THE CONTRIBUTION OF CRINOIDS TO EARLY PALEOZOIC ECHINODERM MORPHOLOGIC DIVERSITY

Morphological diversity (disparity) is a powerful tool in deciphering macroevolutionary patterns. This metric can capture the appearance of new forms, changes within preexisting forms, as well the loss of features resulting from extinction. Capturing morphological patterns at higher taxonomic levels is particularly difficult because of the wide array of forms. However, exploring trends at the phylum-level allows the documentation of body plan evolution and the relative contributions of those body plans during the evolution of the entire clade. Echinoderms are a taxonomically and morphologically diverse phylum containing many distinctive groups. Within echinoderms, crinoids were the most successful Paleozoic group and although their morphologic diversity has been intensely studied, understanding their disparity in context of the phylum may yield a new perspective on their evolutionary history.

Approximately, 400 genera of Cambrian and Ordovician Echinoderms were coded using a novel character suite that encompasses the features found within the phylum. This data set was analyzed using principle coordinate analysis. Echinoderm disparity trends were examined through time showing a steady increase from their first appearance in Cambrian Series 2 through the Early Ordovician then leveling off. The partial disparity of crinoids was calculated to assess the relative contribution of crinoids to overall echinoderm disparity. During the Early Paleozoic there were 24 described classes of echinoderms, but crinoids contributed approximately a third of the morphologic diversity of the entire phylum. Furthermore, if crinoids were removed from the analysis then echinoderm disparity would decrease following the Cambrian such that the morphologic expansion in echinoderms during the Great Ordovician Biodiversity Event is largely the result of crinoid diversification and evolution.