Sakawa Bölgesindeki (Güneybatı Japonya) Titoniyen-Berriasiyen Yaşlı Torinosu Tipi Kireçtaşlarında Saptanan Epidiceras (Bivalvia, Hippuritoidea)

Excavated specimens of two primitive rudists, Epidiceras speciosum (Goldfuss) and Epidiceras guirandi (de Loriol), are systematically described for the first time from the Tithonian-Berriasian Torinosu-type limestones of the Torinosu Group in the Sakawa area, Central Shikoku, Southwest Japan. E. guirandi was previously known only from the Late Kimmeridgian Mediterranean Tethys, so this occurrence in Southwest Japan significantly extends the recorded biogeographical and stratigraphical distribution of the species. Moreover, the documentation of E. speciosum from the Torinosu-type limestones in Japan and the Bau Limestone in Sarawak, Borneo, indicate that this species was already widespread throughout the Tethyan Realm at that time. Such records of early rudists in the eastern Tethys and the western Pacific enhance our understanding of the early evolutionary history of rudists.