892. SYSTEMATICS OF THE AMMONITE GENUS TETRAGONITES FROM THE UPPER CRETACEOUS OF HOKKAIDO

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  • 892. 北海道上部白亜系産 Tetragonites 属アンモナイトの分類
  • 北海道上部白亜系産Tetragonites属アンモナイトの分類〔英文〕
  • ホッカイドウ ジョウブ ハクアケイサン Tetragonitesゾク アンモナ

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Description

The ammonite genus Tetragonites (Lytoceratina, Lytocerataceae) from the Upper Cretaceous (Turonian to Maastrichtian) of Hokkaido includes three species : T. glabrus (Jimbo), T. minimus, n. sp. and T. terminus, n. sp., based on the shell size, apertural shape and early internal shell structure. Although the three species are similar in external shell shape, they are clearly distinguishable from one another on the basis of the above characters without any intermediate form. T. glabrus, occurring in the Middle Turonian to the Upper Campanian, exhibits remarkable variation in shell form, and the previously described species T. sphaeronotus (Jimbo), T. crassus (Jimbo), T. popetensis Yabe and T. glabrus problematicus Matsumoto should all be regarded as synonyms of this species. The specimens illustrated as T. superstes Van Hoepen and T. epigonus Kossmat from Hokkaido are also identified with this species. T. glabrus shows a chronocline in which the sizes of initial chamber and ammonitella decrease with time. T. minimus, n. sp., occurring from the Lower Turonian to the Upper Campanian, is much smaller in adult shell size (less than 30 mm in diameter) than T. glabrus, and shows also a chronocline of miniaturization in adult shell size and of narrower umbilicus toward later forms. T. terminus, n. sp., restricted to the Lower Maastrichtian, has an unusually large embryonic shell and a peculiar early internal shell structure.

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