PALEONTOLOGICAL TOPIARY: THE NORTH AMERICAN EARLY CAMBRIAN ECHINODERM TREE AND IMPLICATIONS FOR COMPETITIVE EXCLUSION DURING THE "CAMBRIAN EXPLOSION"

Class Echinodermata is represented in Lower Cambrian rocks of North America by helicoplacoids, two species of edrioasteroids, two species of imbricate eocrinoids, and the eocrinoid genus Gogia. New research relevant to the generation of an Early Cambrian echinoderm tree justifies a reduction in number of helicoplacoid species from nine to two. Furthermore, six new eocrinoids attributable to Gogia have been collected, four of which are sufficiently preserved to warrant naming. The resulting tree for the 11 Early Cambrian taxa shows that helicoplacoids, Imbricata, and Gogia each constitute monophyletic groups, as do the eocrinoids (Imbricata + Gogia). Class Edrioasteroidea, represented in the Early Cambrian by Camptostroma roddyi and Stromatocystites walcotti, is a polyphyletic group.

Reduction in the number of helicoplacoid species and addition of several new species of Gogia are both in accord with a trend commonly seen in all “non-carpoid” echinoderms from the Early and Middle Cambrian. For each locality, there exists only one (frequently) or two (rarely) echinoderm species that occupy the same morphospace, suggesting fierce within-group competition to the point of exclusion of all other echinoderm species. As the known Early and Middle Cambrian non-carpoid echinoderms are exclusively low- to mid-level suspension feeders that attached to skeletal debris or semi-lithified substrates by means of a suction pad, it follows that substrates were a limiting resource. Competition for optimal attachment sites therefore resulted in the dominance of a single echinoderm species per locality. The two cases of co-occurring species may both be temporal snapshots showing overturn from one species to another. Competition from non-echinoderm taxa that occupied echinoderm morphospace may explain why some Early and Middle Cambrian faunas lack echinoderms. Furthermore, competition may have forced the development of echinoderms with long stalks and cemented holdfasts in the Late Cambrian, as these structures allowed a greater number of echinoderm attachment sites per equivalent substrate surface area, and allowed the animals to reach further away from the sea floor.