THE ESTABLISHMENT OF ECHINODERM BODY PLANS

The pattern and timing of the establishment of animal body plans have long-term ramifications for the evolutionary history of a phylum. Insight into this formative process can be achieved through the study of morphological disparity. However, the quantification of biologic form at higher taxonomic levels is difficult given the vast range of morphologic features present within a phylum as well as the biases produced by variations in the preservation potential of structures and organisms. A novel character suite was constructed that encompasses the range of morphology present within Early Paleozoic echinoderms. Characters were added from a combination of literature descriptions, examinations of specimens, published phylogenetic matrixes, and discussions with specialists in particular echinoderm groups. Features of the Extraxial-Axial Theory (EAT) and the Universal Elemental Homology (UEH) model were also incorporated into the character matrix to recognize similarities in disparate organisms.

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.