Abstract
A large fossil frog was discovered along with many fish and plant fossils from the Lower Pleistocene deposits of the Katanoyama Member of the Masuda Formation at Katanoyama, in the southwestern part of Nishino-omote City in Tanega-shima of Osumi Islands, South Kyushu, Japan. Comparative morphological study of the fossil frog was conducted with skeletal specimens of living frogs (73 individuals belonging to 17 species of 6 genera) from Kyushu and the Nansei Islands, in order to clarify the taxonomic position of the fossil frog. As a result of this study, the fossil frog from the Katanoyama Member was identified with Ishikawa's frog Rana ishikawae presently endemic to Okinawa and Amami Oshima Islands of the Nansei Islands, based on morphologic characters such as ilium and omosternum. It is estimated by age measurement of its pumice bed that the Katanoyama Member deposited at 1.3±0.2 F.T. Ma. It is geologically evident that there was a remarkable regression around the Nansei Islands from the end of sedimentation of the Shimajiri Group in the latest Pliocene until the beginning of the Ryukyu Group in the Early Pleistocene. These facts indicate that land animals including the ancestor of Ishikawa's frog could have migrated to Osumi Islands from Amami Oshima Island over a land bridge in the latest Pliocene.
