New species of Yuomys (Rodentia, Ctenodactyloidea) from the upper Eocene of eastern Ningxia, China

A new species, Yuomys robustus of the ctenodactyloid rodent Yuomys, is described in the paper. It is from the Western margin of the Ordos Basin District in eastern Ningxia Hui Autonomous Region, China. Yuomys robustus is characterized by a combination of features: large size, high tooth crown, having a postparacrista on M2 and lacking hypocone on P4. We also emended the diagnosis of type species of Yuomys, Y. cavioides. It is characterized by the absence of hypocone on P4, having a distinct ridge connecting the metaconule to the protocone on M1-M3, a postparacrista on M1, a mesostyle on M2, and a small ridge or spur on the mesial side of the protoloph on P4 and M1; the hypoconid smaller than the protoconid and is elongated, the paraconid absent and the mesostylid faintly visible on p4, but well developed on m1-m3; the talonid basin, sinusid, and posteroflexid are large and open on lower cheek teeth. The occurrence of Lophiomeryx angarae in the same stratigraphic layer as Y. robustus indicates that the horizon is possibly late Eocene in age, not early Oligocene as suggested by previous workers. Body mass estimations of Y. cavioides, Y. eleganes, and Y. robustus show that their weights are roughly in the range of 485-880 g, which is in between those of extant Myospalax and Ratufa. From the middle Eocene to the late Eocene, Yuomys exhibited a trend of gradually enlarging the cheek teeth, and increasing the tooth crown height and body mass.