THE ESTABLISHMENT OF ECHINODERM BODY PLANS
A preliminary analysis of 92 Early Paleozoic echinoderms using this character suite resulted in a morphospace that captures the three well-recognized body plans within echinoderms (Pelmatozoans, Eleutherozoans, and Homalozoans). An analysis of morphologic diversity through the Early Paleozoic shows a large and consistent seven-fold increase in disparity during the Cambrian followed by relative stability in the Ordovician despite a large increase in taxonomic diversity. The rise in disparity during the Cambrian was the result of morphological expansion of and divergence between the Pelmatozoans and the Eleutherozoans, whereas Homalozoans remained relatively stable in position and disparity. The abrupt end of morphological expansion during the Early Ordovician is consistent with either the establishment of genetic regulatory networks that prohibit the evolution of radically different body plans, the filling of ecological niches that result in the loss of ecological opportunity, or a combination thereof.