Fish fossils from the Middle Pleistocene Saginota Formation of the Ihara Group, Fuji, Shizuoka, Japan

抄録

While the Middle Pleistocene (Chibanian) ichthyofauna in Japan has been reconstructed from only a few fossil records to date, three well-preserved, articulated marine fish fossils were newly found from the middle part (ca. 0.58–0.57 Ma) of the Middle Pleistocene Saginota Formation of the Ihara Group in Shizuoka, central Japan. The fossil assemblage from the Saginota Formation comprises two fish species (Lateolabrax cf. japonicus and Mugil sp.) and two mollusks (Musculista senhousia and Corbicula japonica). In one fossiliferous block, the fossil remains identified as Lateolabrax cf. japonicus are characterized by: (1) ctenoid scales with a subrectangular outline and rounded corners; (2) a dentary bifurcating posterodorsally with a swollen upper ramus; and (3) a preopercle bearing five or more spines along the ventral margin. The skeletal remains of the other specimens are identified as Mugil sp. based on the presence of: (1) a pelvic-fin formula of I,5; (2) a pectoral fin consisting of 16 soft rays; and (3) an opercle that is rounded-subtriangular, with a small process anterior to the articular grooves and posteriorly radiating striae. The burial conditions of the fossil specimens, combined with sedimentary-facies observations, indicate that the deposit of the Saginota Formation occurred in a semi-enclosed brackish setting with intermittent marine inflow and possibly with dysoxic or anoxic bottom waters. This marine environment is supported by the occurrence of extant fish genera, Lateolabrax and Mugil, primarily inhabiting inner-bay to brackish areas.