RESOLVING A GROWTH SERIES WITHIN HELICOPLACOIDS: IMPLICATIONS FOR FUNCTIONAL MORPHOLOGY AND SYSTEMATICS

The helicoplacoid bauplan constitutes the most radical thecal design of any Early Cambrian echinoderm group. They are predominantly found in shales of the Poleta Formation in California and Nevada, along with edrioasteroid and eocrinoid debris. Whereas the two latter groups exhibit the pentameral symmetry diagnostic of Echinodermata, helicoplacoids have a medial triradiate food groove (amb) imbedded within pleated interambulacral (IA) plates, which spiral in a left-handed fashion into a spindle or elongate globular test shape. Past misinterpretations of a growth series have resulted in an overestimation of morphotypes, which has further complicated functional morphology associated with an expandable test.

Diagnosis of the two most common helicoplacoid genera, Helicoplacus and Westgardella, was originally based on tube foot pores, plate ornament, and plate dimensions. Analysis of the ambs of several specimens of both morphotypes indicates identical biserial floor plating with pores between adjacent plates. The floor plate arrangement is nearly identical to that of Early Cambrian Camptostroma and Stromatocystites, suggesting a close relationship with the edrioasteroid echinoderms. Furthermore, recent examination of new large specimens of Westgardella with patches of IA plating diagnostic of Helicoplacus suggests that the two genera can be resolved into a growth series, with plate addition at the poles and at a few other places on the test. Helicoplacoid IA plates form a complex of erect primary plates underlain by two rows of recumbent secondary plates, which allowed for accordion-like expansion of the test. Despite the presence of secondary IA plates in juveniles, only adult helicoplacoids are found in the expanded state, indicating that the capability to expand and contract developed later in ontogeny. Eight to twelve rows of IA plates separate the A amb near the lower pole. Medially, only two IA separate the C and D ambs, accompanied by a reduction in IA elsewhere. Amb reduction from the ancestral pentameral arrangement is necessitated by the mechanics of coiling and volumetric change.