MORPHOMETRICS OF PSYCHOCIDARID ECHINOIDS SHOWING TRENDS IN SPINE MORPHOLOGY

Selective pressures can produce morphological trends that can be explored using geometric morphometric analyses to test for adaptive evolution. In echinoids, spine shape has been understood to be ecologically important and therefore is likely under significant selective pressures. Here we present preliminary data on trends and spine shape of the cidaroid echinoid Family Psychocidaridae. We utilized landmark analysis with 4 anatomical landmarks and 50 semilandmarks to quantify the outline of 44 psychocidarid spines representing 9 species of both fossil and extant forms. We find morphological variability in spine shape within individuals and this variability manifests across PC1. Along PC2, we find delineation of Jurassic to Danian forms and post-Danian forms. This suggests a turnover in spine morphology in response to some early Paleogene biotic or abiotic event. In terms of environmental change, post-Cretaceous climatic instability may have played a role in psychocidarid evolution as these echinoids tend to occupy shallow bioherm environments and are therefore susceptible to thermal events. Radiation of several important echinoid predator groups, such as durophagus crustaceans, fish, and predatory gastropods, may have also directed change in psychocidarid spine shape. Further work is needed to identify specific biotic and abiotic drivers and future directions for this are an expanded taxon list and specimen count.