896. EVOLUTION OF SOME LATE CRETACEOUS DESMOCERATINE AMMONOIDS

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  • 896. 後期白亜紀デスモセラス亜科アンモナイト類の進化
  • 後期白亜紀デスモセラス亜科アンモナイト類の進化〔英文〕
  • コウキ ハクアキ デスモセラスアカ アンモナイトルイ ノ シンカ エイブン

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The Cretaceous sediments of Hokkaido accumulated in an island arc setting that was part of a plate convergence margin in the Northwest Pacific. Many differences have been recognized between such sediments and those of epicontinental seas, and there are also faunal differences. A land with a coastline extending to north and south laid to the west of the forearc basin in which lateral changes of lithofacies and biofacies occurred. The desmoceratine taxa Desmoceras (Pseudouhligella) ezoanum, Desmoceras (Pseudouhligella) japonicum, Tragodesmoceroides subcostatus and Tragodesmoceroides matsumotoi, n. sp. to be described below flourished in this forearc basin sea during Cenomanian and Turonian times. A phylogenetic progression from D. (P.) japonicum through T. subcostatus to T. matsumotoi is proposed, based upon investigations of relative growth, sutural ontogeny, shell ornamentation, stratigraphical ranges and geographical distribution. In this lineage the eastern limit of the habitat moved towards the west with decreasing adult size at each speciation event. The ontogeny of sutural complexity, however, gradually accelerated with time. The details of the evolution of this lineage and T. matsumotoi are described and discussed together with the relationship to such events as a marine transgression/regression, oceanic anoxic events and speciation/extinction.

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