Synchronous Diversification of Laurentian and Baltic Rhynchonelliform Brachiopods: Implications for Regional Versus Global Triggers of the Great Ordovician Biodiversification Event

The profound global impact of marine radiations during the Great Ordovician Biodiversification Event (GOBE) is widely appreciated; however, diversification varied among paleocontinents and these individual trajectories are less understood. Here we present a new species-level diversity curve for rhynchonelliform brachiopods from midcontinental Laurentia based on bed-by-bed analysis of the Simpson Group of Oklahoma (USA). Diversity and abundance data span the Dapingian through Sandbian Stages, which encompass the interval of maximum global diversification. A rapid, statistically significant increase in brachiopod diversity was observed in the early Darriwilian Histiodella holodentata Biozone. We interpret this as a biological signal because the increase cannot be explained by sampling intensity, facies types, or position along depositional gradient. Diversifications on Laurentia and Baltica were temporally synchronous at the biozone level, and cumulative diversity curves for the regions show similar patterns, suggesting a global driver for the radiations. The taxonomic composition of the brachiopod faunas, however, differs substantially, highlighting the importance of regional controls on diversification. Thus both global and local factors controlled diversity increase during the GOBE.