野尻湖層産カワニナ胎児殼化石について : 現生カワニナとの比較研究 On the fossil embryonic shell of Semisulcospira libertina (GOULD) (Mesogastropoda: Pleuroceridae) from the latest Pleistocene Nojiri-ko Formation, Nagano Prefecture, Central Japan : A comparative study of recent and fossil Semisulcospira
The fossil embryonic shells of the genus Semisulcospira (Mesogastropoda: Pleuroceridae) had been obtained from the latest Pleistocene Nojiri-ko Formation, lacustrine deposits, seasonally exposed on the bottom of Lake Nojiri, Nagano Prefecture, Central Japan. The fossil-bearing horizon is the upper part of Section H of the Middle Nojiri-ko Formation and the age is approximately estimated at 27,000 y. B. P. by the ^<14>C method. The fossil embryonic shells preserved in a mature female whorl are 12, a well-preserved one of which is 0.87mm in height and 0.76mm in width; the sculpture consists of spiral and growth lines or striae; the body whorl has a round periphery. By comparison with the embryonic shell of the Recent Japanese Semisulcospira libertina group including S. libertina (Goutn), S. reiniana (BROT), S. kurodai KAJIYAMA et HABE, and S. trachea (WESTERLUND), this fossil embryonic shell is identified with S. libertina (GOULD) which is widely distributed over the Japanese Islands, eastern area of China, Formosa, and southern area of Korea. Also the fossil that the embryonic shell ratio height/width is 1.14 was inferred to be 2.25 whorl stage in a brood pouch of the female by statistical analysis. From the occurrence of S. libertina (GOULD) from the Nojiri-ko Formation, this species shows to have bred in Lake Nojiri of the latest Pleistocene, and the temperature conditions necessary for S. libertina (GOULD) to breed can be beduced from the annual birth behavior of S. cancellata (BENSON) and S. libertina (GOULD). It is assumed that the population of Nojiri-ko libertina in the last glacial period corresponds to a population of higher latitude than that of Lake Nojiri in present day. Furthermore, we discuss on the ecology of S. libertina (GOULD) that helps to know the paleoenvironment of Lake Nojiri in this age.