Cenozoic vertebrate evolution and paleoenvironment in Tibetan Plateau: Progress and prospects

Due to its lofty height, the Tibetan Plateau features some of the harshest environments in the world with extreme coldness, low oxygen, high UV radiation, and in places, severe aridity. These harsh environments have served as the main driver of vertebrate evolution during the Cenozoic that produced a low productivity, low diversity, high endemicity community environmental forcing seems to be a dominant mechanism for mammalian evolution in marginal habitats. Mammals have acquired special adaptations for thermal insulation, oxygen transport, DNA damage repair, and others. While broadly similar to faunal assemblages in North China and Central Asia, Tibetan faunas often show initiation of cold-adapted lineages that predate Ice Age megafauna. Our out of Tibet hypothesis thus postulates that cold-tolerant species in Pliocene high Tibet were pre-adapted to conditions that were to become widespread during the subsequent Pleistocene Ice Age and Tibet had thus become a cradle for Ice Age megafauna. The best examples of the out of Tibet scenario include the Tibetan woolly rhinoceros (Coelodonta thibetana), Tibetan bharal (Pseudois), Asian hunting dog (Sinicuon), and ancestral snow leopard (Panthera blytheae). Our Third Pole to North Pole linkage of an extinct Pliocene Tibetan fox, Vulpes qiuzhudingi, with the late Pleistocene and extant arctic fox, Vulpes lagopus, is a subset of the out of Tibet hypothesis. One of the iconic Tibetan mammals, the endemic Tibetan antelope (Pantholops) has the longest residence in Tibet going back to the late Miocene, suggesting a long history of perfecting its adaptations to cold environments. Stable carbon and oxygen isotope analyses of fossils from the Tibetan Plateau reveal significant spatial and temporal variations in the evolution of herbivores' diets and environments during the late Cenozoic. The diets of modern Tibetan herbivores consist primarily of C-3 plants, reflecting the present-day C-3 dominance in high-elevation cold habitats. The available carbon isotope data, however, show that warm-climate C-4 grasses were a significant dietary component of ancient herbivores in the Gyirong Basin (central Himalayas) in the late Miocene and in Shangri-La (southeastern Tibetan Plateau) in the Pliocene, and likely in the Qaidam Basin (NE Tibetan Plateau) from late Miocene to early Pliocene and in Kunlun Pass Basin (northeastern Tibetan Plateau) in the Pliocene. This suggests that C-4 grasses spread into the Tibetan Plateau in the late Miocene and had persisted in some places to early Pleistocene, but have since disappeared or become insignificant in the region. The retreat of C-4 grasses from the Tibetan Plateau is most likely driven by decreases in local temperatures to below the cross-over temperature unfavorable to C-4 grasses, as a result of tectonic uplift and/or global climate change. The available isotope data also show that high-elevation cold climatic conditions that favor C-3 over C-4 grasses arrived earlier in southwestern Tibetan Plateau than in southeastern and northeastern parts of the plateau. This would imply that the Tibetan Plateau had not uniformly reached its present height as recent as early Pleistocene. Future research should focus on determining intra-regional patterns of ecological and climatic changes in order to tease apart the effects of tectonic uplift and global climate change on local and regional environmental changes and to better understand the linkage between tectonics and climatic and biotic changes. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.