Cranial anatomy and relationships of anew ctenodactyloid (Mammalia, Rodentia) from the early Eocene of Hubei Province, China

Two remarkably complete skulls and associated mandibles from the late early Eocene of the Yuhuangding Formation of Hubei Province, central China, are described as a new genus and species of rodent, Exmus mini, and referred to the early ctenodectyloid family Cocomyidae. This is a small rodent in the size range of the early Eocene Asian Cocomys. Dental characters that link Exmus with other cocomyids include: non-molariform P4; cheek teeth increasing in size posteriorly; lower molar ectolophid weak or absent; and lower molar hypoconulid distinct, enlarged on m3. Cranial characters linking Exmus and Cocomys include: skull shape and proportions, including a short rostrum and large orbit; sagittal crest absent; large optic canal, confluent between the orbits; and no petrosal shelf posterior to the caudal tympanic process of the petrosal. Differences from Cocomys are numerous and include: absence of conules on P4 and loss of the protoconule on the upper molars; tiny metacone and hypocone on P4; molariform p4: and auditory bulla tightly attached, rather than loose as in Cocomys. Exmus also displays a number of autapomorphies that distinguish it from all other Eocene rodents examined in this study, such as substantial orbital roof formed by lacrimal and frontal, and a suture between the parietals that is nearly completely fused. A phylogenetic analysis based on characters of the skull, mandible, and dentition supports the concept of two clades within the ingroups, one composed of Eocene paramyids, sciuravids, and theridomyids and the other composed of Bandaomys, Exmus, Cocomys, Tamquammys, Advenimus, and Yuomys as stem taxa to etenodactylids and hystricognaths. This dichotomy, which has been suggested by others prior to the discovery of Exmus, is here considered to represent an accurate assessment of Eocene rodent evolution and makes good paleogeographic sense. However, the desirability of adding more taxa to a phylogenetic analysis of early rodents is clearly recognized.