DECONSTRUCTING HELICOPLACOIDS: CUTTING OUT TWO-DIMENSIONAL PLATING DIAGRAMS FOR THE MOST PUZZLING CAMBRIAN ECHINODERMS

Helicoplacoids, the most enigmatic group of Early Cambrian echinoderms, have a spirally plated, spindle-shaped to elongate globular theca with numerous interambulacral (IA) pleats and a long branched ambulacrum (amb). They occur in the western part of Laurentia along with the earliest edrioasteroids and epispire-bearing eocrinoids(?). Unlike these other groups, helicoplacoids show no pentameral symmetry around a centrally located mouth, and it is uncertain where major body openings were located, where new plates were added during growth, and where the axial, perforate extraxial, and imperforate extraxial plate regions are located.

We made side-layout plating diagrams of different helicoplacoid species by cutting the theca apart half-way between the spiraling ambulacra, leaving 4-6 IA pleats on each side. Unrolling this thecal strip resulted in an elongate plating diagram with a long central single or paired ambulacrum that is interpreted as 3 amb branches around a central mouth (A down, C and D up). The unrolled plating diagram resembles an elongate French curve drawing template with nearly parallel sides, a highly sigmoidal top (where the theca is rounded and the IA pleats converge), and an open deviated bottom, where the IA pleats turn down to a truncated conical attachment or insertion surface. The two upward-extending ambs stay only 1-2 IA pleats apart so as not to greatly increase the plating strip width. New IA plates are apparently added at the top and bottom ends, and IA pleats rarely branch along their length. New amb plates are apparently added at the top and bottom distal amb tips. If this plating model is correct, the anus (which has still not been identified) should be located near the thecal summit where the C and D ambs end, and all imperforate extraxial plating has apparently been eliminated from the theca.