinoderm taphonomy, taphofacies, and Lagerstätten[END_REF], these two groups can be classified as 'type 1 echinoderms' in that they possess a skeleton composed of loosely articulated plates, and require exceptional preservation to prevent fragmentation. In spite of comparable taphonomic attributes, brittlestars are much better understood owing to being present in modern environments and will thus be discussed first. However, the preservation and palaeoecology of stylophorans have been compared with those of ophiuroids [START_REF] Lefebvre | Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms[END_REF]. The fossil record of ophiuroids is considered so patchy that their former diversity is often misunderstood. This is primarily owing to their calcareous endoskeleton being composed of ossicles, which completely disarticulate and disassociate in a very short period after death. Such ossicles have long been thought to be unclassifiable (see [START_REF] Mortensen | Über die stratigraphische verwendbarkeit der mikropischen Echinodermen-Reste[END_REF]. However, subsequent studies using fragmentary remains have tried to readdress this balance in the Mesozoic (see [START_REF] Jagt | Late Cretaceous-Early Palaeogene echinoderms and the K/T boundary in the southeast Netherlands and northeast Belgium-Part 1: Introduction and stratigraphy[END_REF][START_REF] Jagt | Late Cretaceous-Early Palaeogene echinoderms and the K/T boundary in the southeast Netherlands and northeast Belgium-Part 3: Ophiuroids; (with a chapter on: Early Maastrichtian ophiuroids from Rügen (Germany) and Møn (Denmark)[END_REF][START_REF] Hunter | Facies control on distribution of crinoid habitats in the Middle Jurassic (Bathonian) of England: implications for other echinoderm groups[END_REF]. As yet only [START_REF] Hotchkiss | Isolated skeletal ossicles of a new brittlestar of the Family Cheiropterasteridae SPENCER, 1934 (Echinodermata: Ophiuroidea) in the Lower Devonian of Bohemia (Czech Republic)[END_REF] have applied this treatment to Palaeozoic echinoderms. This study looks at the exceptional instances when ophiuroids are preserved in obrution deposits. Owing to the apparent similarities between extant and fossil ophiuroids it would be expected that the history of preservation would be well understood, especially in such obrutions. However, this is not the case, although [START_REF] Schäfer | Ecology and Paleoecology of Marine Environments[END_REF] noted that the arms of extant ophiuroids begin to disintegrate within 15 h of death and noted that catastrophic burial would be reasonable for articulated brittlestar assemblages, such as those that are observed in the Lower Jurassic in Dorset, UK. More rigorous studies were not conducted until recently by [START_REF] Kerr | Experimental decay and disarticulation of Ophiura texturata: implications for the fossil record of ophiuroids[END_REF]. It is this study that is critical to understanding the Middle Ordovician assemblage from Gourin. Using experiments analogous to those carried out by [START_REF] Briggs | Decay and mineralization of shrimps[END_REF], specimens of the extant ophiuroid Ophiura texturata were decayed for 14 days at temperatures ranging from 15 to 25 °C, with frequent tumbling of specimens to represent the turbidity of the environment. These experiments produced several stages of decay that could be quantified ranging from fully articulated carcases to complete disarticulation. The ophiuroids in the present study could be classified in two of the decay stages identified in [START_REF] Kerr | Experimental decay and disarticulation of Ophiura texturata: implications for the fossil record of ophiuroids[END_REF] study. Some specimens (including the paratype) could be classified as stage 1 (i.e. loss of ventral plates, oral shields, tooth papillae and jaws, and full articulation of the arms) (Figs 2a,d & 3a, MHNN.P.025955 and MHNN.P.025956), while most of them (including the holotype) would belong to stage 2 (i.e. arms begin to break off at/near the disk, and at least one arm remains attached) (see Figs 2b,c & 3c, MHNN.P.025954b and MHNN.P.025954a). However, interpretation of stage 2 preservation is difficult, as ophiuroids can sometimes autotomize their arms under environmental stress, so that forms lacking arms could have been buried before decay or shed arms during the catastrophic burial event itself. [START_REF] Kerr | Experimental decay and disarticulation of Ophiura texturata: implications for the fossil record of ophiuroids[END_REF] observed this type of preservation in brittlestars of the British Lower Jurassic and considered that burial occurred around the time of death, which implies the existence of a single community. However, such an observation could be a result of collector bias (see [START_REF] Hunter | Field sampling bias, museum collections and completeness of the fossil record in echinoderms from the Campanian-Maastrichtian of Northern Europe[END_REF]. In this study we have to assume an error caused by dealing with more distant Palaeozoic forms where the Recent brittlestar morphology is not strictly analogous with those in the Lower Palaeozoic.

The most fossiliferous starfish bed known in the Lower Palaeozoic is the Lady Burn starfish beds of Girvan, Scotland, UK. This assemblage is so diverse that in the most recent survey of Palaeozoic asterozoans (Shackleton 2003) approximately 60% of the taxonomic data came from this locality. The Lady Burn starfish beds have lenticular thinning from 15 to 1 cm within 1.5 m. This occurrence was interpreted by [START_REF] Goldring | The depositional environment of three starfish beds[END_REF] as resulting from the rapid entombment by storm deposits of a living, shallow-water assemblage, with limited transport and redeposition in a region where the fossilization potential is much higher than the environment of origin. However, more recent analyses based on the regional geological context, the composition of the faunal associations, as well as available sedimentological and taphonomic features of the Lady Burn starfish beds, all suggest that these assemblages more probably correspond to the rapid downslope transportation (by turbiditic currents), accumulation and burial of shallower, but relatively deep (i.e. outer shelf), living assemblages (e.g. [START_REF] Ingham | Geology of a continental margin. 2. Middle and late Ordovician transgression, Girvan[END_REF][START_REF] Harper | The stratigraphy of the Drummuck Group (Ashgill), Girvan[END_REF][START_REF] Harper | Late Ordovician brachiopod biofacies of the Girvan district, SW Scotland[END_REF][START_REF] Daley | The anatomy of the solute Girvanicystis batheri (?Chordata) from the Upper Ordovician of Scotland and a new species of Girvanicystis from the Upper Ordovician of south Wales[END_REF]). In the Ordovician, the geological context of Gourin (i.e. continental shelf, with relatively shallow deposits, at or below storm wave base) was clearly distinct from that documented in Girvan (i.e. outer edge of the Laurentian plate continental shelf). Moreover, contrary to the situation in the Lady Burn starfish beds, the Guernanic unit does not contain any evidence of lenticular thinning or several episodes of transport. Instead the whole horizon (including the echinoderm level) seems to be laterally continuous suggesting that the fauna was preserved in situ. Consequently, it is proposed here that the Guernanic fauna must have been entombed by a rapid influx of sediment as seen in ophiuroids from the Lower Lias of SW England (Kerr & Twichett 2004). It is most probable that the ophiuroid-stylophoran association represented the termination of a period of echinoderm colonization extinguished by sediment influx (as possibly suggested by the local abundance of floated micas). This would best explain the odd faunal content of the isolated echinoderm level atypical of the sampled horizon, primarily composed of trilobites, associated with rare brachiopods and molluscs (see Table 1). This trilobite-dominated assemblage would be more akin to the assemblages of the Middle Ordovician of Spain, where T. ibericus occurs at the base of a channel (5x2 cm). These channels are considered to be traps for the ophiuroids caught in the channel and covered quickly by channel fill [START_REF] Hammann | Depositional environment and systematics of a new ophiuroid, Taeniaster ibericus n. sp., from the Middle Ordovician of Spain[END_REF]. Although there is an absence of such channels in Guernanic, there are considerable similarities both in the monogeneric nature of the assemblage and the sedimentology.

In Guernanic, the stylophorans are well preserved, and occur in 'clusters' on the rock surface (see Fig. 3b). Like the ophiuroids, stylophorans can also occur in deep low-energy distal shelf or slope environments [START_REF] Lefebvre | Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms[END_REF]. In most Gourin specimens, the preservation of the whole theca is likely to be the result of a quick burial to prevent disarticulation seen in type 1 skeletons. It is also likely that the plates of the theca are preserved in the same manner as regular echinoid tests (see [START_REF] Aslin | Echinoid preservation in the Upper Estuarine Limestone of Blisworth, Northamptonshire[END_REF], where the theca is quickly filled with sediment through the openings in the cavity, thus preventing crushing of the theca. Another indicator of swift preservation is the absence in several specimens of the typical post-mortem kinking of their appendage (e.g. MHNN.P.025958 and MHNN.P.025960, see Fig 3b). In mitrates, such a flexed position of the aulacophore is generally interpreted as a distressed posture and/or as resulting from the postmortem contraction of this structure [START_REF] Dehm | Cystoideen aus dem rheinischen Unterdevon[END_REF][START_REF] Jefferies | Locomotion, shape, ornament, and external ontogeny in some mitrate calcichordates[END_REF][START_REF] Parsley | Feeding and respiratory strategies in Stylophora[END_REF][START_REF] Ruta | A redescription of the anomalocystitid mitrate Rhenocystis latipedunculata from the Lower Devonian of Germany[END_REF] 1998; Lefebvre 2003a). Consequently, the preservation of several specimens showing both their arm in the extended position and their cover plates ajar strongly suggests that these individuals were probably buried alive.

In Gourin, the presence of some slightly disarticulated specimens of Mitrocystella (e.g. MHNN.P.025957, see Fig. 3b) suggests that all mitrates were not alive when buried. However, the low degree of disarticulation of the Gourin Mitrocystella, combined with the absence of epibionts on their thecal plates, are both indicative of a short time of exposure on the sea floor. This preservation is clearly different from that observed in specimens of Mitrocystella from the Dobrotiva Formation (Llandeilian) of Bohemia. Although they are usually exquisitely preserved, and not much disarticulated, Bohemian mitrates are frequently relatively intensively bioeroded (e.g. numerous Arachnostega-like tubes on both the internal and external side of their lower thecal surface: see [START_REF] Bruthansová | Pellets independent of or associated with Bohemian body fossils[END_REF][START_REF] Lefebvre | Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms[END_REF]). These taphonomic features suggest that carcasses of Bohemian Mitrocystella were exposed for a long time on the seafloor, in quiet palaeoenvironmental conditions. Consequently, contrary to the situation in Gourin, the Bohemian assemblage clearly represents a taphocoenosis (accumulation of dead organisms).

To sum up, several lines of evidence suggest instantaneous preservation of the Gourin echinoderm specimens (fully articulated, with even the most fragile portions still intact) and are suggestive of rapid burial in an otherwise low-energy environment. With such quick preservation, it seems extremely likely that both ophiuroids and stylophorans occurred together in the same environment.

Autecology of ophiuroids and stylophorans

The above taphonomic section has established that fully articulated specimens of Taeniaster and Mitrocystella from Gourin are very probably preserved in life position and correspond to an in situ assemblage, suddenly and deeply buried by obrution deposits, in a moderately deep-shelf environment, below, but close to, storm wave base. In Palaeozoic sediments, the frequent simultaneous occurrences of abundant stylophorans and ophiuroids in the same layers could indicate that these two groups of echinoderms shared comparable palaeoecology or functional morphology, or at least inhabited the same habitat. Mixed assemblages of ophiuroids and stylophorans are relatively common in the late Ordovician-early Devonian time interval. Notable examples that will be discussed here are the Izegguirene Formation of Anti-Atlas, Morocco (early Caradoc: Lefebvre 2006), the South Threave Formation of Girvan, Scotland, UK (Ashgill: Spencer 1950; [START_REF] Donovan | Echinoderms[END_REF][START_REF] Jefferies | Calcichordates[END_REF], the Ponta Grossa Formation of Parana, Brazil (Emsian: [START_REF] Caster | A new carpoid from the Parana Devonian[END_REF] and the Hunsrück Slate, Bundenbach, Germany (Emsian: [START_REF] Dehm | Cystoideen aus dem rheinischen Unterdevon[END_REF][START_REF] Ruta | A redescription of the anomalocystitid mitrate Rhenocystis latipedunculata from the Lower Devonian of Germany[END_REF][START_REF] Glass | Two isorophid edrioasteroids (Echinodermata) encrusting conulariids from the Hunsrück Slate (Lower Devonian, Emsian; Rheinisches Schiefergebirge) of Germany[END_REF]. Such an association is not entirely unexpected, as both stylophorans and ophiuroids were small, flattened, free-living, unattached, gregarious, epibenthic or infaunal echinoderms, which were using their flexible arms for feeding and anchoring to the substrate for limited mobility [START_REF] Lefebvre | Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms[END_REF].

Comprising approximately 1800 species in 250 genera, ophiuroids are the largest group of Table 1. Faunal list of macrofossils found in Guernanic quarry, Gourin (Morbihan, France), in the Guernanic unit, Upper Member of the Schistes de Postolonnec Formation (Llandeilian), with an indication of their abundance (C, common; R, rare)

Brachiopods Trilobites lingulids indet. (R) Colpocoryphe (C) Aegiromena (R) Coplacoparia (R) Eorhipidomella (Crozonorthis)?(R) Illaenid indet (C) articulate forms indet. (R) Isabelinia (C) Neseuretus (C) Echinoderms Nobiliasaphus (C) Calix (C) Panderia (C) crinoids (C) Parabarrandia (R) Diamphidiocystis (R) Phacopidina (R) Mitrocystella (C) Prionocheilus (R) Taeniaster (C) Salterocoryphe (C) Selenopeltis (R) Molluscs Zeliszkella (R) Trocholites (R) nautiloids indet. (R) Glyptarca (R) Other fossils Praenucula (R) conularilds (R) Redonia (R) hyolithids (R) bivalves indet. (R) Ribeiria (R)
extant echinoderms and today range across all modern environments from the Arctic regions to the Equator [START_REF] Simms | Chapter 25, Echinodermata[END_REF]. In terms of ecology, modern ophiuroids can be epifaunal suspension feeders or infaunal detritus feeders. However, the morphology of even the modern fauna is not mutually exclusive to one feeding habit, although many interpretations have been proved to be ambiguous. For instance, [START_REF] Spencer | Early Palaeozoic starfish[END_REF] suggested that aboral paxillae in Palaeozoic asteroids could indicate an infaunal behaviour. However, in modern brittlestars, such a character occurs in both epifaunal and infaunal forms [START_REF] Gale | Phylogeny and classification of the Asteroidea (Echinodermata)[END_REF]. Although it is possible that Palaeozoic ophiuroids were probably low-level suspension feeders, with some of them adapted to occasional scavengers (see [START_REF] Petr | Revision of morphology and ecology of Bohemura jahni J AEKEL, 1903 (Ophiuroidea, Protasteridae) from Bohemian Middle Ordovician[END_REF], they however lacked the mobility of the arms seen in more recent forms, so that a more advanced filter-feeding habit may have not been possible [START_REF] Shackleton | Skeletal homologies, phylogeny and classification of earliest asterozoan echinoderms[END_REF].

With many anatomical characters common to both ophiuroids and stylophorans [START_REF] Nichols | The water-vascular system in living and fossil echinoderms[END_REF][START_REF] Lefebvre | Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms[END_REF], it is probable that comparable feeding techniques were used in these two groups of echinoderms. Thus, observations of modern ophiuroid meadows can probably help to understand not only the palaeoecology of fossil brittlestars, but also that of stylophorans. Indisputable ophiuroid remains have not been documented before the Lower Ordovician (Tremadocian [START_REF] Thoral | Contribution à l'étude paléontologique de l'Ordovicien inférieur de la Montagne Noire et révision sommaire de la faune cambrienne de la Montagne Noire[END_REF][START_REF] Spencer | Asterozoa and the study of Palaeozoic faunas[END_REF][START_REF] Spencer | Early Palaeozoic starfish[END_REF][START_REF] Fell | The phylogeny of sea-stars[END_REF], and a possible gregarious or social mode of life for this echinoderm group (so typical of their modern relatives) was not evident before the Late Ordovician (Caradoc: [START_REF] Petr | Revision of morphology and ecology of Bohemura jahni J AEKEL, 1903 (Ophiuroidea, Protasteridae) from Bohemian Middle Ordovician[END_REF][START_REF] Fujita | Dense beds of ophiuroids from the Paleozoic to the Recent: the significance of bathyal populations[END_REF]Mikulás et al. 1995). The oldest dense assemblages of ophiuroids known so far have been documented in the Caradoc of Morocco and Bohemia [START_REF] Petr | Revision of morphology and ecology of Bohemura jahni J AEKEL, 1903 (Ophiuroidea, Protasteridae) from Bohemian Middle Ordovician[END_REF][START_REF] Lefebvre | Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms[END_REF]). On the other hand, stylophorans have been described as far back as the Middle Cambrian, and a gregarious mode of life seems to be the rule for most of them [START_REF] Ubaghs | Stylophora[END_REF][START_REF] Ubaghs | Homalozoan echinoderms of the Wheeler Formation (Middle Cambrian) of Western Utah[END_REF].

The distribution of modern dense beds of epibenthic ophiuroids is relatively substrateindependent, but strongly controlled by depth (and water temperature). Extensive, dense populations of modern epibenthic ophiuroids are nearly exclusively restricted to the bathyal zone (depths of between 200 and 3000 m: [START_REF] Blaber | Population density and predators of Ophiacantha fidelis (KOEHLER, 1930) (Echinodermata: Ophiuroidea) on the continental slope of Tasmania[END_REF][START_REF] Fujita | Spatial structure within a dense bed of the brittle star Ophiura sarsii (Ophiuroidea: Echinodermata) in the bathyal zone off Otsuchi, northeastern Japan[END_REF][START_REF] Fujita | Dense beds of ophiuroids from the Paleozoic to the Recent: the significance of bathyal populations[END_REF][START_REF] Gage | P/B ratios in deep-sea brittle stars[END_REF]. In shallower environments, very rare, small, patchy aggregations of epibenthic ophiuroids have been documented in the infralittoral zone (depths of between 10 and 50 m), as for example on the SW coast of England [START_REF] Warner | On the ecology of a dense bed of the brittle-star Ophiotrix fragilis[END_REF][START_REF] Broom | Aggregation behaviour of the brittle-star Ophiotrix fragilis[END_REF], or in a salt lake of the Bahamas [START_REF] Aronson | Ophiuroids in a Bahamian saltwater lake: the ecology of a Paleozoic-like community[END_REF][START_REF] Aronson | Evolutionary paleoecology of dense ophiuroid populations[END_REF].

As argued above, ophiuroids can occur in dense aggregations (also known as 'brittlestar beds'), in which representatives of other echinoderm groups (e.g. echinoids, asteroids) are frequently present [START_REF] Hunter | Facies control on distribution of crinoid habitats in the Middle Jurassic (Bathonian) of England: implications for other echinoderm groups[END_REF]), whilst members of other major groups of marine invertebrates (e.g. crustaceans, molluscs, sea anemones, sponges) are generally rare and poorly diversified [START_REF] Fujita | Spatial structure within a dense bed of the brittle star Ophiura sarsii (Ophiuroidea: Echinodermata) in the bathyal zone off Otsuchi, northeastern Japan[END_REF]. A gregarious mode of life, with very high densities of individuals and lowdiversity-associated faunas are features, which are not only observed in modern and fossil ophiuroid assemblages (see [START_REF] Petr | Revision of morphology and ecology of Bohemura jahni J AEKEL, 1903 (Ophiuroidea, Protasteridae) from Bohemian Middle Ordovician[END_REF]Mikulás et al. 1995) but seem to be a conservative characteristics of most echinoderm dense populations as far back as the early Palaeozoic [START_REF] Lefebvre | Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms[END_REF]. Such a gregarious mode of life can be highly advantageous not only for reproductive success [START_REF] Lawrence | A Functional Biology of Echinoderms[END_REF][START_REF] Emson | Making the best of it: the ecology of deep sea echinoderms[END_REF], but also for a greater efficiency of filter feeding: in large aggregations of modern ophiuroids the presence of numerous arms facing the current locally slows down the current flow, and thus increases the deposition of food particles into the feeding structures [START_REF] Warner | On the ecology of a dense bed of the brittle-star Ophiotrix fragilis[END_REF][START_REF] Fujita | Dense beds of ophiuroids from the Paleozoic to the Recent: the significance of bathyal populations[END_REF]. A gregarious mode of life may also provide a better resistance against both high current flows [START_REF] Warner | On the ecology of a dense bed of the brittle-star Ophiotrix fragilis[END_REF][START_REF] Broom | Aggregation behaviour of the brittle-star Ophiotrix fragilis[END_REF]) and predation ('principle of strength in numbers' [START_REF] Warner | On the ecology of a dense bed of the brittle-star Ophiotrix fragilis[END_REF][START_REF] Fujita | Spatial structure within a dense bed of the brittle star Ophiura sarsii (Ophiuroidea: Echinodermata) in the bathyal zone off Otsuchi, northeastern Japan[END_REF][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].

Several studies based on modern and fossil ophiuroids have indicated that the presence and spatial extension of brittlestar beds were both strongly inversely correlated with the abundance of putative predators (e.g. sea-stars, teleost fishes [START_REF] Aronson | Biology of a scale-independent predator-prey interaction[END_REF][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]. More generally, the distribution of dense populations of both fossil and modern sessile, suspension-feeding, epibenthic echinoderms (e.g. crinoids, ophiuroids) appears to be strongly influenced by the intensity of predation pressure [START_REF] Bottjer | Paleoenvironmental patterns in the evolution of post-Paleozoic benthic marine invertebrates[END_REF][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] Meyer | Implications of research on living stalked crinoids for paleobiology[END_REF]. This important interpretation suggests that, in the Palaeozoic, dense assemblages of stylophorans and/or other free-living epibenthic suspensionfeeding echinoderms (e.g. cinctans, ophiuroids, solutes) were probably also restricted to palaeoenvironments where predators were rare and even absent [START_REF] Lefebvre | Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms[END_REF]). However, observation of modern and fossil brittlestar beds indicate that in such echinoderm meadows not only predators are rare, but the whole associated fauna is also scarce and poorly diversified. Consequently, the comparison with modern brittlestar beds demonstrates that the low diversity observed in most dense assemblages of ophiuroids and stylophorans (e.g. in the Middle Ordovician of Gourin or Traveusot, or in the Upper Ordovician of Bohemia: see [START_REF] Petr | Revision of morphology and ecology of Bohemura jahni J AEKEL, 1903 (Ophiuroidea, Protasteridae) from Bohemian Middle Ordovician[END_REF][START_REF] Mikulas | The first occurrence of a 'brittlestar bed' (Echinodermata, Ophiuroidea) in Bohemia (Ordovician, Czech Republic)[END_REF] probably does not result from a preservational bias, but corresponds to a general feature of most gregarious echinoderm faunas [START_REF] Lefebvre | Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms[END_REF].

Echinoderm meadows represent major bioaccumulations and are in urgent need of further consideration. Such bioaccumulations are not only important in tropical waters, but in modern Arctic waters as well. This study shows that such gregarious occurrences of echinoderms are not uncommon in the fossil record.

Conclusions

•

In the Llandeilian Guernanic unit (Upper Member of the Schistes de Postolonnec Formation) of Gourin (Brittany), sedimentological, taphonomic (e.g. bivalves preserved with their valves in connection) and palaeontological evidence (e.g. composition of trilobite assemblages) all suggest that deposition was at or below storm wave base.

•

In Gourin, the fossils are largely complete and some of them in life position (e.g. mitrates with extended aulacophore), suggesting rapid sedimentation.

• An application of experimental taphonomy on ophiuroids has allowed the greater understanding of the origin of bioaccumulations and whether they are allochthonous (e.g. Girvan) or autochthonous (e.g. Gourin). Thus, establishing that the ophiuroids like the mitrates were preserved quickly after death • Large echinoderm bioaccumulations are recorded early in the fossil record and could have been at least as extensive as those observed in modern temperate and Arctic waters.

• It is becoming increasingly apparent that stylophorans occurred in association with ophiuroids, and that the Gourin assemblage represents the oldest indisputable evidence of such an association.

This paper is a contribution to IGCP project No. 503 'Early Palaeozoic Palaeography and Palaeoclimate', and of the team 'Macroévolution et dynamique de la biodiversité' of the UMR CNRS 5561 Biogéosciences (Université de Bourgogne, Dijon). The authors are particularly grateful to P. Jean (Muséum d'Histoire naturelle de Nantes) and J. Thomas (Biogéosciences, Université de Bourgogne) for photographs, D. Lewis (Natural History Museum, London), J. Plaine (Géosciences Rennes), R. Prokop (Narodni Muzeum, Prague) and D. Vizcaïno (Carcassonne) for access to specimens of Mitrocystella, and P.-A. Jaouen (Riecsur-Belon), L. Le Berre (Morlaix) and P. Réaudin (Le Faouet) for access to the material they have collected in Gourin. The authors are also very grateful to D. le Solliec, maire de Gourin, for providing authorization for sampling, and to F. Barazer (Gourin) for providing detailed maps and various important documents on the former quarries of Gourin. The Conseil Régional de Bourgogne is greatly acknowledged for funding A.W. Hunter's research through a postdoctoral fellowship. The authors thank J.J. Álvaro (Zaragoza), D.A.T. Harper (Copenhagen) and an anonymous reviewer for their informed and useful comments. Finally, A.W. Hunter's wishes to thank E. Clarkson (Edinburgh) for introducing him to the importance of Palaeozoic asterozoans and in particular the Ordovician of the Girvan district, Scotland.

Appendix: Systematic palaeontology

Class OPHIUROIDEA [START_REF] Gray | A synopsis of the genera and species of the class Hypostoma (Asterias Linn.)[END_REF] (see [START_REF] Simms | Chapter 25, Echinodermata[END_REF]) Order OEGOPHIUROIDEA Matsumoto, 1915 Suborder LYSOPHIURINA Gregory, 1897 Family PROTASTERIDAE S. A. [START_REF] Miller | North American Geology and Palaeontology for the Use of Amateurs, Students and Scientists[END_REF] (Diagnosis of family modified by Petr 1989) Genus TAENIASTER Billings, 1858 Type species, Palaecoma spinosa [START_REF] Billings | Descriptions of new fossils from Silurian rocks of Canada[END_REF] Diagnosis. Modified from [START_REF] Hammann | Depositional environment and systematics of a new ophiuroid, Taeniaster ibericus n. sp., from the Middle Ordovician of Spain[END_REF] and [START_REF] Petr | Revision of morphology and ecology of Bohemura jahni J AEKEL, 1903 (Ophiuroidea, Protasteridae) from Bohemian Middle Ordovician[END_REF]: interradial outline of the disc straight or concave (but may be convex). Oral and aboral faces of the disc with small scales of uncertain outline covered with granulated integument. Oral face covered with spines. Probably no true marginalia present. Disc margins often thickened on aboral faces (possible post-mortem folding of the skin after death). Mouth angle plates short, with oral side covered with large papillae. Ambulacrals with typical boot-shaped ridge on the oral side of arms. Median suture between alternate ambulacrals straight, with sinuous appearance between neighbouring boot-shaped ridges. Excavations for the dorsal longitudinal muscles not very broad. Aboral outline of ambulacrals trapezoidal to quadrate. Laterals ear-shaped, with vertical and groove spines. Remarks. Taeniaster is considered a valid genus in this study despite being placed by [START_REF] Shackleton | Skeletal homologies, phylogeny and classification of earliest asterozoan echinoderms[END_REF] within the genus Protaster along with Bohemura. Despite good arguments by [START_REF] Shackleton | Skeletal homologies, phylogeny and classification of earliest asterozoan echinoderms[END_REF] we consider that Taeniaster has too many distinct characters to be included within a single Protaster genus. The oral sides of arms possess a straight median suture between the alternate ambulacrals. Straight median suture is unique among the protasterid ophiuroids. A post-mortem condition allows the tips of the arms to appear whip-like. However, following [START_REF] Petr | Revision of morphology and ecology of Bohemura jahni J AEKEL, 1903 (Ophiuroidea, Protasteridae) from Bohemian Middle Ordovician[END_REF], this character is considered here as an unreliable character, and not included in the diagnosis above. Although [START_REF] Hammann | Depositional environment and systematics of a new ophiuroid, Taeniaster ibericus n. sp., from the Middle Ordovician of Spain[END_REF] noted that the area where the open ambulacral groove grades into the closed part does not vary within one species, [START_REF] Kesling | Strataster ohioensis, a new Early Mississipian brittle-star, and the paleoecology of its community[END_REF]see Petr 1989) showed considerable variation. The area where the open ambulacral groove grades into the closed part was considered to be very stable at species level by [START_REF] Hammann | Depositional environment and systematics of a new ophiuroid, Taeniaster ibericus n. sp., from the Middle Ordovician of Spain[END_REF], but highly variable in position by [START_REF] Kesling | Strataster ohioensis, a new Early Mississipian brittle-star, and the paleoecology of its community[END_REF]. The degree of closure of the ambulacram possibly represents a post-mortem feature; this is also true in Taeniaster lacking a whip-like tail, as this is simply not preserved in most specimens [START_REF] Petr | Revision of morphology and ecology of Bohemura jahni J AEKEL, 1903 (Ophiuroidea, Protasteridae) from Bohemian Middle Ordovician[END_REF].

Taeniaster armoricanus sp. nov. Figs 2a-d & 3a, c.
Derivation of name. Both, the holotype and paratype are from the Department of Morbihan, in the ancient Gallo-Roman territory of Armorica (modern Brittany). Holotype. Specimen MHNN.P.025954b. Paratype. Specimen MHNN.P.025955.

Material. Six specimens preserved on four slabs, housed in the Muséum d'Histoire Naturelle de Nantes.

Diagnosis. Medium-sized pentagonal-round disc. Shortened arms with small short straight ambulacra with square boots. Very large spaces between the ambulacra and laterals, weakly lobed bowed reduced laterals that tend to be larger than the ambulacra.

Description. The disc small and lightly pentagonal curved edges (Fig. 2a, MHNN.P.025955), these edges range from straight to very round in some specimens making the disk appear almost circular (Fig. 2b, MHNN.P.025954b). Only aboral surface is observed in the specimens, the thickening of the disc is observed in one specimen (Fig. 2d, MHNN.P.025956). Like many specimens of ophiuroids it is difficult to see the detail of the integument covering the aboral sides of the disc, although the remains of the papillae can be seen. The arms are well preserved. In most specimens the arms taper evenly (Fig. 2a, MHNN.P.025955), but there is no evidence of whip-like terminal section on any of the arms. From five to seven pairs of the ambulacral ossicles are incorporated into the disc (Fig. 2b, MHNN.P.025954b). About 16-17 pairs are present in the free arm; no ambulacra in the arm tips are recorded (Fig. 2a, MHNN.P.025955). Ambulacral plates of the left and right of the arm alternate and the median suture (also known as the median groove) is straight and narrow. The ambulacra are boot-shaped in morphology (Fig. 2c , MHNN.P.025954a). In the boot, the toe or foot is thinner and shorter and not bluntly developed compared to the leg section making the foot portion to appear larger in most specimens. Laterals are curved and are thinner and slightly reduced in size compared with the ambulacrals. Ambulacrals are not ear-shaped (i.e. depressed down) with clear space in between the lateral plates and those of the ambulacra (Fig. 2b, MHNN.P.025954b) Remarks. Like in many early ophiuroid species, the differences seen are very slight but significant to diagnose a new species (without having to infer significant geographical separation as a proxy for new species diagnosis). Having a smaller and more rounded disk with shorter arms, this species has more in common with Taeniaster ibericus with reduced and more rounded disk and reduced laterals or adambulacrals. However, T. armoricanus is separated by the further reduction of laterals and a more pentagonal disk than that observed in Taeniaster ibericus. The laterals are not ear-shaped as seen in Taeniaster bohemicus specimens, where laterals are larger than ambulacrals, while the short arms seems to be a unique feature of T. armoricanus. Occurrence. Upper Member, Postolonnec Formation (Llandeilian), Guernanic quarry, Gourin, Morbihan, Brittany, France.

Class STYLOPHORA Gill & Caster, 1960 Order MITRATA Jaekel, 1918Suborder MITROCYSTITIDA Caster, 1952 Genus MITROCYSTELLA Jaekel, 1901 Type species, Anomalocystites incipiens [START_REF] Barrande | Système Silurien du centre de la Bohême[END_REF] Remarks. Two species were assigned to the genus Mitrocystella by [START_REF] Jaekel | Uber Carpoideen; eine neue Klasse von Pelmatozoen[END_REF][START_REF] Jaekel | Phylogenie und System der Pelmatozoen[END_REF]: M. incipiens (type species) and M. barrandei. In his cladistic analysis of mitrocystitid mitrates, Lefebvre (2000b) placed M. barrandei (with 13 marginals, Z, and two infracentrals) in a distinct genus (Promitrocystites), so as to avoid the paraphyly of the genus Mitrocystella (defined by the possession of 12 marginals, Z, and two infracentrals).

In his original description of 'Anomalocystites' incipiens, [START_REF] Barrande | Système Silurien du centre de la Bohême[END_REF] illustrated eight specimens from the Šárka and Dobrotivá formations (Middle Ordovician) of Bohemia, but no type was implicitely designated. In his revision of mitrocystitid mitrates, [START_REF] Chauvel | Recherches sur les cystoïdes et les carpoïdes armoricains[END_REF] re-examined Barrande's original material of Mitrocystella incipiens and chose one specimen as lectotype (NMP.L9292; illustrated in Barrande 1887, pl. 5, fig. 1.3-4). Chauvel (1941, p. 174) also pointed out that two of the specimens figured by [START_REF] Barrande | Système Silurien du centre de la Bohême[END_REF] do not correspond to Mitrocystella incipiens. Re-examination of the original material of Barrande by one of the authors (B. Lefebvre) confirms that the two specimens NMP.L9297 (Barrande 1887, pl. 5 fig. 1.5-6) and NMP.L9300 (Barrande 1887, pl. 5 fig. 1.15-16) should be assigned to Mitrocystites mitra, and not to Mitrocystella incipiens. Moreover, the re-examination of Barrande's original material also shows that the morphology of the lectotype of Mitrocystella incipiens differs from that of all other specimens assigned to this species, and reported so far from Bohemia, Brittany, Portugal and Spain: four plates (instead of three) are present in central position on the lower thecal surface of NMP.L9292 (two posterior infracentral platelets, instead of one, are located between Z, PP1, PP2 and Mp5). Although relatively rare, irregularities in plating sometimes occur in mitrocystitid mitrates (Lefebvre 2003b). Consequently, the specimen chosen as lectotype of M. incipiens by [START_REF] Chauvel | Recherches sur les cystoïdes et les carpoïdes armoricains[END_REF] more probably corresponds to an abnormal (teratological) individual, rather than to a different species.

Mitrocystella incipiens (Barrande, 1887) Figs 2e-g & 3b

Material. Nine specimens preserved on five slabs are deposited in the collections of the Muséum d'Histoire Naturelle de Nantes (MHNN.P.025957-61). Additional material, present in private collections, was also examined.

Description. Measurements made on the theca of complete specimens follow the definitions proposed by Chauvel (1941, p. 156). Thecal length (L) is comprised between about 21 and 27 mm (L is estimated between the M1-Mp1 suture, at the aulacophore insertion, and the posterior edge of PP2). Thecal width (l) is comprised between about 15.7 and 19 mm (l corresponds to the largest width measured perpendicularly to L). The ratio l/L is between 0.70 and 0.77. In some individuals, the theca is relatively elongate and narrow (l/L<0.75), with gently curved lateral margins, a straight to slightly concave posterior edge, and asymmetric outlines (e.g. MHNN.P.025960 and MHNN.P.025961). Thecal asymmetry is due to: (1) the forward extension of the right anterior lobe (right of the aulacophore insertion), clearly longer and more prominent than the left one; and (2) the course of the left thecal side, straighter than that of the right one. In other specimens, the theca is apparently less elongate and larger (l/L>0.75), with more parallel-sided lateral margins, less asymmetric outlines (left and right anterior lobes of comparable extension), and a gently curved, rounded posterior extremity (e.g. MHNN.P.025957 and MHNN.P.025959). Specimens from Gourin can be grouped into three categories based on their preservation: (1) individuals with partly disarticulated lower thecal surface, no upper surface elements (adorals, supracentrals) and no aulacophore (e.g. MHNN.P.025957); (2) specimens with fully articulated lower thecal surface, partly preserved upper surface (adorals and a few supracentrals) and no aulacophore (e.g. MHNN.P.025959 and MHNN.P.025961); and (3) individuals with wellpreserved lower and upper thecal surfaces, and complete, articulated aulacophore in extended position (e.g. MHNN.P.025958 and MHNN.P.025960). Although they are flattened and pyritized, most specimens are relatively well preserved, and most external and internal features generally documented in three-dimensional individuals preserved in silicic-aluminous concretions can be observed.

Two large adorals of comparable shape and dimensions form the anterior edge of the upper surface. The external surface of each adoral bears, anteriorly, a strong anterior transverse crest, and posteriorly to this crest, numerous (about 12-15), sinuous, transverse cuesta-shaped ribs (e.g. MHNN.P.025960 and MHNN.P.025961). The hydropore (cross-shaped slit anterior to the adoral crest) and the internal surface of adorals have not been observed. Posteriorly to the pair of adorals, the upper thecal surface is paved by numerous (more than 70), slightly imbricated, unorganized supracentral platelets (e.g. MHNN.P.025960). The size of supracentrals regularly decreases in a posterior direction. Several (at least 12), elongate and narrow anal plates are preserved in some specimens (e.g. MHNN.P.025958 and MHNN.P.025960).

The lower thecal surface is composed by 12 large plates in marginal position (M1, M2, M3, M4, G and PP1 on the right; Mp1, Mp2, Mp3, Mp4 and Mp5 on the left; PP2 in terminal position), and three elements in central position (the zygal plate Z, and two left infracentrals). No orifices are present on the lower thecal surface (e.g. lateripores, paripores). Several features of the internal (upper) side of the lower thecal surface can be observed in the Gourin specimens: (1) on M1 and Mp1, a deep, well-marked furrow located on the posterior wall of the two aulacophore apophyses : the transverse anterior groove (e.g. MHNN.P.025957 and MHNN.P.025958); (2) on Mp1 and Z, a strong oblique ridge, asymmetrical in cross-section (right flank steeper than the left one): the zygal crest or septum (e.g. MHNN.P.025957 and MHNN.P.025959); (3) on Mp1, a small circular depressed area located above the contact between the left apophysis and the zygal crest: the left scutula (e.g. MHNN.P.025957 and MHNN.P.025958); (4) on Mp1, a short vertical ridge located left of the zygal crest and connected to the left scutula: the accessory septum (e.g. MHNN.P.025958); and (5) on Mp5 and PP1, traces of insertion for rectal muscles (e.g. MHNN.025958 and MHNN.P.025959).

Morphology of the aulacophore insertion is apparently variable within Gourin specimens: relatively deep and narrow (e.g. MHNN.P.025958), or shallower and more widely open (e.g. MHNN.P.025957). In bestpreserved specimens, at least seven-eight imbricated rings have been observed in the proximal portion of the aulacophore (e.g. MHNN.P.P.025958 and MHNN.P.025960). Each proximal ring is made of four plates (two inferolaterals and two tectals). The lower surface of the stylocone bears two strong, laterally compressed spines, connected by a sharp, longitudinal ridge (e.g. MHNN.P.025960). Ornamentation rapidly decreases in height and strength on the lower surface of more distal brachials (ossicles). In some specimens, ajar cover plates are preserved articulated to underlying ossicles (e.g. MHNN.P.025958). Remarks. The morphology of all mitrocystitid mitrates collected in Gourin is identical to that of specimens of Mitrocystella incipiens reported from elsewhere in Bohemia, Brittany, Portugal and Spain, and thus totally supports their assignment to this species. [START_REF] Chauvel | Les Hétérostèlées armoricaines[END_REF][START_REF] Chauvel | Recherches sur les cystoïdes et les carpoïdes armoricains[END_REF][START_REF] Chauvel | Etude critique de quelques échinodermes stylophores du Massif armoricain[END_REF]) placed all specimens of M. incipiens from Brittany in a different subspecies (M. incipiens miloni) from those from Bohemia (M. incipiens incipiens). This distinction is mostly based on: (1) the aspect of thecal outlines (in M. incipiens incipiens, lateral margins are parallel-sided, rather than curved, and the posterior thecal edge is not straight, but rounded);

(2) the morphology of the aulacophore insertion (large and widely open in M. incipiens incipiens, narrower and deeper in M. incipiens miloni); and (3) the asymmetry of the two anterior lobes (right anterior lobe more developed in M. incipiens miloni). For the same reasons, all specimens collected in Portugal and Spain were also assigned to M. incipiens miloni (e.g. [START_REF] Gutiérrez-Marco | Nuevos hallazgos de Estilóforos (Homalozoos) en los materiales ordovícicos de la zona Centroibérica[END_REF]. Re-examination of the six Bohemian specimens of M. incipiens illustrated by [START_REF] Barrande | Système Silurien du centre de la Bohême[END_REF] confirms that their morphology appears slightly different from that of most specimens collected in Brittany, Portugal and Spain. However, it should be pointed out that: (1) the mean size of Barrande's specimens is smaller than that of most individuals sampled in Britanny, Spain; and Portugal; and (2) the morphology of small Breton specimens is closely comparable to that of Bohemian forms. It is thus possible that the differences pointed out by [START_REF] Chauvel | Recherches sur les cystoïdes et les carpoïdes armoricains[END_REF] more probably reflect growth allometries, rather than actual regional differences between Bohemian forms on one side, and Ibero-Armorican forms on the other side. The slight morphological differences observed within the specimens collected in Gourin seems to support this interpretation. Occurrence. Upper Member, Postolonnec Formation (Llandeilian), Guernanic quarry, Gourin (Morbihan), Brittany (France); Traveusot Formation (Llandeilian), Bain-de-Bretagne, Guichen, Guignen and Langon (Ille-et-Vilaine), Brittany (France); Valongo Formation (Llandeilian), San Pedro de Cova (Portugal); Guindo Shales (Llandeilian), Almaden-Campos de Calatrava, Almodovar del Campo and Calzada de Calatrava, Ciudad Real (Spain); Sarka Formation (Abereiddian), Praha-Sarka and Rokycany (Osek), Bohemia (Czech Republic); Dobrotiva Formation (Llandeilian), Malé Prilepy and Svata Dobrotiva (='Sancta Benigna' in Barrande 1887), Bohemia (Czech Republic).

Fig 1 .

 1 Fig 1. (a) Location of the Armorican Massif. (b) Geological map of the Armorican Massif focusing on the areas of Ordovician outcrop. (c) Synthetic stratigraphic column of Middle Ordovician sections in Gourin area, showing identified fossiliferous horizons (star signifies the echinoderm bed).

Fig. 2 .

 2 Fig. 2. (a)-(d) Morphology of the ophiuroid Taeniaster armoricanus and (e)-(g) of the mitrate stylophoran Mitrocystella incipiens, Upper Member, Postolonnec Formation (Llandeilian), Guernanic quarry, Gourin (Morbihan), Brittany, France. (a) MHNN.P.025955. (b) MHNN.P.025954b. (c) MHNN.P.025954a. (d) MHNN.P.025956. (e) MHNN.P.025957, partly disarticulated lower thecal surface (upper aspect) showing the zygal crest. (f) MHNN.P.025961, articulated lower thecal surface (upper aspect), with adorals and some supracentrals. (g) MHNN.P.025960, articulated upper thecal surface, with adorals, supracentrals, and aulacophore (in extended position). Scale bar represents 1 cm.

Fig. 3 .

 3 Fig. 3. (a) & (c) Taphonomy of the ophiuroid Taeniaster armoricanus and (b) of the mitrate stylophoran Mitrocystella incipiens, Upper Member, Postolonnec Formation (Llandeilian), Guernanic quarry, Gourin (Morbihan), Brittany, France. (a) MHNN.P.025955, articulated specimen. (c) MHNN.P.025955a-b, semi-articulated specimens. (c) MHNN.P.025958, cluster of articulated specimens. Scale bar represents 1 cm.