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

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