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The earliest Tertiary mammals from Japan have been found in the Akasaki Formation, Amakusa area in Kumamoto Prefecture, western Japan. The Akasaki Formation is meandering river deposits and is conformably overlain by the Fukuregi Formation which yields the early Middle Eocene nannofossils (Cp13a subzone, approximately 47.3-46.1 Ma).
The Akasaki mammalian fauna is represented by seven taxa belonging to the four orders : Tillodontia, Pantodonta, Perissodactyla, and Rodentia. They are three trogosines (Higotherium hypsodon, cf. Trogosus sp. A, and B), two coryphodontids (Asiocoryphodon cf. Conicus and Coryphodontidae gen. Et sp. Indet.), a primitive tapiroid (Orientolophus sp.), and an unidentified rodent. Higotherium hypsodon is one of the most derived tillodonts among the order, and inhabited East Asia together with Trogosus-like tillodonts (cf. Trogosus sp. A and B). Asiocoryphodon cf. Conicus is represented by nearly complete skull and mandible with extremely large 13/3 ; this derived 13/3 condition is probably an autapomorphy based on our reexamination of Coryphodontidae. Orientolophus sp., represented by a fragmentary left maxilla with DP4/ and M1/, closely resembles O. hengdongensis from the Early Eocene of China, but is sistringuished in absence of paraconule, metaconule, and posterior cingulum on the M1/.
Judging from the mammal fossils and the stratigraphic correlation with marine strata, the Akasaki Formation is most likely placed in the Early to early Middle Eocene. Although material from the formation are never plentiful, the mammalian fauna is very significant for corroborating the geochronology of the Eocene land mammals in East Asia.