Revision of Four Species of Turbo (Marmarostoma) (Gastropoda: Turbinidae) from the Lower-Middle Miocene of Japan

Abstract

The turbinid gastropod species Turbo (Lunella) ozawai Otuka, 1938 was described based on two opercula from the lower to middle Miocene Korematsu Formation in Southwest Japan. This inadequate proposal of the new species without the shell character information has confused subsequent taxonomic and faunal studies on the early to middle Miocene species of Turbo in Japan. For the first time, we clarify the shell characters of T. (M.) ozawai based on the type series and newly obtained materials from the Korematsu Formation in the Shobara area, Hiroshima Prefecture, and the Yoshino Formation in the Tsuyama area, Okayama Prefecture, Southwest Japan. The clarification of the shell and opercular characters of T. (M.) ozawai has enabled a taxonomic revision of the previously described four nominal species from early to middle Miocene species of Turbo (Marmarostoma) from the Honshu Arc of the Japanese Islands. Consequently, these four species are reclassified into two species, T. (M.) ozawai and T. (M.) tochiyensis Kanno, 1958; T. (M.) parvuloides Nomura, 1940 and T. (M.) minoensis Itoigawa, 1960 are junior synonyms of T. (M.) ozawai and T. (M.) tochiyensis, respectively. Both species are rare examples of mollusks that thrived along the Honshu Arc throughout the Miocene Climatic Optimum (∼16.9 to 14.7 Ma). The species richness of Turbo (Marmarostoma) is similar to that of the modern species in the warm water region of the Japanese Islands.