Fossil Fishes of Ashiya Group –(I). Late Oligocene Elasmobranchs from Islands of Ainoshima and Kaijima, Kitakyushu

Abstract

On the islands of Ainoshima and Kaijima, which are located off the northern coast of the City of Kitakyushu, various vertebrate fossils have been collected including many shark teeth. The age of the bed is considered as the Late Oligocene. There we found at least 13 species of sharks and rays. They are Hexanchus sp. A, Hexanchus sp. B, Carcharodon angustidens, Isurus desori, Odontaspis sp., Cetorhinus sp., Carcharhinus spp., Negaprion sp., Galeocerdo sp., Galeorhinus sp., Squalus sp., Squatina sp., Myliobatis sp. Here we briefly described the specimens representing these species. Oligocene assemblages of elasmobranchs are meager in Japan and the shark fauna from Ainoshima and Kaijima are important to compare with shark faunas from various Miocene deposits which are common around Japan. Crowns of Hexanchus sp. A and H. sp. B, are comparable to H. griseus and H. vitulus, though Oligocene species have much elevated thick roots on the inner surface. Specimens of Carcharodon angustidens include well preserved specimens showing well developed tooth neck as in Carcharodon megalodon. Abundant teeth of Hexanchus which are found with sharks of the upper layer of oceans probably indicate that Hexanchus spp. lived in the upper layer in Oligocene, instead of deep water as in the present day.