白亜紀二枚貝Sphenoceramus(イノセラムス科)の殻彫刻の変異

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, cha...

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Bibliographic Details
Format: e - Departmental Bulletin Paper
Published in:静岡大学地球科学研究報告 26 1-15
Main Authors: 松田, 昌之, 生形, 貴男
Language:Japanese
Published: 静岡大学地球科学教室 2017-12-14
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Online Access:http://hdl.handle.net/10297/360
Description
Summary: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.