A SPECIES-LEVEL PHYLOGENY OF ARCHAEOCIDARID ECHINOIDS FROM NORTH AMERICA: IMPLICATIONS FOR BASIN EXCHANGE AND ECOLOGICAL INTERACTIONS

After the Late Devonian mass extinction, archaeocidarid echinoids diversified rapidly to become common members of marine ecosystems. Carboniferous and Permian paleocommunities contained up to four coexisting echinoid species; this diversification and increased abundance of shallow water sea urchins had important ramifications for community organization and ecological interactions. A cladistic analysis of Late Paleozoic archaeocidarids reveals not only the evolutionary relationships of the group but also provides insight into ecological relationships of echinoid clades with epibionts and into Carboniferous-Permian interbasin circulation.

Using Devonechinus and Nortonechinus as outgroups, 6 binary and 20 multistate characters of 32 taxa were analyzed with PAUP* using global parsimony. The heuristic search with 10,000 iterations resulted in 120 equally parsimonious trees. The majority consensus tree of North American archaeocidarids is stratigraphically well-ordered, with no reversals in stratigraphic position and only two nodes with ghost lineages. The results reveal: (1) Archaeocidarid echinoids evolved increasingly complex spine ornament. Using the comparative method with preliminary data on encrustation of spines indicates a correlation between the evolution of highly ornate spines and encrustation of spines by epibionts. However, results are yet too tentative to discern the process behind the pattern of epibiosis and the evolution of ornate spines: whether echinoid spine ornament enhances encrustation, epibionts drive ornate spine evolution, or positive feedbacks from both. (2) When the paleogeographic distribution of species is mapped onto the phylogeny, archaeocidarid clades evolved mainly in the Midcontinent of the U.S. and repeatedly migrated into basins of the western U.S. and Texas. This suggests an intermittent but major interbasin circulation from the Midcontinent basin outward, which sporadically dispersed echinoid larvae into distal basins.