A new species of <i>Eubostrychoceras</i> (Ammonoidea, Nostoceratidae) from the Santonian (Upper Cretaceous) of Hokkaido, Japan

A new species of nostoceratid ammonoid, Eubostrychoceras perplexum sp. nov., is described from the Santonian (Upper Cretaceous) in the Tappu, Kotanbetsu, and Haboro areas of Hokkaido, northern Japan. The diagnostic features of E. perplexum sp. nov. are tightly coiled, slowly enlarging whorls with a small apical angle, a small umbilicus, and a rounded rectangular whorl section. In addition to Hokkaido, the new species is also recorded from Wakayama, southwestern Japan. Given that the occurrence of this new species might be limited to the uppermost Santonian, it may thus serve as a useful indicator of stratigraphic correlation in the northwestern Pacific region. Although the new species cannot be assumed to have a direct phylogenetic relationship with any of the described Eubostrychoceras species, Hyphantoceras orientale (Yabe) is established to have multiple characters in common with E. perplexum sp. nov., and both species occur sympatrically in the upper Santonian. However, as E. perplexum sp. nov. lacks rows of tubercles, it does not satisfy the definition of the genus Hyphantoceras and accordingly cannot be placed within this genus. Nonetheless, the plausibility of a phylogenetic relationship between the two species cannot be ruled out. This contradiction between taxonomic evaluation and phylogenetic estimation raises questions regarding the current taxonomic criteria used to classify genera within the family Nostoceratidae.