Extinction Vulnerability of Large Foraminiferans at Mass Extinction Events

Throughout Earth's history, numerous extinction events have occurred, with mass extinctions revealing a notable pattern: different biological groups experience distinct extinction risks. Paleontologists have long been curious about which traits determine the likelihood of species extinction. This study investigates the role of three specific traits—body size, species richness, and geographic range—in influencing the extinction risk of foraminifera across a total of 12,701 specimens. The results highlight body size as a critical determinant of extinction risk, with larger foraminifera exhibiting heightened vulnerability. This size selectivity was particularly pronounced during the Guadalupian-Lopingian (259.51 Ma), Permian-Triassic (251.902 Ma), and Cretaceous-Paleogene (66.1 Ma) mass extinctions. One possible explanation for this pattern is the increased difficulty of oxygen diffusion within larger foraminifera, which may have made them more susceptible to extinction. In contrast, species richness and geographic range showed no statistically significant effects on extinction risk. We hence propose a hypothesis to explain size-selective extinction and the Lilliput effect in diffusion-dependent organisms in past extinctions, which may also apply to future anoxic conditions. We also use newly compiled data to explore the temporal patterns of foraminiferan size evolution beyond mass extinction events.