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  1. With the dawn of the Paleogene, the mammalian survivors of the Cretaceous–Paleogene mass extinction, 66 million years ago, found themselves in an emptied landscape. Within a million years of the bolide impact, placental mammals reached a diversity and abundance never seen during the Age of Dinosaurs. The North American ‘condylarths’ were amongst the first mammals to diversify during the early Paleogene and are often considered the ancestral ‘stock’ from which other euungulate groups evolved. Amongst these, Phenacodontidae are often regarded to lie at the base of the perissodactyl family tree, but their phylogenetic position, and that of other ‘condylarths’, remain contentious. Tetraclaenodon, a medium-sized herbivorous phenacodontid from the Torrejonian (~64 to ~62 Ma) of North America is generally recognized as the oldest member of Phenacodontidae, and thus is instrumental for untangling the evolutionary relationships of ‘condylarths’ and perissodactyls. Here we present new information on Tetraclaenodon based on a description of new and previously known fossil material from the San Juan Basin of New Mexico, U.S.A., which we studied using high-resolution computed tomography (CT) scanning. From CT scans of the cranium, we segmented the brain endocast, which is relatively small and smooth (lissencephalic), similar to that of other Paleocene mammals. The petrosal lobules, which are involved in eye movement coordination, are small. The semi-circular canals associated with balance, provide an agility score of 3 indicating that Tetraclaenodon was probably moderately agile, similar to the extant raccoon dog or the aardwolf. A multivariate analysis of tarsal measurements for a sample of Paleocene and extant mammals, which informs locomotor style, indicates that Tetraclaenodon was most suited to terrestrial locomotion. This is in line with anatomical and myological features of the limbs of Tetraclaenodon and other phenacodontids, early perissodactyls and extant mammals. These findings contradict previous studies that designated Tetraclaenodon as a scansorial mammal, capable of habitually climbing trees. Our results shed light on the locomotory adaptations of Tetraclaenodon in comparison to more cursorial phenacodontids and perissodactyls, such as Phenacodus and Hyrachyus. The earliest member of the perissodactyl stem lineage apparently lacked the more cursorial adaptations of their relatives in the late Paleocene and onwards. 
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  2. After the end-Cretaceous mass extinction, approximately 75% of life on land and in the sea disappeared. The mammals of the early Cenozoic rapidly diversified and dispersed, rising to numerical and ecological dominance beyond their Mesozoic norms. Among those initial groups that ushered in the Age of Mammals, Paleocene and Eocene ‘condylarths’ are thought to include the ancestors of modern odd-toed ungulates (horses, tapirs, rhinos). Tetraclaenodon is the oldest genus of the ‘condylarth’ group Phenacodontidae and one of the most abundant fossils from the San Juan Basin (SJB) of New Mexico. Tetraclaenodon was a medium sized (mean body mass ca. 10kg), terrestrial mammal which was lightly built and had an omnivorous to herbivorous bunodont dentition. Here we use multivariate and statistical analyses to investigate body mass and dental variation in 110 teeth of Tetraclaenodon spanning the Torrejonian (Paleocene) interval of the SJB. The specimens were grouped into six time bins by their biostratigraphical reference, from Tj1 (~63.8 Ma) through Tj6 (~62.7 Ma). Measurements of the length, mesial and distal width of the lower first molars (m1) were subject to principal component analysis (PCA), and m1 area was used to predict body mass using a regression equation. The PCA morphospace ordinates specimens along a PC1 axis that accounts for 90.05% of total variance and is significantly correlated with body size. A PERMANOVA test finds a significant difference in morphospace occupation (non-overlap) between clusters of specimens from Tj1-3 and Tj4-6, but there are no significant differences between the individual time bins within each cluster. This trend is also seen in the body size estimates: Mann-Whitney tests recover significant differences between the two clusters. These results suggest that Torrejonian populations of Tetraclaenodon were relatively constant in size throughout Tj1-3, but between Tj3 and Tj4 underwent an increase in body mass and subsequently stabilized (at this resolution) for the remainder of the Torrejonian. A similar trend is seen in contemporary populations of the periptychid ‘condylarth’ Periptychus, suggesting that there were selective environmental pressures acting on these herbivorous species. These body size differences may reflect the emergence of a new, larger Tetraclaenodon species in Tj4, or may be associated to an environmental change, perhaps relating to climate or vegetation. In either case, this illustrates dynamic evolution of mammals during the few million years after the extinction. Grant Information: European Research Council Starting Grant (ERC StG 2017, 756226, PalM), National Science Foundation (EAR- 1654952) 
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