The Middle Eocene Arshanto fauna (Mammalia) of Inner Mongolia

Abstract

Review of the Eocene deposits in Inner Mongolia has shown that the classic "Arshanto Formation" forms the lower beds of the Irdin Manha Formation. Mammalian faunas from the Arshanto beds as well as from the overlying Irdin Manha beds are middle Eocene in age, roughly equivalent to the early and late Bridgerian, respectively. Two other units, formerly included in the "Arshanto Formation" are the Lower Eocene Bayan Ulan beds and the Upper Paleocene Nomogen beds, both here referred to the Nomogen Formation. Comparisons between the Inner Mongolian Arshanto fauna and the approximately time equivalent faunas of eastern and southern Asia suggest the existence of two Asiatic paleobiogeographic districts during the middle Eocene.