Paleogene mammals from the Iwaki Formation in Japan: Their implications for the geologic age and paleobiogeography of this formation

摘要

The mammalian fauna and geologic age of the lwaki Formation of the Paleogene Shiramizu Group (lwaki, southern Fukushima, northeastern Japan) are reviewed and previously undescribed specimens are described. The Maki mammalian fauna consists of three artiodactyl species: Bothriogenys sp. cf. B. hui (Anthracotheriidae), Entelodon gobiensis (Entelodontidae), and cf. Notomeryx sp. (Ruminantia). These three genera indicate an Ergilian Asian Land Mamma Age (=late Eocene [Priabonian] equivalent) correlation for the Iwaki Formation, demonstrating that the Eocene-Oligocene boundary exists within the Shiramizu Group. 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 lwaki Formation implies that Bothriogenys, Entelodon, and perhaps also Notommyx can be useful late Eocene indicators in terrestrial eastern Asia. It also suggests that the lwaki mammalian fauna is paleobiogeographically located between the northern and southern late Eocene faunas of eastern Asia, showing some faunal mixture. The lwaki fauna is also unique in comprising diverse faunas of marine sharks and seashore birds together with terrestrial mammals. The lwaki vertebrate fauna is key for reconstructing the faunas of the eastern coastal margin of the Asian Continent during the late Eocene. (C) 2015 Elsevier Ltd. All rights reserved.