白亜紀二枚貝Sphenoceramus(イノセラムス科)の殻彫刻の変異

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  • ハクアキ 2マイガイ Sphenoceramus イノセラムスカ ノ カラ チョウコク ノ ヘンイ
  • Variation of shell sculpture in a Cretaceous bivalve Sphenoceramus(Inoceramidae)

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Abstract

Variation of the external shell sculpture in three species of Late Cretaceous inoceramid bivalve Sphenoceramus [S. naumanni (YOKOYAMA), S. orientalis (SOKOLOW) and S. schmidti (MICHAEL)] from the Upper Yezo Group in northern Hokkaido were studied ontogenetically. Three growth stages β, γ, and δ, characterized by the pattern of commarginal costae, were recognized in S. naumanni and S. schmidti, while the γ-stage is absent in S. orientalis. The size of the growth stage boundaries, the onset size of the divergent ribs or commarginal undulations of the shell, and the rib density in each growth stage were analyzed in a total of 252 specimens belonging to the three species. S. orientalis is distinguished by the early appearance of commarginal undulations of the shell in the β-stage. Large size and lack of divergent ribs in the β-stage is characteristic of S. naumanni. Low rib density in the δ-stage, occasionally found in S. schmidti, helps to identify younger individuals lacking the divergent ribs.

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