CEPHALOPOD DISPARITY OVER OAE 2

Changes in the biodiversity and morphological disparity of a clade have been used widely in paleontology to assess the group’s response to environmental change, its evolutionary potential, and its extinction risk. However, a more in-depth examination of different groups within the same time period is needed to better understand how different organisms responded to similar environmental stressors. We examined how the disparity of three Cretaceous ammonoid cephalopod clades (Acanthoceratidae, Baculitidae, and Desmoceratoidea) compared to each other along with how disparity changed over time, particularly through the environmental perturbations associated with Ocean Anoxic Event 2 at the Cenomanian-Turonian boundary. Specimens of each ammonoid group were examined and measured to assemble character matrices, including suture sinuosity and overall shell shape, and then a pairwise dissimilarity metric was computed for each group at varying time periods. Acanthoceratidae showed a higher disparity than both the baculitids and desmoceratoids. The three clades also showed differences in distribution of pairwise disparities, with the acanthoceratids clustering around the upper bounds of their range in disparities while the baculitids and desmoceratoids showed more of an even range over their distributions. Both baculitids and desmoceratoids increase in disparity over time, indicating that they do not quickly exhaust their available character spaces. On the other hand, acanthoceratids do not increase in disparity over time, rather staying relatively the same. This study shows that different clades of ammonoid cephalopods responded differently over a time of high environmental stress.