Abstract

The evolutionary history of the major boring-bivalve superfamily Pholadoidea remains unclear. Opertochasma somaensis n. sp., preserved in situ in fossilized wood from upper Kimmeridgian to lower Tithonian strata within the Upper Jurassic shallow marine Nakanosawa Formation, northeast Japan, is described. This new species represents one of the oldest body fossils of pholadoidean boring-bivalves and the first fossil record from the Jurassic in the circum-Pacific regions. The authochthonous occurrence in fossilized wood, the presence of microscopic file-like sculpture on the anterior shell slope, the short clavate burrow, and other shell features demonstrate that O. somaensis n. sp. was a filter-feeding, obligate wood-borer with anterior-boring locomotion, and also that the complex shell morphology unique to Pholadoidea was established by the Late Jurassic. The documentation of the new species and taxonomic review on the Jurassic species previously described, show that the superfamily Pholadoidea was widely distributed along the Northern Hemisphere middle latitudes in the latest Jurassic. The Pholadoidea likely evolved by exploiting woody substrata which had become abundant during the Late Jurassic with floral diversification in the middle latitudes. It is notable that the xylophagous mode of life, referable to modern Teredinidae and Xylophagaidae, was most probably established in the Jurassic and provided an important background for the establishment of chemosynthesis-based, sunken wood-associated communities.

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