An enigmatic ammonite population from the late Callovian of Montreuil-Bellay (Maine-et-Loire, France)

An enigmatic ammonite (Ammonoida, Ammonitida) population represented by ten immature specimens from the late Callovian (Pseudopeltoceras leckenbyi Horizon) of Montreuil-Bellay is documented in this work. No evident similarities can be found between the studied material and any previously described species overall, suggesting at first glance that this population represents a new taxon. However, a thorough literature review on ammonoid pathologies and a detailed investigation on the morphology and ornamentation of the specimens lead us to instead support the hypothesis that the studied population reflects abnormal specimens. More specifically, we suggest that the specimens described herein are genetically-induced abnormal (forma aegra circumdata) Taramelliceras taurimontanum Erni, 1934 specimens (or alternatively – although more unlikely – Granulochetoceras hungaricum Lóczy, 1915 specimens). In our opinion, additional material providing information on the adult morpho­logy of this remarkable and extremely rare ammonite assemblage is required before completely ruling out the hypothesis that this population represents a new taxon.