Japan abstract The mammalian fauna and geologic age of the Iwaki Formation of the Paleogene Shiramizu Group Iwaki, southern Fukushima, northeastern Japan are reviewed and previously undescribed specimens are described.
The Iwaki mammalian fauna consists of three artiodactyl species: Bothriogenys sp. Notomeryx sp. These three genera have never co-occurred in a single formation, although in Asia they have been recorded only in the late Eocene. In Asia, Bothriogenys has been recorded in the southern and middle regions, Entelodon has been mostly recorded in the northern and middle regions with one exception from the southern region, and Notomeryx has been recorded in the southern region. The co-occurrence of these three genera in the Iwaki Formation implies that Bothriogenys,Entelodon, and per- haps also Notomeryx can be useful late Eocene indicators in terrestrial eastern Asia.
It also suggests that the Iwaki mammalian fauna is paleobiogeographically located between the northern and southern late Eocene faunas of eastern Asia, showing some faunal mixture. The Iwaki fauna is also unique in comprising diverse faunas of marine sharks and seashore birds together with terrestrial mammals.
The Iwaki vertebrate fauna is key for reconstructing the faunas of the eastern coastal margin of the Asian Continent during the late Eocene. All rights reserved. Introduction Paleogene land mammals have rarely been found in the Japanese Islands Tomida, ; Miyata et al. These recent studies have indicated that Japanese Paleogene land mammals contribute greatly to the age determination of terrestrial deposits in Japan and also to our understanding of the mammalian biochronology and evolution in continental Asia because the Japanese Islands were part of the eastern coastal margin of the Asian Continent until the middle Miocene Taira et al.
The Iwaki Formation contains coal seams and has yielded plant, molluscan, and many vertebrate fos- sils, but its geologic age has been determined only by a few pieces of evidence and old interpretation Takai, ; Tomida, ; Yanagisawa et al. E-mail address: tsubamoto sci. The present article describes the anthracotheriid and ruminant specimens reported by Koda and discusses the geologic age of the Iwaki Formation and the paleobiogeography of the Iwaki mammalian fauna. In particular, the anthracothere contributes greatly to the age determination of the Iwaki Formation and to the mammalian paleobiogeographical history in the Paleogene of the Asian Continent because anthracotheres are considered to be useful for the study of terrestrial biochronology and paleobiogeography from the middle Eocene to the Pliocene Pickford, , ; Rasmussen et al.
