Deconstructing helicoplacoids: Reinterpreting the most enigmatic Cambrian echinoderms

Abstract

Helicoplacoids, the most enigmatic group of Early Cambrian echinoderms, have a spirally plated, inverted pear- to spindle-shaped theca with numerous interambulacral pleats and a long triradiate ambulacrum. They occur in western Laurentia along with the earliest edrioasteroids and epispire-bearing eocrinoids(?), but unlike these other groups, helicoplacoids do not show pentameral symmetry around a centrally located, upward-facing mouth. It is uncertain where the major body openings were located in helicoplacoids, where new plates were added during thecal growth, and whether helicoplacoids were primitive, prepentameral, stem-group echinoderms or derived, relatively specialized forms that had lost several ambulacra. We made highly modified, side-layout, plating diagrams of different helicoplacoid species by cutting the theca apart half-way between the spiraling ambulacra, leaving four to six interambulacral pleats on each side. This deconstructed thecal strip resulted in an elongate sigmoid-shaped plating diagram with a long central single or paired ambulacrum that is interpreted as three ambulacral branches (A down, C and D up) around a central mouth on the side of the theca. The unrolled plating diagram resembles an elongate French curve drawing template with nearly parallel sides, a large or small sigmoidal top (where the theca is rounded and the interambulacral pleats converge), and an open deviated bottom, where the interambulacral pleats turn down to a truncated conical base. The two upward-extending ambulacra are only one to two interambulacral pleats apart, which only slightly increases the plating strip width. New interambulacral plates are added at the summit and base, and interambulacral pleats rarely branch elsewhere. New floor and cover plates are added at the top and bottom distal ambulacral 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 ambulacra end, and all imperforate extraxial plating has apparently been lost from the theca. If upright, spirally pleated helicoplacoids with three ambulacra were derived from attached, domal or pyrgate, pentameral edrioasteroids, numerous changes must have occurred in the thecal shape, number of ambulacra, and interambulacral areas. These changes either occurred gradually in a sequence, or a single major change (spiralled body form?) caused all the others to rapidly follow. Copyright © 2005 John Wiley & Sons, Ltd.