New Fossil Record of Two Warm-water Bivalves from the Pleistocene Itaya Formation, Toyama Prefecture, Central Japan

摘要

Two warm-water species, Pecten (Pecten) byoritsuensis Nomura, 1933 and Oxyperas (Oxyperas) bernardi (Pilsbry, 1904) have been obtained from the Lower Pleistocene Itaya Formation in Toyama Prefecture. Based on the fossil occurrences recorded by previous studies as well as this study, these species are rare and first appeared in the Early Pleistocene in the Japan Sea borderland. P. (P.) byoritsuensis became extinct at the end of the Early Pleistocene as a result of deteriorating environments associatedwith glacial periods in the Japan Sea. O. (O.) bernardi survived these conditions because of its wide distribution, as with Pecten (Notovola) naganumanus Yokoyama, 1920 and P. (N.) albicans (Schroter, 1802). Detailed examination of molluscan fossils from the Itaya Formation and their ages indicates that the Tagawa assemblage from the Lower Pleistocene Omma Formation in Oyabe City can be correlated with the Itaya assemblage, not with the Zukawa.