Small mammal fossils are efficient indicators of the paleoclimate and are often used in biostratigraphy. However,
the Paleogene small mammal fossil record is very rarely identified in Japan. In this study, we report that a new
rodent assemblage from Kyushu, Japan, has been found in the Late Oligocene Fukui Formation, Sasebo Group, in
Nagasaki Prefecture. The fossiliferous layer is approximately 30 cm thick and contains transgressive lag deposit
comprising granule-sized mudstone fragments, which unconformably overlie the coal-bearing nonmarine shale of
the Sechibaru Formation of the Sasebo Group. The rodent specimens mainly comprise isolated teeth associated with
the fragmental skeletal remains of crocodiles, tortoises, and freshwater fish. The assemblage includes four taxa,
Cricetidae (Eucricetodon sp.), Dipodidae, Sciuridae, and Castoridae (Steneofiber sp.). This rodent fauna, including
abundantly occurring dipodids and cricetids and rare sciurids, resembles the Oligocene northern Xinjiang, Gansu
and Inner Mongolian faunae of China, and the Valley of Lake fauna of Mongolia. In the Late Oligocene, the Japanese
Islands were situated in the eastern margin of Asia and, at the time, the Sea of Japan had not yet been widened.
Hence, in the Late Oligocene, the Cricetidae- and Dipodidae-dominant faunae were extensively found in East Asia.
However, East Asian faunae are not associated with Castoridae. Furthermore, although Ctenodactylidae, Eomyidae,
and Gliridae are common taxa in the Eurasian Oligocene, they are not included in this rodent assemblage. It must
be considered that even though these faunal compositions are affected by biases of the depositional and taphonomic
settings of fossiliferous layers, the difference in faunal composition between this assemblage and other East Asian
Oligocene rodent faunae implies the difference in paleoenvironments between marginal areas and inner-continental
regions of East Asia.
