Global factors constrain body-size trends across the Great Ordovician Biodiversification Event at a regional scale: a case study from the Arbuckle Mountains of Oklahoma

The Great Ordovician Biodiversification Event (GOBE) records a global increase in marine biodiversity that reached maximum diversification rates during the Middle Ordovician. The degree to which the causes of the GOBE are regional or global is a question that must be addressed through analysis of regional data. In this study, stratigraphically constrained field-based data from the Middle Ordovician Simpson Group of Oklahoma were collected to identify temporal trends in body volume and determine whether body volume trends are more closely associated regional or global environmental and diversity changes. Anteroposterior–transverse (AT) volume estimations were produced for rhynchonelliform brachiopods at a bedding-plane level of resolution. Time-series analysis was used to establish temporal trends in brachiopod volume. Volume data were then analyzed alongside paired δ ¹⁸ O, Δ ¹³ C, ⁸⁷ Sr/ ⁸⁶ Sr, taxonomic diversity, and lithologic data using a boosted regression model to identify their relative influence on shell volume through time. Results of these analyses indicate that (1) a rapid pulse of brachiopod volume increase occurred coincident with the main diversification pulse in Simpson Group strata and (2) volume increase was not coupled with an increase in brachiopod volume variance. Volume increase was primarily associated with global-scale factors such as age, δ ¹⁸ O (temperature), ⁸⁷ Sr/ ⁸⁶ Sr (tectonics), and taxonomic diversity trends; whereas local-scale factors of Δ ¹³ C (carbon cycle) and lithologic trends were more weakly associated with local volume trends. Notably, all factors had a nonzero influence over brachiopod volume, indicating that local diversification was influenced by multifaceted interactions among abiotic and biotic controls. These results support the argument that Ordovician diversification included a substantial biotic shift during the Middle Ordovician and support the hypothesis that global factors were dominant, influencing diversification patterns during the main phase of the GOBE.