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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RBINS Staff Publications 2025 OA
