Ecological revolution of Oklahoma's rhynchonelliform brachiopod fauna during the Great Ordovician Biodiversification Event

Abstract

During the Great Ordovician Biodiversification Event (GOBE), the number of marine invertebrate families more than tripled. Fundamental shifts in clade dominance and ecological structure led to the rise of the Palaeozoic Fauna and the second Palaeozoic Ecological Evolutionary Unit (EEU P2 of Sheehan), which revolutionized benthic ecosystems for the next 200 million years. Although general global signals of increased diversity and ecosystem restructuring are known, direct links between these changes are more poorly constrained on high-resolution local scales. To address this, rhynchonelliformean brachiopod assemblages of the Middle Ordovician Simpson Group (Oklahoma, USA) were assessed before, during and after maximum local diversity increase related to the second global pulse of the GOBE. Stratigraphically constrained brachiopod species occurrence and abundance data were collected for Dapingian-lower Sandbian strata via field study. Brachiopod assemblages exhibited declining dominance and increasing evenness from the Dapingian through middle Darriwilian stages followed by high volatility in index values during and after the GOBE interval. These patterns document local faunal turnover and shift from EEU P1 to P2. Furthermore, the average species body volume statistically increased over this interval, a pattern that is independent of phylogenetic constraints. Paired increase in mean body size and size disparity is evidence of niche specialization and utilization of additional food resources. Together, changes in body size, evenness and dominance indicate that increasing ecological complexity was coincident with diversification in Oklahoma. This suggests that a common mechanism, such as climate optimization or increased food abundance, was locally responsible for building both taxonomic diversity and ecological complexity during the GOBE.