Heterocarinella, a new ammonoid genus: Turrilitidae survived into the Santonian (Late Cretaceous) in the northwestern Pacific realm

Abstract

A new genus and species of the family Turrilitidae, Heterocarinella japonica gen. et sp. nov., is described from the Santonian (Upper Cretaceous) of the Haboro and Kotanbetsu areas in Hokkaido, northern Japan. This taxon is characterized by a persistently turriconic shell, a siphuncle positioned near the upper edge of the flank, and a vertically asymmetrical keel at or below the middle flank of the whorls. Notably, the keel differentiates Heterocarinella japonica gen. et sp. nov. from all other genera belonging to the Turrilitidae. The family Turrilitidae has previously been considered to range from the middle Albian to the late Cenomanian. However, the occurrence of Heterocarinella japonica gen. et sp. nov. demonstrates that this family survived until at least the Santonian. In addition, Tridenticeras from the Turonian to Coniacian of Europe, which is often assigned to the Nostoceratidae, is here re-evaluated as more appropriately belonging to the Turrilitidae. The previous misclassification likely resulted from limited knowledge of the position of the siphuncle, an internal structure, and from the tendency to prioritise stratigraphic consistency over morphological characters in taxonomic assignments. The discovery of Heterocarinella japonica gen. et sp. nov. suggests that the Turrilitidae survived the Cenomanian–Turonian Oceanic Anoxic Event 2 (OAE-2) while progressively narrowing their palaeogeographic range, ultimately contracting only in the northwestern Pacific realm.

ZooBank registration: urn:lsid:zoobank.org:pub:7980761E-A428-4B6D-842B-82CD1F83BABB