Tanabeceras horokanaiense sp. nov. is described from the lowest Cenomanian (Upper Cretaceous) in the Horokanai area, Hokkaido, northern Japan. This new taxon, which represents an intermediate form between T. yezoense (Shigeta, 1996) and T. mikasaense (Shigeta, 1996), suggests that Tanabeceras evolved and radiated in the Northwest Pacific realm during the early Cenomanian. This evidence together with the presence of many endemic gaudryceratid ammonoids strongly suggests that there was something to separate distribution of the gaudryceratids between the Northwest Pacific realm and other regions during early Cenomanian time.
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Vol. 17 • No. 4