GSA Connects 2024 Meeting in Anaheim, California

Paper No. 65-13
Presentation Time: 5:15 PM

THE SHIFTING MORPHOSPACE OF CIDAROID ECHINOID SPINES


WATTS, Jerrad, Geosciences Department, Baylor University, One Bear Place #97354, Baylor University, Waco, TX 76798 and PETSIOS, Elizabeth, Department of Geosciences, Baylor University, One Bear Place #97354, Waco, TX 76798

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. This was implemented across numerous cidaroid genera with a focus in the basal family, Psychocidaridae, with a fossil record spanning the Jurassic to Holocene. Variability in the extent of morphospace occupation of psychocidarid spines 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 genera Prionocidaris and Goniocidaris, suggesting a convergence and constriction of spine shapes in the recent evolutionary history of the Cidaroida. Alongside the diversification in morphologies is the diversification in ornamentation in which the extant pyschocidarid falls. Some ornamentation types are more commonly found on specific spine morphologies, displaying a potential connection between the two. Further work is necessary to elucidate the likely selective pressures that have resulted in these trends in cidaroid spine shape and the trends in spine ornamentation.