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The Linxia Basin in northwestern China lies topographically at the northeastern margin of the Tibetan Plateau and climatically at the western margin of the East Asian monsoon region, and bears abundant mammalian fossils with environmental significance on central Asian aridification and east Asian monsoon evolution. However, these mammals have not been precisely dated, hindering our ability to address the ecologic environment changes associated with the evolution of the Asian monsoon and multiple uplifts of the Tibetan Plateau. We present a detailed paleomagnetic study of a sedimentary sequence from the Laogou section in the Linxia Basin containing many mammalian fossils with ages ranging between middle and late Miocene. The 203 m thick section revealed 11 normal and 10 reversed zones that correlate well with chrons 3An to 5An of the Geomagnetic Polarity Time Scale, constraining its age to similar to 12.56-6.0 Ma, and suggesting that the middle Miocene Platybelodon fauna existed at about 11.5 Ma and the late Miocene Hipparion fauna occurred at about 6.4 Ma. These ages, combined with other magnetostratigraphic results from the whole basin, show that these two major mammal faunas lived during 12.5-11.1 Ma and 11.5-6.3 Ma in the Linxia Basin. The presence of well-developed floodplain paleosols, the color change of the fluviolacustrine sediments and the transition of large bodied mammals suggest that strong climatic and environmental changes happened in the Linxia Basin during late - middle Miocene, in the form of a transition from the warm and humid forest to the warm and subarid steppe at about 11.1 Ma.