PHYLOGENETIC ANALYSIS OF THE MIOCIDARIDAE AND TRIADOTIARIDAE WITH IMPLICATIONS FOR THE EVOLUTION OF EARLY POST-PALEOZOIC ECHINOIDS

The Miocidaridae and Triadotiaridae are the two most basal clades of crown group cidaroid echinoids. Furthermore, species from both families comprise the earliest occurring post-Paleozoic echinoids, being found in Lower Triassic strata globally. Both clades display features such as imbricate test plating, which differentiate them from the crown group, and have also led to a rather poor fossil record. The Miocidaridae, in particular, have received much attention in recent years, because they are the only published family of echinoids which has constituent species occurring both before and after the end-Permian mass extinction. Although understanding the relationships of both members of these two clades is key for understanding the initial post-Paleozoic diversification of echinoids, little is known of the precise phylogenetic relationships of species of the Miocidaridae and Triadotiaridae. Because there are relatively few taxa within both families, however, an exhaustive phylogenetic treatment of all species is possible. We therefore undertook a phylogenetic analysis of all valid miocidarid and triadotiarid taxa, which includes taxa from the Permian to Jurassic. Key analyzed taxa included Eotiaris, Miocidaris, Lenticidaris, Procidaris, and Triadotiaris, while the analyses were rooted on the Paleozoic taxon Archaeocidaris. Analyses were run in both parsimony-based and Bayesian frameworks. With a phylogenetic framework established, it was possible to examine the number of lineages of both of these taxa which crossed the Permian-Triassic boundary, and furthermore the end-Triassic mass extinction to gain an understanding for the early post-Paleozoic diversification of cidaroids. Because the early Mesozoic fossil record of echinoids is poor, the determination of ghost lineages from the phylogenies will be key for determining post-Paleozoic estimates of standing cidaroid diversity. Additionally, with a sound phylogenetic backbone, revision of both families based upon synapomorphies was possible, resulting in firmer diagnoses for both the Miocidaridae and Triadotiaridae, and better constraints on the stratigraphic ranges of both families.