Dietary niche reconstruction of Pliocene and Pleistocene Equidae from the Linxia Basin of northwestern China based on stable isotope analysis

In this paper, stable isotope (613C, 618O) analyses of five equid species from the Linxia Basin, northwestern China, were conducted to investigate dietary niche turnover during a critical Pliocene-Pleistocene phase in the evolution of Old World Equidae. In the Early Pliocene Shilidun fauna, Proboscidipparion pater is inferred to have mainly fed on C3 grasses in open grasslands as indicated by their relatively high 613C values (-9.1 & PLUSMN; 0.4%o, five teeth, n = 61), whereas Cremohipparion licenti and Sivalhippus platyodus were possibly mixed feeders inhabiting woodlands and grasslands because of their comparatively low 613C values (-10.8 & PLUSMN; 0.6%o, seven teeth, n = 36; -10.6 & PLUSMN; 0.6%o, nine teeth, n = 35). Inhabiting the same environments, both C. licenti and S. platyodus went extinct, possibly, because climatic changes led to a decline in woodlands resulting in greater competition. In the Early Pleistocene Longdan fauna, Proboscidipparion sinense had higher 613C values (-9.1 & PLUSMN; 0.5%o, four teeth, n = 23) than coexisting Equus eisenmannae (-10.2 & PLUSMN; 0.5%o, eight teeth, n = 57), implying that P. sinense had a stronger grazing preference compared to E. eisenmannae. Ecomorphological analyses (body size, tooth crown height, and enamel surface complexity) reveal that P. pater and P. sinense had dental characteristics consistent with a strong grazing preference. Therefore, P. pater might have been preadapted to open grasslands during the warm and humid Early Pliocene, which helped its descendent P. sinense persist into the Pleistocene. The results of this study reveal distinct dietary niches for these equids for the first time and shed light on some aspects of the evolutionary history of Equidae in East Asia.