Identifying Driving Factors of Extinction and Body Size for Molluscs During the Late Ordovician Mass Extinction

While trends between extinction and body size have been explored extensively, few have studied them within the Late Ordovician Mass Extinction (LOME). None have attempted to observe it with Mollusca as a focal point. Additionally, our research investigated ecological factors and their relationship with body size and extinction risk.

Using R, we subsetted data from Heim et al. 2015 into three main classes, Cephalopoda, Bivalvia, and Gastropoda. First, we conducted logistic regression analyses to determine the odds of extinction based on calculated fossil biovolume. Next, we ran Pearson's Correlation tests to compare the calculated odds to the sea level and pO2 of each Ordovician stage. Secondary logistic regression analyses were performed on the entire molluscan dataset, comparing body size and extinction to circulation, feeding, motility, and tiering data, followed by correlation tests between these odds and sea level and pO2. While our first test for the main classes yielded no significant results, our second tests involving ecological traits found significant (p < 0.05) and strongly positive coefficients. For example, there were high odds of extinction for molluscs with closed circulation. The regression coefficient for extinction risk and genera with closed circulation was 1.06 in the LOME, indicating that such organisms were more likely to go extinct. Our analysis revealed this likelihood had actually reduced from earlier stages. Additionally, there were strong positive correlations between coefficients for extinction and body size for the traits of predatory feeding, water column tiering, and a closed circulation. These are all features of cephalopods, indicating that the high coefficients are due to cephalopod extinction. Furthermore, the correlations between the calculated coefficients of body size, extinction risks, and ecological factors to pO2 and sea levels yielded mixed yet significant results.

While there exists no clear relationship between body size and extinction for molluscs in the LOME, alternative physiological and ecological information act as better predictions for body size and extinction risk during this time. These factors clearly play a role in the underpinning mechanisms of mass extinctions, indicating that extinction trends are more complex than simple correlations.