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Palaeoecology of Akebiconcha kawamurai (Bivalvia: Vesicomyidae) from the Pliocene Tamari Silt Formation in the Kakegawa area, central Japan

1993; Elsevier BV; Volume: 102; Issue: 1-2 Linguagem: Inglês

10.1016/0031-0182(93)90003-2

1872-616X

Takami Nobuhara, Toshio Tanaka,

Aquatic Invertebrate Ecology and Behavior

The Japanese vesicomyid clam Akebiconcha kawamurai is taxonomically very close to Calyptogena species. Living specimens of A. kawamurai have been collected from bathyal depths but its ecology is not clear. A fossil Akebiconcha kawamurai assemblage is found in the uppermost part of the Tamari Silt Formation, Early Pliocene in age, cropping out in the Kakegawa area, central Japan. The assemblage occurs in several shell beds, scattered through the massive silt. The silt matrix between shell beds yields sporadic shells of another molluscan fossil assemblage, an “Empleconia” sp. A-Neilonella coix assemblage. The Akebiconcha kawamurai assemblage commonly is associated with Solemya johnsoni, Conchocele bisecta, and Lucinoma aff. acutilineata. These three genera prefer a reducing environment and possess endosymbiotic sulphide-oxidising bacteria. Therefore, the co-occurrence and the large biomass suggest that Akebiconcha kawamurai possesses such endosymbiotic bacteria as do many other Calyptogena species. By contrast, the species composition of the “Empleconia” sp. A-Neilonella coix assemblage indicates an oxic environment in the bathyal zone. We do not think that the ecosystem of the Akebiconcha assemblage was supported by cold-seepage water from a subduction zone, because the assemblage is enclosed by the massive fine silt. The mode of fossil occurrence and the living depth of Recent Akebiconcha kawamurai make us presume rather that conditions supporting Akebiconcha kawamurai colonies are a reducing mud bottom at bathyal hypoxic depths. Alternation between the reducing and oxic assemblages in the outcrop is likely to have been formed by fluctuation of the water mass controlling the dissolved oxygen in the water.