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The early Palaeocene (66–61.6 mya) witnessed the establishment of mammal-dominated terrestrial ecosystems after the extinction of the non-avian dinosaurs. Understanding the mammals that formed these communities is crucial not only for disentangling the origin of living mammal clades, but also the forces that structured these first precursors of modern ecosystems. The potential role of life history as a driving factor in the composition of early mammalian ecosystems has long been appreciated but has historically been difficult to evaluate. A central focus thus far has been on differences in reproductive strategy between three major North American mammal clades, specifically multituberculates, metatherians, and eutherians. However, virtually no work has considered whether reproductive strategy was uniform within these clades. Recent advances combining paleohistology with geochemistry have opened a new window into reproduction in extinct mammals, revealing a highly precocial lifestyle in the eutherian pantodont Pantolambda, but it is unclear whether this life history style characterized early eutherians more broadly. Results from another eutherian, the phenacodontid Tetraclaenodon, challenge this notion. Both cementochronology and osteohistology indicate a drastically slower life history in the slightly smaller Tetraclaenodon, at virtually the opposite end of the eutherian spectrum from Pantolambda. After a relatively short gestation period (~2 months), Tetraclaenodon retained slow-growing deciduous teeth for as long as four years. The oldest individual in our sample grew exceptionally slowly towards the end of its life, which spanned at least 8–9 years. The ratio of gestation period to body size (10–15 kg) in Tetraclaenodon is similar to small-bodied carnivorans like the coyote (Canis latrans), Caracal (Caracal caracal), and African civet (Civettictis civetta). However, these extant species vary significantly in the duration of suckling (1.5–4 months), and therefore total maternal investment. 

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.