Asian gliriform origin for arctostylopid mammals.

Arctostylopids are common in Late Palaeocene and Early Eocene mammal faunas of Asia, but they are rare in North America. From the time of their first discovery, arctostylopids were grouped with the strictly endemic South American Notoungulata based on their strikingly similar dental morphology. This relationship was challenged based on dental morphology of more recently discovered arctostylopids and on supposed tarsal morphology (based on unassociated material) of the Asian arctostylopid Palaeostylops. Therefore, Arctostylopidae were placed in a separate order, Arctostylopida, of enigmatic affinities. Many authors, however, continue to unite Arctostylopidae and Notoungulata based on dental similarities. In the Late Palaeocene Subeng site in Inner Mongolia, we identified the Palaeostylops tarsals based on their size and abundance. This identification is supported by comparison to unpublished Arctostylops tarsals from the North American Clarkforkian, derived from a semi-articulated skeleton also including dental material. Tarsal morphology shows moderate similarity to the gliriform Pseudictops, and strong resemblance to the tarsally conservative gliroid Rhombomylus. Hence, Arctostylopidae may best be grouped with Asian non-gliroid Gliriformes, which we interpret as having dispersed into North America in the Late Palaeocene.