Taxonomic reassessment of a large fossil trionychid turtle from the Miocene Bihoku Group, Miyoshi City, Hiroshima Prefecture, Japan

Akio Takahashi, Ryunosuke Goto, Tomomi  Kiyoshi, and Takanori Miyanaga

The Japan-Taiwan Joint Conference on Herpetology

Herpetological Society of Japan

Taipei Zoo

https://herpetology.jp/meeting/2025/HSJ64Joint_abstract.pdf

A large trionychid turtle with a carapace length of about 65 cm has been reported from the Miocene Bihoku Group in Kimita, Miyoshi City, Hiroshima Prefecture, western Japan. This specimen was described as a new species, Trionyx ishiharaensis, in 1984, based on comparisons of carapace size and morphology with two other trionychid fossils known from the Lower Miocene of Japan. However, the original description did not provide comparisons using appropriate taxonomic characters or diagnostic features, and it also contained clear errors in the identification of carapacial elements, thus necessitating a re-evaluation of its taxonomic attribution. In addition, although many species of trionychids were once placed in Trionyx, subsequent taxonomic studies since the 1980s have divided the group into several genera, leaving the paleobiological significance of this fossil inadequately assessed. In this study, we removed sediments covering the ventral side of the specimen and re-examined its taxonomic position mainly on the basis of currently recognized diagnostic characters. The results indicate that the Kimita specimen can be referred to the genus Rafetus, as it exhibits features such as a strongly reduced eighth costal and rectangular fifth and sixth neurals. Compared with the two extant species of Rafetus and a fossil species from the Lower Miocene of the Czech Republic, the Kimita specimen differs in having a notched anterior margin of the carapace and a relatively broader shell. These findings suggest that an undescribed species of Rafetus occurred in western Honshu during the Middle Miocene.