The Diatomyidae (Mammalia, Rodentia) and Bilophodonty in Middle Eocene Asian Rodents

The Asian endemic rodent family Diatomyidae is known to have inhabited eastern and southern Asia since the Oligocene. Its current distribution is limited to karstic regions of Laos. So far as known these hystricomorphous-sciurognathous rodents have some degree of transverse bilophodonty of the cheek teeth. The earliest recognized diatomyids, which are from the Oligocene of Pakistan, retain some traces of cusps on the cheek teeth, overlying the basically bilophodont tooth structure. By the end of the Oligocene there is sonic dental diversity within the family. Miocene and later diatomyids are relatively rare in the fossil record. Molecular evidence unites the diatomyids in the Ctenohystrica, an assignment that receives some support from the molar morphology of Eocene ctenodactyloids. Other than this connection, little is clear regarding the origin of the diatomyids. The middle Eocene Asian Hydentomys exhibits a slight degree of bilophodonty, but is otherwise unlike diatomyids. Another taxon of bilophodont rodent, Dolosimus n. gen. from the middle Eocene of Jiangsu Province, has still more precocious development of bilophndonty, especially in the lower molariform teeth. The incomplete record of this new taxon as well as its morphology cannot answer the question of whether this taxon is allied to such later appearing strongly bilophodont rodents as diatomyids and pedetids, or is an early experiment of this striking morphological development that left no successors.