Nanpoping fauna of the Lanzhou Basin and its environmental significance

季风干旱的环境系统开始在渐新世期间开发了的亚洲人，而是干旱地区边界的准确位置在渐新世是不明确的。石块哺乳动物集合能被用来估计一个区域的环境。有在从 Lanzhou 盆的渐新世阶层的 Nanpoping 动物志的哺乳动物的二种尺寸：小的是象 Ctenodactylidae 那样的草地啮齿类动物；大的是象 Paraceratherium 和 Paraentelodon 那样的森林居民。在 Nanpoping 动物志识别的哺乳动物的 23 种类， ~40% 是森林居民和剩余的(~60%) 是草地 taxa。Nanpoping 动物志在到南方的 Linxia 区域与在到北方和 Jiaozigou 动物志的 Danghe 区域的同时期的 Dingdanggou 动物志相比。Nanpoping 动物志包含两另外的动物志的元素并且因此代表在森林和草地之间的一个过渡集合。那么，在中国的干旱地区的南部的边界可能在渐新世期间在 Lanzhou 盆附近，这能被推断。

The Asian monsoon-arid environment system began to develop during the Oligocene, but the exact position of the arid zone boundary is uncertain in the Oligocene. Fossil mammal assemblages can be used to assess the environment of an area. There were two sizes of mammals in the Nanpoping fauna from the Oligocene strata of the Lanzhou Basin： the small ones were grassland rodents such as Ctenodactylidae; the large ones were forest dwellers such as Paraceratherium and Paraentelodon. Of the 23 species of mammals identified in the Nanpoping fauna, -40% were forest dwellers and the rest （-60%） were grassland taxa. The Nanpoping fauna was compared with the contemporaneous Dingdanggou fauna in the Danghe area to the north and Jiaozigou fauna in the Linxia area to the south. The Nanpoping fauna contains elements of both other faunas and thus represents a transitional assemblage between forest and grassland. So, it can be inferred that the southern boundary of the arid zone in China during the Oligocene was probably near the Lanzhou Basin.