The Dongwan section is located near Dongwan Village,Qin'an County,Gansu Province (34°58′N,105°47′E).The section is a continuous eolian red clay sequence and is dated from 7.3 to 3.5 Ma by paleomagnetic study.The 74.8 m long section is divided into 21 layers(0 to 20)and 39 species of small mammals are found from 40 fossil levels.Based on faunal composition of small mammals,especially the myospalacines,four biozones are recognized. Biozone I covers layers 0~7,ranges magnetic polarities from upper part of C3Br...