CRYPTO-HELICAL BODY PLAN IN EARLY ECHINODERMS: NEW EVIDENCE FROM THE CAMBRIAN STRATA OF SOUTH CHINA

All living echinoderms bear a pentaradial symmetry that is unique within the Bilateria. However, the Cambrian ancestry of derived echinoderms that exhibit radial/pentaradial symmetry is a long-standing problem. During the Cambrian (542-488 Ma), gogiids are the most common stalked echinoderms characterized by an “irregularly” plated body. Based on recently discovered new material from the Balang Formation (Cambrian Series 2), eastern Guizhou, China, seven unusual specimens of a well-described stalked echinoderm have clear evidence for partial disarticulation in helical pattern, prompting us to rethink the fundamental plating principles for some echinoderms. The nature of the helix plating is best observed in spiral brachioles among Cambrian stalked echinoderms (e.g., Guizhoueocrinus yui, Globoeocrinus globulus, Gogia spiralis, Gogia granulosa, Gogia guntheri). By considering the crypto-helical body construction gogiids can be linked directly to helicoplacoids, which are enigmatic Cambrian echinoderms with definite spiral plating in the body plan. With new material presented here, some rare specimens of the oldest gogiid probably had disarticulated in helix without signs of secondary causes (e.g., bioturbation, current alignment, tectonic distortion), reflecting its original plating principle for constructing the body architecture. Thus, a crypto-helical body construction is interpreted as a key feature to understand the early evolution of gogiid echinoderms. This hypothesis implies a phylogenetic connection between helicoplacoids and eocrinoids. The crypto-helical body construction in gogiids marks as an important evolutionary innovation evolved during the transition from pre-radiate echinoderms to pelmatozoans and, eventually, radiate echinoderms. A new transitional form has been reported recently from Morocco. This finding indicates that helical plating is not as uncommon among Cambrian echinoderms as previously thought.