Invasion, Diversification and Extinction of Scapharca (Bivalvia: Arcidae) in the Japan Sea Borderland during the Plio-Pleistocene

Abstract

A semi-enclosed marginal sea such as the Japan Sea was an important locus for species diversification in the benthos, along with climate change. The genus Scapharca Gray, 1847 represents an example of such diversification after its first appearance in the Japan Sea in the Late Pliocene (Piacenzian) via the inflow of the Tsushima warm current. In this paper, S. satowi Dunker, 1882 is newly recorded and described from the uppermost Lower Pleistocene (Calabrian) Omma Formation in Ishikawa Prefecture. S. pseudosubcrenata (Ogasawara, 1977) is redescribed based on newly obtained specimens from the type locality and the Lower Pleistocene (Calabrian) Uonuma Group (Middle Formation) in Niigata Prefecture. By summarizing the geologic range of all Plio-Pleistocene species of Scapharca in the Japan Sea, it has been revealed that two latest Early Pleistocene glacial stages (MIS 22 and MIS 20) caused the extinction of S. ommaensis (Otuka, 1936) and S. pseudosubcrenata respectively. In the glacial-interglacial age of the Middle Pleistocene (Chibanian), S. akitaensis (Noda, 1966) appeared and S. broughtonii (Schrenck, 1867) also adapted itself to colder conditions. Four Recent species, S. satowi, S. broughtonii, S. kagoshimensis and S. akitaensis, also survived the glacial ages since the latest Early Pleistocene (Calabrian) because fossils occur on both the Japan Sea side and the Pacific Ocean side.