At the beginning of the Neogene a remarkable faunal turnover occurred within the North American carnivore community. The dominant larger Oligocene carnivores (creodonts, nimravid cats, the amphicyonid Daphoenus) became extinct during the late Oligocene and were replaced in the early Miocene by amphicyonine amphicyonids and hemicyonine ursids that entered North America from Eurasia. During a five million-year interval from ∼23 to 18 Ma, large amphicyonines appear in late Arikareean and early Hemingfordian faunas of the North American midcontinent. Although most fossils are from western Nebraska and southeastern Wyoming, occurrences of amphicyonines at several sites in the eastern United States (Delaware, Florida) indicate that they rapidly established a broad geographic distribution in North America during the early Miocene.

This report describes and summarizes the North American specimens of the rare immigrant amphicyonine Ysengrinia, the first large amphicyonine to enter the New World. Ysengrinia exists in North America from ∼23 to 19 Ma, becoming extinct at the end of the late Arikareean. A single species of Ysengrinia is recognized in North America: Y. americana (Wortman, 1901), comb. nov., restricted to the late Arikareean of western Nebraska and southeastern Wyoming. The North American hypodigm includes the only known complete skull of the genus, associated with a mandible and partial postcranial skeleton. Because most Eurasian occurrences of Ysengrinia are limited to mandibles or isolated teeth of single individuals, intraspecific variation in teeth and skeleton in these carnivores has been difficult to determine. The more complete North American specimens provide estimates of dental and osteological variation in Ysengrinia, and suggest that the North American species is dimorphic. Skeletal remains of early Miocene New World Ysengrinia are most often found in riparian and waterhole environments.