Nothernmost Record of Dinocerata (Mammalia: Eutheria) in North America from the Middle Eocene Kishenehn Formation of Montana

Dinoceratans or uintatheres are a phylogenetically enigmatic clade of extinct mammalian herbivores that were among the first eutherians to obtain truly massive body size. Uintatheres were reasonably common elements of North American mammal faunas spanning from the late Paleocene (late Tiffanian, Ti-5) through the middle Eocene (Uintan). However, prior records of North American uintatheres have been geographically restricted to the central Rockies, Texas, and California. Here, we report the first occurrence of a uintathere from the Kishenehn Formation in northern Montana, ∼750 km northwest of their previously known range. The Kishenehn uintathere is currently documented by a single upper molar bearing several characters that may indicate it pertains to a new taxon. Because various lineages of uintatheres are known to have inhabited North America and Asia, additional fossils from higher latitudes of North America could be essential in untangling their evolution, dispersal, and biogeographic history.