Maeva J. Orliac

Early Miocene hippopotamids (Cetartiodactyla) constrain the phylogenetic and spatiotemporal settings of hippopotamid origin

The affinities of the Hippopotamidae are at the core of the phylogeny of Cetartiodactyla (even-toed mammals: cetaceans, ruminants, camels, suoids, and hippos). Molecular phylogenies support Cetacea as sister group of the Hippopotamidae, implying a long ghost lineage between the earliest cetaceans (∼53 Ma) and the earliest hippopotamids (∼16 Ma). Morphological studies have proposed two different sister taxa for hippopotamids: suoids (notably palaeochoerids) or anthracotheriids. Evaluating these phylogenetic hypotheses requires substantiating the poorly known early history of the Hippopotamidae. Here, we undertake an original morphological phylogenetic analysis including several “suiform” families and previously unexamined early Miocene taxa to test previous conflicting hypotheses. According to our results, Morotochoerus ugandensis and Kulutherium rusingensis, until now regarded as the sole African palaeochoerid and the sole African bunodont anthracotheriid, respectively, are unambiguously included within the Hippopotamidae. They are the earliest known hippopotamids and set the family fossil record back to the early Miocene (∼21 Ma). The analysis reveals that hippopotamids displayed an unsuspected taxonomic and body size diversity and remained restricted to Africa during most of their history, until the latest Miocene. Our results also confirm the deep nesting of Hippopotamidae within the paraphyletic Anthracotheriidae; this finding allows us to reconstruct the sequence of dental innovations that links advanced selenodont anthracotheriids to hippopotamids, previously a source of major disagreements on hippopotamid origins. The analysis demonstrates a close relationship between Eocene choeropotamids and anthracotheriids, a relationship that potentially fills the evolutionary gap between earliest hippopotamids and cetaceans implied by molecular analyses.

Open data and digital morphology

Over the past two decades, the development of methods for visualizing and analysing specimens digitally, in three and even four dimensions, has transformed the study of living and fossil organisms. However, the initial promise that the widespread application of such methods would facilitate access to the underlying digital data has not been fully achieved. The underlying datasets for many published studies are not readily or freely available, introducing a barrier to verification and reproducibility, and the reuse of data. There is no current agreement or policy on the amount and type of data that should be made available alongside studies that use, and in some cases are wholly reliant on, digital morphology. Here, we propose a set of recommendations for minimum standards and additional best practice for three-dimensional digital data publication, and review the issues around data storage, management and accessibility.

Phylogenetic relationships of the Suidae (Mammalia, Cetartiodactyla): new insights on the relationships within Suoidea

Orliac, M. J., Antoine, P. ‐O., Ducrocq, S. (2010). Phylogenetic relationships of the Suidae (Mammalia, Cetartiodactyla): new insights on the relationships within Suoidea. —Zoologica Scripta, 39, 315–330.

Virtual endocranial cast of earliest Eocene Diacodexis (Artiodactyla, Mammalia) and morphological diversity of early artiodactyl brains

The study of brain evolution, particularly that of the neocortex, is of primary interest because it directly relates to how behavioural variations arose both between and within mammalian groups. Artiodactyla is one of the most diverse mammalian clades. However, the first 10 Myr of their brain evolution has remained undocumented so far. Here, we used high-resolution X-ray computed tomography to investigate the endocranial cast of Diacodexis ilicis of earliest Eocene age. Its virtual reconstruction provides unprecedented access to both metric parameters and fine anatomy of the most complete endocast of the earliest artiodactyl. This picture is assessed in a broad comparative context by reconstructing endocasts of 14 other Early and Middle Eocene representatives of basal artiodactyls, allowing the tracking of the neocortical structure of artiodactyls back to its simplest pattern. We show that the earliest artiodactyls share a simple neocortical pattern, so far never observed in other ungulates, with an almond-shaped gyrus instead of parallel sulci as previously hypothesized. Our results demonstrate that artiodactyls experienced a tardy pulse of encephalization during the Late Neogene, well after the onset of cortical complexity increase. Comparisons with Eocene perissodactyls show that the latter reached a high level of cortical complexity earlier than the artiodactyls.

Endocranial morphology of Palaeocene Plesiadapis tricuspidens and evolution of the early primate brain.

Expansion of the brain is a key feature of primate evolution. The fossil record, although incomplete, allows a partial reconstruction of changes in primate brain size and morphology through time. Palaeogene plesiadapoids, closest relatives of Euprimates (or crown-group primates), are crucial for understanding early evolution of the primate brain. However, brain morphology of this group remains poorly documented, and major questions remain regarding the initial phase of euprimate brain evolution. Micro-CT investigation of the endocranial morphology of Plesiadapis tricuspidens from the Late Palaeocene of Europe—the most complete plesiadapoid cranium known—shows that plesiadapoids retained a very small and simple brain. Plesiadapis has midbrain exposure, and minimal encephalization and neocorticalization, making it comparable with that of stem rodents and lagomorphs. However, Plesiadapis shares a domed neocortex and downwardly shifted olfactory-bulb axis with Euprimates. If accepted phylogenetic relationships are correct, then this implies that the euprimate brain underwent drastic reorganization during the Palaeocene, and some changes in brain structure preceded brain size increase and neocortex expansion during evolution of the primate brain.

The petrosal of the earliest elephant-shrew Chambius (Macroscelidea: Afrotheria) from the Eocene of Djebel Chambi (Tunisia) and the evolution of middle and inner ear of elephant-shrews

Macroscelidea (elephant-shrews or sengis) are small insectivorous mammals restricted to Africa; they belong to the super-cohort Afrotheria along with other insectivorans (aardvarks, tenrecs and golden moles) and ungulates (elephants, sea cows and hyraxes). Though their fossil record extends back to the Eocene, cranial remains of Palaeogene elephant-shrews, including the middle and inner ear structure, remain unknown. Two macroscelid isolated petrosal bones are described from the late Early–early Middle Eocene Djebel Chambi locality in Tunisia. Chambius kasserinensis is the only macroscelid represented in this locality. A cladistic analysis based on petrosal and inner ear characters highlights the crucial interest of the petrosal and inner ear morphology for understanding the evolution of Macroscelididae. It confirms the attribution of these isolated petrosals to C. kasserinensis. This hypothesis is supported by a common pattern of circulatory system, the morphology of the rostral and caudal tympanic processes, and the shape of the cochlea. In addition, Chambius appears to be the basal-most taxon among the macroscelid sample; this position is supported by the lack of some specializations of the middle ear such as inflated ossicular bones and pneumatized bulla. The presence of a secondary common crus in Chambius suggests a convergent loss of this structure, at least in Macroscelidea and Tenrecoidea. The petrosal and inner ear characters support the clade Afroinsectivora, which gathers macroscelids with other endemic African insectivorans (tenrecs and golden moles), reinforcing the hypothesis of an African origin of macroscelids. The petrosal bone and inner ear characters provide further morphological support for the debated clade (Petrodromus, Elephantulus rozeti, Macroscelides). New data underlines the fact that the cranial arterial pattern of the Eocene macroscelid Chambius was already similar to that of modern macroscelid species. It also suggests that early elephant-shrews were probably not as capable of hearing low frequencies as their extant representatives.

The petrosal bone of extinct Suoidea (Mammalia, Artiodactyla)

The morphology of the petrosal is often employed in mammalian systematics, yet this bone has not been described in detail for a comparative sample of extinct members of Suoidea (the clade encompassing pigs, peccaries and their extinct relatives), a total clade that represents one of the major divisions within Artiodactyla. Here, the petrosal osteology of fossil Suoidea ranging from their early representatives (Late Eocene) up to Late Miocene is described. A sample of petrosal specimens documenting eight genera from the four suoid families (Suidae, Tayassuidae, Sanitheriidae and Palaeochoeridae) has been collected from in situ mechanical preparation, CT-based reconstruction or identification of isolated petrosals. The diagnostic significance of petrosal characters for suoid systematics is assessed by the inclusion of the new petrosals data in a phylogenetic analysis. As in other mammal groups, petrosal characters are shown to be of primary phylogenetic interest. The monophyly of the included families and subfamilies shown by craniodental and postcranial characters is supported by petrosal characters, and the problematic New World suoid Perchoerus is interpreted as the first offshoot of the Suoidea clade. This work allows definition of Suoidea on the basis of petrosal morphological characters and reveals the shaping over time of the peculiar petrosal morphology observed in extant suoids, notably the drastic reduction of the mastoid of this ‘amastoidean’ group, convergently present in hippos.

Digital cranial endocast of hyopsodus (Mammalia, "condylarthra"): A case of paleogene terrestrial echolocation?

We here describe the endocranial cast of the Eocene archaic ungulate Hyopsodus lepidus AMNH 143783 (Bridgerian, North America) reconstructed from X-ray computed microtomography data. This represents the first complete cranial endocast known for Hyopsodontinae. The Hyopsodus endocast is compared to other known “condylarthran” endocasts, i. e. those of Pleuraspidotherium (Pleuraspidotheriidae), Arctocyon (Arctocyonidae), Meniscotherium (Meniscotheriidae), Phenacodus (Phenacodontidae), as well as to basal perissodactyls (Hyracotherium) and artiodactyls (Cebochoerus, Homacodon). Hyopsodus presents one of the highest encephalization quotients of archaic ungulates and shows an “advanced version” of the basal ungulate brain pattern, with a mosaic of archaic characters such as large olfactory bulbs, weak ventral expansion of the neopallium, and absence of neopallium fissuration, as well as more specialized ones such as the relative reduction of the cerebellum compared to cerebrum or the enlargement of the inferior colliculus. As in other archaic ungulates, Hyopsodus midbrain exposure is important, but it exhibits a dorsally protruding largely developed inferior colliculus, a feature unique among “Condylarthra”. A potential correlation between the development of the inferior colliculus in Hyopsodus and the use of terrestrial echolocation as observed in extant tenrecs and shrews is discussed. The detailed analysis of the overall morphology of the postcranial skeleton of Hyopsodus indicates a nimble, fast moving animal that likely lived in burrows. This would be compatible with terrestrial echolocation used by the animal to investigate subterranean habitat and/or to minimize predation during nocturnal exploration of the environment.

Endocranial morphology of Microchoerus erinaceus (Euprimates, Tarsiiformes) and early evolution of the Euprimates brain

OBJECTIVES Innovations in brain structure and increase in brain size relative to body mass are key features of Primates evolutionary history. Surprisingly, the endocranial morphology of early Euprimates is still rather poorly known, and our understanding of early euprimate brain evolution (Eocene epoch) relies on a handful of specimens. MATERIALS AND METHODS In this article, we describe the endocranial cast of the tarsiiform Microchoerus erinaceus from the late Early Eocene of Perrière (Quercy fissure filling, France) based on a virtual reconstruction extracted from CT scan data of the endocranial cavity of the complete, undeformed specimen UM-PRR1771. RESULTS The endocast of M. erinaceus shows the derived features observed in other Euprimates (e.g. sylvian fissure and temporal lobe), with limited neocortical folding, and a telencephalic flexure comparable to that of extant primates. DISCUSSION Comparison with the endocasts of other available late Eocene primates shows that they already exhibited a variety of brain morphologies, highlighting the complex history of the external features of the primate brain, as early as the Eocene. M. erinaceus was a fruit and gum eater considered as nocturnal based on its orbit size. However, its brain showed small olfactory bulbs--smaller than in the coeval diurnal taxa Adapis parisiensis--and a neocorticalization similar to folivorous taxa. These observations contrast with patterns observed in primates today where nocturnal taxa have larger olfactory bulbs than diurnal taxa, and call into question a direct correlation between frugivory and neocorticalization increase in primates.

The inner ear morphology of the ‘condylarthran’ Hyopsodus lepidus

We describe the bony labyrinth morphology of the Eocene ‘archaic ungulate’ Hyopsoduslepidus (Bridgerian, North America) reconstructed from micro computed tomography scan data. Comparisons with the inner ear of the Eocene early diverging artiodactyl Diacodexis and perissodactyl Xenicohippus allow refining the picture of the ancestral inner ear morphology of Euungulates. These taxa are very close morphologically and mostly differ by slight differences in their semicircular canal angulations and profile. They all present a secondary crus and a low position of the plane of the lateral semicircular canal relative to the posterior semicircular canal. These two characters, considered as ancestral features for Theria, might be symplesiomorphies of Euungulata as well. Hyopsodus and Xenicohippus share characters also observed in other basal Equoidea, which would support the close relationship between these two taxa previously proposed in the literature. A functional study of the cochlea of Hyopsodus lepidus is also realised to discuss its putative ability of using terrestrial echolocation previously proposed in the literature. The morphology of the cochlea of Hyopsodus lepidus does not indicate a specialisation to sophisticated echolocation such as observed today in microchiropteran bats. However, its estimated audible range of frequencies (208 Hz to 76.8 KHz) would be compatible with terrestrial echolocation.