Testing the Relationship of Brachiopoda Generic Richness, Origination, and Body Size During the Great Ordovician Biodiversification Event

During the Great Ordovician Biodiversification Event (GOBE), the phylum Brachiopoda went through a rapid diversification of genera. Prior research, while limited, suggests a bias exists towards the origination of larger-bodied organisms. To explore possible relationships between Brachiopoda body size, origination, generic richness, and oxygen during the GOBE, we conducted statistical tests (correlation and logistic regression) on a data set of the three most data-rich Brachiopoda classes: Lingulata, Rhynchonellata, and Strophomenata.

Using Pearson's correlation test, we found a positive correlation between body size and the origination of genera within Strophomenata (r ≈ 0.8, p-value ≈ 0.034) and Rhynchonellata (r ≈ 0.81, p-value ≈ 0.054). Additionally, correlation coefficients indicate a strong negative correlation between biodiversity and oxygen levels in both Strophomenata (r ≈ -0.88, p-value ≈ 0.022) and Rhynchonellata (r ≈ -0.80, p-value ≈ 0.063) genera. We found no statistically significant relationships within Lingulata, possibly a reflection of unstandardized data or limited sampling.

The correlation between genera origination and body size suggests preferential origination of larger genera during the Ordovician, which may be a result of selective advantages that come with increased size. Furthermore, a negative correlation between generic richness and oxygen levels reveals that increased oxygen may have led to a decrease in Strophomenata and Rhynchonellata diversity. These findings suggest that body size increased with genera origination, but oxygen did not play a role in body size patterns during the GOBE. The limited statistically-supported correlations discovered may reflect shortcomings in the data sampled or the influence of other factors.

Our findings show significant differences between Brachiopoda classes with regard to origination based on body size. To further clarify these differences, more rigorous analyses and additional research is required.