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Dryad

Data from: Evolutionary–phylogenetic pathway of the Cretaceous ammonite genus Aegocrioceras and its relationship to Juddiceras spp. and Crioceratites spp.

Abstract

The systematics of ammonoids are complicated by their large degree of intra-specific variation, which complicates a stable validation of species. Aegocrioceras is 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 on Aegocrioceras species from an assemblage collected in the clay pit Resse (north-west Germany) to evaluate the systematics and phylogeny of this Cretaceous genus. We simplify the systematic of the genus into the three entities A. bicarinatum [m]/A. semicinctum [M] complex (which potentially contains A. quadratum as well), A. raricostatum and A. spathi. The most likely phylogeny coincides very well with the stratigraphic record of the species and implies anagenetic adaptations in A. raricostatum and A. spathi after the origin of the species. Aegocrioceras most likely derived from warm-water adapted Tethyan Crioceratites species, and Boreal Crioceratites are potentially warm-water adapted descendants of the cold-water adapted Aegocrioceras but may alternatively represent renewed Tethyan invasions. Our data imply that Aegocrioceras’ success against incumbent ammonites in the Boreal was rooted in abiotic change (Court Jester) processes due to its high adaptability, while selection within the Aegocrioceras clade was more likely based on biotic interaction (Red Queen) processes.