Habitat Reconstruction of Oligocene Elasmobranchs from Yamaga Formation, Ashiya Group, Western Japan

Abstract

Remains of 11 species of extinct elasmobranch were obtained from the Oligocene Yamaga Formation on Ainoshima Island in northern Kitakyushu City, Japan. More than 360 gill rakers belonging to a basking shark (Cetorhinus sp.) individual were collected from a stratigraphic horizon in the northern part of the island. This report represents the highest concentration of basking shark gill rakers in the fossil record of Japan.Based on sedimentary facies analysis combined with habitat data of extant sharks, habitats of Oligocene elasmobranchs were reconstructed. The result indicates that Oligocene sharks probably lived in an environment similar to that of extant sharks at the genus level. This does not support the previous hypotheses that some shark taxa (e.g., Hexanchus) existed within shallow environments during the Oligocene, and have since migrated into deeper water habitats. Analysis of other genera supports this hypothesis of stability in elasmobranch habitat preference, at least at the genus level, between the Oligocene and present day.