临夏盆地晚新生代哺乳动物群演替与青藏高原隆升背景

临夏盆地的晚新生代沉积中富含哺乳动物化石,以晚渐新世巨犀动物群、中中新世铲齿象动物群、晚中新世三趾马动物群和早更新世真马动物群的化石最为丰富。晚新生代是青藏高原快速隆升的时期,临夏盆地的4个主要哺乳动物群在构造剧烈变化的背景下发生了显著的更替。通过对不同动物群所代表的生态特征的分析,恢复了临夏盆地晚新生代以来的气候环境演变过程:晚渐新世以温暖湿润的森林环境为主,间杂有一些开阔地带;中中新世的森林更加茂密,水体更加丰富;晚中新世演变为炎热半干旱的稀树草原环境,季节性变化加强;早更新世气候寒冷而干燥,并伴有显著的海拔升高。青藏高原在晚渐新世的隆升幅度还不足以阻挡大型哺乳动物在高原南北的交流,但到中中新世已成为明显的障碍,至晚中新世对动物迁徙的阻碍作用更加突出,而临夏盆地在早更新世已经达到相当大的高度,产生了一个高原或高山的动物群

EVOLUTION OF THE LATE CENOZOIC MAMMALIAN FAUNAS IN THE LINXIA BASIN AND ITS BACKGROUND RELEVANT TO THE UPLIFT OF THE QINGHAI-XIZANG PLATEAU

A continuous sedimentary sequence from the Oligocene to the Holocene is well developed and exposed in the Linxia Basin (Gansu, China), in which very abundant mammal fossils are discovered. The Linxia Basin is one of the regions with the richest Late Cenozoic mammal fossils in China as well as the whole Eurasia. The Late Oligocene Dzungariotherium fauna, the Middle Miocene Platybelodon fauna, the Late Miocene Hipparion fauna, and the Early Pleistocene Equus fauna are the most representative in the Linxia Basin. Mammals are very sensitive to climatic and environmental changes. The strong uplift of the Qinghai-Xizang Plateau during the Late Cenozoic greatly influenced climate and environment, which would be reflected from the evolution of mammalian faunas. The Linxia Basin is situated at the transitional zone between the Qinghai-Xizang Plateau and Loess Plateau. During the important period of the uplift of the Qinghai-Xizang Plateau in the Late Cenozoic, the Linxia Basin has the mammal fossil assemblages, which can determine geological ages, and the thick deposits, which can accurately reflect climatic variations. As a result, the Linxia Basin is a best site to study the uplift process of the Qinghai-Xizang Plateau and its influence to climate and environment. For the study to the Qinghai-Xizang Plateau, the Linxia Basin can correspond to or be better than the classical Siwaliks on the south margin of the Qinghai-Xizang Plateau. The ecological features of the distinct faunas indicate that the Linxia Basin came through great climatic and environmental changes: warm and humid forest mixed with some open lands in the Late Oligocene, denser forest with richer water bodies in the Middle Miocene, tropical semiarid savanna with strong seasonal variation in the Late Miocene, to cold and arid climate with high elevation in the Early Pleistocene. The particular geographical position of the Linxia Basin implies that the evolution of its mammalian faunas is tightly related to the uplift of the Qinghai-Xizang Plateau in the Late Cenozoic. The uplift of the Qinghai-Xizang Plateau was not enough to baffle the dispersal of large mammals between north and south of this plateau in the Late Oligocene, but it became an obvious barrier for the mammal migration in the Middle Miocene and more obvious in the Late Miocene. The Linxia Basin reached a relatively high elevation in the Early Pleistocene, so that a plateau or alpine mammalian fauna appeared in this area.