Late Jurassic-earliest Cretaceous rudists from the Torinosu-type limestones in Southwest Japan - preliminary report

A synoptic guide to rudist bivalves from the Late Jurassic-earliest Cretaceous Torinosu-type limestones is provided for future research. Three taxa of early rudists, Epidiceras speciosum, Epidiceras guirandi and Monopleura sp. are recorded from the Torinosu-type limestones in Southwest Japan. E. speciosum is recognized in all four areas: the Youra area in Kyushu Island, and the Shirokawa, Sakawa and Kahoku areas in Shikoku Island, encompassing a stratigraphical range of late Kimmeridgian-early Tithonian to Tithonian-Berriasian. E. guirandi is found in the Tithonian-Berriasian limestone bodies in the Sakawa area, and Monopleura sp. from the late Kimmeridgian-early Tithonian limestone blocks in the Shirokawa area. In the Kahoku area, E. speciosum occurs in oolitic facies, but in other localities, the rudists occur in micritic facies. In the Youra area, E. speciosum co-occurs with several species of corals. The rudist records of the Torinosu-type limestones extend the biogeographic and stratigraphic distributions of each of these species, and may also contribute to understanding of the early evolutionary history of rudists.