TRANSFORMATION OF BRACHIOPOD COMMUNITY STRUCTURE AND MORPHOLOGICAL TRENDS DURING THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT IN OKLAHOMA

During the Great Ordovician Biodiversification Event (GOBE), fundamental shifts in clade dominance and community structure led to the rise of the Paleozoic Evolutionary Fauna and second Paleozoic Ecological Evolutionary Unit (EEU P2 of Sheehan), which revolutionized the taxonomic composition and community ecology of benthic marine invertebrate ecosystems for the next 200 million years. The Cambrian Fauna was diluted and replaced by the Paleozoic Fauna, which achieved higher epifaunal tiers, increased infaunal activity, and more densely-packed guild structures. Although ecological impacts of the GOBE are well-known on a global scale, a direct relationship between diversity, morphologic disparity, and inter-community structure has yet to be firmly established on regional scales. To address this, brachiopods of the Oklahoma Basin were assessed using (1) community structure and (2) body size and morphological disparity. Field work was conducted to obtain stratigraphically-constrained brachiopod species occurrence and abundance data from the Simpson Group (Dapingian-Early Sandbian). Rarefaction indicates that the GOBE began locally in the Middle Darriwilian (H. holodentata biozone). Communities exhibited declining dominance and increasing evenness until the Middle Darriwilian, followed by high variability in these indices during and after the GOBE. This shift to a more volatile structural regime is local evidence of the faunal turnover and shift from EEU P1 to P2. Furthermore, the average species body volume nearly doubled during this interval. Time series analyses indicate that species size trends are phylogenetically independent and best fit a directional change model. Thus, size increase may be attributable to favorable environmental changes. Increased morphological disparity was also observed during this interval, and is evidence of brachiopod niche specialization that could have provided a mechanism for building diversity and complexity during the GOBE.