Taxonomy and Distribution of Cyclocardia (Bivalvia: Carditidae) of the Plio-Pleistocene Omma-Manganji Fauna in the Japan Sea Borderland

Abstract

In the Plio-Pleistocene Omma-Manganji fauna, the carditid genus Cyclocardia comprises four extant species: Cyclocardia (Cyclocardia) rotunda (Tokunaga, 1906), C. (Cy.) isaotakii (Tiba, 1972), C. (Cy.) crebricostata (Krause, 1885), C. (Crassicardia) crassidens (Broderip & Sowerby, 1829), and one extinct species, C. (Cy.) myogadaniensis (Itoigawa, 1958). Among them, C. (Cy.) rotunda is newly designated for the species formerly identified as "C. (Cy.) ferruginea (Clessin, 1888)". This common species likely first appeared in the Early Pliocene in Japan. However, C. (Cy.) rotunda became part of the Omma-Manganji fauna in the Late Pliocene. The extinct species, C. (Cy.) myogadaniensis, first appeared in the Late Pliocene. The other three extant species, C. (Cy.) isaotakii, C. (Cy.) crebricostata and C. (Cr.) crassidens, initially appeared in the Miocene of Kamchatka or Sakhalin and became part of the Omma-Manganji fauna after the climate cooling at Datum A (2.75 Ma). These three species in the Omma-Manganji fauna are smaller than the largest individuals in the present-day fauna, likely due to the warm temperature in the southern limit area.