SHORT OR LONG FUSE: TIMING THE ORDOVICIAN RADIATION USING THE FOSSILIZED BIRTH DEATH MODEL

Numerous environmental forcing mechanisms have been proposed as potential causes of the Ordovician Radiation, ranging from bolide impacts to geochemical changes. However, any proposed cause of the radiation must align temporally with the changes observed in the biological data. Narrowing down the timing of the radiation can pair down the list of potential causes. Studies based on taxonomic data (the temporal and geographic occurrence of species and genera) have constrained the main phase of diversification to the Early and Middle Ordovician. These analyses are an excellent first step towards dating the Ordovician radiation, but the results could be obscured by fossil preservation. Taxonomic estimates of diversification rates do not take into account the evolutionary history of the species sampled. It is possible that some of the species may have diverged from their ancestors far earlier than the available evidence suggests, due in part to a failure to sample an ancestor species. This would suggest a longer fuse for the radiation. Phylogenetic methods can be used to more accurately constrain the timing of diversification by incorporating the evolutionary history of a biological group with statistical models of the probability of fossil preservation across each time period. These methods are collectively referred to as the Fossilized Birth Death Model. We present a phylogenetic study of brachiopods from the superfamily, Strophomenoidea, that suggests diversification in this large group of brachiopods was concentrated in the Dapingian. We generate a Fossilized Birth Death Model that incorporates temporal variation in diversification and sampling of the entire order, Strophemenata, to better constrain the timing of diversification within the strophomenoids. Our data largely corroborates the Dapingian timing of the strophomenoid radiation within the strophomenates, and suggests that global sampling across the Ordovician is generally quite good for this group, with some potential biogeographic gaps in sampling across North America. Additional studies are needed from other clades to see if a similar burst of diversification across the Dapingian is common across all animal groups.