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

Abstract

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.

New Yuomys rodents from southeastern Qinghai-Tibet Plateau indicate low elevation during the Middle Eocene

“…Known species of Yuomys show a clear evolutionary tendency of developing lophodont and hypodont teeth and increasing body size. The evolutionary sequence of Yuomys clearly correlates with the biochronology sequence of Asia (Gong et al, 2021). Apart from the earliest species, Yuomys huheboerhensis, later Yuomys species are significantly large compared to contemporary rodents, such as ctenodactylids and myomorph rodents.…”

“…Yuomys are moderately diverse Eocene rodents. The taxonomy and distribution of Yuomys have been revised recently (Gong et al, 2021). Previously, Yuomys was considered as a rodent with a hystricomorphous skull but with a hystricognathous jaw.…”

“…Yuomys weijingensis was discovered in the Ulan Shireh area of Inner Mongolia. The locality was originally reported as late Eocene, but recent revision of the stratigraphic and mammalian fauna correlations suggest that Y. weijingensis should be middle Eocene Irdinmanhan in age (Li and Meng, 2015;Li, 2019;Gong et al, 2021). Yuomys minggangensis was discovered from the middle Eocene at the Tuanshan locality near the Xinji Village of Minggang Town, Xinyang City of Henan Province (Wang and Zhou, 1982).…”

“…Yuomys cf. Y. cavioides was present at the Ula Usu locality of Inner Mongolia (Li, 1975;Gong et al, 2021), which is Sharamurunian in age. Yuomys records of similar age are Y. eleganes, Y. huangzhuangensis, Y. altunensis, and Y. magnus from a locality near Dalishu village of Wucheng Town, Tongbai County of Henan Province (Wang, 1978), the Langtougou locality near Huangzhuang village of Qufu City of Shandong Province (Shi, 1989), the Altyn Tagh of Xinjiang Province (Wang, 2017), and the Erden Obo locality of Nomogeng, Siziwangqi County of Inner Mongolia (Li, 2019).…”

“…The youngest known Yuomys, Y. robustus, was discovered from the late Eocene at a locality near the Bujiamiaozi village, Lingwu City of Ningxia Province. A specimen from the Zhanglizi Gou locality near Chengliu village of Jiyuan County, Henan Province was recently referred to Y. robustus (Gong et al, 2021).…”