COMPARISON OF EXTINCTION SELECTIVITY STRENGTH BETWEEN GEOGRAPHIC RANGE AND BODY SIZE OF FOSSIL MARINE ANIMALS ACROSS THE PHANEROZOIC

Numerous traits are selective during background intervals and mass extinctions across the Phanerozoic. Geographic range and body size are perhaps the two traits most commonly hypothesized to influence extinction risk, due to both their biological importance and their ease of measurement and comparison across disparate taxa. Despite much research on each trait, prior studies have not directly established which trait is more important in determining extinction risk during background extinction and whether their relative importance changes during mass extinction. Here, we measure extinction selectivity with respect to geographic range and body size for genera across five classes of fossil marine animals (Bivalvia, Gastropoda, Rhynchonellata, Strophomenata, and Trilobita), and compare selectivity during background intervals with the Big Five mass extinction events covering 485 Ma-1 Ma. We calculate a standard score for geographic range and body size to enable direct comparison of selectivity coefficients. We measure selectivity using capture-mark-recapture models and compare the log odds of extinction for geographic range and body size between background and mass extinctions. Extinction is strongly selective with respect to geographic range during background intervals, where genera with narrower geographic range exhibit higher selectivity across all classes. In contrast, selectivity on body size is statistically significant but substantially weaker for four classes, with the direction of selectivity varying across classes. During mass extinctions, selectivity on geographic range is reduced for all five classes, with three classes (Trilobita, Rhynchonellata, and Strophomenata) demonstrating no statistically significant association. Selectivity on body size varies by class, where the only significant association observed is against larger trilobite and smaller gastropod genera. The magnitude and pervasiveness of selectivity on geographic range during background intervals suggest that this effect dominates extinction patterns even when there is significant selectivity on other traits, such as body size. The reduction in selectivity on geographic range during mass extinction may enable other traits to become more important in shaping the surviving biota across mass extinction events.