EXPLORING MORPHOLOGICAL DISPARITY IN THE CASSIDULOIDA (ECHINODERMATA, ECHINOIDEA) USING GEOMETRIC MORPHOMETRICS

The Cassiduloida are irregular echinoids with a relatively rich fossil record dating back to the Cretaceous and a paucity of modern representatives. Although globally distributed in the tropics and subtropics, most cassiduloid species have a small geographic range and live buried in coarse-grained sediment. Cassiduloids reached their peak of diversity in the Eocene and since then the group has been in decline. The mechanisms behind their diversity loss are not well understood, but several hypotheses have been proposed—one of them referring to the lack of morphological innovation within the group. Cassiduloids are well-known for their conserved morphology, with most of the variation among taxa being related to shape. Their shape variation, however, has never been quantified. In this study, we used 2D geometric morphometrics to quantify the morphological disparity within the cassiduloids and evaluate the relationship between shape, functional morphology and extinction risk. We collected images from over 400 specimens from digitized museum collections, vetted websites, published works, and private databases. The specimens belong to seven historically classified cassiduloid groups and stem lineages, ranging from the Late Jurassic to the recent and geographically distributed in the six continents. We digitized landmarks and semi-landmarks along the test margin and petals using tpsDig. Preliminary results show a significant shape variation among the groups analyzed. The highest overall disparity was found in the Neolampadoida, a group of cassiduloids that have the broadest bathymetric range and the highest number of living genera (n=6, all monotypic). Extreme areas of the morphospace were occupied by the Eurhodiidae (straight margins), the Echinolampadidae (narrow posterior region) and the Faujasiidae and the Echinolampadidae (round tests). Our analysis of disparity through time indicates a selective loss of disparity following the K-Pg extinction, when species with round tests went extinct. This area of the morphospace remained unoccupied until the Neogene, when echinolampadids evolved round tests. Test outline is thought to be related to burrowing ability, with elongated tests being more adapted to burrowing. Analysis of petal shape will yield results related to breathing efficiency in this group.