NEW PROSPECTS ON THE DEEP EVOLUTIONARY HISTORY OF STALKED ECHINODERMS

Echinoderms represent the only calcareous branch of deuterostomes and have a long fossil record since Cambrian (542-488 Ma). However, the Cambrian ancestry of modern echinoderms that exhibit radial/pentaradial symmetry is a long-standing problem. Among extinct groups, gogiids are the most common stalked echinoderms characterized by an “irregularly” plated body in the Cambrian. Based on recently discovered new material from South China, one unusual specimen of a well-described gogiid species has clear evidence for a helical body plan, prompting us to rethink the fundamental plating principles for echinoderms. A crypto-helical body construction is proposed as a new key synapomorphy for the new family Gogiidae based on parsimony analyses of 17 Cambrian taxa with 149 characters. This study shows that using Helicoplacoidae as outgroup is critical for interpreting the transformation series of Cambrian stalked echinoderms. Thus, a close phylogenetic relationship between helicoplacoids and gogiids is suggested here for the first time. Reduction and specialization of thecal plates, resulting in suppressing the helical body plan and compartmentalization of internal body chamber via differentiation of connective tissues are major evolutionary events for stalked echinoderms during the Cambrian explosion.