Functional morphology of the first known skeleton of Leptarctus (Mustelidae, Carnivora) clarifies the palaeoecology of a distinctive predator lineage

Leptarctine mustelids are Neogene carnivorans with fossil records in Asia and North America. In the 170 years since the clade was erected, all taxa within this clade have been characterised based on craniodental traits. Despite having geographically extensive and well-preserved fossil records, no postcranial elements of leptarctines are known other than a single atlas and axis reported from Inner Mongolia. Here, I describe a partial skeleton of Leptarctus wortmani , with associated lower dentition, partial cranium and partial postcranial elements representing fore-limb, hind-limb and pre-sacral vertebral column. I conduct a functional morphological analysis of the external proportions and internal epiphyseal trabeculae of limb elements to provide information for the locomotor mode of Leptarctus . Functional indices of external limb morphology and internal trabeculae both indicate that L. wortmani was likely a generalist in its locomotor mode; there is no support for an arboreal lifestyle as previously suggested, based on dental similarities to kinkajou or koala. Overall, the postcranial morphology of L. wortmani most closely resembles that of extant Eurasian badgers Meles , albeit with reduced ungual phalanges and more elongate long bones for its size. These palaeoecological inferences suggest that L. wortmani occupied a badger-like niche during the middle and late Miocene, pre-empting the appearance of true North American badgers, such as Pliotaxidea and Taxidea . Competitive replacement by extant badgers may be responsible for the extinction of leptarctines by the Pliocene.