AN EARLY BURST IN BRACHIOPOD EVOLUTION CORRESPONDING WITH SIGNIFICANT CLIMATIC SHIFTS DURING THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT

Paleobiologists have long pointed to major environmental shifts as potential drivers of evolutionary radiations. Multiple environmental shifts transpired over a roughly 30 million year span in the Ordovician, all of which have been linked to at least some of the many radiations happening in that time span. Here, we employ modified tip-dating methods to date divergence times within the Strophomenoidea, one of the most abundant and species-rich brachiopod clades to radiate during the Great Ordovician Biodiversification Event (GOBE), to determine if significant environmental changes at this time correlate with the diversification of the clade. Models using origination, extinction and sampling rates to estimate prior probabilities of divergence times strongly support both high rates of anatomical change per million-years and rapid divergences shortly before the clade first appears in the fossil record. These divergence times indicate much higher rates of cladogenesis than typical of brachiopods during this interval. The correspondence of high speciation rates and high anatomical disparity suggests punctuated (speciational) change drove the high frequencies of early anatomical change, which in turn suggests increased ecological opportunities rather than shifting developmental constraints account for high rates of anatomical change. The pulse of rapid evolution began coincident with cooling temperatures, the start of major oscillations in sea level, and increased levels of atmospheric oxygen. Our results suggest that these factors permitted major geographic and ecological expansion of strophomenoids with intervals of geographic isolation, resulting in elevated speciation rates and corresponding elevated frequencies of punctuated change.