ANCIENT HYPERTHERMAL EVENTS INFLUENCE ORIGINATION SELECTIVITY MORE THAN EXTINCTION SELECTIVITY WITH RESPECT TO BODY SIZE

Ancient hyperthermal events in Earth’s history, including the end-Permian and end-Triassic mass extinction events, can be used to isolate the evolutionary consequences of climate change and other environmental factors from background geologic intervals. A key biological trait hypothesized to be sensitive to climate change and straightforward to quantify in fossil data is body size, as ectotherms modulate their physiological response to temperature and oxygen change in part through their body size. Here, we measure genus-level extinction and origination selectivity with respect to body size for six extant and ectothermic Linnean classes with robust fossil records (Rhynchonellata, Cephalopoda, Echinoidea, “bony fish”, Bivalvia, and Gastropoda) using occurrences from the Paleobiology Database. We compare selectivity during background intervals with those during hyperthermal events and their associated recovery intervals spanning the Early Permian to the Recent using capture-mark-recapture models. Using the best-fitting model for each class, we find that background extinction preferentially affects genera with smaller body sizes, whereas hyperthermal events do not show a consistent association between extinction probability and body size. Conversely, genera originating during background intervals are typically larger than average, whereas genera originating during hyperthermal recovery intervals are typically smaller than average, with the exception of bony fish exhibiting preferential origination of larger genera. The exact cause(s) for the greater effect of hyperthermal events on origination versus extinction selectivity are not certain, but could indicate that marine ectotherms at the clade level are largely avoiding extinction with respect to body size by adapting to hyperthermal conditions. These adaptations could result in the origination of smaller genera to compensate for the deleterious effects of increased temperatures and deoxygenation on the biochemical reactions that govern ectothermic physiology. Climate-associated perturbations change extinction and origination dynamics relative to background intervals, suggesting that climate-associated evolutionary dynamics in the modern and future ocean may differ from geological background in predictable ways.