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

Abstract

Yuomysare medium-sized Hystricomorpha rodents. They are known for coming from areas of low elevation in China during the middle and late Eocene. Two newYuomyswere discovered from a locality near Xueshuo village in Litang County, Sichuan Province. The locality lies in the Gemusi pull-apart basin formed in the Litang Fault System (LTFS) in the Hengduan Mountains. The current average elevation is about 4200 m. One of the two newYuomysis larger and shows clear lophodont and unilateral hypsodont morphology, similar toYuomys yunnanensis, which was discovered as being from the early middle Eocene (Irdinmanhan, Asian Land Mammal Ages) in the Chake Basin of Jianshui County, Yunnan Province. The Chake Basin is one of the small pull-apart basins formed in the Xianshuihe-Xiaojiang Fault system (XSH-XJF). The other newYuomysrodent is smaller, brachydont, and less lophodont than the larger new species. The small newYuomysis smaller than all knownYuomysexceptYuomys huheboerhensis, which is from the early middle Eocene Irdinmanhan of Inner Mongolia in Northern China. Given their narrow biochronological distribution and presumably preferred living environment, the occurrence ofYuomysin the pull-apart basins in LTFS and XSH-XJF suggests that the two deep fault systems probably started strike-slip movement by the early middle Eocene, about 49–45 million years ago. Well-studied middle Eocene mammalian faunas from Henan and Inner Mongolia includeYuomys, primates, and other low elevation forest mammals. We suggest that the two newYuomysspecies reported here probably also lived in a similar low elevation forest environment.