Biotic Changes in Terrestrial Environments around the Eocene–Oligocene Transition

Yuomys are medium-sized Hystricomorpha rodents. They are known for coming from areas of low elevation in China during the middle and late Eocene. Two new Yuomys were 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 new Yuomys is larger and shows clear lophodont and unilateral hypsodont morphology, similar to Yuomys 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 new Yuomys rodent is smaller, brachydont, and less lophodont than the larger new species. The small new Yuomys is smaller than all known Yuomys except Yuomys 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 of Yuomys in 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 include Yuomys, primates, and other low elevation forest mammals. We suggest that the two new Yuomys species reported here probably also lived in a similar low elevation forest environment.