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

在日本西部熊本县Amakusa地区的赤崎组中发现了来自日本最早的第三纪哺乳动物。赤崎组为曲流河沉积，并被产生中新世早期超微化石(Cp13a亚带，约47.3-46.1 Ma)的Fukuregi组整合。赤崎哺乳动物动物群由Tillodontia、Pantodonta、Perissodactyla和啮齿目4个目的7个分类群组成。它们是三种金鱼藻(Higtherium hysodon，cf.)特罗戈斯属(Trogous)A，和B)，两个冠齿目(Asiocoryphdon cf.Conicus和Coryphodontidae Gen.ET sp.Indet.)，一种原始的卷毛虫(Orientolopus sp.)，以及一种身份不明的啮齿动物。Higtherum hysodon是该目中最具衍生性的斜齿动物之一，并与类Trogous的斜齿动物一起栖息于东亚地区(参见：特罗戈斯属(Trogous)A和B)。Asiocoryphdon cf.圆锥表现为几乎完整的头骨和下颌骨，13/3非常大；这种13/3的情况可能是基于我们对冠齿科的重新检查而得出的同种异型。东方藻(Orientolopus sp.)与中国始新世早期的横断体十分相似，但M1/上无副锥体、后扣带和后扣带。从哺乳动物化石和与海相地层的地层对比来看，赤崎组最有可能发生在始新世早期至中新世早期。尽管地层中的物质从来都不是很丰富，但哺乳动物动物群对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.