Systematic and evolutionary studies of the oldest known large mammalian fauna from Kumamoto, Japan

日本最早的第三纪哺乳动物发现于日本西部熊本县阿马库萨地区的赤崎组。赤崎组为曲流河沉积，与Fukuregi组整合覆盖，产出中始新世早期超微化石（Cp 13 a亚带，约47.3- 46.1Ma）。赤崎哺乳动物群由Tillodontia、Pantodonta、Perissodactyla和Rodentia四个目的7个分类群代表。它们是三个trogosines（Higotherium hypsodon，cf. Trogosus sp. A，and B），两种冠齿兽科（Asiocoryphodon cf. Conicus和Coryphodontidae gen. Et sp. Indet.），一种原始的貘类（Orientolophus sp.），和一只身份不明的啮齿动物Higotherium hypsodon是该目中最具衍生性的tillodonts之一，与Trogosus相似的tillodonts一起生活在东亚（参见。Trogosus sp. A和B）。无冠齿龙属锥齿兽的头骨和下颌骨几乎完整，13/3非常大;这种推导出的13/3条件可能是基于我们对Coryphodontidae的重新检查的自形性。Orientolophus sp.，以一个带有DP 4/和M1/的不完整的左上颌骨为代表，非常类似于O。hengdongensis，但在M1/上缺失副小锥、后小锥和后扣带，根据哺乳动物化石及与海相地层的对比，赤崎组最可能位于始新世早期至中始新世早期。尽管该地层的物质并不丰富，但哺乳动物群对于证实东亚始新世陆地哺乳动物的年代学具有重要意义。

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.