Balcarcel A Gilissen

PALEONEUROLOGY OF AMNIOTES: New Directions in the Study of Fossil Endocasts

Keywords: endocast, Neopallium, Cetacea, operculization

This chapter presents a detailed review of works published on Artiodactyla endocasts and provides a comprehensive examination of artiodactyl brain evolutionary history, including Cetacea, from the early Eocene (c.a. 45 Ma) onwards. Artiodactyl endocasts have been actively studied from the second half of the 19 th century to the 1970's. These works on natural or artificial endocasts widely took place outside the frame of phylogenetic concerns. We compile here the data available, including recent works using µCT-scan imagery techniques, and place them in a phylogenetic framework. We also provide new additional data regarding paleogene representatives of North American extinct clades (Homacodon, Helohyus, Leptauchenia, Agriochoerus), endemic European clade (Mouillacitherium, Dichobune), and Suoidea (Palaeochoerus) to complete the picture. The brain of modern artiodactyls is remarkable by the expansion and by the folding of the neopallium. We highlight the diversity of neopallium patterns at the Artiodactyla scale and their convergent nature on the last 45 millions years and show that encephalization increases with time, but with different modes between terrestrial and fully aquatic taxa (i.e. Cetacea). Each clade shows a mosaic pattern of derived and plesiomorphic features that now has to be put in perspective with both the history and the ecology of taxa.

Systematic and Phylogenetic context

The name "artiodactyl" [START_REF] Owen | Description of teeth and portions of jaw of two extinct anthracotherioid quadrupeds (Hyopotamus vectianus and Hyop. bovinus) discovered by the Marchioness of Hastings in the Eocene deposits of the N.W. coast of the Isle of Wight: with an at[END_REF]) comes from the ancient Greek άρτιος, pair, and δάκτυλος, finger, and unites ungulate mammals that have an even number of digits and paraxonian limbs (i.e. the axis of the limb is between digits III and IV; [START_REF] Thewissen | Postcranial osteology of the most primitive artiodactyl: Diacodexis pakistanensis (Dichobunidae)[END_REF][START_REF] Luckett | Phylogenetic relationships between the orders Artiodactyla and Cetacea: a combined assessment of morphological and molecular evidence[END_REF]. They are generally characterized by a "double pulley" astragalus with a distal trochlea and a large articular surface for the cuboid (Schaeffer 1947, Thewissen and[START_REF] Thewissen | Postcranial osteology of the most primitive artiodactyl: Diacodexis pakistanensis (Dichobunidae)[END_REF][START_REF] Martinez | The astragalus of Paleogene artiodactyls: comparative morphology, variability and prediction of body mass[END_REF][START_REF] Rose | On the origin of the order Artiodactyla[END_REF][START_REF] Luckett | Phylogenetic relationships between the orders Artiodactyla and Cetacea: a combined assessment of morphological and molecular evidence[END_REF][START_REF] Thewissen | Ankle morphology of the earliest cetaceans and its implications for the phylogenetic relations among ungulates[END_REF][START_REF] Geisler | New morphological evidence for the phylogeny of Artiodactyla, Cetacea, and Mesonychidae[END_REF][START_REF] Gingerich | Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan[END_REF]Thewissen et al. 2001b;[START_REF] Geisler | Phylogenetic relationships of Cetaceans to Terrestrial Artiodactyls[END_REF], and a trilobed lower deciduous fourth premolar (e.g., [START_REF] Luckett | Phylogenetic relationships between the orders Artiodactyla and Cetacea: a combined assessment of morphological and molecular evidence[END_REF][START_REF] Geisler | Phylogenetic relationships of Cetaceans to Terrestrial Artiodactyls[END_REF]. Behind the name "artiodactyl" lies one of the greatest mammalian evolutionary successes. Indeed, artiodactyls occupy today the second rank in terms of family diversity after rodents, and rank third in terms of generic and specific diversity [START_REF] Burgin | How many species of mammals are there?[END_REF], overtaken there by chiropterans. Artiodactyla today encompass five major crown groups: the tylopods (Tylopoda Illiger, 1811), the suoids (Suoidea Gray, 1821 sensu [START_REF] Gentry | (ed) The phylogeny and classification of the tetrapods[END_REF], the ruminants (Ruminantia Scopoli, 1777), the hippopotamoids (Hippopotamoidea Gray, 1821 sensu [START_REF] Gentry | (ed) The phylogeny and classification of the tetrapods[END_REF], and the cetaceans (Cetacea Brisson, 1762). Artiodactyls appeared quite abruptly in the fossil record ca. 55.8 Ma ago in the Holarctic, followed by an intense adaptive radiation in the early-middle Eocene (50-45 Ma; [START_REF] Theodor | The earliest artiodactyls: Diacodexeidae, Dichobunidae, Homacodontidae, Leptochoeridae, and Raoellidae[END_REF][START_REF] Rose | Earliest Eocene mammalian fauna from the Paleocene-Eocene thermal maximum at Sand Creek Divide, southern Bighorn Basin, Wyoming[END_REF][START_REF] Boivin | New material of Diacodexis (Mammalia, Artiodactyla) from the early Eocene of Southern Europe[END_REF]. Past generic diversity is at least seven times larger than today's, with more than 40 extinct families and nearly 950 fossil genera recognized [START_REF] Janis | Evolution of Tertiary mammals of North America: volume 1, Terrestrial carnivores, ungulates, and ungulate like mammals[END_REF][START_REF] Uhen | Middle to late Eocene Basilosaurines and Dorudontines[END_REF][START_REF] Williams | Synopsis of the Earliest Cetaceans Pakicetidae, Ambulocetidae, Remingtonocetidae, and Protocetidae[END_REF][START_REF] Uhen | Evolution of marine mammals: back to the sea after 300 million years[END_REF]Prothero and Foss 2007;[START_REF] Gingerich | Cetacea[END_REF][START_REF] Marx | (ed) Amphibian Evolution, the life of early land vertebrates[END_REF].

Despite a major breakthrough in Artiodactyla phylogenetic relationships made through molecular analyses, that is, the identification of the close affinities between hippopotamids and cetaceans (e;g., Miyamoto and Goodman 1986;[START_REF] Irwin | Evolution of the cytochromeb gene of mammals[END_REF]Montgelar et al. 1997;[START_REF] Hassanin | Pattern and timing of diversification of Cetartiodactyla (Mammalia, Laurasiatheria), as revealed by a comprehensive analysis of mitochondrial genomes[END_REF], basal relationships of the group remain largely unresolved and the origin of modern clades is still problematic when morphological characters are considered (e.g., O'Leary and [START_REF] O'leary | Impact of increased character sampling on the phylogeny of Cetartiodactyla (Mammalia): combined analysis including fossils[END_REF][START_REF] Geisler | Hippopotamus and whale phylogeny[END_REF]. According to most recent phylogenetic analyses, Tylopoda would be the first modern artiodactyl group to differentiate, followed by Suoidea; Ruminantia shares a close relationship with the Cetancodonta clade gathering hippos and cetaceans [START_REF] Arnason | The mitochondrial genome of the sperm whale and a new molecular reference for estimating eutherian divergence dates[END_REF], 2002[START_REF] Hassanin | Pattern and timing of diversification of Cetartiodactyla (Mammalia, Laurasiatheria), as revealed by a comprehensive analysis of mitochondrial genomes[END_REF]Gatesy et al. 2016). Totalevidence analyses, combining morphological and molecular characters, have been performed in the 2000s in order to enhance resolution of the early radiation and clarify the relatioship of extinct artiodactyl groups relative to modern ones, although with unsuccessful results (e.g., [START_REF] Geisler | Phylogenetic relationships of extinct cetartiodactyls: results of simultaneous analyses of molecular, morphological, and stratigraphic data[END_REF][START_REF] O'leary | Impact of increased character sampling on the phylogeny of Cetartiodactyla (Mammalia): combined analysis including fossils[END_REF][START_REF] Geisler | Hippopotamus and whale phylogeny[END_REF][START_REF] Spaulding | Relationships of Cetacea (Artiodactyla) among mammals: increased taxon sampling alters interpretations of key fossils and character evolution[END_REF]. Since 2009, no new comprehensive contribution to artiodactyl phylogeny based on morphological characters including all four modern families and fossils has been published, and major questions remain about the placement of extinct taxa in the artiodactyl phylogenetic tree. The huge diversity of artiodactyls is perceptible in their brain pattern. Modern representatives show a wide array of brain morphologies (Fig. 1), including some of the biggest and most convoluted mammalian brains [START_REF] Welker | Why does cerebral cortex fissure and fold?[END_REF]) in those of delphinid cetaceans (e.g., Tursiops truncatus), where brain size expressed as a function of the body mass ('encephalization quotient = EQ', [START_REF] Jerison | Brain evolution: new light on old principles[END_REF] ranks second after that of humans [START_REF] Marino | A comparison of encephalization between odontocete cetaceans and anthropoid primates[END_REF][START_REF] Marino | Convergence of complex cognitive abilities in cetaceans and primates[END_REF].

Historical background

It is worth noting that the first definition of the endocranial cast was provided by [START_REF] Cuvier | Recherches sur les ossemens fossiles: où l'on rétablit les caractères de plusieurs animaux dont les révolutions du globe ont détruit les espèces[END_REF] after an observation based on a natural endocast of an artiodactyl, Anoplotherium commune, from Montmatre gypsum. He wrote: "… it was moulded in the cavity of the skull; and as this cavity itself in the living animal was moulded on the brain, the clay necessarily represents the true shape of the latter…" Like many other mammalian groups, the first descriptions of artiodactyl endocranial casts mainly date from the second half of the 19 th century and are based on natural endocasts. Among them are included those of European cainotheriids (Gratiolet 1858) and ruminants [START_REF] Gaudry | Vertébrés fossiles du Mont Luberon[END_REF], North American oreodontids [START_REF] Leidy | The extinct mammalian fauna of Dakota and Nebraska, including an account of some allied forms from other localities, together with a synopsis of the mammalian remains of North[END_REF][START_REF] Bruce | Observations upon the brain casts of Tertiary mammals[END_REF], and North African archaeocete whales [START_REF] Gervais | Remarques sur l'anatomie des cétacés de la division des balénidés[END_REF]. Descriptions of these endocranial features are directly related to the discovery and scientific exploitation of major fossil outcrops that yielded abundant and well-preserved remains in a taphonomic context that allowed for natural preservation of casts of the endocranial cavity. Thus, the historical knowledge of endocranial anatomy is much contrasted depending on the taxonomic group and the geological period considered. The brain of modern artiodactyls is remarkable by the expansion and by the folding of the neopallium. Modern artiodactyl groups are all highly gyrencephalic [START_REF] Kazu | Cellular scaling rules for the brain of Artiodactyla include a highly folded cortex with few neurons[END_REF]) and differ in their neocortical pattern (Fig. 1), and most of the early works on fossil artiodactyl endocranial casts pay special attention to the foldings of the neopallium and their identification. Detailed identification of the different sulci of the neopallium of artiodactyls was established during the end of the 19 th century [START_REF] Krueg | Über die Furchung der Grosshirnrinde der Ungulaten[END_REF]) and the first half of the 20 th century based on the study of embryologic/ontogenetic series by [START_REF] Anthony | Le neopallium des suidés: étude de son développement et interprétation de ses plissements[END_REF][START_REF] Anthony | Le Neopallium du Boeuf. Étude de son Développement et Interprétation de ses Plissements[END_REF], 1936) for suids (Sus scrofa) and domestic bovids (sheep Ovis aries, and cow Bos taurus), and by [START_REF] Friant | Interprétation du cerveau de l'hippopotame par l'étude d'un stade embryonaire[END_REF][START_REF] Friant | Le Télencéphale des Hippopotamidae[END_REF] for hippopotamids. The study of artiodactyl endocasts during the 19 th and 20 th centuries can be divided into two approaches: qualitative studies focusing on anatomy from the second half of 19 th century until the 1970's (see for instance the chapter of [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF] in the "Traité de Paléontologie" tome 6 of Piveteau for a remarkable summary of the knowledge of artiodactyl endocasts in the 1960's), and quantitative focusing on the increase of brain size through time that mainly developed in the 1980's (e.g., [START_REF] Jerison | Brain evolution: new light on old principles[END_REF][START_REF] Jerison | Evolution of brain and Intelligence[END_REF][START_REF] Radinsky | The evolution of vertebrate design[END_REF]. Most of these observations were performed in a systematic framework separating artiodactyls in two categories on the basis of tooth morphology: Bunodontia (pigs, hippos and extinct relatives) and Selenodontia (ruminantsincluding camelidsand extinct relatives). The lack of a clear phylogenetic context (or basically the lack of phylogenetic concepts for earliest works) has limited the scope of the thorough, highly detailed, first observations performed on artiodactyl endocasts (see next section 2.1).

Endocranial studies did not progress much until the democratization of CT-scan investigations due to the small number of natural endocranial casts available (and the limitation of their taxonomic attribution if not associated with dental remains) and to the often destructive nature of preparation of plaster/silicone endocasts. Yet descriptions of fossil artiodactyl endocasts based on virtual intracranial investigations and 3D reconstructions of the internal mould of the cranial cavity remain scarce and are only available for early representatives of the group (Oreodontoidea, Macrini 2009;Diacodexis, Orliac and Gillisen 2012), for early Hippopotamoidea [START_REF] Thiery | Endocasts and brain evolution in Anthracotheriidae (Artiodactyla, Hippopotamoidea)[END_REF], for Cetacea and their closest relative (Raoellidae, [START_REF] Orliac | The endocranial cast of Indohyus (Artiodactyla, Raoellidae): the origin of the cetacean brain[END_REF]Remingtonocetidae, Bajpail et al. 2011;crown Cetacea, Marino et al. 2003;[START_REF] Racicot | Endocranial anatomy of a new fossil porpoise (Odontoceti, Phocoenidae) from the Pliocene San Diego Formation of California[END_REF][START_REF] Boessenecker | New records of the dolphin Albertocetus meffordorum (Odontoceti: Xenorophidae) from the lower Oligocene of South Carolina: encephalization, sensory anatomy, postcranial morphology, and ontogeny of early odontocetes[END_REF], and for crown Ruminantia (Cervidae, [START_REF] Fontoura | Virtual brain endocast of Antifer (Mammalia: Cervidae), an extinct large cervid from South America[END_REF].

Documentation of artiodactyl endocasts in the fossil record

Endocranial morphology of extinct Artiodactyla clades

The endocast of Diacodexis, earliest Artiodactyla -The oldest known artiodactyl endocasts described in the literature belong to the genus Diacodexis and originate from the Early Eocene of North America (Diacodexis ilicis, earliest Wasatchian, ca. 55 Ma; Orliac and Gilissen 2012, 3D reconstruction of the virtual endocast) and from the early Middle Eocene of Pakistan (Diacodexis pakistanensis, ca. 48 Ma; Sigogneau-Russel and Russel 1983, partial composite reconstruction based on nine skull fragments). The presence of a simple neopalleal pattern in Diacodexis, shared by other early artiodactyls, allowed for proposing a reconstruction of the ancestral neopalleal pattern for Artiodactyla.

Endemic European artiodactyls (EEA) -Several extinct artiodactyl genera and families (including Cebochoeridae, Mixtotheridae, Robiacinidae, Cainotheriidae, Choeropotamidae, Anoplotheriidae, Xiphodontidae, and Amphimerycidae) are geographically restricted to Europe. They derive from several endemic radiations that took place during the late early and middle Eocene when Europe was geographically isolated from other land masses in a general context of high sea level [START_REF] Prothero | The late Eocene-Oligocene extinctions[END_REF]. Phylogenetic relationships among the basal families of artiodactyls remain unclear and greatly differ between analyses (e.g., [START_REF] Geisler | Phylogenetic relationships of Cetaceans to Terrestrial Artiodactyls[END_REF][START_REF] Luccisano | Revision of the Eocene artiodactyls (Mammalia, Placentalia) from Aumelas and Saint-Martin-de-Londres (Montpellier limestones, Hérault, France) questions the early European artiodactyl radiation[END_REF]Weppe et al. 2020a, b). Endocasts of European endemic artiodactyls were among the first mammalian endocasts to be studied [START_REF] Cuvier | Recherches sur les ossemens fossiles: où l'on rétablit les caractères de plusieurs animaux dont les révolutions du globe ont détruit les espèces[END_REF][START_REF] Gratiolet | Sur l'encéphale de l'Oreodon gracilis[END_REF]. Most of them have been described based on exceptionally well-preserved fossil material from the early Miocene lacustrine deposits of Saint-Gérand-le-Puy (Allier department, France), and from Paleogene karstic infillings from the Quercy Phosphorite Formation (Southwestern France). Among the first taxa described were the cainotheriids, considered then to be early ruminant representatives [START_REF] Gratiolet | Sur l'encéphale de l'Oreodon gracilis[END_REF][START_REF] Milne-Edwards | Recherches anatomiques, zoologiques et paléontologiques sur la famille des chevrotains[END_REF]Gervais 1873;[START_REF] Hürzeler | Osteologie und odontologie der Caenotheriden[END_REF]. The endocast of Cainotherium was extensively studied by [START_REF] Anthony | Recherches sur le télencéphale des ruminants primitifs. Le Cainotherium[END_REF] who concluded that it represented the primitive pattern of the ruminant brain. [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF][START_REF] Dechaseaux | Les débuts de l'histoire de la fissuration du néopallium chez les carnivores fissipèdes et chez les artiodactyles[END_REF]Dechaseaux ( , 1969aDechaseaux ( , b, 1970[START_REF] Dechaseaux | Essais de paléoneurologie[END_REF] described then in great detail the endocranial casts of a variety of European endemic artiodactyls (Tapirulus, Mouillacitherium, Cebochoerus, Dichobune, Dacrytherium, Cainotherium, Oxacron, Diplobune, Amphimeryx, Pseudamphimeryx, Mixtotherium), and the general trends in neocortical fissuration patterns in Artiodactyla. These works from the second half of the 20 th century benefitted from the identification of cortical patterns established by [START_REF] Anthony | Le neopallium des suidés: étude de son développement et interprétation de ses plissements[END_REF][START_REF] Anthony | Le Neopallium du Boeuf. Étude de son Développement et Interprétation de ses Plissements[END_REF], 1936) and from the sharp reflections on external brain features by the works of [START_REF] Friant | Interprétation du cerveau de l'hippopotame par l'étude d'un stade embryonaire[END_REF][START_REF] Friant | Morphologie, développement et évolution du Cerveau des Ongulés artiodactyles sélénodontes: exposé des recherches originales du Laboratoire d'Anatomie comparée[END_REF][START_REF] Friant | Le Télencéphale des Hippopotamidae[END_REF], together with a more integrative evolutionary context (e.g., [START_REF] Dobzhansky | Genetic nature of species differences[END_REF][START_REF] Mayr | Systematics and the origin of species, from the viewpoint of a zoologist[END_REF][START_REF] Huxley | Evolution. The modern synthesis[END_REF]Simpson 1944;[START_REF] Stebbins | Variation and evolution in plants[END_REF].

Oreodontoidea -Among the earliest works on artiodactyl endocranial casts are the studies on oreodontoids from North America. Oreodontoidea include two families, Agriochoeridae and Merycoidodontidae, they include a wide variety of subfamilies documented from the middle Eocene to the middle Miocene. The phylogenetic relationships of, and within, Oreodontoidea are still unclear (Ludke 2007;[START_REF] Stevens | The family Mericoidodontidae In: Prothero DR, Foss SE (edds) The evolution of artiodactyls[END_REF]. Natural endocasts of multiple oreodontoid taxa are particularly well described in the literature of the second half of the 19 th century and the first half of the 20 th [START_REF] Gratiolet | Sur l'encéphale de l'Oreodon gracilis[END_REF][START_REF] Leidy | The extinct mammalian fauna of Dakota and Nebraska, including an account of some allied forms from other localities, together with a synopsis of the mammalian remains of North[END_REF][START_REF] Bruce | Observations upon the brain casts of Tertiary mammals[END_REF][START_REF] Scott | The Mammalia of the Uinta Formation. Part I. The geological and faunal relations of the Uinta Formation. Part II. The Creodonta, Rodentia, and Artiodactyla[END_REF][START_REF] Moodie | On the sinus paranasales of two early Tertiary mammals[END_REF][START_REF] Moodie | On the endocranial anatomy of some Oligocene and Pleistocene mammals[END_REF][START_REF] Black | Studies on endocranial anatomy. II. On the endocranial anatomy of Oreodon (Merycoidodon)[END_REF][START_REF] Thorpe | Natural brain casts of merycoidodonts[END_REF][START_REF] Thorpe | The Merycoidodontidae: an extinct group of ruminant mammals[END_REF][START_REF] Friant | Morphologie, développement et évolution du Cerveau des Ongulés artiodactyles sélénodontes: exposé des recherches originales du Laboratoire d'Anatomie comparée[END_REF][START_REF] Friant | Interprétation du cerveau de l'Oreodon[END_REF]. Most of these natural endocasts came from the Oligocene White River fauna of the Western U.S. [START_REF] Scott | The mammalian fauna of the White River Oligocene. Part IV. Artiodactyla[END_REF], and the Eocene of West Texas [START_REF] Wilson | Early Tertiary vertebrate faunas, Vieja Group, Trans-Pecos, Texas: Agriochoeridae and Merycoidodontidae[END_REF]). Among the taxa described or mentioned are the agriochoerid Protoreodon [START_REF] Macrini | Description of a digital cranial endocast of Bathygenys reevesi (Merycoidodontidae; Oreodontoidea) and implications for apomorphy-based diagnosis of isolated, natural endocasts[END_REF]) and the merycoidodontids Merycoidodon [START_REF] Gratiolet | Sur l'encéphale de l'Oreodon gracilis[END_REF][START_REF] Bruce | Observations upon the brain casts of Tertiary mammals[END_REF][START_REF] Black | Studies on endocranial anatomy. II. On the endocranial anatomy of Oreodon (Merycoidodon)[END_REF][START_REF] Friant | Interprétation du cerveau de l'Oreodon[END_REF]Leidy 1959), Eporeodon [START_REF] Marsh | Dinocerata: a monograph of an extinct order of gigantic mammals[END_REF][START_REF] Thorpe | Natural brain casts of merycoidodonts[END_REF], Merycochoerus [START_REF] Moodie | A new fish brain from the Coal Measures of Kansas, with a review of other fossil brains[END_REF][START_REF] Moodie | On the endocranial anatomy of some Oligocene and Pleistocene mammals[END_REF], Promerycochoerus [START_REF] Thorpe | Natural brain casts of merycoidodonts[END_REF], and Leptauchenia (originally described as Cyclopidius Cope 1878; Prothero and Sanchez 2008). Among these descriptive works, the incredible preservation of a specimen and the thorough description of the smallest details of its brain allowed Black (1920:fig.23-24) to propose a restoration of the "Oreodon" brain (in fact rather derived Merycoidodon species) with unrivalled precision. Black noticed the strikingly complex cerebellum contrasting with the small and relatively simply arranged neopallium and questioned, as early as the beginning of the 20 th century, the possible independent evolution of these parts of the brain. [START_REF] Black | Studies on endocranial anatomy. II. On the endocranial anatomy of Oreodon (Merycoidodon)[END_REF] and [START_REF] Moodie | On the sinus paranasales of two early Tertiary mammals[END_REF][START_REF] Moodie | On the endocranial anatomy of some Oligocene and Pleistocene mammals[END_REF] noticed the association of ruminant and "suilline" characters in merycoidodontid brains and qualified them as "pig-like ruminants." Oreodontid brain features were then discussed in a wider comparative frame by [START_REF] Friant | Morphologie, développement et évolution du Cerveau des Ongulés artiodactyles sélénodontes: exposé des recherches originales du Laboratoire d'Anatomie comparée[END_REF] and [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF]Dechaseaux ( , 1969a)). More recently, the endocranial cast of Bathygenys reevesi was described by [START_REF] Macrini | Description of a digital cranial endocast of Bathygenys reevesi (Merycoidodontidae; Oreodontoidea) and implications for apomorphy-based diagnosis of isolated, natural endocasts[END_REF] based on µCT-scan data, together with a general discussion about morphological diversity within Merycoidodontidae.

Endocasts of modern artiodactyl groups

Tylopoda -The evolution of brain morphology in Tylopoda is documented mainly by camelids from the Eocene to the Miocene; whose origin is the Holarctic zone of North America. In geochronological order, the camelid brain is represented by endocasts of Protylopus (Eocene), Eotylopus (Eocene), Poebrotherium (early and late Oligocene), and lastly, Protolabis (late Miocene) and Procamelus (late Miocene). As the earliest known member of Camelidae, the brain of Protylopus (ca. 40 Ma) is considered the precursor to the modern camelid brain. The Oligocene camelid brain is known from two specimens: Poebrotherium wilsoni of the early Oligocene, and the relatively larger Poebrotherium labiatum in the late Oligocene [START_REF] Bruce | Observations upon the brain casts of Tertiary mammals[END_REF]Cope 1886;[START_REF] Jerison | Quantitative analysis of the evolution of the camelid brain[END_REF]. [START_REF] Edinger | Brains from 40 million years of camelid history[END_REF] briefly described the basic pattern of evolution from these earlier forms to the late Miocene, the latter stage represented by two endocasts of Procamelus. The smaller of the two, first recognized as Procamelus occidentalis (see Cope 1877a, b), was later re-assigned to the genus Protolabis [START_REF] Jerison | Quantitative analysis of the evolution of the camelid brain[END_REF]). The larger endocast, which remains as Procamelus, was referred to as a late Pliocene form by many researchers [START_REF] Edinger | Brains from 40 million years of camelid history[END_REF][START_REF] Jerison | Quantitative analysis of the evolution of the camelid brain[END_REF]Repérant 1971a, b;[START_REF] Kruska | Hirngroßenanderungen bei Tylopoden wahrend der Stammesgeschichte und in der Domestikation[END_REF]. Recent investigations have clarified, as previously considered by [START_REF] Jerison | Quantitative analysis of the evolution of the camelid brain[END_REF], that this is actually a Miocene Procamelus (ca. 12-10 Ma) (Balcarcel et al. unpublished data). The major trends in neocortical evolution, spanning the Eocene to late Miocene, were described by [START_REF] Repérant | Moulages endocrâniens de tylopodes fossiles[END_REF]Repérant ( , 1971a)). Recently, newly described endocasts including that of Camelops hesternus (artificial), a giant camelid, and one of what is likely a "Palaeolama" (natural), highlight the degree of brain complexity reached during the Pleistocene (Balcarcel et al. unpublished data).

Suoidea -The external features of the brain of Sus scrofa, the domestic pig, are well known and described in detail (e.g., [START_REF] Saikali | A three-dimensional digital segmented and deformable brain atlas of the domestic pig[END_REF], including its developmental aspects [START_REF] Krueg | Über die Furchung der Grosshirnrinde der Ungulaten[END_REF][START_REF] Anthony | Le neopallium des suidés: étude de son développement et interprétation de ses plissements[END_REF]. This is not the case for other modern suid genera, which have only been partly documented [START_REF] Anthony | Le neopallium des suidés: étude de son développement et interprétation de ses plissements[END_REF]. The external morphology of the brain of modern Tayassuidae is also described in very few works [START_REF] Krueg | Über die Furchung der Grosshirnrinde der Ungulaten[END_REF][START_REF] Allanson | Observations on the pituitary gland of some members of the suborder Suiformes (Mammalia: Artiodactyla)[END_REF][START_REF] Saraiva | Macroscópia do encéfalo de catetos (Pecari tajacu[END_REF]). To our knowledge, there is no documentation of a fossil suoid endocast in the literature.

Ruminantia -The general morphology of the brain of ruminant artiodactyls and the evolution of brain features has been described and discussed by [START_REF] Friant | Morphologie, développement et évolution du Cerveau des Ongulés artiodactyles sélénodontes: exposé des recherches originales du Laboratoire d'Anatomie comparée[END_REF], and subsequently augmented by [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF]. These works largely build on the studies of embryologic/ontogenetic series in domestic bovids (sheep Ovis aries, and cow Bos taurus) by Anthony andGrzybowski (1934, 1936). Several endocranial casts (natural or plaster) of fossil ruminants from the Quaternary and the Neogene are described and figured in the literature, and the morphological features are generally described in great detail. Among the first descriptions are those of Gazella deperdita (Bovidae), described [START_REF] Gaudry | Vertébrés fossiles du Mont Luberon[END_REF] from the late Miocene deposits of Mont Luberon (France), and Samotherium (Giraffidae), described by [START_REF] Black | A study of the endocranial casts of. Ocapia, Giraffa and Samotherium, with special reference to the convolution pattern in the family Giraffidae[END_REF] from the late Miocene of the Samos Island. The endocranial morphology of extinct Bovidae was further documented by natural and silicone endocasts of the Plio-Pleistocene insular bovid Myotragus [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF](Dechaseaux :fig. 16, 1962;;Köhler and Moyá Solá 2004;[START_REF] Palombo | Brain versus body mass in endemic ruminant artiodactyls: a case studied of Myotragus balearicus and smallest Candiacervus species from Mediterranean Islands[END_REF]) and of the Pliocene ovine Megalovis [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF]. The endocast of the cervoid Palaeomerycidae Aletomeryx was described from the late Miocene of Nebraska [START_REF] Lull | New Tertiary artiodactyls[END_REF][START_REF] Friant | Morphologie, développement et évolution du Cerveau des Ongulés artiodactyles sélénodontes: exposé des recherches originales du Laboratoire d'Anatomie comparée[END_REF][START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF]) and the endocranial morphology of the fossil stem Cervidae Dicrocerus and Megaceros is known from the middle Miocene and the Pliocene of France [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF]. More recently the small Candiacervus from the Plio-Pleistocene of the Mediterranean islands (Angeleti 1980;Palombo et al. 2007[START_REF] Palombo | Brain versus body mass in endemic ruminant artiodactyls: a case studied of Myotragus balearicus and smallest Candiacervus species from Mediterranean Islands[END_REF], and the late Pleistocene Antifer ensenadensis from southern Brazil [START_REF] Fontoura | Virtual brain endocast of Antifer (Mammalia: Cervidae), an extinct large cervid from South America[END_REF]) further documented the brain morphology of extinct cervids. The earliest ruminant endocranial casts described in the litterature are, to our knowledge, that of Dremotherium (unknown family) and Amphitragulus? (unknown family) from the late Oligocene-early Miocene deposits of Saint-Gérand-le-Puy [START_REF] Sigogneau | Le genre Dremotherium (Cervoidea): anatomie du crâne, denture et moulage endocranien[END_REF][START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF]Dechaseaux , 1969)).

Hippopotamoidea -A first illustration and very brief mention of a fossil hippopotamid endocast (artificial) was made by Friant (1940:fig.4). To our knowledge, studies of fossil Hippopotamidae endocasts in the literature are limited to Hippopotamus protamphibius from the Pleistocene of Ethiopia (uncertain locality from Omo valley), H. madagascariensis and H. lemerlei from the ?late Pleistocene to Holocene of Madagascar, and H. minor from the Pleistocene of Cyprus (Anthony 1948). The latter notice the thickness of the dura mater preventing access to most of the cerebellar structures, and the presence of a large "K lobe", characteristic of the brain of Hippopotaminae (see section 3.1.3). This work on natural and artificial plaster endocasts was then integrated by [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF] in her discussions of adaptation to amphibiosis in hippopotamuses. The fossil record of Hippopotamidae only goes back to the Oligocene (ca. 30 Ma, [START_REF] Lihoreau | Hippos stem from the longest sequence of terrestrial cetartiodactyl evolution in Africa[END_REF] whereas hippopotamoids (i.e. "Anthracotheriidae") appear in the fossil record in Asia during the middle Eocene [START_REF] Lihoreau | Family Anthracotheriidae[END_REF]. The endocranial morphology of Hippopotamoidea is still poorly documented. Only two "anthracotheriid" taxa are described in the literature, a representative of the Microbunodontinae Microbunodon minimum and of a more derived Botriodontinae Merycopotamus medioximus [START_REF] Thiery | Endocasts and brain evolution in Anthracotheriidae (Artiodactyla, Hippopotamoidea)[END_REF]. Description of the endocast of M. minimum (late Oligocene from La Milloque, France) relies on a virtual reconstruction of the endocranial cavity of a well-preserved specimen, allowing a precise description of the external brain features of this small-sized animal. The external features of the brain of M. medioximus are only partly documented from a partial natural cranial endocast from Potwar Plateau, Pakistan, dated from the late Miocene [START_REF] Lihoreau | A new species of Anthracotheriidae, Merycopotamus medioximus nov. sp. from the Late Miocene of the Potwar Plateau, Pakistan[END_REF]). Given the scarcity of available data, the evolutionary history of brain features of hippopotamoids remains widely undocumented.

Cetacea -The first descriptions of cetacean endocasts are, to our knowledge, by Gervais who described plaster endocasts of extant Mysticeti (Gervais 1871), a partial braincase referred to the Eocene basilosaurid Zeuglodon cetoides (Watchita river, Louisiana, U.S.), and one of the Miocene delphinid Glyphidelphis sulcatus (Hérault, France, now referred to as Schizodelphis sulcatus) [START_REF] Gervais | Remarques sur les formes cérébrales propres aux thalassothériens[END_REF]. Natural endocranial casts of basilosaurids, fully aquatic archaeocetes sister taxa to Neoceti, have been described based on Dorudon by [START_REF] Smith | The brain of the Archaeoceti[END_REF]), Andrews (1906[START_REF] Stromer | Zeuglodon-reste aus dem Oberen Mitteleocan des Fajum[END_REF][START_REF] Dart | The brain of the Zeuglodontidae (Cetacea)[END_REF] who provided an extensive description of Zeuglodontidae endocasts. These works on basilosaurids were integrated with those of [START_REF] Edinger | Hearing and smell in cetacean history[END_REF] and [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF]. Following the concerns of Marple (1949) about the interpretation of fossil cetacean endocasts, [START_REF] Breathnach | Observations on endocranial casts of recent and fossil cetaceans[END_REF] revised the brain/endocast shape correspondence for a sample of modern cetaceans and concluded that regarding the cerebellar region of odontocetes and the brain of mysticetes in general, the endocranial cast is "little less than a poor and misleading caricature" (Breathnach 1955:541). The earliest evolutionary history of the cetacean brain is only partly documented back to the early middle Eocene by early diverging, non-fully aquatic archaeocetes, pakicetids [START_REF] Nummela | Cranial anatomy of Pakicetidae (Cetacea, Mammalia)[END_REF][START_REF] Kishida | Aquatic adaptation and the evolution of smell and taste in whales[END_REF], protocetids (Indocetus sp., cf. I. ramani, [START_REF] Bajpai | Indocetus (Cetacea, Mammalia) endocasts from Kachchh (India)[END_REF], and remingtonocetids [START_REF] Bajpai | Cranial anatomy of middle Eocene Remingtonocetus (Cetacea, Mammalia) from Kutch, India[END_REF][START_REF] Kishida | Aquatic adaptation and the evolution of smell and taste in whales[END_REF]. The endocranial morphology of Indohyus belonging to the Raoellidae, the sister taxon to Cetacea, recently provided some additional insights into the onset of cetacean brain characteristics [START_REF] Orliac | The endocranial cast of Indohyus (Artiodactyla, Raoellidae): the origin of the cetacean brain[END_REF]. The endocranial morphology of Neoceti, the clade gathering modern cetaceans, is still partially documented relative to their diversity. Odontocetes are the best documented and include records of Xenorophidae (Albertocetus from early Oligocene, South Carolina, [START_REF] Boessenecker | New records of the dolphin Albertocetus meffordorum (Odontoceti: Xenorophidae) from the lower Oligocene of South Carolina: encephalization, sensory anatomy, postcranial morphology, and ontogeny of early odontocetes[END_REF] Xenorophus from late Oligocene North Carolina, [START_REF] Marino | Reconstructing cetacean brain evolution using computed tomography[END_REF], Eurhinodelphinidae (Xiphiacetus sp. described as Eurhinodelphis morrisi by [START_REF] Marino | Reconstructing cetacean brain evolution using computed tomography[END_REF], Middle Miocene of Maryland), Eoplatanistidae (Pilleri and Gihr 1982, from Italy, originally referred to Schizodelphis, transferred to Eoplatanista by [START_REF] Muizon | Le polyphylétisme des Acrodelphidae, odontocètes longirostres du Miocène européen[END_REF], Delphinidae (Globicephalinae [START_REF] Boessenecker | Globicephaline whales from the mio-pliocene purisima formation of central California, USA[END_REF], Phocoenidae [START_REF] Racicot | Endocranial anatomy of a new fossil porpoise (Odontoceti, Phocoenidae) from the Pliocene San Diego Formation of California[END_REF], and several records of indeterminate odontocetes. These include "Squalodon" from the Oligocene of New Zealand [START_REF] Marples | Two endocranial casts of cetaceans from the Oligocene of New Zealand[END_REF]specimen C.34.7; see Fordyce 1978 for systematic reassessment), "Prosqualodon davidi" by [START_REF] Dart | The brain of the Zeuglodontidae (Cetacea)[END_REF] from the Miocene of Tasmania, and a specimen from the middle Miocene of Poland [START_REF] Stefaniak | Natural endocranial cast of a delphinid (Cetacea, Delphinidae) from the Pińczów limestones (middle Miocene; Holy Cross Mountains, Central Poland)[END_REF], originally referred to as Delphinidae indet.). [START_REF] Bisconti | A natural endocast of an early Miocene odontocete and its implications in cetacean brain evolution[END_REF] recently restudied the natural endocast of an early Miocene Odontocete from Piedmont, Italy, previously studied by Dal [START_REF] Piaz | Sugli avanzi di Cyrtodelphis sulcatus dell'arenaria di Belluno[END_REF], [START_REF] Parona | Fossili rari[END_REF] and [START_REF] Pilleri | Das Zentralnervensytem der Zahn und Bartenwale[END_REF]. Natural endocasts of extinct mysticetes are very scantily documented in the fossil record, which is, to our knowledge, limited to the cetotherids Cetotherium (Strobel 1881:pl1, fig2) and Imerocetus [START_REF] Mchedlidze | Fossil Cetacea of the Caucasus[END_REF], to Pinocetus polonicus (middle Miocene of Poland; Czyzewska, 1988, originally placed in the Cetotheriidae but recently found outside this clade by [START_REF] Marx | A large Late Miocene cetotheriid (Cetacea, Mysticeti) from the Netherlands clarifies the status of Tranatocetidae[END_REF], to the Llanocetidae Llanocetus denticrenatus [START_REF] Mitchell | A new cetacean from the late Eocene La Meseta formation Seymour Island, Antarctic peninsula[END_REF]

Problematics

As exposed above, endocasts of artiodactyls have been actively described and studied from the second half of the 19 th century to the 1970's. These works widely take place outside the frame of phylogenetic concerns, or in a paradigm mainly splitting Artiodactyla in two/three groups based on their dental morphology (bunodont/selenodont/bunoselenodont), and, naturally, separating cetaceans from artiodactyls. The phylogenetic context has drastically evolved since the end of the 20 th and the beginning of the 21 st centuries, and the relationships between modern artiodactyl groups are now pretty consensual. Integrating Cetacea within a broader definition for Artiodactyla is now necessary, as is the clarification of the evolutionary history of brain structures within this new phylogenetic context. In the meantime, phylogenetic relationships at the order scale including fossil taxa remains highly debated and no consensus has yet been reached today. Major artiodactyl groups can be differentiated by their endocast morphology (Dechaseaux 1969;[START_REF] Macrini | Description of a digital cranial endocast of Bathygenys reevesi (Merycoidodontidae; Oreodontoidea) and implications for apomorphy-based diagnosis of isolated, natural endocasts[END_REF], and inclusion of endocranial characters will certainly bring a source of relevant characters to define clades and clarify basal relationships within Artiodactyla. Gathering an increasing corpus of data for artiodactyl endocasts is now crucial to address these evolutionary issues. Indeed, documentation of endocranial morphology of the various artiodactyl groups remains scant compared to their amazing diversity. Most data regarding endocranial morphology of artiodactyls derives from natural and artificial endocasts and the number of available virtual models remains limited so that quantitative parameters such as relative volume of the different components (e.g., olfactory bulb, cerebrum, and cerebellum) or relative neocortical surface cannot currently be discussed at the order scale, hampering a quantitative assessment of brain evolution in the group.

Among major questions that are currently investigated is to what extent ecological specialisation has shaped the neocortical pattern and different components of the artiodactyl brain. The general overview of published endocasts suggests a complicated pattern of evolutionary history of the different structures with decoupling of evolutionary stages between cerebrum and cerebellum; each clade shows a mosaic pattern of derived and plesiomorphic features that has to be put in perspective with both the history and the ecology of taxa. Artiodactyla therefore appears to be a perfect group for a case study of brain evolution and its associated drivers because of their broad temporal and spatial repartition, and their incredible diversity within mammals, including a wide aray of body masses and ecological specialisations (e.g., terrestrial and aquatic).

Domestication is also a crucial aspect of the evolution of the artiodactyl brain. Artiodactyls comprise an impressive proportion of today's domestic livestock, including suids, camelids, and a wide variety of ruminants (cervids, bovids). Each group represents a model for exploring morphological changes to the brain in correlation with the domestic niche: selection for tameness, environmental and dietary alterations, and life cycle changes [START_REF] Zeder | The domestication of animals[END_REF]. Brain size differences have already been noted between many wild and domestic mammals, particularly in artiodactyls [START_REF] Kruska | Effects of domestication on brain structure and behavior in mammals[END_REF]), but neuroanatomical differences are less known. This is currently an area of great scientific interest, as behavioral and cognitive abilities are increasingly being inferred from brain morphology (e.g., [START_REF] Balcarcel | Intensive human contact correlates with smaller brains: differential brain size reduction in cattle types[END_REF], Hetch et al. 2019).

In this chapter, we provide a first step to discussing brain morphology and evolutionary history at the Artiodactyla scale, including Cetacea, based on a first compilation of available data and including recent, yet still very scant, 3D models deriving from µCT-scan acquisitions.

Overview of general and comparative anatomy

Characterization of cranial endocast morphology

Overview of modern artiodactyl brain morphology, primary identification of structures

Modern Artiodactyla are characterized by an important extension of their neopallium that covers most of the cerebral surface and extends posteriorly over the midbrain, partially hiding the cerebellum. If terrestrial groups mostly differ by the sulcal pattern of their neopallium, Cetacea stand as an exception with a highly morphologically divergent brain, with, among others, drastic reduction or lack of olfactory bulbs, very strong telencephalic flexure, and special cortical characteristics (i.e., simple cortical organization, [START_REF] Glezer | Implications of the "initial brain" concept for brain evolution in Cetacea[END_REF][START_REF] Morgane | Visual cortex of the dolphin: an image analysis study[END_REF][START_REF] Glezer | Implications of the "initial brain" concept for brain evolution in Cetacea[END_REF][START_REF] Glezer | Calcium-binding protein-containing neuronal populations in mammalian visual cortex: a comparative study in whales, insectivores, bats, rodents, and primates[END_REF][START_REF] Raghanti | A comparison of the cortical structure of the bowhead whale (Balaena mysticetus), a basal mysticete, with other cetaceans[END_REF]. Due to these unique features, Cetacea have always been studied outside the Artiodactyla framework. The literature relating to the study of gyrencephaly abounds with diverse and often contradictory nomenclatures. We principally use here the revised nomenclature of Repérant (1971b), mainly built on the work of [START_REF] Smith | On the homologies of the cerebral sulci[END_REF] and [START_REF] Anthony | Le Neopallium des Équidés: Étude du Développement de ses Plissements[END_REF] and based on various homology criteria (topographical, morphological, anatomical, histological, ontological, phylogenetic). We add here some other support to homology based on the morphological intermediate criteria as defined by Repérant (1971b). The main neocortical sulci of Artiodactyla observed on endocasts (i.e., exposed on the external aspect of the brain) are, from the rhinal fissure on the ventral margin of the neopallium (delimiting the paleopallium from the neopallium, Smith 1902) to the interhemispheric fissure (at the sagittal plane): the ectosylvia, the suprasylvia, the coronal and the lateral (Figs 1, 2). The cruciate and the splenial sulci, originating from the internal aspect of the neopallium at the interhemispheric fissure, are also exposed on the dorsal surface of the hemispheres in some taxa (see section 3.1.3). There are also typical secondary grooves: the diagonal sulcus ("sillon γ" of [START_REF] Anthony | Le neopallium des suidés: étude de son développement et interprétation de ses plissements[END_REF][START_REF] Anthony | Le Neopallium du Boeuf. Étude de son Développement et Interprétation de ses Plissements[END_REF], 1936;[START_REF] Friant | Morphologie, développement et évolution du Cerveau des Ongulés artiodactyles sélénodontes: exposé des recherches originales du Laboratoire d'Anatomie comparée[END_REF][START_REF] Anthony | Encéphale. Morphologie encéphalique et moulages endocraniens[END_REF][START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF], the arcuate sulcus ("sillon α" of Friant 1952observed in Camelidae), and the oblique sulcus ("sillon β" of [START_REF] Anthony | Le néopallium du mouton-Étude de son développement et interprétation de ses plissements[END_REF][START_REF] Friant | Morphologie, développement et évolution du Cerveau des Ongulés artiodactyles sélénodontes: exposé des recherches originales du Laboratoire d'Anatomie comparée[END_REF][START_REF] Sigogneau | Contribution à l'étude du télencéphale des « elaphoïdes » (Elaphoidea Frechkop[END_REF][START_REF] Anthony | Encéphale. Morphologie encéphalique et moulages endocraniens[END_REF][START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF]Dechaseaux , 1969[START_REF] Dechaseaux | Essais de paléoneurologie[END_REF]. In all modern artiodactyls, the pseudosylvia, observed in gyrencephalic mammal groups such as carnivorans [START_REF] Smith | On the homologies of the cerebral sulci[END_REF]) is very small or lacking (Repérant 1971b). If the homology of the various artiodactyl sulci has been discussed based on the primitive carnivoran pattern (as exemplified by the dog; e.g., [START_REF] Krueg | Über die Furchung der Grosshirnrinde der Ungulaten[END_REF][START_REF] Smith | On the homologies of the cerebral sulci[END_REF], it has never been discussed at the Artiodactyla scale based on a global comparison of extant and extinct taxa. A hallmark of the complexity of the brain of artiodactyls is the operculization, often incomplete, of the central area (the region delimited by the rhinals, the suprasylvia and the presylvia sulci) of the neopallium. It consists of an invagination of a more or less large surface of neopallium, localized between the rhinal and the ectosylvia and recognized as the gyrus arcuatus I (yellow area on Fig. 3) based on topographical similarities with the carnivoran neocortical pattern (see Repérant 1971b for considerations on homologies). The sylvian complex resulting from this operculization has various orientation depending of artiodactyl clades (Fig. 3).Modern representatives show different patterns, recognizable from each other, and with different degrees of operculization (Fig. 3). It is, for example, almost complete to complete in suoids [START_REF] Anthony | Le neopallium des suidés: étude de son développement et interprétation de ses plissements[END_REF], while the gyrus arcuatus I remains variously exposed in hippopotamids [START_REF] Friant | Morphologie, développement et évolution du Cerveau des Ongulés artiodactyles sélénodontes: exposé des recherches originales du Laboratoire d'Anatomie comparée[END_REF][START_REF] Friant | Le Télencéphale des Hippopotamidae[END_REF], camelids (Repérant 1971b), cetaceans [START_REF] Hof | Structure of the cerebral cortex of the humpback whale, Megaptera novaeangliae (Cetacea, Mysticeti, Balaenopteridae)[END_REF][START_REF] Knopf | The neocortex of Indian River dolphins (genus Platanista): comparative, qualitative and quantitative analysis[END_REF], and ruminants (Anthony andGrzybowski, 1934, 1936;[START_REF] Friant | Morphologie, développement et évolution du Cerveau des Ongulés artiodactyles sélénodontes: exposé des recherches originales du Laboratoire d'Anatomie comparée[END_REF]. Operculization is related to the expansion of the surface of the neopallium, the part of the brain dedicated to higher cognitive functions. As such, this phenomenon has been central to the study of artiodactyl endocasts and to the evolutionary history of their brains. The endocranial cast only gives access to the external structures of the brain, providing no clues as to the internal folds of the neopallium. This partial access to the morphology of the brain sometimes makes it difficult to identify first steps of operculization based on extinct taxa.Most works on the brain cavity of artiodactyls have focused mainly on the morphology of the neopallium, while studies on the cerebellum are scarce. Modern artiodactyls exhibit a complicated cerebellum with highly convoluted and folded vermis and hemispheres (e.g., Suidae, domestic pig Sus scrofa, [START_REF] Saikali | A three-dimensional digital segmented and deformable brain atlas of the domestic pig[END_REF]Hippopotamidae, common hippopotamus Hippopotamus amphibius, Garrod 1880;Camelidae, Repérant 1971b), but most of these delicate foldings are generally not visible on endocasts. Given the particular attention devoted to the study of the artiodactyl neopallium in the literature and the paucity of data available concerning the cerebellum, this chapter focuses essentially on the former.

Endocranial morphology of extinct Artiodactyla clades

Earliest artiodactyls / primitive pattern -The endocranial morphology of earliest artiodactyls is documented by the early and early middle Eocene Diacodexis (Sigogneau-Russel and Russel 1983;[START_REF] Orliac | Virtual endocranial cast of earliest Eocene Diacodexis (Artiodactyla, Mammalia) and morphological diversity of early artiodactyl brains[END_REF], Homacodon (Fig. 4A-C; Orliac, in press) and Helohyus (Fig. 4D-F; Orliac, in press). Olfactory bulbs. These are only documented from the specimen of Diacodexis ilicis [START_REF] Orliac | Virtual endocranial cast of earliest Eocene Diacodexis (Artiodactyla, Mammalia) and morphological diversity of early artiodactyl brains[END_REF] and from fragmentary specimens of Diacodexis pakistanensis (Sigogneau-Russel and Russel 1983). This genus exhibits very large olfactory bulbs (13% of the total endocast volume), so far the largest observed in Artiodactyla (Tab. 1). Cerebral hemispheres. In Diacodexis, Homacodon and Helohyus, the extension of the neopallium on the surface of the cerebral hemispheres is reduced (43-44% of the surface of the cerebral hemisphere) and the paleopallium is visible in dorsal view. The ventral extension is weak and the caudal part of the cerebrum does not abut the cerebellum, letting a wide portion of the midbrain dorsally exposed (Fig. 4A-F). Their groove pattern is very simple with two deep sulci, the suprasylvia and the lateral sulcus (closer to the interhemispheric fissure), converging in their anterior-most part and delimiting an almond-shape gyrus III. A small groove interpreted as the presylvia is also observed merging to the anterior segment of the rhinal fissure in the anterior most portion of the neopallium. The pyriform lobes are large. Diacodexis and Homacodon also show large olfactory tubercles. The lateral aspect of Helohyus endocast cannot be described due to deformations (Fig. 4F). It is worth to note that in earliest artiodactyls the orbitotemporal canal lies ventral to the rhinal fissure and is not a hallmark of the paleopallium/neopallium limit like it is the case in Primates (Microsyopidae, [START_REF] Silcox | Endocasts of Microsyops (Microsyopidae, Primates) and the evolution of the brain in primitive primates[END_REF]; although in some cases it is suggested dorsally, Proprimates, Gingerich and Gunnell 2005) and rodents (Ischyromyidae, [START_REF] Bertrand | First virtual endocasts of a fossil rodent: Ischyromys typus (Ischyromyidae, Oligocene) and brain evolution in rodents[END_REF]. Midbrain exposure. The midbrain is widely exposed in Diacodexis, Homacodon and Helohyus. Midbrain exposure is observed in few other early artiodactyls (Figs 4,5), but it is clearly wider in early and middle Eocene taxa (see also Indohyus). Cerebellum. Fine morphological details of the cerebellum are not visible on the endocasts, but all three taxa show a rather wide vermis compared to paramedian lobes. On Diacodexis [START_REF] Orliac | Virtual endocranial cast of earliest Eocene Diacodexis (Artiodactyla, Mammalia) and morphological diversity of early artiodactyl brains[END_REF] and Helohyus (Fig. 4D-F), the fissure prima can be identified on the dorsal aspect of the vermis, its position makes the paleocerebellum (lobus rostralis) widely exposed on the anterior part of the vermis. The lobus caudalis bears few fissures, and the fissure secunda is identifiable as a deep groove on the vermis endocast.

Endemic European artiodactyls (EEA) -Endocranial casts of EEA are documented from the late Eocene until the late Oligocene, most of them known from Quercy localities or from Saint-Gérand-le-Puy and described by [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF]Dechaseaux ( , 1969aDechaseaux ( ,b, 1973)). They show a wide array of morphologies reflecting the radiation of the group in the "Island Europe" context. Olfactory bulbs: Cebochoerus, Dichobune, Mouillacitherium as well as Anoplotherium present a rather similar relative size of the olfactory bulb with values comprised between 5.7 and 7.5 % (Tab. 1). These values are smaller than what is observed in Diacodexis (13.8%). Among EEA, the cainotherid Caenomeryx exhibits the smallest olfactory bulbs, representing less than 4% of the total volume of the endocast (Tab. 1). In all these taxa, olfactory bulbs are joined on most of their length and separated from the anterior margin of the neopallium by a short circular fissure. Cerebral hemispheres: The EEA show a great disparity of neocortical folding pattern, with a very simple pattern (two sulci delimiting the almond shapes gyrus) observed in Cebochoeridae (Dechaseaux 1969a:fig.13) and in Mouillacitherium (Figs 4 G-I; Orliac, in press), and more complicated ones found in other groups documented. The dichobunoid Dichobune shows a slightly more complex pattern, with a long presylvia widely visible on the dorsal aspect and three accessory sulci notching the gyrus III (entolaterals, Figs 4 J-L; Orliac, in press).

Cainotheriidae also present additional sulci, with a long coronal sulcus linked to both the lateral sulcus and the suprasylvia, and a diagonal sulcus, sometimes branched. The cainotheriids taxa analized Caenomeryx filholi (Fig. 4M-O; Orliac in press), and Cainotherium [START_REF] Anthony | Recherches sur le télencéphale des ruminants primitifs. Le Cainotherium[END_REF][START_REF] Friant | Morphologie, développement et évolution du Cerveau des Ongulés artiodactyles sélénodontes: exposé des recherches originales du Laboratoire d'Anatomie comparée[END_REF]Dechaseaux 1969a:fig.12) shows a small sulcus linking the rhinal fissure, here interpreted as a pseudosylvia, and most probably had partial operculization of the central area, highlighted by the pathway of the midle meningeal artery, partly hidden by the gyrus II [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF](Dechaseaux , 1969a; Fig. 4O). Anoplotherioidea exhibit another type of folding pattern. The dacrytherid Dacrytherium (Dechaseau 1969a:fig. 2) is very close to the cainotherioid plan, with elongated lateral sulcus, suprasylvia and coronal sulcus, and also a small oblique sulcus. Anoplotheriidae exhibit more elongated, parallel sulci, and a coronal sulcus preferentially linked to the lateral sulcus. Anoplotherium presents a complex folding pattern of the neopallium with additional ramifications (Fig. 4 PR); an additional sulcus located anterior to the coronal sulcus ("sulcus γ" of Dechaseaux 1969a) would correspond to a diagonal sulcus according to the nomenclature of Repérant (1971b). Anoplotherium shows a clear operculization of the central area, with a deep sylvian complex. The oblique sulcus is sinuous and ramified. The extension of the neopallium on the surface of the cerebral hemispheres of Dichobune and Mouillacitherium is similar to Diacodexis and Homacodon (ca. 43%, see Tab. 1), but the paleopallium is not visible in dorsal view for the former (Fig. 4G,J). The relative sizes of the neopallium of the Oligocene taxa Caenomeryx and Anoplotherium is larger than that of other EEA and exceeds 60% (see Tab. 1). Midbrain exposure: Midbrain exposure varies greatly depending on taxa. Although the exposure is smaller than in early Eocene taxa, the midbrain is exposed in Cebochoerus and colliculi are even visible (Dechaseaux 1969a:fig.13). Despite a relatively simple folding pattern and general weak extension of the neopallium, the midbrain exposure is small in Dichobune (Fig. 4G) and Mouillacitherium (Fig. 4J), Tapirulus (Dechaseaux 1969:fig.12) and Mixtotherium (Dechaseaux 1973:fig.3), smaller than in Diacodexis, Homacodon and Helohyus. The midbrain is not exposed in Cainotheriidae (Caenomeryx; Fig. 4M) and Anoplotheriidae (Anoplotherium; Fig. 4P).

Cerebellum: Just like in other groups, the cerebellum of EEA has received little attention in the literature. They differ by the folding pattern, size and shape of their vermis (Fig. 4). Compared to the size of the cerebrum, the cerebellum is relatively large in Anoplotherioidea where the anterior extension of the vermis reaches that of the cerebellar hemispheres (Dechaseaux 1969:figs.2,6,10; Fig. 4P). It is relatively smaller in other EEA taxa such as cainotheriids, Mouillacitherium or Dichobune. In these taxa, the cerebellar hemispheres extend anteriorly to the anterior margin of the vermis, like in early Eocene taxa (Fig. 4G,J,M).

Oreodontoidea -Endocranial casts of oreodontoids are best illustrated in the literature by the late Eocene oreodontine Bathygenys reevesi [START_REF] Macrini | Description of a digital cranial endocast of Bathygenys reevesi (Merycoidodontidae; Oreodontoidea) and implications for apomorphy-based diagnosis of isolated, natural endocasts[END_REF], and by natural endocasts of the early Oligocene merycoidodontine Merycoidodon culbertsoni (Leidy 1869:pl. 14, fig. 11;Black 1920:fig. 18: specimen IV;Moodie 1922:fig. 22) and middle Eocene Merycoidodon gracilis (Dechaseaux 1961:fig.10). We complete here the picture with 3D models of the leptauchenine merycoidodontid Leptauchenia sp. from Washington County, South Dakota (AMNH-FAM 45508; Fig. 4S-U) and of the agriochoerid Agriochoerus sp. from Sage Creeck, Chadron Mont.

(AMNH-FAM-95330, Late Eocene; Fig. 4V-X) (see Orliac and Gilissen 2012:fig. 3). Olfactory bulbs: Olfactory bulbs are proportionally smaller than in early artiodactyls described above (values comprised 3.2-5.5%, Tab. 1), but larger than in crown Artiodactyla. Unfortunately, most of the oreodontoid specimens are natural endocasts and the volume of the olfactory bulbs is not provided in their original description. Cerebral hemispheres. Oreodontoids described in the literature present different degrees of neopallium sulcal complexity, from few sulci arranged in a rectilinear disposition, without ramifications, to complex, ramified, sulci and an operculization of the central area. The simplest pattern illustrated is described in Bathygenys which only presents a suprasylvia prolonged by a coronal sulcus, and a lateral sulcus delimiting a gyrus III (Macrini 2009:fig.3). A more complex pattern is observed in Merycoidodon culbertsoni and Miniochoerus gracilis from the middle Oligocene. An ectolateral sulcus divides the gyrus III in Mi. gracilis (Dechaseaux 1961:fig. 10).

Me. culbertsoni (Black 1920:fig.18) presents several ramifications branching on the suprasylvia, including a small sulcus x, originating from the suprasylvia and joining the interhemispheric cleft, and an oblique sulcus located on the lateral margin, posterior to the sylvian complex. We agree with the conclusions of [START_REF] Friant | Morphologie, développement et évolution du Cerveau des Ongulés artiodactyles sélénodontes: exposé des recherches originales du Laboratoire d'Anatomie comparée[END_REF][START_REF] Friant | Interprétation du cerveau de l'Oreodon[END_REF] and [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF] that the gyrus I is operculized in those species. Yet, there is no indication of operculization of the central area in Bathygenys (see [START_REF] Macrini | Description of a digital cranial endocast of Bathygenys reevesi (Merycoidodontidae; Oreodontoidea) and implications for apomorphy-based diagnosis of isolated, natural endocasts[END_REF]. A noticeable variability of the neocortical pattern has been described in the literature in Merycoidon and Bathygenys [START_REF] Macrini | Description of a digital cranial endocast of Bathygenys reevesi (Merycoidodontidae; Oreodontoidea) and implications for apomorphy-based diagnosis of isolated, natural endocasts[END_REF]. The virtual endocranial cast of Leptauchenia reveals a rather simple cortical pattern with two main sulci, a long oblique suprasylvia/coronal and a small lateral sulcus delimiting a closed gyrus III, and two accessory sulci, an oblic and a diagonal (Fig. 4S-U). There is no indication of operculization. The endocranial morphology of Agriochoeridae is here documented by Agriochoerus (Fig. 4V-X). It shows a simple sulcal pattern with only a suprasylvia, a coronal sulcus and a lateral sulcus delimiting a closed gyrus III, without accessory sulcus. It is worth noting that in this taxon, the coronal sulcus is in line with the lateral sulcus and not with the suprasylvia. The extension of the neopallium on the surface of the cerebral hemispheres in Leptauchenia and Agriochoerus is more important than in Diacodexis and Homacodon and exceeds 50% (Tab. 1). The pyriform lobes and the olfactory tubercles are proportionally smaller in oreodontids than in Diacodexis (Orliac and Gilissen 2012:fig. 1c,fig. 2d) and Homocadon (Fig. 4A).

Midbrain exposure: The posterior development of the neopallium and the transverse sinus cover the midbrain in oreodontoid endocasts. Cerebellum: All oreodontoid taxa show a wide vermis compared to the cerebellar hemispheres. Black (1920:fig.9-10) illustrated an incredibly well preserved cerebellum of an oreodont from middle Oligocene of the White River group, South Dakota. The different lobules and crura can be identified in great detail. The fissura prima lies rather anteriorly, especially compared to Bathygenys, and the exposition of the lobus rostralis on the anterior part of the vermis is reduced compared to Diacodexis and Helohyus. Details of the vermis structures cannot be observed on the endocasts of Leptauchenia and Agriochoerus (see Fig. 4).

The endocranial casts of oreodontoids show a diversity of shape and folding pattern. Our sample is restricted to the Late Eocene/Early Oligocene and it would be very interesting to document endocranial casts of all Merycoidodontidae subfamilies and more recent taxa.

-Endocranial morphology of crown Artiodactyla clades

Tylopoda -Endocranial casts of the Camelidae are known from the late Eocene to the late Pleistocene. At least one endocast is known from each epoch in this timespan, allowing the observation of a tremendous brain size increase in this lineage [START_REF] Edinger | Brains from 40 million years of camelid history[END_REF][START_REF] Jerison | Quantitative analysis of the evolution of the camelid brain[END_REF][START_REF] Jerison | 20 How Can Fossils Tell us About the Evolution of the Neocortex?[END_REF]. Basic qualitative descriptions for this group were made by [START_REF] Edinger | Brains from 40 million years of camelid history[END_REF]. Olfactory bulbs: Descriptions of relative sizes of different brain regions, particularly of the anterior region and olfactory bulbs, are largely lacking for this group. These structures are missing from the known Protylopus endocast. However, if its brain proportions parallel those of Oligocene camelids, olfactory bulbs would have been about 8mm in length [START_REF] Edinger | Brains from 40 million years of camelid history[END_REF]. Cerebral hemispheres: Detailed descriptions of gyrification patterns across multiple fossil taxa and modern camelids were made by [START_REF] Repérant | Moulages endocrâniens de tylopodes fossiles[END_REF]Repérant ( , 1971a, b), b). The late Eocene Protylopus brain is more complex than that of earliest artiodactyls and bears three longitudinal sulci: a corono-suprasylvian sulcus prolonged medially by a short ansate sulcus, a lateral sulcus and an entolateral sulcus (Repérant 1971a). By the Oligocene, in Poebrotherium, the suprasylvian sulcus moved toward the midline and developed a descending branch at its anterior end. Lateral and entolateral sulci get close to the sagittal plane as new sulci, ectolateral and the sylvian complex, began to form and move dorsally from the lateral edges of the brain. At this time, the presylvian sulcus is faintly visible in the anterior brain, and the appearance of sulcus "oblique 1" begins to define the oblique posterior brain. Operculization is observed in Poebrotherium where a long ectosylvia and a short sylvian complex occur (Repérant 1971a:fig. 2). More sulci developed by the early Pliocene in Procamelus, when we observe the greatest expansion of the forebrain. The suprasylvian becomes more complex as its anterior portion couples with the ansate and coronal sulci, forming a branching pattern. All longitudinal sulci continue to move closer to the sagittal plane, as outer sulci develop further, and the rhinal fissure has moved further down the lateral edges of the brain (Repérant 1971a). Operculization is more extended. The main distinction in camelid brain evolution is the pattern of gradual introversion of posterior longitudinal sulci into the sagittal plane, and simultaneous eversion of sulci from the frontal region (Repérant 1971a). Cerebellum: Enlargement of the cerebral hemispheres was paralleled by enlargement of the cerebellum, whereas the vermis becomes less dominant [START_REF] Edinger | Brains from 40 million years of camelid history[END_REF]. Fine details of cerebellar surface anatomy are scarce in this clade.

Suoidea -As mentioned above there is, to our knowledge, no endocast of fossil suoid described in the literature. We document here the endocranial cast of Palaeochoerus sp. from an unknown locality in Quercy, France (ancient collections from the Museum d'Histoire Naturelle of Toulouse; MHNT PAL2014.0.3075.1; Fig. 5A-C). Given the biochronological repartition of this taxon in Quercy, it is most likely of late Oligocene age. Phylogenetic analyses [START_REF] Orliac | Phylogenetic relationships of the Suidae (Mammalia, Cetartiodactyla): new insights on the relationships within Suoidea[END_REF][START_REF] Orliac | Osteology of the petrosal bone of Suoidea (Artiodactyla, Mammalia)[END_REF]) support a basal position of Palaeochoeridae, before the split between Suidae and Tayassuidae. Olfactory bulbs: The palaeochoerid specimen consists of a partial braincase and unfortunately does not preserve the olfactory bulbs. The small portion preserved indicated that they were of decent size, separated from the cerebral hemispheres by a short circular fissure. Cerebral hemispheres: The neopallium sulcal complexity of Palaeochoerus sp. already strongly recalls that of modern suoids (Fig. 5A-C). The lateral sulcus is rather short and close to the interhemispheric fissure, the suprasylvia is prolonged anteriorly by an anterior branch and medially by an ansate sulcus that joins a long coronal sulcus. The later extends anteriorly and bifurcates in its anterior-most portion. On the lateral aspect of the neopallium, there is a clear sylvian complex that forms an open area, letting part of the gyrus I be exposed. There is a short deep oblique sulcus. Contrary to the domestic pig, Sus scrofa, there is no trace of a cruciate sulcus (we follow here the identification of Anthony and[START_REF] Anthony | Le neopallium des suidés: étude de son développement et interprétation de ses plissements[END_REF][START_REF] Barone | Neurologie I: Système nerveux central[END_REF][START_REF] Barone | Neurologie I: Système nerveux central[END_REF], that the anterior most sulcus in Sus scrofa is the anterior prolongation of the splenial; for alternative interpretation see [START_REF] Krueg | Über die Furchung der Grosshirnrinde der Ungulaten[END_REF]. The neopalleal pattern of Palaeochoerus is closer to that of Tayassuidae that do not seem to present a dorsal exposure of the anterior portion of the splenial sulcus (Saraiva 2017:figs.2,5). The neopallium is widely expanded on the cerebral hemispshere and covers 55% of it, a slightly inferior value compared to Tayassu (Tab. 1). There are wide, salient, olfactory tubercles on the Palaeochoerus endocast (Fig. 5B-C), just like in modern representatives of the group (Saraiva 2017:fig.4; Barone and Bortolami, 2004:pl.158) Midbrain exposure and cerebellum: In Palaeochoerus, the midbrain is not exposed dorsally and the posterior expansion of the neopallium covers the anteriormost part of the vermis and the paramedian lobes. It is also the case in modern Tayassuidae (Tayassu pecari, Saraiva 2017:fig. 7) and Suidae (Sus scrofa domesticus, [START_REF] Saikali | A three-dimensional digital segmented and deformable brain atlas of the domestic pig[END_REF]. The vermis of Palaeochoerus is wide relative to the paramedian lobes, and its subdivisions are impossible to identify; the shape of the paramedian lobes is masked by the sinuses surrounding the petrosal region. In modern suoids, the vermis appears smaller than the paramedian lobes (Barone and Bortolami 2004:pl.113;[START_REF] Saraiva | Macroscópia do encéfalo de catetos (Pecari tajacu[END_REF].

Knowledge of the endocranial morphology of Suoidea remains highly limited, even for modern taxa. The general morphology of the brain of Palaeochoerus indicates an advanced stage of neopalleal expansion (Fig. 5A,C), as soon as the lower Oligocene, even if it is less convoluted than that of modern suoids. Its pattern is closer to Tayassuidae than to Suidae.

Ruminantia -Compared to their great specific diversity, documentation of ruminant endocasts remains limited, especially for Paleogene times. We briefly describe and illustrate here the endocast of Leptomeryx sp. (AMNH 53596; Fig. 5G-I) from the Early Oligocene of South Dakota (Brule Formation, Orellan stage). In addition, we report the endocast of the Early Pleistocene cervid Antifer ensenadensis [START_REF] Fontoura | Virtual brain endocast of Antifer (Mammalia: Cervidae), an extinct large cervid from South America[END_REF]. Olfactory bulbs. The endocast of Leptomeryx, earliest Ruminantia from our sample (early Oligocene) presents large olfactory bulbs (Fig. 5G-I) compared to modern representatives of the group (Tab. 1). The olfactory bulb chamber, or ethmoidal fossa, is globular and the two bulbs are closely apposed on their whole length. The endocasts of Dremotherium (Sigogneau 1968:fig.16-18) and of Amphitragulus? (Sigogneau 1968:fig.28-29) from the late Oligoceneearly Miocene of Saint-Gérand-le-Puy show less globular, divergent olfactory bulb chambers. Unfortunately, 3D models of these specimens are not available yet, and the olfactory bulb chamber volume cannot be measured. Regarding olfactory bulb chamber morphology, Leptomeryx is closer to modern tragulids (e.g., Moschiola memmina Fig. 5J-L), than to extant or extinct bovids or cervids (for illustrations of the latter see Palombo et al. 2008:figs.7-9, 11). The endocast of the cervoid Palaeomerycidae Aletomeryx from the late Miocene of Nebraska [START_REF] Lull | New Tertiary artiodactyls[END_REF] does not preserve the olfactory bulbs portion. As for Dremotherium and Amphitragulus?, the endocast of the late Miocene giraffid Samotherium [START_REF] Black | A study of the endocranial casts of. Ocapia, Giraffa and Samotherium, with special reference to the convolution pattern in the family Giraffidae[END_REF]) also presents set apart, rather slender olfactory bulb chambers. The olfactory lobes of Antifer ensenadensis are strongly separated, similar to those of the current cervid species with which they are compared (Mazama, Blastocerus, Ozoterus; [START_REF] Fontoura | Virtual brain endocast of Antifer (Mammalia: Cervidae), an extinct large cervid from South America[END_REF]. Cerebral hemispheres: Modern Ruminantia show a wide panel of neopalleal pattern from rather simple patterns like in Tragulidae (e.g., Moschiola memmina Fig. 5J-L; Hyaemoschus aquaticus Friant 1939:fig. 21), to complex, highly convoluted neopallium (e.g., Giraffidae, Bovidae, Cervidae, [START_REF] Friant | Le Télencéphale des Hippopotamidae[END_REF][START_REF] Palombo | Brain versus body mass in endemic ruminant artiodactyls: a case studied of Myotragus balearicus and smallest Candiacervus species from Mediterranean Islands[END_REF][START_REF] Graïc | The brain of the giraffe (Giraffa camelopardalis): surface configuration, encephalization quotient, and analysis of the existing literature[END_REF]). The gyrification pattern has been demonstrated to be relevant at a systematic level, between families [START_REF] Palombo | Brain versus body mass in endemic ruminant artiodactyls: a case studied of Myotragus balearicus and smallest Candiacervus species from Mediterranean Islands[END_REF]) and within [START_REF] Oboussier | Evolution of the brain and phylogenetic development of African Bovidae[END_REF]. Regarding the main neopalleal sulci, the early Oligocene Leptomeryx shows the simplest pattern with a small, faint, lateral sulcus, and a long suprasylvia prolonged by an anteriorly extended coronal sulcus. A short cruciate and a short oblique sulci are also present, as well as a short presylvia (Fig. 5G-I). On the lateral aspect, the rhinal fissure lies fairly high and shows a marked inflexion between the pre-and postsylvia; however, no pseudosylvia, the hallmark of an incipient operculization, is visible. The simple pattern observed in Leptomeryx is close to that of modern tragulids (Moschiola memmina, Fig. 5J-L) that also show a very faint lateral sulcus and a long suprasylvia anteriorly prolonged by the coronal sulcus. However, tragulids show a long dorsal exposure of the splenial sulcus and a clear operculization of the central area with a marked sylvian complex. The neopalleal pattern of the late Oligocene/early Miocene Amphitragulus? (Sigogneau 1968:fig.28-29) is slightly more complex than that of Leptomeryx and shows a long anterior suprasylvia and a diagonal sulcus; pathway of the middle cerebral artery indicates that some part of the ventral margin of the neopallium is operculized. The pattern of Dremotherium (Sigogneau 1968:fig.16-18), of similar age, is even more complex, with more ramifications and elongation of the diagonal and oblique sulci. Operculization is also more important and the sylvian complex is visible as a deep depression on the surface of the endocast. A complex pattern is also observed in the late Miocene palaeomerycidae Aletomeryx (Lull 1920:fig.6) where the ansate also seems to be present. Highly complex, branched, neopalleal pattern are observed in late Miocene and plio-pleistocene Bovidae, Giraffidae and Cervidae. The neopallium of Leptomeryx covers ca. 56% of the cerebral hemisphere; this value is slightly inferior to that of the modern tragulid Moschiola (Tab. 1). The olfactory tubercles are not salient on ruminant endocasts. Midbrain exposure and cerebellum: The midbrain is not exposed in ruminant endocasts documented so far. In Leptomeryx the posterior extension of the neopallium abuts the cerebellum and most probably covers part of the paramedian lobes. The vermis is slender and posterior divisions are visible on the endocast (Fig. 5G-I). The vermis is also slender in Amphitragulus? [START_REF] Sigogneau | Le genre Dremotherium (Cervoidea): anatomie du crâne, denture et moulage endocranien[END_REF]:fig. 28) and Dremotherium (Sigogneau 1968:fig.16). In bovids and cervids, the paramedian lobes seem to be smaller in dorsal view, the vermis looking relatively wider in these taxa.

Hippopotamoidea -As mentioned above the number of endocasts of fossil hippopotamoids described in the literature remains very limited. Recent data about two phylogenetically distant taxa, the Microbunodontinae Microbunodon minimum and the Bothriodontinae Merycopotamus medioximus shed some light on important steps in their brain evolution [START_REF] Thiery | Endocasts and brain evolution in Anthracotheriidae (Artiodactyla, Hippopotamoidea)[END_REF]. Again, most of the discussions is focused on the evolution of the neopallium. Regarding modern hippopotamines, some morphological specificities of their endocast are related to modifications of their skull for semiaquatic habits. Indeed, focus has been made on the angle between the two optic nerves (Fig. 5Q), considered as an indicator of laterally projected, dorsally protruding eyes related to amphibiosis (Anthony 1948;[START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF]. Documentation of hippopotamoid endocasts remains very fragmentary and nothing is known prior to the late Oligocene. Olfactory bulbs. Hippopotaminae present among the smallest relative size of the olfactory bulb chamber of our artiodactyl sample (2 to 3 %; Tab. 1). Compared to other Artiodactyla, olfactory bulbs of modern hippopotamines are separated along their entire length (e.g., pygmy hippopotamus, Choeropsis liberiensis, Fig. 5P-R). Despite deformations of the fossil specimens, the olfactory bulbs of Microbunodon are obviously proportionally larger than that of modern and subfossil hippopotamines (Fig. 5M-O). Cerebral hemispheres: The neopallium of modern hippopotamines is extended on a very large portion of the cerebral hemisphere (Tab. 1). It shows a complex folding pattern [START_REF] Garrod | On the brain and other parts of the Hippopotamus (H. amphibius)[END_REF]Anthony 1948;[START_REF] Pilleri | Zur Anatomie des Gehirnes von Choeropsis liberiensis Morton (Mammalia, Artiodactyla)[END_REF][START_REF] Butti | The cerebral cortex of the pygmy hippopotamus, Hexaprotodon liberiensis (Cetartiodactyla, Hippopotamidae): MRI, cytoarchitecture, and neuronal morphology[END_REF], with a partial operculization of the gyrus I. Identification of the cortical foldings in Choeropsis liberiensis differs slightly depending on the authors (see [START_REF] Friant | Le Télencéphale des Hippopotamidae[END_REF][START_REF] Pilleri | Zur Anatomie des Gehirnes von Choeropsis liberiensis Morton (Mammalia, Artiodactyla)[END_REF][START_REF] Butti | The cerebral cortex of the pygmy hippopotamus, Hexaprotodon liberiensis (Cetartiodactyla, Hippopotamidae): MRI, cytoarchitecture, and neuronal morphology[END_REF]. [START_REF] Friant | Le Télencéphale des Hippopotamidae[END_REF] describes a rather linear organisation of the sulci, and characterizes modern hippos by the presence of a "K lobe" corresponding to a special folding induced by a bifid sylvian complex [START_REF] Friant | Le Télencéphale des Hippopotamidae[END_REF]:fig. 2). Instead, [START_REF] Butti | The cerebral cortex of the pygmy hippopotamus, Hexaprotodon liberiensis (Cetartiodactyla, Hippopotamidae): MRI, cytoarchitecture, and neuronal morphology[END_REF] highlight a pattern similar to cetaceans, with a concentric organization of the three main gyri of the lateral aspect (suprasylvian gyrus, ectosylvian gyrus, and perisylvian gyrus) around an almost vertical Sylvian fissure (Butti et al. 2014:675). Organization of the foldings relative of the Sylvian fissure in Choeropsis rather seems to be close to that of H. amphibius and we follow here the interpretation of [START_REF] Friant | Le Télencéphale des Hippopotamidae[END_REF] that main foldings have a mostly longitudinal organization. Our observations indicate that the recording of neopalleal structure on endocasts is quite blunt in C. liberiensis (Fig. 5P) and H. madagascariensis due to meninges thickness and sulci only leaving faint depressions depressions (see Anthony 1940:pl4,figs.4-5). The rhinal fissure is only distinct in its posterior part and the cleft visible anterior to the rhinal angle corresponds to the pseudosylvia (Fig. 5R). Neopalleal structures are almost completely obscured in H. amphibius. A complex neopalleal pattern is retrieved on the natural endocast of the "advanced bothriodontine" Merycopotamus medioximus with the potential presence of a "K lobe" [START_REF] Thiery | Endocasts and brain evolution in Anthracotheriidae (Artiodactyla, Hippopotamoidea)[END_REF]. The partial preservation of the specimen, however, does not allow for precise description. The microbunodontine Microbunodon minimum, phylogenetically more distant from modern hippopotamines than is Merycopotamus, shows a much simpler pattern with only two main elongated parallel sulci: a long lateral sulcus and a long corono-sylvia (Thiery and Ducrocq 2015:fig. 2; Fig. 5M-O). Anterior to the coronal sulcus, joining the interhemispheric fissure is a small cruciate sulcus, and, on the lateral aspect, the neopallium shows a shallow oblique sulcus. The specimen is damaged at the level of the rhinic angle making the presence of a Sylvian complex uncertain, the presence of a K lobe seems most unlikely. There is no apparent olfactory tubercles on hippopotamoid endocasts (Fig. 5N-O; Q-R), even though modern hippopotamines do present large tubercles on the ventral aspect of actual brains (Pilleri 1962:figs.2, 4). Midbrain exposure and cerebellum: The packing of the posterior part of the cranium in modern hippos drastically impacts the braincase; besides, the presence of wide sinuses dorsal to the cerebellum completely hides the dorsal aspect of the latter. In modern hippopotamuses, the anterior part of the cerebellum is covered dorsally by the posterior extension of the cerebrum (e.g., Garrod 1880:fig. 2; Butti et al. 2014:fig.1). The late Oligocene Microbunodon minimum shows no midbrain exposure (Fig. 5M). Its vermis is salient and protrudes dorsally above the level of the cerebrum (Fig. 5O). The cerebellum is not preserved in Merycopotamus and cannot be described.

Cetacea -Endocranial morphology of cetaceans is only partly documented back to late

Eocene. However, correspondence between the brain size and shape and the endocranial cast is not complete due to the presence of extensive adnexa. Olfactory bulbs: The brain of modern cetaceans is characterized by a great reduction of the olfactory apparatus (e.g., [START_REF] Edinger | Hearing and smell in cetacean history[END_REF][START_REF] Pihlström | Comparative anatomy and physiology of chemical senses in aquatic mammals[END_REF][START_REF] Godfrey | On the olfactory anatomy in an archaic whale (Protocetidae, Cetacea) and the minke whale Balaenoptera acutorostrata (Balaenopteridae, Cetacea)[END_REF][START_REF] Berta | Review of the cetacean nose: form, function, and evolution[END_REF][START_REF] Kishida | Aquatic adaptation and the evolution of smell and taste in whales[END_REF], and modifications of the olfactory tract occur in the earliest phase of cetaceans' history (e.g., [START_REF] Orliac | The endocranial cast of Indohyus (Artiodactyla, Raoellidae): the origin of the cetacean brain[END_REF]). An elongation of the olfactory tract has been described in archaeocetes (Pakicetidae, [START_REF] Kishida | Aquatic adaptation and the evolution of smell and taste in whales[END_REF]Remingtonocetidae, Bajpai et al. 2011;Basilosauridae, Edinger 1955;[START_REF] Uhen | Form, function, and anatomy of Dorudon atrox (Mammalia, Cetacea): an archaeocete from the middle to late Eocene of Egypt[END_REF][START_REF] Godfrey | On the olfactory anatomy in an archaic whale (Protocetidae, Cetacea) and the minke whale Balaenoptera acutorostrata (Balaenopteridae, Cetacea)[END_REF] and in raoellids [START_REF] Orliac | The endocranial cast of Indohyus (Artiodactyla, Raoellidae): the origin of the cetacean brain[END_REF]. Regarding Neoceti, early mysticetes present massive, elongated, olfactory peduncles (Llanocetus, Mitchell 1989); these peduncles and associated ethmoidal chambers are still present, though relatively small in modern representatives of the group (e.g., Balaena mysticetus, Duffiel et al. 1992;Balaenoptera acutorostrata, Godfrey et al. 20013;Megaptera novaeangliae, Hof and Van der Gucht 2007). In early foetal stages of odontocetes, the olfactory bulbs, nerve, and tracts are present, but these structures then degenerate and are completely absent from mature odontocete brains [START_REF] Ridgway | The cetacean central nervous system[END_REF]e.g., Platanista, Kamiya and Pirlot 1980). The olfactory apparatus is present in archaic odontocetes and has been described in Oligocene taxa (Simocetus, Fordyce 2002;USNM 299482,fig. 5E;, platanistoid odontocete, Hoch 2000, figs.6-7), in the early Miocene prosqualodontid Prosqualodon davidi [START_REF] Flynn | Description of Prosqualodon davidi Flynn, a fossil cetacean from Tasmania[END_REF], and in the Miocene Squalodon sp. [START_REF] Dart | The brain of the Zeuglodontidae (Cetacea)[END_REF].

Cerebral hemispheres: Increased neocortical size and complexity is one of the hallmarks of the modern cetacean brain [START_REF] Marino | Endocranial volume of mid-late Eocene archaeocetes (Order: Cetacea) revealed by computed tomography: implications for cetacean brain evolution[END_REF][START_REF] Marino | Origin and evolution of large brains in toothed whales[END_REF][START_REF] Marino | Cetaceans have complex brains for complex cognition[END_REF], however, the presence of endocranial vascular networks and other adnexa makes it impossible to access the neopalleal pattern of most fossil cetaceans, as evidenced by basilosaurids in which the pattern could not be described [START_REF] Pilleri | Betrachtungen uber das Gehirn der Archaeoceti (Mammalia, Cetacea) aus dem Fayum Agyptens[END_REF]. In protocetids and remingtonocetids, the extension of the retia is limited; three faint oblique sulci have been mentioned on a natural endocranial cast of the protocetid Indocetus [START_REF] Bajpai | Indocetus (Cetacea, Mammalia) endocasts from Kachchh (India)[END_REF]) and a faint one, close to the cerebral midline, is visible on the 3D reconstruction of the endocast of Remingtonocetus harudiensis (Bajpai et al. 2011:fig.7). The raoellid Indohyus, sister taxon to the Cetacea clade, shows a very simple neocortical pattern, limited to two major sulci (suprasylvia and lateral sulcus) plus a small coronal sulcus [START_REF] Orliac | The endocranial cast of Indohyus (Artiodactyla, Raoellidae): the origin of the cetacean brain[END_REF]. This pattern is similar to the primitive neocortical pattern retrieved in Eocene terrestrial artiodactyls (see [START_REF] Orliac | Virtual endocranial cast of earliest Eocene Diacodexis (Artiodactyla, Mammalia) and morphological diversity of early artiodactyl brains[END_REF]. Endocasts of extant and extinct Neoceti mainly give access to the overall shape of the cerebral hemispheres and to the cranial vasculature. Some major neocortical sulci might be observed too, such as the Sylvian fissure, and sulci in the anterior portion [START_REF] Pilleri | Betrachtungen uber das Gehirn der Archaeoceti (Mammalia, Cetacea) aus dem Fayum Agyptens[END_REF][START_REF] Racicot | Morphology and variation in porpoise (Cetacea: Phocoenidae) cranial endocasts[END_REF][START_REF] Bisconti | A natural endocast of an early Miocene odontocete and its implications in cetacean brain evolution[END_REF], however, the groove pattern cannot be described nor compared with accuracy. The brain-stem flexure is highly pronounced in the modern representatives of Cetacea [START_REF] Kruger | Specialized Features of the Cetacean Brain[END_REF], and they show a concentric organization of the three main gyri of the lateral side of the cortex (suprasylvian gyrus, ectosylvian gyrus, and perisylvian gyrus) around an almost vertical Sylvian fissure. As far as we know, this organization, potentialy linked to the higly derived conformation of the cranium of crown cetaceans, is not documented in the earliest archaeocetes (e.g., [START_REF] Dart | The brain of the Zeuglodontidae (Cetacea)[END_REF][START_REF] Bajpai | Indocetus (Cetacea, Mammalia) endocasts from Kachchh (India)[END_REF][START_REF] Bajpai | Cranial anatomy of middle Eocene Remingtonocetus (Cetacea, Mammalia) from Kutch, India[END_REF]. Midbrain exposure and cerebellum: In modern cetaceans, the caudal extension of the neopallium covers a large part of the cerebellum, both in mysticetes (e.g., Balaena, Raghanti et al. 2019:fig.1B) and in odontocetes (e.g., Stenella, Kamiya and Pirlot 1974:pl.1;Platanista, Kamiya and Pirlot 1980:fig. 10D). Regarding extinct taxa, the midbrain area is impossible to visualize on endocasts of Neoceti and basilosaurids because of the presence of endocranial vascular networks and other adnexa. In archaeocetes, Indocetus [START_REF] Bajpai | Indocetus (Cetacea, Mammalia) endocasts from Kachchh (India)[END_REF]:fig. 1C) and Remingtonocetus (Bajpai et al. 2011:fig.7) have a posteriorly expended neopallium that most probably covers the midbrain and abuts the cerebellum. Compared to archaeocetes, Indohyus indirae has a much limited extension of the neopallium and a wide midbrain exposure (Orliac and Thewissen 2021:fig.2), greater than in middle and late Eocene endemic European artiodactyls (e.g., Dichobune, Cebochoerus, [START_REF] Dechaseaux | Encéphales d'artiodactyles fossiles[END_REF]Dechaseaux , 1969a;;[START_REF] Orliac | Virtual endocranial cast of earliest Eocene Diacodexis (Artiodactyla, Mammalia) and morphological diversity of early artiodactyl brains[END_REF]; Fig. 4J-K), and Eocene North American taxa (e.g., Agriochoerus, Bathygenys, [START_REF] Whitmore | Cranial morphology of some Oligocene Artiodactyla[END_REF][START_REF] Macrini | Description of a digital cranial endocast of Bathygenys reevesi (Merycoidodontidae; Oreodontoidea) and implications for apomorphy-based diagnosis of isolated, natural endocasts[END_REF][START_REF] Orliac | Virtual endocranial cast of earliest Eocene Diacodexis (Artiodactyla, Mammalia) and morphological diversity of early artiodactyl brains[END_REF] and Leptauchenia Fig. 4S-U). The brain of modern Cetacea shows a small and narrow vermis compared to the two voluminous, highly convoluted, cerebellar hemispheres (e.g., [START_REF] Ries | A study of the surface structure of the brain of the whale (Balaenoptera physalus and Physeter catodon)[END_REF]Pilleri 1966a,b;[START_REF] Hanson | Quantitative examination of the bottlenose dolphin cerebellum[END_REF]Bisconti et al. 2021). Regarding archaeocete endocasts, most of the surface of the cerebellum of basilosaurids is covered with a large rete mirabile (e.g., [START_REF] Breathnach | Observations on endocranial casts of recent and fossil cetaceans[END_REF][START_REF] Pilleri | Betrachtungen uber das Gehirn der Archaeoceti (Mammalia, Cetacea) aus dem Fayum Agyptens[END_REF][START_REF] Geisler | Relationships of Cetacea to terrestrial ungulates and the evolution of cranial vasculature in Cete[END_REF][START_REF] Uhen | Form, function, and anatomy of Dorudon atrox (Mammalia, Cetacea): an archaeocete from the middle to late Eocene of Egypt[END_REF], making it impossible to accurately observe the relative size of the vermis. The endocasts of Indocetus [START_REF] Bajpai | Indocetus (Cetacea, Mammalia) endocasts from Kachchh (India)[END_REF] and Remingtonocetus [START_REF] Bajpai | Cranial anatomy of middle Eocene Remingtonocetus (Cetacea, Mammalia) from Kutch, India[END_REF] show no major shift in the proportions of the different elements of the cerebellum compared to other artiodactyl groups, with a wide vermis relative to the plausible representation of the cerebellar hemispheres. Indohyus has a relatively large vermis compared to its cerebellar hemispheres (Orliac and Thewissen 2021:fig.2), as in earliest artiodactyls, Diacodexis [START_REF] Orliac | Virtual endocranial cast of earliest Eocene Diacodexis (Artiodactyla, Mammalia) and morphological diversity of early artiodactyl brains[END_REF] or Dichobune (Fig. 4J).

Space associated with cranial blood vessels

Mentions of cranial blood vessels in fossil artiodactyls are mostly based on descriptions of the external aspect of the basicranium or on petrosal morphology (e.g., [START_REF] Coombs | Anatomy of the ear region of four Eocene artiodactyls, Gobiohyus, ?Helohyus, Diacodexis and Homacodon[END_REF][START_REF] O'leary | An anatomical and phylogenetic study of the osteology of the petrosal of extant and extinct artiodactylans (Mammalia) and relatives[END_REF]. Works describing endocasts of terrestrial artiodactyls based on intracranial investigations and µCT-scan data (Merycoidontidae, Macrini 2009; early artiodactyls, [START_REF] Orliac | Virtual endocranial cast of earliest Eocene Diacodexis (Artiodactyla, Mammalia) and morphological diversity of early artiodactyl brains[END_REF]Hippopotamoidea, Thiery and Ducrocq 2015) do not include detailed descriptions of cranial blood supply beyond mention of the orbito-temporal canal. Studies on endocasts of crown cetacean and their relatives do focus more on circulatory casts and sinuses (e.g., [START_REF] Racicot | Endocranial anatomy of a new fossil porpoise (Odontoceti, Phocoenidae) from the Pliocene San Diego Formation of California[END_REF]Bajpail et al. 2011;[START_REF] Orliac | The endocranial cast of Indohyus (Artiodactyla, Raoellidae): the origin of the cetacean brain[END_REF], mainly in relation with the presence and extension of the retia mirabilia ("wonderful net", [START_REF] Slijper | Die Cetaceen. Vergleichend-anatomisch und systematisch[END_REF][START_REF] Ridgway | Comparison of dolphins' body and brain measurements with four other groups of cetaceans reveals great diversity[END_REF].

Modern artiodactyls indeed present a selective brain cooling system [START_REF] Baker | Carotid rete and brain temperature of cat[END_REF] enabling them to lower their brain temperature below their body temperature. This system is mediated by the carotid rete, a subdural arterial meshwork that anatomically and functionally replaces the internal carotid artery (O'Brien 2018). It lies on the basisphenoid roof, slightly posterior to the hypophyseal fossa (O'Brien 2015:fig. 1; O'Brien and Bourke 2015). The arterial meshwork of the carotid rete is housed within the venous cavernous sinus, a large pool of venous blood that drains from the sphenoparietal and frontal regions of the cerebrum and from the nasal area of the face (O'Brien 2017) and that receives blood that has been evaporatively cooled by the nasal turbinates. This contact between cooled venous blood and the high surface area of the arterial rete enables rapid heat exchange and cooling of the arterial blood destined for the brain. Nearly all modern artiodactyls possess a carotid rete and perform selective brain cooling. However, major arteries supplying the rete are derived from different embryonic aortic arches in the different artiodactyl families (for review see O'Brien 2018). The results of O'Brien (2020) based on ancestral character reconstructions support that this pattern of variation results from independent evolutionary processes and suggest that different modern artiodactyls groups developed a carotid rete convergently. Unfortunately, presence of a carotid rete cannot be determined using endocasts for most artiodactyls, and the presence of a carotid rete is mostly correlated with the absence of major correlates for the internal carotid artery (O'Brien and Bourke 2015). Bony correlates therefore mostly imply the petrosal morphology or the region close to the petrosal and are best seen on bony material than on endocasts.

The intracranial retia of modern cetaceans greatly differ from that of other artiodactyl groups by their size and position: i) the rostral arterial rete is more extensive (e.g., McFarland 1979;[START_REF] Vogl | Arterial circulation of the spinal cord and brain in the Monodontidae (Order Cetacea)[END_REF], ii) there is a caudal endocranial arterial and venous rete mirabile (in mysticetes only ;[START_REF] Breathnach | Observations on endocranial casts of recent and fossil cetaceans[END_REF][START_REF] Pilleri | Betrachtungen uber das Gehirn der Archaeoceti (Mammalia, Cetacea) aus dem Fayum Agyptens[END_REF][START_REF] Melnikov | The arterial system of the sperm whale (Physeter macrocephalus)[END_REF], and iii) there are various retia in the basicranium and thus a high vascularization of the pneumatic sinuses around the petrosal and the ectotympanic bulla [START_REF] Fraser | Anatomy and function of the cetacean ear[END_REF]. The presence of these retia widely masks the external morphology of the brain, and therefore greatly impacts the shape of the endocasts of modern cetaceans and in basilosaurids (i.e., Pelagiceti; see section 3.1.3). The presence and extension of retia is variable in non-basilosaurid archaeocetes: the protocetid Indocetus sp. presents a venous rete, dorsal to the cerebellum, and a caudal rete, medial and dorsomedial to the petrosal (Bajpai et al. 1996:fig.lA), whereas in the remingtonocetids Dalanistes ahmedi [START_REF] Gingerich | New archaeocetes (Mammalia, Cetacea) from the middle Eocene Domanda Formation of the Sulaiman Range, Pun Jab (Pakistan)[END_REF] and Remingtonocetus harudiensis (Bajpail et al. 2011), a rete also probably fills the region dorsomedial to the petrosals, but there is little evidence of the presence of caudal or rostral rete. The raoellid Indohyus indirae has no extensive rostral or lateral retia mirabilia, but an intraosseous space dorsal to the cerebellum might have housed a network of diploic veins and arteries (Orliac and Thewissen 2021:fig. 34) that might represent the first steps of an incipient caudal venous rete mirabile.

Brain evolution and paleobiological inferences based on endocast morphology

Artiodactyls follow the same broad lines of evolution of the brain as other gyrencephalic mammals, with general increase of the relative size of the brain, and of the size and complexity of the neopallium, from the early Eocene to modern times. Yet, despite a decent number of endocasts description in the literature, the picture of brain evolution at the Artiodactyla scale remains limited and major gaps remain to be filled regarding the earliest history of modern groups such as Suoidea, Hippotamoidea and Cetacea. Besides, quantitative data remain very scanty and the trends described in the following paragraphs are built on a very limited sample and only provide a very first step to understand brain evolution at the Artiodactyla scale.

Morphological brain diversity: General picture of brain evolution in Artiodactyla

Olfactory bulbs

Still very few data are available regarding the relative size of the olfactory bulbs in Artiodactyla, or in mammal in general. The data collected for terrestrial artiodactyls (Tab. 1) based on 3D models suggest a general trend toward a reduction of relative olfactory bulb size through time at the Artiodactyla scale (Fig. 6A). Morphologically speaking, the earliest artiodactyls show bulbous olfactory chambers, joined on most of their length (Figs. 4,7). The shape of the olfactory chamber is quadrangular in modern taxa and they are separated on most of their length in most extant representatives of modern groups (Figs. 5,7). In the latter, the olfactory chambers appear anteroposteriorly compressed and a large portion of their surface corresponds to the cribriform plate and bears imprints of the foramina for olfactory nerves.

Cetacea show progressive lengthening of the olfactory tract and concurrent reduction of the relative size of the olfactory bulb chamber. Indeed, the nose of cetaceans underwent great modifications as a result of their adaptation to obligate aquatic lifestyle. The raoellid Indohyus shows an elongated olfactory tract, with narrow olfactory bulbs and peduncles [START_REF] Orliac | The endocranial cast of Indohyus (Artiodactyla, Raoellidae): the origin of the cetacean brain[END_REF]). An elongation of the olfactory tract has also been described in archaeocetes, Pakicetidae [START_REF] Kishida | Aquatic adaptation and the evolution of smell and taste in whales[END_REF], Remingtonocetidae [START_REF] Bajpai | Cranial anatomy of middle Eocene Remingtonocetus (Cetacea, Mammalia) from Kutch, India[END_REF], and Basilosauridae [START_REF] Edinger | Hearing and smell in cetacean history[END_REF]. This narrowing and lengthening of the olfactory tract might be directly related to modifications of the intertemporal region in early cetaceans and relatives and has been proposed as a synapomorphy of the clade (Orliac and Thewissen 2021:fig. 5). These authors also suggests that the modification of the postorbital morphology and the concurrent elongation of the olfactory tract would primarily originate from modifications of the masticatory apparatus, related to specialized diet. Neoceti exhibit a reduction of the major olfactory structures (ethmoturbinates, cribriform plate and maxilloturbinates) including olfactory bulbs, with further reductions and subsequent losses within Odontoceti [START_REF] Berta | Review of the cetacean nose: form, function, and evolution[END_REF].

Neopallium size and complexity

The representation of a global picture of artiodactyl brain evolution in a rough phylogenetic and temporal context shows that the size of the neopallium relative to other components of the brain shows an increase with time in Artiodactyla in general and within all artiodactyl clades (Tab. 1; Fig. 7). The expansion of the neopallium surface can notably be appreciated through i) its posterior extend, ii) its lateral extent and the location of the rhinal fissure on the lateral aspect of the endocast, iii) the operculization of the central area and the invagination of the external part with increasing neopalleal surface. Earliest artiodactyls from the early Eocene show the smallest neopallium surface relative to the surface of the cerebral hemisphere (Tab. 1, Fig. 6B), and a concurrent wide midbrain exposure (Fig. 7). Indohyus indirae from the middle Eocene also shows a widely exposed midbrain and concurrent small neocortex expansion [START_REF] Orliac | The endocranial cast of Indohyus (Artiodactyla, Raoellidae): the origin of the cetacean brain[END_REF], and some middle Eocene European endemic taxa also show small midbrain exposure (Figs. 6B,7). All artiodactyls from the Oligocene and on, have extended neopallium and no midbrain exposure. The area of the neopallium relative to the total surface of the cerebral hemisphere shows a trend to increase with time (Tab. 1, Fig. 6B), with the lowest values retrieved in Early Eocene and middle/Late Eocene taxa, and highest values observed for modern taxa. Yet, relatively high values are also found in endemic European Paleogene taxa (Fig. 6B) such as the large bodied Anoplotherium (Anoplotheriidae; body mass estimate = 150 kg, Hooker 2007) and the small Caenomeryx (Cainotheriidae, body mass estimate = 1.5 kg) highlighting that the picture will get more complicated as the data available increase and that independent tempos are to be expected in the various artiodactyl clades. Besides, as soon as operculization occurs, the external surface of the neopallium is necessarily an underestimation of the total neopalleal surface, which tempers the relevance of direct comparisons of values.

Earliest artiodactyls exhibit a very simple sulcal pattern of the neopallium with only two sulci, the suprasylvia and the lateral sulcus, converging in their anterior-most part and delimiting an almond-shape gyrus III + presylvia. This simplest pattern, found in early Eocene diacodexeids, homacodontids, and helohyids, is also observed in some middle/late Eocene endemic European taxa ("Dichobunoidea" with Mouillacitherium Fig. 4G, Amphimerycidae with Pseudamphimeryx, Amphimeryx, Dechaseaux 1969a:fig. 17). A slightly more complicated pattern with the addition of a coronal sulcus occurs in middle/late Eocene taxa such as early oreodontoids, some endemic European taxa (Tapirulus, [START_REF] Orliac | Virtual endocranial cast of earliest Eocene Diacodexis (Artiodactyla, Mammalia) and morphological diversity of early artiodactyl brains[END_REF]Cebochoerus, Dechaseaux 1969:fig.13), and the raoellid Indohyus. Small additional sulci such as ectolaterals (Dichobune Fig. 4J), diagonal sulcus and/or oblique sulcus (Leptauchenia Fig. 4J; Dacrytherium Dechaseaux 1969a:fig. 2) also occur in middle/late Eocene taxa. Anoplotheriidae, which include the largest of endemic European taxa, Anoplotherium, present, as soon as the late Eocene, a rather complex and extended neopallium with: i) a longitudinal organization of the coronolateral sulcus and suprasylvia and additional sulci, ii) the presence of a sylvian complex, hallmark of an operculization. A growing cortical complexity and diversity is observed in the different artiodactyl clades in the Oligocene, with branched patterns resulting from ramification of the main sulci and from the extension of the anterior part of the suprasylvia (suoids, oreodontids, ruminants, camelids), of the ansate sulcus (suoids, oreodontids, camelids), and of the oblique sulcus (cainotheriids, suoids, oreodontids, ruminants, camelids). Dorsal exposure of the cruciate and splenial sulci is variously present in ruminants, suids and hippopotamoids. Camelids, on the contrary, experience gradual introversion of posterior longitudinal sulci into the sagittal plane, and simultaneous eversion of sulci from the frontal region (Repérant 1971a). The presence of a small neopallium and very simple neopalleal pattern in raoellids, sister taxon to Cetacea and therefore lying very high in the Artiodactyla tree, implies that a simple pattern was most probably present at the base of all modern groups and that neopallium complexity arose independently in the different crown clades of artiodactyls. This is consistent with the differences in neopalleal patterns observed today. The overview of neopalleal pattern in Artiodactyla through time highlights their diversity and the relevance of a neocortical pattern blueprint for phylogenetic and taxonomic purpose. The same is true for neopalleal extension and concurrent operculization of part of the central territory. Based on the available sample, operculization seems to be present in all artiodactyl taxa from the Oligocene onward. But the extension of the neopallium in the various artiodactyl clades did not imply the same areas, and operculization occurred independently in all crown clades and in major extinct clades. This is highlighted by the variety of operculization patterns observed, with the Sylvian complex following the rhinal margin plan only (Cainotheriidae), forming an open triangle on the rhinal (Anoplotheriidae), not connecting the rhinal (Hippopotaminae), or with the anterior ectosylvia pointing anteriorly (e.g., ruminants), or posteriorly (e.g., suines).

Brain-size evolution and Encephalization Quotient

Encephalization corresponds to the increase in brain size beyond that expected from the allometric brain-body relation (see [START_REF] Jerison | Brain evolution: new light on old principles[END_REF][START_REF] Jerison | Evolution of brain and Intelligence[END_REF][START_REF] Jerison | The evolution of biological intelligence[END_REF] for historical reviews). It is often determined by calculating the residual value for a given species relative to the allometric regression line [START_REF] Jerison | Animal intelligence and encephalization[END_REF]. Specifically, the encephalization quotient (EQ, Jerison 1970) has been defined as the ratio of the observed brain mass over the expected brain mass for a given body mass. The EQ is therefore rather straightforward to interpret; if a species/specimen has a greater brain mass than expected, the ratio is above 1, and if not, the ratio is under 1. Before going further, a point must be raised. In a large brain a high quotient of encephalization might correspond to the addition of many grams of brain tissue (and a correspondingly large number of brain cells), whereas in a small brain the same quotient of encephalization would correspond to the addition of a comparatively small amount of brain tissue. When comparing species, brain size is often considered as proportional to neural information processing capacity, and the evolution of encephalization as the evolution of an increase in information processing capacity. It would therefore appear that large brains require more tissue than small brains to achieve the same increase in information processing capability. This paradox, raised by H.B. Barlow in [START_REF] Jerison | Animal intelligence and encephalization[END_REF] has not yet been answered satisfactorily and must be kept in mind when interpreting trends in brain size evolution.

The main challenge in calculating the EQ is to estimate the expected brain mass, which is commonly calculated by an allometric formula with defined parameters. The general allometry formula Y = aX b can be used to calculate an expected brain mass (Y) for a given body mass (X). As a power function, this relationship can be illustrated by plotting the logarithm of the measurement against the logarithm of the size, thus transforming the previous equation into a linear relationship: log(Y) = b log(X) + log(a). Authors have used various values for the parameters a and b, and at various scales: some at the class level (e.g., Mammalia, Jerison 1970;[START_REF] Eisenberg | Relative Brain Size and Feeding Strategies in the Chiroptera[END_REF], others at the order scale (e.g., rodents, [START_REF] Pilleri | Cephalization in rodents with particular reference to the Canadian beaver (Castor canadensis)[END_REF]primates, Martin 1990). The parameters used are assumed to represent the group of interest (i.e., that they do not vary much within this group); the relevance of parameters a and b therefore depends of the group of interest, and may not be relevant/suitable to another group or scale. More precise and complex methods often deal with phylogenetic comparative methods (PCM; Cornwell and Nakagawa 2017) but these require a well-supported phylogenetic context. The phylogenetic relationships within Artiodactyla are still debated and if the relationships between crown groups are rather consensual on molecular grounds [START_REF] Hassanin | Pattern and timing of diversification of Cetartiodactyla (Mammalia, Laurasiatheria), as revealed by a comprehensive analysis of mitochondrial genomes[END_REF], the position of extinct taxa are still highly disputed (e.g., [START_REF] Geisler | Hippopotamus and whale phylogeny[END_REF]. This lack of a clear/robust phylogenetic framework deprives us of the use of phylogenetic comparative methods on residuals at the artiodactylan scale. We therefore use here the EQ to provide (limited) state-of-the-art of the data. 1C). Color code: light orange, cerebellum; turquoise, midbrain exposure; violet, olfactory bulbs and peduncles; neopalleal sulci: blue, lateral; orange, coronal; yellow, suprasylvia (+ anterior suprasylvia and ansate for some taxa); green, cruciate; purple, splenial; pink, oblique and diagonal. Turning arrows indicate operculization. Abbreviations: Ce, Cetacea; EA, endemic artiodactyls; Hi, Hippopotamoidea; Or, Oreodontoidea; Ru, Ruminantia; Su, Suoidea; Ty, Tylopoda. Phylogenetic relationships are based on [START_REF] Hassanin | Pattern and timing of diversification of Cetartiodactyla (Mammalia, Laurasiatheria), as revealed by a comprehensive analysis of mitochondrial genomes[END_REF] for crown groups branching, fossil taxa relationships rely on [START_REF] Thewissen | Whales originated from aquatic artiodactyls in the Eocene epoch of India[END_REF] for Cetacea, [START_REF] Geisler | Phylogenetic relationships of Cetaceans to Terrestrial Artiodactyls[END_REF] for Tylopoda, [START_REF] Métais | Basal Ruminants[END_REF] for Ruminantia, [START_REF] Orliac | Osteology of the petrosal bone of Suoidea (Artiodactyla, Mammalia)[END_REF] for Suoidea, Oreodontoidea and endemic European taxa are here considered as monophyletic groups, relationships within endemic European taxa rely on Weppe et al. (2020a). Not to scale.

Brain size in Artiodactyla

To accurately describe the evolution of brain size through time within Artiodactyla, we define and use an empirical EQ formula based on a sample of 113 extant artiodactyl species. Fig. 8A shows a plot of log (brain mass) against log (body mass) with the regression line based on extant species only. The equation for the regression is 0.6008x -1.1278. The expected brain mass equation is thus 0.3237(body mass) 0.6008 and the corresponding EQ formula for Artiodactyla (EQartio) is as follows: observed brain mass / 0.3237(body mass) 0.6008 . The previous scaling exponents of [START_REF] Jerison | Brain evolution: new light on old principles[END_REF] and of [START_REF] Eisenberg | Relative Brain Size and Feeding Strategies in the Chiroptera[END_REF] for Mammalia are, respectively, 0.667 and 0.74, which are slightly higher than the scaling exponent of 0.6008 proposed here for artiodactyls. With a lower slope value, a negative allometry between brain and body mass in artiodactyls is thus clear in our study compared to previous works at the Mammalia scale. A striking fact about artiodactylans relative brain size (Fig. 8A) is the duality between Cetacea and non-cetacean artiodactyls. Indeed, convex hulls for both groups of extant species do not overlap, and both extant and fossil cetaceans are generally above the regression line of Artiodactyla, while non-cetaceans are below this line. We therefore calculated two separate EQ formulas, one for non-cetacean artiodactyls with EQnoncet = observed brain mass / 0.3405(body mass) 0.5603 , and one for cetacean artiodactyls with EQcet = observed brain mass / 16.0007(body mass) 0.3490 . Linear models of log(brain mass) relative to log(body mass) for extant species of non-Cetacea and Cetacea reveal a correlation between log(brain mass) and log(body mass) (p<2.2x10 -16 ) and a difference between both groups (p<2.2x10 -16 in both tests). Additionally, there is a difference between the slopes of each group over time (p=6.053x10 -7 in a t-test of slope comparisons), highlighting a difference in allometry equations between each group (Fig. 8A). In a comparable analysis (but using phylogenetic generalized least squares -GLS -regressions), [START_REF] Smaers | The evolution of mammalian brain size[END_REF] also found a lower encephalization slope for cetaceans and a higher encephalization intercept for cetaceans (both odontocetes and delphinids compared to ferungulates). Brain/body mass allometry is more negative in Cetacea (scaling exponent of 0.3490) than in other artiodactyls (scaling exponent of 0.5603, closer to that of the whole order, 0.6008). This does not imply a lower encephalization (their EQ is greater than other artiodactyls, as is their EQ variation; Fig. 8B), but may rather results from a massive increase in body mass for this clade (see [START_REF] Montgomery | The evolutionary history of cetacean brain and body size[END_REF]; Fig. 8A) and/or may simply reflect the greater homogeneity of Cetacea when compared to other artiodactyls.

Encephalization differences between cetaceans and non-cetacean artiodactyls is even clearer in a temporal framework (Fig. 8B). In both groups, there is a significant linear increase in EQ through time until the present (p=9.401x10 -4 and R²=0.13 in cetaceans, p<2.2x10 -16 and R²=0.58 in non-cetacean artiodactyls). Cetaceans EQ appears to increase particularily between late Eocene and Oligocene, while the EQ increase in non-cetacean artiodactyls seems to only start in the Oligocene. Moreover, residuals of both regressions are not homoscedastic (p=0.00305 in cetaceans and p=0.01022 in non-cetacean artiodactyls for Breusch-Pagan tests) and their variance increases through time (p=2.669x10 -3 in cetaceans and p=9.956x10 -3 in noncetacean artiodactyls for linear models). This highlights two differing and diachronic patterns of EQ increase for cetacean and non-cetacean artiodactyls during the Cenozoic.

Brain size in non-cetacean artiodactyls

Figure 8C illustrates the evolution of EQ in non-cetacean artiodactyls over time, based on the non-cetacean artiodactyl equation. There is a significant difference between EQ values of Paleogene (Eocene-Oligocene), and Neogene (Miocene-Pliocene) and Quaternary noncetacean artiodactyls (p<2.2x10 -16 in a t-test). There are no significant differences between Paleogene modalities in EQ (p=0.7104 in an ANOVA, p=0.4122 in a Kruskal-Wallis test, p=0.4212 in a linear model), translating a stasis in EQ values during Paleogene times, while there is an increase in EQ values from the Oligocene onward supported by differences between each time section (p=1.546x10 -12 in an ANOVA, p=1.384x10 -8 in a Kruskal-Wallis test, 1.538x10 -13 in a linear model). There also seems to be an increase in EQ variation during the Neogene and Quaternary, but this is not confirmed by linear models based on squared residuals of each linear model against time (p=0.1775 in ANOVA and p=0.5652 in Kruskal-Wallis test for Paleogene, p=0.1258 and p=0.0461 since Oligocene). The Oligocene Epoch corresponds to the massive appearance of artiodactyl crown groups in the fossil record (except for Cetacea) that progressively replaced stem artiodactyl lineages. Considering EQ variation over time within crown groups, Ruminantia is the only crown group that indubitably shows an increase in EQ (p=2.4x10 -4 in an ANOVA, p=0.0053 in a Kruskal-Wallis test, p=4x10 -5 in a linear model). EQ increase for Suoidea and Tylopoda, as illustrated in Fig. 8C by the median line, is statistically not as well supported as in Ruminantia. Tylopoda shows significant increase according to ANOVA and linear models (p=0.0032 and p=2.7x10 -4 ), while Suoidea do not (p=0.0918). By contrast, Kruskal-Wallis test results are in conflict: Suoidea are significant (p=0.0455), and Tylopoda are not (p=0.0687). Hippopotamoidea EQ seems to remain constant over time (p=0.1896 in ANOVA and linear model, p=0.1213 in a Kruskal-Wallis test). However, EQ increase over time in non-cetacean artiodactyl crown groups (Fig 8C) is based on unbalanced fossil records and has to be interpreted with caution. Further documentation of extinct taxa may change the picture.

Brain size in Cetacea

Studies relative to brain size evolution in cetaceans describe two major encephalization pulses related to two major events: the onset of odontocetes near the Eocene-Oligocene boundary, and within odontocetes, the differentiation of Delphinoidea (15 Ma; e.g., [START_REF] Marino | Origin and evolution of large brains in toothed whales[END_REF]). Regarding the trends accompanying the early steps of their evolutionary history, which comprises the transition to a fully aquatic environment, [START_REF] Marino | Origin and evolution of large brains in toothed whales[END_REF] concluded that there was no significant increase in brain size during archaeocete evolution, ruling out the B, evolution of encephalization through time in Artiodactyla using the EQ artio; bold central line is the median, thin lateral lines are the quartiles. C, evolution of encephalization through time in noncetacean artiodactyls using the corresponding EQ formula; colored lines are the median in each non-cetacean crown group; values above the boxes indicate the total number of taxa for the corresponding time section. D, evolution of encephalization through time in cetacean artiodactyls using the corresponding EQ formula; interquartile range is represented by a semi-transparent range in non-mysticete cetaceans and by a segment in mysticetes (due to the absence of fossil values in this group); values above the boxes indicate the total number of taxa for the corresponding time section. Brain and body data for extant non-cetaceans artiodactyls are from Perez-Barberia et al. (2007), Shultz and[START_REF] Schultz | Encephalization is not a universal macroevolutionary phenomenon in mammals but is associated with sociality[END_REF], and [START_REF] Weston | Insular dwarfism in hippos and a model for brain size reduction in Homo floresiensis[END_REF], for fossil non-cetaceans artiodactyls from [START_REF] Jerison | Evolution of brain and Intelligence[END_REF], [START_REF] Orliac | Virtual endocranial cast of earliest Eocene Diacodexis (Artiodactyla, Mammalia) and morphological diversity of early artiodactyl brains[END_REF], [START_REF] Lyras | Brain changes during phyletic dwarfing in elephants and hippos[END_REF], [START_REF] Thiery | Endocasts and brain evolution in Anthracotheriidae (Artiodactyla, Hippopotamoidea)[END_REF], [START_REF] Radinsky | Evolution of brain size in carnivores and ungulates[END_REF], [START_REF] Berthet | Le genre Cainotherium (Mammalia, Artiodactyla): étude morphométrique, révision systématique, implications évolutives et paléogéographiques, extinction[END_REF], [START_REF] Macrini | Description of a digital cranial endocast of Bathygenys reevesi (Merycoidodontidae; Oreodontoidea) and implications for apomorphy-based diagnosis of isolated, natural endocasts[END_REF], [START_REF] Janis | Evolution of horns in ungulates: ecology and paleoecology[END_REF]), Shultz (2009), and [START_REF] Köhler | Reduction of brain and sense organs in the fossil insular bovid Myotragus[END_REF]; data for cetacean mainly come from [START_REF] Marino | Origin and evolution of large brains in toothed whales[END_REF][START_REF] Gingerich | New partial skeleton and relative brain size in the Late Eocene Archaeocete Zygorhiza kochii (Mammalia, Cetacea) from the Pachuta Marl of Alabama, with a note on contemporaneous Pontogeneus brachyspondylus[END_REF]. Detailed information about EQ dataset are provided in Supplementary Information.

hypothesis of a correlation between relative brain size increase and aquatic habitats. These conclusions were challenged by [START_REF] Gingerich | New partial skeleton and relative brain size in the Late Eocene Archaeocete Zygorhiza kochii (Mammalia, Cetacea) from the Pachuta Marl of Alabama, with a note on contemporaneous Pontogeneus brachyspondylus[END_REF], based on revised and more robust body mass estimates for archaeocetes but with a smaller sample, who concluded that the relative size of the brain in archaeocetes had doubled between the middle and late Eocene.

Figure 8D illustrates EQ variation over time for Archaeoceti, Mysticeti, non-delphinoid Odontoceti, and Delphinoidea, using the EQcet equation. It shows the general EQ increase during the Cenozoic for Cetacea (p=3.604x10 -4 in an ANOVA, p=3.489x10 -6 and R²=0.24 in a linear model) and crown cetaceans (Neoceti; p=1.503x10 -3 in an ANOVA, p=4.983x10 -4 and R²=0.15 in a linear model). In the literature, scaling patterns for these groups are based on other (less specific) formulas or other statistical treatment (e.g., [START_REF] Marino | Origin and evolution of large brains in toothed whales[END_REF][START_REF] Montgomery | The evolutionary history of cetacean brain and body size[END_REF][START_REF] Gingerich | New partial skeleton and relative brain size in the Late Eocene Archaeocete Zygorhiza kochii (Mammalia, Cetacea) from the Pachuta Marl of Alabama, with a note on contemporaneous Pontogeneus brachyspondylus[END_REF][START_REF] Serio | Macroevolution of toothed whales exceptional relative brain size[END_REF][START_REF] Mccurry | Brain size evolution in whales and dolphins: new data from fossil mysticetes[END_REF][START_REF] Smaers | The evolution of mammalian brain size[END_REF]. Some results are nonetheless congruent in an increase of cetacean relative brain size (see [START_REF] Serio | Macroevolution of toothed whales exceptional relative brain size[END_REF][START_REF] Mccurry | Brain size evolution in whales and dolphins: new data from fossil mysticetes[END_REF]. It also shows an EQ increase over time in Archaeoceti (p=0.01939 in an ANOVA, p=7.693x10 -3 and R²=0.91 in a linear model), as supported by [START_REF] Gingerich | New partial skeleton and relative brain size in the Late Eocene Archaeocete Zygorhiza kochii (Mammalia, Cetacea) from the Pachuta Marl of Alabama, with a note on contemporaneous Pontogeneus brachyspondylus[END_REF], but contra [START_REF] Marino | Origin and evolution of large brains in toothed whales[END_REF], and indicates a very early pulse of encephalization during cetacean evolutionary history. Regression parameters of archaeocetes and neocetes are not significantly different (t-test; p=0.2603 for slope, p=0.6329 for intercept), indicating no special pulse at the onset of Neoceti. Similarly, the encephalization pulse corresponding to the differentiation of odontocetes near the Eocene-Oligocene boundary is not supported here because the t-tests of the archaeocetes vs. odontocete and neocetes vs. odontocete regression paramaters as a whole are not significant (p=0.1190 and p=0.3859, and p=0.7173 and p=0.5672 for slope and intercept of each comparison). An EQ increase over time is nevertheless observed in odontocetes as a whole (p=4.127x10 -4 in an ANOVA, p=1.417x10 - 4 and R²=0.19 in a linear model).

The differentiation of Delphinoidea (at ca. 15 Ma) is assumed to coincide with an encephalization pulse at the onset of this superfamily [START_REF] Marino | Origin and evolution of large brains in toothed whales[END_REF]). In our analyses, Delphinoids and non-delphinoid odontocetes are normally distributed groups regarding their EQ (p=0.1561 and p=0.075 in Shapiro-Wilk tests respectively), which has not increased over time (p=0.05785 and p=0.3098 in linear models respectively). The EQ however differ in their means (p=1.334x10 -6 in a bilateral t-test, mean EQs are 1.23 and 0.71 respectively) and variance (p=9.775x10 -5 in a bilateral F-test, EQ variances are 0.23 and 0.05 respectively). Even though EQ did not increased over time in each odontocete subgroup, delphinoids have a general higher EQ together with a larger EQ range (as seen in Figure 8D). The hypothesis of the initial encephalization pulse at the onset of delphinoids is thus supported, and the increase of EQ over time described at the Odontoceti scale (since Oligocene) might reflect that EQ pulse (since the Miocene). The more thorough analyses of [START_REF] Serio | Macroevolution of toothed whales exceptional relative brain size[END_REF] found the high relative brain mass of odontocetes to be plesiomorphic, meeting partially our results: they found a high relative brain mass at the onset of odontocetes (as found here), but without increase since (as also found by [START_REF] Mccurry | Brain size evolution in whales and dolphins: new data from fossil mysticetes[END_REF]. [START_REF] Smaers | The evolution of mammalian brain size[END_REF], using phylogenetic GLS regressions, also found that encephalization increases in mean at the onsets of odontocetes (both body and brain size decreasing, the former faster than the latter), but tightens in variance (variance of brain size diminishing faster than do that of body size). They also found, concurrently to our results, another encephalization increase at the onset of delphinids, both in mean (with decreasing body size and increasing brain size) and in variance (variance of brain size increasing faster than do that of body size). Finally, within Neoceti, EQ values of extant mysticetes do not differ from those of Oligocene odontocetes (p=0.8594 in a t-test and p=0.8438 in a Mann-Whitney test). Of the two clades, the odontocetes are particularly diverse and more frequently documented in the fossil record than mysticetes. Assuming that Oligocene odontocetes are the closest estimates for ancestral EQ values in Neoceti would imply that EQ did not change during mysticete evolution. As a corollary, non-delphinoid odontocetes and extant mysticetes do not differ in EQ. Based on an EQ formula built solely on cetacean artiodactyls, there would only be two EQ increases/pulses in Cetacean history: a first one during archaeocete evolutionary history (in agreement with Gingerich 2015) and a second one at the onset of delphinoids (in agreement with [START_REF] Marino | Origin and evolution of large brains in toothed whales[END_REF].

Future directions: outstanding questions and perspectives

A long tradition of paleoneurological studies in artiodactyls, based both on rich fossil documentation and solid neontological foundations, provides a substantial body of data to discuss the evolutionary history of brain structures in artiodactyls. µCT-scan imagery techniques now allow for a detailed investigation of all endocranial structures as well as for quantitative studies of the evolution of the brain components. Yet, the number of these studies remains small and entire sections of Artiodactyla brain history remain undocumented, particularly concerning modern crown groups, especially Hippopotamoidea and Suoidea. Increasing the documentation of extinct and present-day species endocasts, both for qualitative and quantitative perspectives, is now crucial to complete the picture of artiodactyl brain evolutionary history and fully exploit this promising source of data for phylogenetic and palaeoecological reconstructions.

The nomenclature used to describe the artiodactyl neopallium is largely based on the resemblance between the sulcal pattern of present-day artiodactyls and that of the dog, but it is clear that the first artiodactyls have a very different pattern and that the recognition of the different cortical areas is only partially applicable. This is all the more difficult for large clades without extant representatives because i) following the placement of the sulci during ontogeny is impossible or highly improbable (mainly used to formulate homologies between crown groups), and ii) some patterns have no equivalent in modern artiodactyls, which themselves most probably have convergent complex patterns. Increasing the documentation of endocasts morphology of extinct representatives of crown artiodactyl clades is necessary to refine homology hypotheses. Finally, the study of the brain of artiodactyls has so far been largely synonymous with the study of their neopallium, but the cerebellum also shows great variation in form and structure and its study will allow for comparing cerebrum and cerebellum evolutionary trajectories.

Regarding quantitative prospects, quantitative works focused on the size of the brain as a whole, notably through EQ calculation, to discuss cognitive abilities [START_REF] Jerison | Evolution of brain and Intelligence[END_REF], or habitat predation intensity [START_REF] Jerison | Evolution of brain and Intelligence[END_REF], or to question the impact of socialization [START_REF] Schultz | Encephalization is not a universal macroevolutionary phenomenon in mammals but is associated with sociality[END_REF][START_REF] Pérez-Barbería | Evidence for coevolution of sociality and relative brain size in three orders of mammals[END_REF]), domestication (e.g., Ballarin et al. 2016;[START_REF] Minervini | Brain mass and encephalization quotients in the domestic industrial pig (Sus scrofa)[END_REF], or differential locomotor faculties [START_REF] Pilleri | Cephalization in rodents with particular reference to the Canadian beaver (Castor canadensis)[END_REF] on brain size would be needed. Consideration of other quantitative data such as olfactory bulb size, cerebellum size, or neopalleal surface will allow for addressing questions relating to species abilities that could be put into perspective with their ecology. Again, the constitution of a large database on modern taxa endocranial casts is necessary to be able to exploit further the data collected on fossils.

Concluding remarks

We provide here a very first glance into artiodactyl brain evolutionary history, including Cetacea. The diversity highlighted in this first overview of the external morphology of the brain at the scale of Artiodactyla underlines the potentiality of a "neopalleal blueprint" and its relevance at the systematic and phylogenetic level. The inclusion of endocast characters to taxon-character matrices will certainly bring a promising new phylogenetic signal. Yet, our understanding of the setting up of neopallium folding in the different artiodactyl groups is still very partial and at this point, the poor resolution of the phylogenetic relationships at the base of the artiodactyl tree, and small number of endocast of extinct representatives of moderns clades limit our understanding of brain evolution at the order scale. A better resolution of the phylogenetic relationships of Artiodactyla and a substantial increase of quantitative and qualitative data for extinct members of crown clades are now needed in order to take advantage of the increasing quality of data extracted from endocasts and to further exploit endocasts for paleobiological inferences.

Figure 1 -

 1 Figure 1 -Phylogenetic relationships and general neocortical pattern of modern artiodactyl groups. Suprasylvia and connected sulci (coronal and ansate sulci) are highlighted in red. Not to scale.

Figure 2 -

 2 Figure 2 -Dorsal (A) and lateral (B) views of the right cerebral hemisphere of the domestic pig (Sus scrofa) illustrating the sulcal nomenclature used in this chapter. Arrow points anteriorwards. Abbreviations: an, ansate sulcus; Co, coronal sulcus; Cr cruciate sulcus; di, diagonal sulcus; ES, ectosylvia; IF; interhemispheric fissure; La lateral sulcus; Ob, oblique sulcus; PS, presylvia; Rh, rhinal fissure; SC, sylvian complex; SS, suprasylvia; SSa suprasylvia anterior; SSp suprasylvia posterior. The inter-hemispheric and the rhinale fissures are in dotted lines.

Figure 3 -

 3 Figure3-Lateral left view of the cerebral hemisphere of crown Artiodactyla, compared to the primitive pattern (highlighted in central position), showing the operculization of the central territory of the neopallium (yellow, gyrus I). Abbreviations: ar, arched sulcus; Co, coronal; Cr Cruciate; di, diagonal; eL, ectolateral; L lateral; ob, oblic; PS, presylvia; SS, suprasylvia; I-III refers to the neopalleal gyri. The sylvian complex, sulci resulting from the operculization, is highlighted in red, diagonal and oblique sulci are highlighted in pink, arcuate is highlighted in orange; yellow, gyrus I; blue, gyrus III; grey sanded area and grey corresponds to the central area.

Figure 4 -

 4 Figure 4 -Endocast morphology of extinct artiodactyl clades. A-C, Homacodon vagans (AMNH 12695); D-F, ?Helohyus (AMNH 13079); G-I, Mouillacitherium elegans (UM ACQ 6625); J-L, Dichobune leporina (MNHN.F.QU16586); M-O, Caenomeryx filholi (UM PDS 2570); P-R, Anoplotherium sp. (3D surface of plaster cast illustrated by Dechaseaux 1969:fig.6); S-T, Leptauchenia sp. (AMNH 45508); V-X, Agriochoerus sp. (AMNH 95330). 3D models are available in Orliac (in press). Illustrations in dorsal (A, D, G, J, M, P, S, V), ventral (B, E, H, K, N, Q, T, W), and lateral (C, F, I, L, O, R, U, X) views. Scale bars = 1cm

Figure 5 -

 5 Figure 5 -Endocast morphology of crown artiodactyl clades: A-F (top line) , Suoidea; G-L (middle line), Ruminantia; M-R (bottom line), Hippopotamoidea with A-C, Palaeochoerus sp. (MHNT_2014_0_3075); D-F, Tajassu pecari (UM V79); G-I, Leptomeryx sp. (AMNH 53596); J-L, Moschiola memmina (UM V68); M-O, Microbunodon minimum (UP LM1967MA300); P-R, Choeropsis liberiensis (MRAC RG 35715). Illustrations in dorsal (A, D, G, J, M, P), ventral (B, E, H, K, N, Q), and lateral (C, F, I, L, O, R) views. Scale bars = 1cm.

Figure 6 -

 6 Figure 6 -Proportions of olfactory bulbs volume (A) and neopallium area (B) through time, generated on the basis of the data in table 1. Colors: light blue, Diacodexis; orange, European Endemic artiodactyls; grey, Oreodontoidea; green, Ruminantia; dark blue, Hippopotamidae; yellow, Suoidea. Asterisks (*) indicate operculized taxa. Time periods abbreviations: EE, early Eocene; M/LE middle /late Eocene; O, Oligocene; MI, Miocene; IV + M, quaternary plus modern; Taxa abbreviations: A, Anoplotherium sp.; Ag, Agriochoerus sp.; Br, Bathygenys reevesi; C, Cebochoerus sp.; Cf, Caenomeryx filholi; Cl, Choeropsis liberiensis; Di, Diacodexis ilicis; Dl, Dichobune leporina; Ha, Hippopotamus amphibius; Hm, Hippopotamus madagascariensis; Hv, Homacodon vagans; L, Leptauchenia sp.; Le, Leptomeryx sp.; Me, Mouillacitherium elegans; Mc, Moschus chrysogaster; M, Microbunodon minimum; Mm, Moschiola memmina; P, Palaeochoerus sp.; Ss, Sus scrofa; Tp, Tayassu pecari; Ts, Tragelaphus scriptus.

Figure 7 -

 7 Figure 7 -Morphology, phylogenetic relationships and temporal distribution of artiodactyl endocasts and brains. Endocasts and brains are figured in dorsal view, anterior tip (olfactory bulbs) pointing upwards. Taxa: 1, Diacodexis ilicis (AMNH 16141), 2, Homacodon vagans (AMNH 12695); 3, ?Helohyus (AMNH 13079), 4, Dichobune leporina (MNHN QU16586), 5, Mouillacitherium elegans (UM ACQ 6625); 6, Anoplotherium sp. (3D surface of plaster cast illustrated by Dechaseaux 1969:fig.6); 7, Caenomeryx filholi (UM PDS 2570); 8, Bathygenys sp. (after Macrini 2009:fig.1); 9, Agriochoerus sp. (AMNH 95330); 10, Leptauchenia sp. (AMNH 45508); 11, Merycoidodon culbertsoni (after Black, 1920:fig. 18); 12, Protylopus (after Edinger 1967:fig.1 and Repérant 1971a:fig.1) ; 13, Poebrotherium (after Repérant 1971a:fig.1); 14, Procamelus (after Edinger 1967:fig.3 and Repérant 1971a:fig.1); 15, Lama glama (after Repérant 1971b:pl.VIID); 16, Palaeochoerus sp. (MHNT_2014_0_3075) ; 17, Tajassu pecari (UM V79); 18, Sus scrofa (after Anthony and Grzybowski 1931); 19, Leptomeryx sp. (AMNH 53596); 20, Dremotherium feignouxi (after Sigogneau 1968); 21, Moschiola memmina (UM V68); 22, Moschus chrysogaster (UM N401); 23, Microbunodon minimum (UP LM1967MA300); 24, Choeropsis liberiensis (after Pilleri 1962:fig.1); 25, Indohyus indirae (NM RR 207); 26, Remingtonocetus harudiensis (after Bajpail et al. 2011:fig.6); 27, Xenorophus sp. (after Marino et al. 2003:fig.6C); 28, Tursiops truncatus (after Morgan et al. 1990:fig.2); 29, Balaena mysticetus (after Duffield et al. 1992:fig.7B, and Ragnanti et al. 2019:fig.1C). Color code: light orange, cerebellum; turquoise, midbrain exposure; violet, olfactory bulbs and peduncles; neopalleal sulci: blue, lateral; orange, coronal; yellow, suprasylvia (+ anterior suprasylvia and ansate for some taxa); green, cruciate; purple, splenial; pink, oblique and diagonal. Turning arrows indicate operculization. Abbreviations: Ce, Cetacea; EA, endemic artiodactyls; Hi, Hippopotamoidea; Or, Oreodontoidea; Ru, Ruminantia; Su, Suoidea; Ty, Tylopoda. Phylogenetic relationships are based on[START_REF] Hassanin | Pattern and timing of diversification of Cetartiodactyla (Mammalia, Laurasiatheria), as revealed by a comprehensive analysis of mitochondrial genomes[END_REF] for crown groups branching, fossil taxa relationships rely on[START_REF] Thewissen | Whales originated from aquatic artiodactyls in the Eocene epoch of India[END_REF] for Cetacea,[START_REF] Geisler | Phylogenetic relationships of Cetaceans to Terrestrial Artiodactyls[END_REF] for Tylopoda,[START_REF] Métais | Basal Ruminants[END_REF] for Ruminantia,[START_REF] Orliac | Osteology of the petrosal bone of Suoidea (Artiodactyla, Mammalia)[END_REF] for Suoidea, Oreodontoidea and endemic European taxa are here considered as monophyletic groups, relationships within endemic European taxa rely onWeppe et al. (2020a). Not to scale.

Figure 8 -

 8 Figure 8 -Encephalization within Artiodactyla. A, log brain mass vs log body mass plot in Artiodactyla; empty circles indicate extant taxa, full circles indicate fossil taxa; lines indicate regression lines of extant groups; convex hulls group extant taxa. B, evolution of encephalization through time in Artiodactyla using the EQ artio; bold central line is the median, thin lateral lines are the quartiles. C, evolution of encephalization through time in noncetacean artiodactyls using the corresponding EQ formula; colored lines are the median in each non-cetacean

  from the latest Eocene of Seymour Island (see[START_REF] Fordyce | Gigantism precedes filter feeding in baleen whale evolution[END_REF] for familial referral), and to Willungacetus from the early Oligocene of South Australia(Pledge 2005, placed in Mysticeti indet. by Fitzgerald 2010). Few endocasts of extant cetaceans are also available in the literature, via physical endocasts (e.g., Balaenoptera rostrate, Balaena mysticetes, Megaptera novaeangliae by Gervais 1871; sperm-whale Physeter microcephalus by Flower 1867; foetal fin-whale, common porpoise Phocaena phocaena by Breathnacht 1955; Balaenoptera musculus by Dechaseaux 1961), or digital reconstructions (e.g., Phocoenidae by Racicot and Colbert 2013; narwhal Monodon monoceros and beluga Delphinapterus leucas by Racicot et al. 2018).

Acknowledgments

We thank R. Lebrun for the access to scanning facilities (MRI platform member of the national infrastructure France-BioImaging supported by the French National Research Agency [ANR-10-INBS-04, «Investments for the future»], the LabEx CEMEB [ANR-10-LABX-0004] and NUMEV [ANR-10-LABX-0020]). We thank J. Meng and M. O'Leary for their help in accessing to the AMNH collections. We are grateful to G. Thiery and S. Ducroqc (PALEVOPRIM) for granting access to the µCT scan data of the Microbunodon minimum cranium from La Milloque, and to O. Lambert for his help with literature related to cetacean endocasts. This work was, in part, financially supported by the ANR program DEADENDER (ANR-18-CE02-0003-01) headed by M. J. Orliac.