BUILDING A BETTER HELICOPLACOID AND THE MEANING OF “EDRIOASTEROID”

Helicoplacoids are a disparate group of bulb- to spindle-shaped echinoderms found in Lower Cambrian rocks of North America. They lack epispires, and have a test composed of expandable pleats arranged in a left-handed spiral, a mouth situated off-axis from the pole of radial symmetry, and three ambulacral areas. These divergences from the body plan of coeval eocrinoids and edrioasteroids can be explained through a series of alterations of the variegated skeletal systems of a generalized Camptostroma-like “hypothetical ancestral echinoderm.”

Camptostroma roddyi, a Lower Cambrian edrioasteroid, has distally curved ambulacra, which conferred greater surface area dedicated to feeding. Ambulacra A, B, D, and E are curved sinistrally, while C is curved dextrally. The remainder of the oral surface is composed of epispire-covered interambulacral areas, and the aboral surface is composed of a suction pad surrounded by a corrugated skirt of skeletal tissue. Loss of the aberrant C ambulacrum and associated loss of either ambulacrum D or E (to preserve symmetry) allowed the oral surface to spiral away from the substrate in a left-handed fashion as it grew. Similar body plans are seen in the convergent edrioasteroid Streptaster vorticellatus and diploporan Gomphocystis tenax. Concurrent with these changes were a reduction of the epispire-covered interambulacral areas and associated orifices to a small ring surrounding the mouth, and replacement by the corrugated aboral surface, allowing respiration by cloacal pumping. This arrangement would have brought into apposition the mouth and anus, forcing the former to migrate toward the lower pole along an ambulacrum. The corrugated aboral pad of Camptostroma is therefore homologous to the interambulacra of the helicoplacoids. Implicit to this argument is that helicoplacoids are highly derived triradiate edrioasteroids. Furthermore, the generalized edrioasteroid body plan may similarly serve as progenitor for the remainder of non-carpoid echinoderms, suggesting that all non-carpoid echinoderms are edrioasteroids in a phylogenetic sense.