Evolutionary–phylogenetic pathway of the Cretaceous ammonite genus<i>Aegocrioceras</i>and its relationship to<i>Juddiceras</i>spp. and<i>Crioceratites</i>spp.

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