952. DIMORPHISM OF LATE CRETACEOUS FALSE-PUZOSIINE AMMONITES, YOKOYAMAOCERAS WRIGHT AND MATSUMOTO, 1954 AND NEOPUZOSIA MATSUMOTO, 1954

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  • 952. 白亜紀後期の偽プゾシア類アンモナイト, Yokoyamaoceras Wright and Matsumoto, 1954 および Neopuzosia Matsumoto, 1954 の二型現象
  • 白亜紀後期の偽プゾシア類アンモナイト,Yokoyamaoceras Wright and Matsumoto, 1954およびNeopuzosia Matsumoto, 1954の2型現象〔英文〕
  • ハクアキ コウキ ノ ギ プゾシアルイ アンモナイト Yokoyamaocer

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Yokoyamaoceras jimboi Matsumoto, previously assigned to the Kossmaticeratidae, and Neopuzosia ishikawai (Jimbo), N. japonica (Spath), and N. haboroensis Matsumoto and Inoma, previously assigned to the Puzosiinae, all co-occurring in Santonian to lower Campanian deposits of the Upper Cretaceous Yezo Group in Hokkaido, Japan, and Sakhalin, Russia, are reinvestigated on the basis of many well-preserved specimens. Observations on the stratigraphic occurrence, morphological variation, and shell ontogeny show these four nominal species to be dimorphs of but a single biospecies. The first two morphospecies represent the macroconch <M>, and the latter two microconch <m> for which the earlier available name is Yokoyamaoceras ishikawai (Jimbo, 1894), which is referred to the Kossmaticeratidae. This is suggested by the mode of ontogenetic shell growth, particularly of the umbilicus. A dimorphic lineage can be traced back to certain upper Turonian-Coniacian Koss-maticeras <M> and Yokoyamaoceras <m> species. The superficial morphological resemblance between the species and Mesopuzosia spp. is mere homeomorphy.

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