THE ROLE OF SUBSTRATES IN THE EARLY DIVERSIFICATION OF ECHINODERMS

Current data on echinoderm diversity suggest that the group originated in Cambrian Stage 3, reached a major peak in diversity during Stage 5, and declined in diversity by the end of the period. The Cambrian substrate revolution (CSR) has been considered by many authors as one of the main drivers of echinoderm diversification throughout the Cambrian. According to this hypothesis, most early Cambrian forms attached directly to the substrate (sediment attacher), or introduced the aboral part into a sediment that was stabilized by microbial activity (sediment sticker); after the early part of the Cambrian, hard substrate attachers were thought to have dominated. This scenario was founded chiefly on the study of three fossil groups: eocrinoids, edrioasteroids and helicoplacoids. To evaluate this model, we analyzed the mode of life of all known Cambrian echinoderms, encompassing 190 species and nine major groups (cinctans, ctenocystoids, stylophorans, solutans, eocrinoids, early rhombiferans, edrioasteroids, helicoplacoids, helicocystoids). This study suggests that many important details on the anatomy and mode of attachment in early echinoderms were previously overlooked. Most sessile Cambrian forms were already attached to hard substrates, like shell fragments, in the early Cambrian, or had aboral parts with morphological adaptations that are suggestive of such a strategy. Nevertheless, the CSR had an important impact on Cambrian ecosystems, creating a mixed layer that was exploited by many early metazoans. Cambrian echinoderms were not an exception, and the analysis of feeding strategies during the Cambrian suggests a considerable increase of detritus feeders adapted to exploit these new resources. Other extrinsic/intrinsic factors like the appearance of hard grounds in proximal and high energetic settings and the development of anatomical novelties are additional factors that had a major impact on the diversification of early echinoderms.