685. PHENOTYPIC SUBSTITUTION OF GAUDRYCERAS : A CRETACEOUS AMMONITE

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  • 685. 後期白亜紀アンモナイト Gaudryceras の表現型置換

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The mutual relations among three nominal species, Gaudryceras denseplicatum, G. intermedium and G. tenuiliratum from Japan and Sakhalien are examined. G. denseplicatum and G. intermedium are here explained as a sexual dimorphic pair from the facts that they coexist, they occur in the same stratigraphic range and a constant numerical ratio of the specimens is kept for the duration in addition to their morphogenetical resemblance. They should be degraded to G. denseplicatum var. denseplicatum and G. d. var. intermedium respectively. Some main characters of G. tenuiliratum are common to those of var. denseplicatum and var. intermedium. The numerical ratio of the specimens of G. tenuiliratum to those of G. denseplicatum var. deseplicatum+G. d. var. intermedium historically changes in the same pattern in Obira, Hokkaido and Naibuchi, Sakhalin. Three possible explanations on the relations of G. tenuiliratum with G. d. var. denseplicatum and G. d. var. intermedium are discussed and transient polymorphism seems to be the most reasonable. Then G. tenuiliratum is degraded to G. denseplicatum var. tenuiliratum.

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