Evolutionary–phylogenetic pathway of the Cretaceous ammonite genusAegocriocerasand its relationship toJuddicerasspp. andCrioceratitesspp.

Abstract

AbstractThe systematics of ammonoids are complicated by their large degree of intraspecies variation, which complicates a stable validation of species.Aegocriocerasis a heteromorph ammonite from the Lower Saxony Basin in the Hauterivian Boreal, and a prime example of a genus with an unstable internal systematic and external relationship to other ammonoids. Here, we use quantitative morphometrics onAegocriocerasspecies from an assemblage collected in the Resse clay pit (north‐west Germany) to evaluate the systematics and phylogeny of this Cretaceous genus. We simplify the systematics of the genus into the three entitiesA. bicarinatum[m]–A. semicinctum[M] complex (which potentially containsA. quadratumas well),A. raricostatumandA. spathi. The most likely phylogeny coincides very well with the stratigraphic record of the species and implies anagenetic adaptations inA. raricostatumandA. spathiafter the origin of the species. It is most likely thatAegocriocerasderived from warm‐water adapted TethyanCrioceratitesspecies, and BorealCrioceratitesare potentially warm‐water adapted descendants of the cold‐water adaptedAegocriocerasbut may alternatively represent renewed Tethyan invasions. Our data imply that the success ofAegocriocerasagainst incumbent ammonites in the Boreal was rooted in abiotic change (Court Jester) processes due to its high adaptability, while it is more likely that selection within theAegocriocerasclade was based on biotic interaction (Red Queen) processes.