Teenage Mutant Helicoplacoid: Growing up during the “Cambrian Explosion”

The enigmatic bulb-shaped helicoplacoids are the oldest named echinoderms. Members of the group are found in the Lower Cambrian Poleta Formation of eastern California and adjacent Nevada, and in equivalent rocks in Washington State and British Columbia. The vast majority of helicoplacoids are diagnosed as Helicoplacus gilberti, while the species Polyplacus kilmeri is known from a single specimen. The test of P. kilmeri is composed wholly of ambulacral cover plates, rather than the interambulacral pleats found in H. gilberti. These cover plates are morphologically identical to those that roof the ambulacra of H. gilberti, which in addition to the rarity of the specimen, suggest that P. kilmeri does not constitute a species, but is a variant of H. gilberti.

According to the Extraxial/Axial Theory of body wall organization, the ambulacra of H. gilberti are a feature of the axial skeleton. Interambulacra are interpreted here to be part of the imperforate extraxial skeleton. At some point in development, the genes coding for the axial skeleton were expressed in lieu of those coding for the imperforate extraxial skeleton, resulting in an individual with one morphologically and developmentally distinct skeletal system substituting for another. The resulting “mutant” nonetheless survived to be at least a late juvenile, as it is nearly as large as the largest known helicoplacoids. The apparent success of this individual suggests plasticity of the echinoderm body plan, such that the skeletal systems were transposable without any apparent deleterious effects in regards to the viability of the animal. This skeletal substitution may therefore serve as a mechanism responsible for the tremendous disparity seen in the Early Cambrian echinoderms (helicoplacoids, edrioasteroids, eocrinoids), and have some role in explaining how novel morphological characteristics arose and persisted during the “Cambrian Explosion.”