“PSYCHO” ECHINOID SPINES – EVOLUTION THROUGH SPINE MORPHOSPACE IN CROWN GROUP CIDAROID ECHINOIDS

Crown group cidaroid echinoids display a variety of spine morphologies and ornamentation thought to have developed as adaptive evolutionary responses to selective pressures such as predation, commensalism, or shelter availability. As spines tend to interact directly with the echinoid’s environment, it stands to reason that spine shape is ecologically meaningful, and that the diversity of spines shapes observed across the fossil record of crown group echinoids tracks the ecological evolution of the group. To quantify morphological trends in cidaroid echinoid spine shape, we implement 2D geometric morphometric analysis of spine outlines to track the occupation of spine morphospace through time and across cidaroid echinoid clades. Four anatomical landmarks and 250 semi-landmarks were used to capture complexly shaped cidaroid spine outlines in the basal Psychocidaridae with a fossil record spanning the Jurassic to Holocene. Variability in the extent of morphospace occupation can be seen between forms during the Jurassic-Cretaceous, Danian, and post-Danian taxa, with Danian taxa exhibiting the least shape variability. Extinction of taxa and changes in selective pressures following the K-Pg mass extinction, such as predation and environmental stability, are likely the causes of this constriction of spine shape variability. The single extant psychocidarid taxon included in this study displays ornamentation most similar to other living cidaroids, like the distantly related and derived Histocidaridae, suggesting a convergence and constriction of spine shapes in the recent evolutionary history of the Cidaroida. Further work is necessary to elucidate the likely selective pressures that have resulted in these trends in cidaroid spine shape.