Characteristics and geological age of Middle Permian brachiopod fauna from Ejin Banner and its vicinities, western Inner Mongolia

Abundant brachiopods have been recorded from the Middle Permian strata of Ejin Banner and its vicinities. This fauna, called Zhesi Fauna, is characterized by a mixed fauna composed of boreal cool-water elements and Tethyan warm-water elements. The brachiopod Spiriferella-Kochiproductus-Yakovlevia assemblage has been recognized from this fauna. In paleobiogeography, Ejin Banner and its vicinities were located in a transitional position between Boreal and Tethyan provinces, called Sino-Mongolia province. The fauna pattern in eastern Yine basin is different from that in western Yine basin. In the shallow-water Beishan of western Yine Basin, the brachiopod is dominated by large-sized and thick-shelled cool-water elements, with a higher abundance and a slightly lower diversity in association with a few bivalves and corals, while in the relatively deep-water Hangwula of eastern Yine basin, the brachiopod is dominated by relatively small-sized cosmopolitan warm-water elements, with a slightly lower abundance and a higher diversity in association with cool-water corals and abundant encrinites. This pattern indicates that the distribution of brachiopods is controlled not only by latitude and temperature but also by water depth. The geological age of Zhesi Fauna is assigned to the Roadian to Kufengian, based on an analysis of its origin, spatial distribution and associated fauna.