Are the erected species in the ammonite genus Aegocrioceras correct?

Morphometric Data of Aegocrioceras spp. from the Clay Pit Resse (Bed 82), NW Germany

Despite the fact that heteromorph ammonites often show a large range of intraspecific variation, that variation has rarely been quantified. Here, we quantify the intraspecific variation and ontogeny of 85 newly collected specimens of the Hauterivian endemic ammonite genus Aegocrioceras. All specimens have been collected from concretions in the newly exposed bed 82 in the clay pit Resse in NW Germany (Lower Saxony Basin). Data for the conch morphology (diameter, whorl height, whorl interspace, rib spacing) have been collected in 45° steps using the new software CONCH. The CONCH software will be made officially available with this publication. All ammonites have been a priori identified as A. bicarinatum or A. semicinctum. An assessment of the intraspecific variation of this population demonstrates the presence of just one species, i.e., A. bicarinatum. Comparison with the type material of A. bicarinatum and A. semicinctum reveals large morphological overlap of different shell parameters and does not support a separation of both forms as independent species. The analyzed population of A. bicarinatum has a high variation in shell morphology including whorl interspace, umbilical width, and whorl expansion. Further, the changes in intraspecific variation through ontogeny have been quantified and the presence or absence of sexual dimorphism within the population is discussed.