PALEOENVIRONMENT OF A LATE PLIOCENE FAUNA OF NORTHERN TIBETAN PLATEAU

A late Pliocene fauna was recently discovered in the Kunlun Mountain Pass area on the northern Tibetan Plateau, at an elevation of about 4700 m above sea level. These fossil materials provide a unique window that allows us to examine the biotic and climatic consequences of the uplift of the Tibetan Plateau. Here we report the initial results from isotopic analyses of this fauna and of modern herbivores in the Kunlun Mountain Pass area. The carbon isotopic ratios of enamel samples from modern horses (Equus kiang and Equus caballus) and yaks (Bos mutus) from the Kunlun Mountain Pass area indicate pure C3 diets consistent with the current dominance of C3 vegetation in local habitats. Serial enamel samples from modern teeth show little δ¹³C variations but significant seasonal changes in δ¹⁸O values within individual teeth, reflecting seasonal variations in the carbon and oxygen isotopic compositions of plant food stuff and water consumed by the animals. In contrast, enamel samples from fossil horse and bovid teeth from the Kunlun Mountain Pass yielded δ¹³C values consistently higher than those of modern herbivores in the area. The higher δ¹³C values indicate that these ancient herbivores, unlike their modern relatives in the area, consumed variable amounts of C4 grasses. Serial samples from a fossil horse tooth and a fossil bovid tooth reveal significant δ¹³C variations within the horse tooth but little δ¹³C variations within the bovid tooth, whereas the bovid tooth displays a larger seasonal δ¹⁸O variation than the horse tooth. These isotopic differences most likely reflect differences in physiology, and dietary and drinking behavior of these animals. Despite limited data, both serial and bulk enamel samples from fossil teeth suggest that C4 grasses were an important component of local ecosystems in the late Pliocene, around 2.1-2.4 Ma. The local ecosystems in the Kunlun Mountain Pass area in the late Pliocene likely included grasslands and/or wooded grasslands that had significant amounts of C4 grasses, very different from today's rock desert and cold steppe environments. The oxygen isotopic difference between fossil and modern teeth most likely reflects climatic and orographic effects on local water isotopic composition.