Abstract
Two specimens of desmostylian fossils have been recently discovered in the Early-Middle Miocene Yatsuo Group in Toyama Prefecture, Central Japan. One is a right lower fourth premolar of Paleoparadoxia tabatai discovered in the uppermost part of the Kurosedani Formation in the Yatsuo area. The other is a right upper second molar of Desmostylus japonicus discovered in the Tsubono Formation in the Namerikawa-Uozu area. It is possible to discuss the paleoenvironment where they inhabited, because both specimens were found associated with abundant molluscan fossils. Furthermore, as a result of studies based on microfossils, the age assignments when two species inhabited have been made clear. According to previous biochronological studies, the Kurosedani Formation yielded the Paleoparadoxia is allocated into 16.5-16.0 Ma, and its molluscan fauna suggests that the Kurosedani Formation was deposited under tropical to subtropical climate. This conclusion is in accordance with previous works. On the other hand, the Tsubono Formation yielded the Desmostylus, stratigraphically overlies the Fukuhira Formation correlated with the Kurosedani Formation and is assignable to 16.0-15.5 Ma, and its mollucan fauna indicates that the Tsubono Formation was deposited under the climatic condition changing from tropical-subtropical to warm-temperrate condition. In general, it has been proposed that Desmostylus were distributed from temperate to subarctic zones. But, from the result of this study, it may be inferred that Desmostylus could inhabit under warmer condition.
