Yantanglestes

Abstract: A new genus and species of carnivorous mammal is described from early Eocene (Wasatchian) sediments of the Clark's Fork and Bighorn Basins in north- western Wyoming. The new form has cheek teeth most closely resembling those of Asian Didymoconidae; however, it retains a full complement of four premolars and three molars. A new subfamily is proposed for primitive generalized didymoconids including the Wyoming genus and, tentatively, Mongoloryctes. Didymoconines Ar- chaeoryctes, Hunanictis, Ardynictis, and Didymoconus, ranging from the late Paleo- cene to Oligocene in Central Asia, represent a radiation characterized by loss of P1' and M33 with progressive simplification of the remaining cheek teeth. The Wyoming didymoconid is known only from early Wasatchian faunas. This restricted temporal distribution, together with its systematic affinities, suggests that the Wyoming form was a short-lived North American immigrant representing a group whose principal radiation occurred in Asia. The Wyoming form is one of the oldest and most primitive didymoconids. Among Paleocene mammals it most closely resembles Yantanglestes conexus, an early mesonychid from the middle(?) Paleocene of South China, suggesting that didymoconids and mesonychids may share a common ancestry. Didymoconidae are provisionally classified with Mesonychidae in the order Condylarthra. Didymoconidae are an extinct family of insectivorous or carnivorous mammals of problematical affinities previously known only from central Asia. The new specimens described below extend the geographic range of this group to North America. More importantly, they document a broader range of morphological variation within Di- dymoconidae than was previously known, thus permitting specialized Eocene and Oligocene elements of the family to be linked to more primitive and generalized Paleocene forms.