THE ORDOVICIAN DIVERSIFICATION OF THE BOTHRIOCIDAROIDA (ECHINODERMATA:ECHINOIDEA)

The echinoids of the order Bothriocidaroida represent the initial burst of sea urchin diversification, which took place in the Ordovician. The bothriocidaroids were the first echinoids to achieve widespread biogeographic dispersal and achieved high levels of species richness compared to other clades of stem-group echinoids. Following long-standing controversy regarding their phylogenetic affinities within the phylum Echinodermata, bothriocidaroids are now confidently regarded as echinoids. The species- and genus-level phylogenetic relationships of the bothriocidaroids, are, however, less clearly known. We herein compiled a database of bothriocidaroid occurrences and undertook a detailed phylogenetic analysis of all named species. Our phylogenetic analysis includes species belonging to three genera, Bothriocidaris, Unibothriocidaris, and Neobothriocidaris. Using phylogenetics, we demonstrated that Unibothriocidaris and Neobothriocidaris are distinct clades, and that Unibothriocidaris appears to have become extinct in the Sandbian, while Neobothriocidaris and Bothriocidaris survived the Late Ordovician extinction and lasted at least until the late Silurian (Ludlow). Furthermore, our phylogenetic analyses and plots of diversity show that bothriocidaroids appear to have diversified relatively early in their evolutionary history, leading to peak Ordovician echinoid diversity in the Sandbian and Katian. This Sandbian and Katian peak is similar to that observed in other echinoderm clades, and we speculate that this may be linked to heightened sea level and expansive tropical shelves present in the Late Ordovician.