Reassessment of the late Campanian (Late Cretaceous) heteromorph ammonite fauna from Hornby Island, British Columbia, with implications for the taxonomy of the Diplomoceratidae and Nostoceratidae
Three heteromorph ammonite families are represented within upper Campanian (Upper Cretaceous) strata of the Northumberland Formation exposed on Hornby Island, British Columbia: Baculitidae, Diplomoceratidae and Nostoceratidae. A variety of species are distinguished within these families, of which only three taxa – Baculites occidentalis Meek, 1862, Diplomoceras (Diplomoceras) cylindraceum (Defrance, 1816) and Nostoceras (Nostoceras) hornbyense (Whiteaves, 1895) – have been reported previously. Over the last decade, large new collections and the further preparation of existing collections has provided new taxonomic and morphometric data for the Hornby Island ammonite fauna, from which new descriptions of heteromorph taxa are formulated. Eleven taxa are recognized, including the new species Exiteloceras (Exiteloceras) densicostatum sp. nov., Nostoceras (Didymoceras?) adrotans sp. nov. and Solenoceras exornatus sp. nov. Morphometric analyses of over 700 specimens demonstrate the considerable phenotypic plasticity of these ammonites, which exhibit a broad spectrum of variability in their ornamentation and shell dimensions. A large population sample of Nostoceras (Nostoceras) hornbyense provides an excellent case study of a member of the Nostoceratidae; the recovery of nearly complete, well-preserved specimens enables the re-evaluation of diagnostic traits within the genus Nostoceras. The north-east Pacific Nostoceras (Nostoceras) hornbyense Zone and the global Nostoceras (Nostoceras) hyatti Assemblage Zone are here regarded as correlative, reinforcing a late Campanian age for the Northumberland Formation.