PHYLOGENETIC ANALYSIS OF THE ARCHAEOCIDARIDAE (ECHINOIDEA) AND IMPLICATIONS FOR THE EVOLUTION OF CROWN GROUP ECHINOIDS

All post-Paleozoic echinoids belong to one of two clades, the Cidaroidea or the Euechinoidea. Both the euechinoids and cidaroids are thought to have arisen from the Paleozoic stem group family the Archaeocidaridae, however, the details of this divergence are unclear. The earliest stem group cidaroids, the miocidarids, are first known from the Permian and are thought to share a common ancestor with the genus Archaeocidaris, however, the exact species of Archaeocidaris to which the miocidarids are most closely related is not known. Although archaeocidarids have been described in the literature for over one hundred and fifty years, little has been done to understand their phylogenetic relationships in a modern cladistic context. A species level phylogenetic analysis of all well-preserved taxa within the Archaeocidaridae was undertaken in order to understand the phylogenetic relationships within the family and to understand the relationships of archaeocidarids to the more derived miocidarids. Although there are more than forty named species of Archaeocidaris that exist in the literature, most of them are based on disarticulated spines or interambulacral plates, and are not easily discernable from each other. Only taxa with well-preserved tests that have preserved characters of the interambulacral, and ambulacral plates and that are relatively articulated have been included in the analysis. This is to avoid the possibility of bias by relying on one part of the morphology, such as spines. Characters were chosen by comparison of fossil skeletal morphology to developmental processes of modern euechinoid and cidaroid echinoids. Four genera of archaeocidarids, Nortonechinus, Archaeocidaris, Lepidocidaris, and Polytaxicidaris and the miocidarid genus Eotiaris were included in the analysis comprising 20 species. Nortonechinus was used as the outgroup as it appears to have the most basal morphology. The resultant phylogeny has revealed which characters appear to be most important in understanding the evolution of the miocidarids from the archaeocidarids, and thus the evolution of crown group echinoids from their stem group ancestors.