RECONSTRUCTING THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT THROUGH BRACHIOPODS OF OKLAHOMA

In this study, we construct the first high-resolution brachiopod species diversity curve for Middle Ordovician strata of Laurentia. Global data aggregates indicate that the number of marine invertebrate families and genera more than tripled during this interval, establishing the highest rate of diversity increase for the Phanerozoic. This pattern is known as the Great Ordovician Biodiversification Event (GOBE). The timing and magnitude of the GOBE varies among geographic areas, and its triggers remain unknown. Efforts in China and Estonia have used brachiopod species diversity to retrace local timing and magnitude of the GOBE, but causal hypotheses cannot be evaluated without taxon-specific data from all relevant paleocontinents. This study generates a symmetrical data set to represent brachiopod diversity patterns on the Laurentian paleocontinent.

Fossil and sedimentological data were collected from one of the most continuous and accurately-dated GOBE-aged exposures in North America: the Simpson Group of central Oklahoma. These strata are exposed in cuts along I-35 in the Arbuckle Mountains. These units preserve a shallow marine siliciclastic and carbonate ramp system. The section was surveyed bed-by-bed for brachiopod species diversity and abundance spanning the Dapingian – Sandbian Stages. Occurrence data were rarefied to mitigate sampling biases and detect any statistically significant changes in diversity throughout the study interval. The diversity data were placed into meter-scale stratigraphic context to analyze environmental associations and limit facies-controlled biases. Diversity changes within the Simpson Group were correlated to strata of other regions using existing conodont biostratigraphy and chemostratigraphy.

Preliminary results indicate the lower Simpson Group exhibits higher brachiopod diversity than previously identified. Based on the raw data, maximum diversity was reached during the early Sandbian Stage in the upper Simpson Group. Similar diversity peaks occurred earlier in China (Floian Stage) and Baltica (Darriwillian Stage). The Simpson Group sections also contain fewer species than the Baltic sections overall. These variable trajectories suggest that local factors were important in determining regional diversity patterns during the GOBE.