Reduction in the number of Early Cambrian helicoplacoid species

Helicoplacoid echinoderms occur in Lower Cambrian shales and mudstones along the northern coast of Laurentia, now western North America. The group originally contained four genera and nine species, diagnosed by differing ambulacra, plate length-to-width ratios, and plate ornament. Ambulacra are identical in all specimens except for the single example of Waucobella nelsoni, and the remaining morphologic differences are caused by ontogeny. The characters used to diagnose seven of the original nine species are therefore invalid, reducing the group to three genera and three species. These are: Helicoplacus gilberti Durham and Caster, 1963, Waucobella nelsoni Durham, 1967, and Polyplacus kilmeri Durham, 1967, which has a test composed of ambulacra, and likely represents a developmental oddity.