UNCOVERING THE DEVELOPMENT OF CAMBRIAN CINCTAN ECHINODERMS

Modern echinoderms undergo a radical transformation during their development, metamorphosing from a bilaterally symmetrical larva to a pentaradiate adult with five ambulacral areas. In contrast, several lower Palaeozoic groups, including ctenocystoids and cinctans, apparently developed quite differently, with no expression of radiality in adults. These taxa are best considered as stem-group echinoderms, exhibiting some, but not all, of the synapomorphies shared by extant species; they are, therefore, of critical importance for reconstructing developmental evolution during the assembly of the echinoderm body plan. Here, we elucidate the postlarval development of cinctans based on a fossilized ontogenetic series of Graciacystis ambigua. In addition, we use computational fluid dynamics to rigorously analyse the functional performance of different ontogenetic stages for this species. As an adult, Graciacystis possessed an asymmetrical pair of marginal food grooves. Study of juveniles revealed that the relative size of these grooves did not change appreciably in later ontogenetic development, suggesting that this asymmetry was established early in ontogeny, perhaps during metamorphosis. The computer simulations demonstrated that juveniles generated less drag than adults, indicating they were more stable on the sediment surface; however, they were also characterized by reduced recirculation downstream of the mouth, meaning that juveniles were not able to capture as much food as adults. Analysing these data in a phylogenetic context confirms that developmental asymmetry was established close to the base of the echinoderm tree.