Chemosynthetic Thraciid Bivalve Nipponothracia, gen. nov. from the Lower Cretaceous and Middle Miocene Mudstones in Japan

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  • 下部白亜系および中部中新統の泥岩より産した化学合成二枚貝・スエモノガイ科の新属 Nipponothracia
  • Chemosynthetic Thraciid Bivalve Nippono

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A new genus Nipponothracia (Bivalvia : Thraciidae) is erected on the basis of Thracidora gigantea Shikama, 1968, from the early Middle Miocene (15 Ma) claystone discovered in the central part of the Miura Peninsula. This new genus also includes N. ponbetsensis from the Early Cretaceous (early Late Albian, 106 Ma) siltstone in the central part of Hokkaido. 1) It is characterized by the following respects : huge, thick and convex shell ; right valve slightly larger than left one ; centrally situated, notched beak ; edentulousness ; non-truncated posterior end ; ellipsoid shell form. 2) Nipponothracia ponbetsensis n. sp. is associated with the species of Solemya, Calyptogena, Conchocele and Yoldia, which are regarded as the chemosynthetic molluscan community. The new locality, however, discovered Calyptogena, Solemya, Acharax, Conchocele, tube worms etc. nearby the place where N. gigantea was found without any record of associating fossils. 3) The modern species of Nipponothracia has not been reported. 4) Most of the bivalves are articulated. The mode of occurrences of mature specimens of Nipponothracia were lying in parallel or oblique to the bedding plane. Considering that their siphons could probably reach at the surface of the muddy sediments of sea bottom, though immature one oriented vertically, and the anterior side was downward like as Solemya and Calyptogena. 5) Thus, Nipponothracia species occurs in association with the species of Solemya, Calyptogena and Conchocele, which are symbiotic with bacteria in their bodies. Therefore, two present species of Nipponothracia are regarded as those of constituents of chemosynthetic fauna.

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