Taxonomic re-evaluation of the genus Canadoceras (Ammonoida) from the middle Campanian of northern Hokkaido, Japan

Abstract

A taxonomic re-evaluation of the genus Canadoceras Spath (Ammonoida, Pachydiscidae) was conducted based on a large sample collected from the Orannai Formation in the Soya area, northern Hokkaido, Japan. Morphometric analyses of 37 specimens from the middle Campanian (Upper Cretaceous) utilizing X-ray computed tomography and statistical tests revealed that four species described from the Sphenoceramus schmidti Zone in Hokkaido can be reduced to three valid taxa: C. yokoyamai (Jimbo), C. kossmati Matsumoto, and C. mysticum Matsumoto. Statistical analysis of relative umbilical size and relative whorl thickness identifies three distinct ontogenetic trajectories corresponding to these species. Notably, Canadoceras minimum Matsumoto and Miyauchi is herein regarded as a junior synonym of C. mysticum, as its holotype represents a juvenile stage within the variation range of the latter. The unique ontogenetic growth patterns observed in each species are established at very early stages, driven by an interplay between modifications in the whorl expansion rate and the underlying geometric constraints of shell construction. This suggests that modifications in these early growth modes were a primary factor in the diversification of the genus. These findings refine paleobiodiversity estimates for the Northwestern Pacific and provide a clearer framework for understanding the phylogenetic and paleobiogeographic history of the genus Canadoceras.