The ecological sequence of Hipparion fauna in the mid-dle reaches of the Yellow River during Neogene

The faunal ecological type and sequence in the Tertiary red clay indicate that Hipparion fauna during the period 7.4-6.1 Ma in the middle reaches of the Yellow River is composed mainly of dorcadoides and mixed types. This represents semiarid veld environment with small fluctuation. The fossil assemblages characterized by the abundance of gaudryi types and even the occurrence of some Hominoid and Chalicotherium indicate an obviously warm and humid period since 5.3 Ma. The fossil assemblages during 3.4 Ma show the typical steppe fauna representing dry climate. It is indicated by the changes of ecological sequence that the climatic conditions in Pliocene are drier than that in late Miocene. The clay-skin content of red clay, which has a synchronous change with faunal features, is a rather good replacing palaeoclimatic index.