858. A SPHENODISCID AMMONITE ACQUIRED RARELY FROM THE CRETACEOUS OF JAPAN

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  • 858. 日本の白亜系から稀に得られたスフェノディスク科アンモナイト
  • 日本の白亜系から稀に得られたスフェノディスク科アンモナイト〔英文〕
  • ニホン ノ ハクアケイ カラ マレ ニ エラレタ スフェノディスクカ アンモ

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Abstract

This paper is the first record of a sphenodiscid ammonite from Japan. While ammonite faunas from the Upper Campanian and Maastrichtian strata of Japan have recently been under intensive investigation, Mr. Masahiko Takada fortunately found a sphenodiscid ammonite from the Zone of Pachydiscus awajiensis in the Izumi Group of Awaji Island and has kindly provided it through Mr. Masahiro Sato for our study. It is fairly large and characterized by the mediolateral and ventrolateral nodes, low fastigiate to nearly tabulate venter of the middle-aged whorl and a nearly smooth, adult body-chamber with the spirally extended mediolateral zone of inflation, shallowly concave spiral zone outside the inflation and broadly rounded venter. Although sutures are not well exposed, it represents a new species, which is allied to Libycoceras afikpoense Reyment but distinguished by its more numerous nodes and less acute venter of the phragmocone than those of that Reyment's species. Morozumi's tentative correlation of the Zone of P. awajiensis to the upper Upper Campanian is approved on the ground of this ammonite and restudied P. awajiensis itself.

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