Paleogene strata in the Huheboerhe area,Erlian Basin,Inner Mongolia,China have been subdivided into three lithological units:the Nomogen Formation,the Arshanto Formation,and the Irdin Manha Formation.At least 12 mammal-bearing horizons have been recognized in these formations,of which 4 in the Nomogen Formation,6 in the Arshanto Formation,and 2 in the Irdin Manha Formation.Recent investigation proved that the “Houldjin Formation” recognized in this area by the Central Asiatic Expeditions(CAE) of the American Museum Natural History in the 1920s is actually the Irdin Manha Formation,while the “Irdin Manha Formation” of CAE is the Arshanto Formation.A recent paleomagnetic study suggests that the upper part of the Nomogen Formation is early Eocene in age and the Arshanto Formation is mainly early Eocene rather than middle Eocene as previously thought.The Gashatan,Bumbanian,and Arshantan land mammal ages are correlated respectively to the Thanetian,the early Ypresian,and the middle Ypresian through earliest Lutetian of the Geological Time Scale.These land mammal ages are also correlated with the late Tiffanian through Clarkforkian,the early Wasatchian,and the middle-late Wasatchian and most of the Bridgerian of the North American Land Mammal Ages.During the early Paleogene,the mammalian history of the Erlian Basin was dominated by the appearance of new mammalian families and the replacement of a variety of genera and species,corresponding to the gradual climatic changes during this time period.The abrupt emergence of several modern mammalian orders at the beginning of the Eocene is probably related to extreme climatic warming in relation to the Paleocene-Eocene Thermal Maximum. Paleogene strata in the Huheboerhe area, Erlian Basin, Inner Mongolia, China have been subdivided into three lithological units: the Nomogen Formation, the Arshanto Formation, and the Irdin Manha Formation. At least 12 mammal-bearing horizons have been recognized in these formations, of which 4 in the Nomogen Formation, 6 in the Arshanto Formation, and 2 in the Irdin Manha Formation. Close investigation proved that the “Houldjin Formation” recognized in this area by the Central Asiatic Expeditions (CAE) of the American Museum Natural History in the 1920s is actually the Irdin Manha Formation, while the “Irdin Manha Formation” of CAE is the Arshanto Formation. A recent paleomagnetic study suggests that the upper part of the Nomogen Formation is early Eocene in age and the Arshanto Formation is mainly early Eocene rather than middle Eocene as previously thought.The Gashatan, Bumbanian, and Arshantan land mammal ages are correlated to the Thanetian, the early Ypresian, and the middle Ypre sian through earliest Lutetian of the Geological Time Scale. The land land ages are also correlated with the late Tiffanian through Clarkforkian, the early Wasatchian, and the middle-late Wasatchian and most of the Bridgerian of the North American Land Mammal Ages. During the early Paleogene, the mammalian history of the Erlian Basin was dominated by the appearance of new mammalian families and the replacement of a variety of genera and species, corresponding to the gradual climatic changes during this time period. The abrupt emergence of several modern mammalian orders at the beginning of the Eocene is probably related to extreme climatic warming in relation to the Paleocene-Eocene Thermal Maximum.