Dimorphism of late Cretaceous false-puzosiine ammonites, Yokoyamaoceras Wright and Matsumoto, 1954 and Neopuzosia Matsumoto, 1954

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. A dimorphic lineage can be traced back to certain upper Turonian-Coniacian Kossmaticeras <M> and Yokoyamaoceras <m> species. The superficial morphological resemblance between the species and Mesopuzosia spp. is mere homeomorphy. -from Author