PHYLOGENY AND ECOLOGY OF THE AMMONITE SUPERFAMILY ACANTHOCERATOIDEA THROUGH THE CENOMANIAN AND TURONIAN

Environmental changes tied to global warming around the Cenomanian/Turonian boundary (C/T) resulted in biotic turnover within marine ecosystems. Phylogenetic analyses and a morphometric analysis were conducted to examine the impact of the C/T event on ammonoid cephalopods. The ammonoid superfamily Acanthoceratoidea was used as a proxy for ammonoids as a whole, focusing on genera found in the Western Interior Seaway of North America, including Texas. Three separate phylogenetic analyses were run using a dataset comprised of 50 discrete morphological characters coded for an exemplar species from each of 35 genera. These analyses recovered the families Acanthoceratidae, Collignoniceratidae, and Vascoceratidae. Coilopoceratidae was recovered as a subfamily of Vascoceratidae; Pseudotissotiidae was not recovered. Spathites, Calycoceras, Choffaticeras, Coilopoceras, and Hoplitoides are all reclassified as a part of Vascoceratidae. Neoptychites remains within the family Vascoceratidae, but is reclassified within the subfamily Coilopoceratinae. Neocardioceras and Hourcqia are both reassigned to Collignoniceratidae.

We used Westermann morphospace, a theoretical space that relates shell shape to mode of life, to understand the impact of the extinction on ammonoid ecology. We predicted a loss of planktonic and less active swimmers and an expansion of more mobile swimmers if ocean anoxia caused the extinction. Shell measurements (diameter, whorl heights and breadth, and umbilical diameter) were taken from 212 specimens from the Middle Cenomanian through Late Turonian. Shape ratios were calculated, normalized, and used to produce ternary diagrams representing Westermann morphospace for each substage. Across the C/T boundary, there was no loss of planktonic specimens but there was an expansion into more active modes of life. Additionally, overall morphological disparity increased, indicating that the extinction did not reduce the range of ammonoid modes of life. Interestingly, a drop in morphological disparity did occur across the Early/Middle Turonian boundary. All shifts in morphospace occupation were driven by members of Vascoceratidae. Other families were stable through time, suggesting Vascoceratidae was uniquely able to shift into novel modes of life in response to environmental change.