New and revised heteromorph ammonites (Family Nostoceratidae) from the Santonian–Campanian (Upper Cretaceous) of the Nanaimo Group of Vancouver Island, British Columbia, Canada

Upper Cretaceous strata of the Nanaimo Group exposed on southeastern Vancouver Island, British Columbia, preserve a diverse assemblage of molluscan fauna, including a wide array of ammonites. This study focuses on five genera of heteromorph ammonites from predominantly Santonian and Campanian strata from the Family Nostoceratidae Hyatt, 1894, including: Eubostrychoceras Matsumoto, 1967, represented by Eu. elongatum (Whiteaves), Eu. indopacificum Matsumoto and Eu. otsukai (Yabe); Ainoceras Matsumoto and Kanie, 1967, represented by Ainoceras boomerangense sp. nov.; Amapondella Klinger and Kennedy, 1997, represented by Amapondella amapondense (van Hoepen) and Am. separoannulum sp. nov.; Limusoceras gen. nov. with species L. japonicum gen. et comb. nov. (Yabe), L. traski gen. et sp. nov. and L. neojaponicum gen. et sp. nov.; and Embreeoceras gen. nov. with species Em. retexum gen. et sp. nov. We also present the Base Whorl Axis Model to serve as a framework for establishing consistency in an approach to measuring from more complete nostoceratids. Of the five genera discussed, only Eubostrychoceras Matsumoto, 1967 and Amapondella Klinger and Kennedy, 1997 have been recorded previously from the Nanaimo Group. Observation of early ontogeny in the Eubostrychoceras type species, Eu. indopacificum , has necessitated an emended diagnosis for the genus. The taxonomic concept of Eu. elongatum is refined and clarified, and ambiguity surrounding the type locality is addressed. Lastly, we emend the Eubostrychoceras elongatum Zone of the eastern North Pacific region to the Eubostrychoceras elongatum Assemblage Zone within a well-constrained interval preceding the appearance of the crinoid Marsupites in the latest Santonian. • Two new genera and five new species are described • The diagnosis of Eubostrychoceras is emended • A new Eubostrychoceras elongatum assemblage biozone is erected • A standardized model for nostoceratid measurement is introduced