日本の中生代汽水生二枚貝化石群に関する研究の現状 : 天草地域の白亜系から産出する二枚貝の生息域とジュラ紀〜白亜紀マガキ類の古生態について(<特集>干潟の自然,その過去と現在)

概要

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Several issues in the methods of reconstructing paleoecology of the Mesozoic brackish-water bivalves, the contents of the bivalve assemblages and their paleoenvironments in Japan are discussed. The brackish-water environments can be reconstructed as shallow marine environments associated with fluvial systems on the basis of facies analysis. Paleoecology of bivalve fossils is inferred from their modes of occurrence in each depositional environment. Here, descriptions are given for two examples of paleoecologic analysis of the Mesozoic bivalve faunas : (1) Comparisons between the "mid"-Cretaceous Goshoura Group and the Upper Cretaceous Himenoura Group ; both groups are characterized by brackish-water-influenced tidal flat deposits containing abundant true oyster (Crassostrea). Infaunal bivalves of the "mid"-Cretaceous assemblage are characterized by neomiodontids, although those of the Upper Cretaceous assemblage are represented by venerids. (2) Another example is the taxonomy and paleoecology of the Upper Jurassic to Cretaceous primitive Crassostrea. These oysters are ecologically similar to recent Crassostrea, but compared with recent species, shell sizes of the Upper Jurassic and Lower Cretaceous materials are small. Their shell structures are characterized by poorly developed chambers and chalky deposits, which suggest more archaic conditions.
日本古生物学会の論文
2004-09-22