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在日本西部熊本县天草地区的赤崎组被发现。赤崎组为曲流河沉积，被福县组整合覆盖，形成中始新世早期纳米化石（Cp13a亚带，约47.3 ~ 46.1 Ma）。赤崎哺乳动物区系有7个类群，分属四目：齿齿亚目、齿齿亚目、齿趾亚目和齿目。它们是三种穴居动物（Higotherium hypsodon, cf. Trogosus sp. A, and B），两种棘齿动物（asiocoryphohodon cf. Conicus and Coryphodontidae gen. Et sp. Indet.），一种原始貘（Orientolophus sp.）和一种身份不明的啮齿动物。下齿高齿（Higotherium hypsodon）是该目中衍生最多的齿形动物之一，与穴居类齿形动物（Trogosus sp. A和B）一起生活在东亚。Asiocoryphodon cf. Conicus的特征是颅骨和下颌骨几乎完整，其大小为13/3；根据我们对棘齿科的重新检查，这种衍生出的13/3条件可能是自异形。东方蛇属（Orientolophus sp.）与中国早始新世的O. hengdongensis非常相似，但在M1/上缺少前小柄、后小柄和后扣带。从哺乳动物化石及与海相地层对比来看，赤崎组最有可能处于始新世早至中始新世早期。虽然来自该地层的材料并不丰富，但哺乳动物区系对东亚始新世陆生哺乳动物的年代学确证具有重要意义。

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.