THE EFFECTS OF BODY SIZE AND LITHOLOGICAL AFFINITY ON EXTINCTION RISK

Body size is intimately linked to metabolic rate, population size, and a number of other important organismal traits. Give the strong link between body size and metabolic rate, it is plausible that body size has a relationship to extinction risk, especially during intervals of geologic time characterized by physiologically stressful environmental conditions. Here I explore the role of body size on extinction risk for Phanerozoic marine animals using logistic regression. Having a large body size is either neutral or it increases the risk of extinction during much of the Phanerozoic. However, during the end-Permian and end-Cretaceous mass extinctions, large body size reduced the risk of extinction. Though there is a sharp drop in extinction risk at the era boundaries, both mass extinctions are preceded by protracted decreases in extinction risk for before the events.

Lithology has also been found to be important for extinction risk. This is especially true with respect to carbonate vs. siliciclastic settings. By adding a nominal variable for lithological preference for each genus in the regression model (coded as carbonate only, siliciclastic only, or both), I can evaluate the relative extinction risks to carbonate-loving, siliciclastic-loving, and broadly tolerant genera while controlling for variations in extinction risk related to body size. Though the overall risk of extinction due to lithological preference is less than that of body size, there are some intriguing results. Being a siliciclastic-loving genus during the Paleozoic increases extinction risk while being carbonate-loving is neutral. During the post-Paleozoic, however, these extinction risks switch and carbonate-loving taxa become more at risk. Among the 10 most diverse classes in the dataset, the gastropods are the only one to show a clear switch from having increased extinction risk in siliciclastic settings during the Paleozoic to having increased extinction risk in carbonate settings during the post-Paleozoic.

These results highlight body size and broad lithological affinity as important factors in extinction risk during times of background and mass extinction. These findings also demonstrate the power of multiple logistic regression in determining the relative of impacts of several biological and physical factors on extinction risk.