Vertebrate paleontology, biostratigraphy, geochronology, and paleoenvironment of Qaidam Basin in northern Tibetan Plateau

Abstract

Qaidam Basin is the largest terrestrial basin of the Tibetan Plateau and has the most continuous sedimentary record in the Cenozoic. Although constituting the first known fossil vertebrates of the Tibetan Plateau and discovered in early scientific expeditions in the 1930s, mammalian fossils from Qaidam Basin have not played a significant role in studying the basin stratigraphy. Our explorations in the Qaidam Basin during the past 8 years have resulted in the first well documented collection of fossil vertebrates from there. In addition to being the best organisms for terrestrial biostratigraphy in the Cenozoic, the new fossil vertebrates offer a unique window into the ancient biotas that chronicle the late Cenozoic uplift of the plateau and associated environmental changes. Here we establish for the first time a faunal sequence of early Oligocene to early Pliocene age. Four mammalian faunas are recognized: Olongbuluk Mammal Fauna (middle Miocene), Tuosu Mammal Fauna (early late Miocene), Shengou Mammal Fauna (early late Miocene), and Huaitoutala Mammal Fauna (early Pliocene). All four mammal faunas can be directly or indirectly tied to a paleomagnetic section in the Olongbuluk–Huaitoutala area, permitting robust calibration of the chronology of these faunas. At least four fish faunas are also found: Lulehe Fish Fauna (late early to early late Oligocene), Shengou Fish Fauna (early late Miocene), Eboliang Fish Fauna (late late Miocene to early Pliocene), and Yahu Fish Fauna (early Pliocene). The fish faunas represent the only records of their respective times in the Tibetan Plateau and thus by themselves cannot be used for age determination. However, these fish faunas are either integrated into paleomagnetic sections or directly tied to mammal faunas of known ages, permitting independent assessment of their chronology. Zoogeographically, Qaidam mammal faunas show increasing endemism through time, presumably related to topographic barriers formed by the growth and uplift of the Tibetan Plateau and to climatic changes triggered by the same tectonic process. Fishes that were trapped in internally drained basins developed pachyostotic morphologies caused by hypersaline environments, which are associated with the increasing aridity of the Tibetan Plateau since early Pliocene.