Perissodactyls make their first appearance in the fossil record during the Paleocene-Eocene Thermal Maximum (PETM), 56 Ma ago, at the same time as most modern placental mammal orders. However, their early diversification remains unclear since all main groups (Equidae, Isectolophidae, Ceratomorpha, Ancylopoda and Brontotheriidae) appear at the same time with numerous genera and are already diverse, contrary to other modern mammal orders, which have a much lower diversity. Moreover, several early perissodactyl genera seem endemic to each of the three Holarctic continents, rather than cosmopolitan as is the case for other mammal groups. We investigate the early evolution of perissodactyls by analyzing a new dataset focusing on early species to obtain a new expansive phylogeny. This large-scale phylogeny shows that the number of early perissodactyl genera was over-estimated and supports the synonymy of several genera. In addition, many species that were named as “Hyracotherium” in the past (or sometimes “Eohippus”, the “dawn-horse”) are not actually closely related to horses, including Hyracotherium itself, but can be considered as basal perissodactyls. Among the latter, at least Pliolophus and Cardiolophus experienced vast and fast dispersals between North America, Europe and Asia, during the PETM. This paleobiogeographic scenario much resembles the one of other earliest Eocene mammals, which comprise genera that are often found in at least two continents, such as in artiodactyls, primates, carnivorans, hyaenodontids, hyopsodontids, phenacodontids or rodents. Here, we show that the paleobiogeographic distribution of earliest perissodactyls does not differ from that of other modern mammals.
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The perissodactyls from the UNESCO World Heritage site of Messel (MP11; around 47 Ma) are among the most well-known fossils from this site, and belong to at least five different genera: three equoids (Propalaeotherium, Eurohippus, and Hallensia) and two tapiromorphs (Lophiodon and Hyrachyus). Here, we investigated by X-ray microtomography at the DTHE (MateIS Laboratory, Lyon) the anatomy of three articulated skeletons from the Royal Belgian Institute of Natural Sciences collection: two specimens of Eurohippus messelensis, as well as the only accessible skeleton of Hallensia matthesi. The segmented 3D models allow us to visualize for the first time some cranial, dental and postcranial features that were previously concealed. Surprisingly, the tooth rows of the two specimens of E. messelensis show two different morphologies. One is typical of E. messelensis whereas the other shows all the diagnostic characters of E. parvulus expressed by Franzen (2006). However, based on the monograph on equoids from Messel by Franzen (2007), E. messelensis was only recognized in Messel, whereas E. parvulus was known from several other localities, notably in the Geiseltal, but not in Messel. We thus suggest that E. parvulus may have also been present in Messel, which raises the question of their potential synonymy. Finally, the preservation of the skeleton of Hallensia matthesi does not permit to observe the fine anatomy of the skeleton, but still permits to discuss important characters such as the number of fingers.
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