Hyperthermal events have a greater effect on body size origination selectivity than extinction

Ancient hyperthermal events in Earth’s history are ideal to isolate the evolutionary consequences of climate change and other environmental factors from other anthropogenic influences. A key biological trait hypothesized to be sensitive to climate change and straightforward to quantify in fossil data is body size. Most ectotherms modulate their physiological response to temperature and oxygen change in part through their body size. As hyperthermal events include both temperature increases and ocean deoxygenation, these events can have a deleterious effect on ectotherms on the ends of the body size spectrum. Large bodied ectotherms are particularly at risk, as oxygen demand increases with both body size and temperature. Previous work has assessed extinction selectivity of body size across hyperthermal events, but origination selectivity has not been assessed, which may be as important as extinction selectivity. Here, we measure extinction and origination selectivity with respect to body size for genera in six Linnean classes with robust fossil records (Rhynchonellata, Cephalopoda, Echinoidea, “bony fish” [an informal class consisting of Osteichthyes, Actinopteri, and Actinopterygii], Bivalvia, and Gastropoda). We compare selectivity during background intervals with those during hyperthermal events and their associated recovery intervals spanning the Middle Triassic to the Recent. We use capture-mark-recapture statistical models to measure extinction and origination selectivity while addressing the effects of variable sampling completeness with respect to time and body size. We find that genera with smaller body size are preferentially lost to extinction during background intervals, whereas body size is not associated with extinction probability during hyperthermal events. Conversely, originating genera are larger than average during background intervals across all size classes, but vary among classes in their body size pattern immediately following hyperthermal events. Rhynchonellate brachiopods, cephalopods, and echinoids exhibit originators that are smaller, on average, than the survivors after hyperthermal events, whereas originating bivalves and gastropods tend to be larger than the survivors. Bony fish do not exhibit size bias in origination after hyperthermal events. Overall, these results show that hyperthermal events affect both extinction and origination dynamics but have a greater effect on body size origination selectivity than extinction. The exact cause(s) for the greater effect of hyperthermal events on body size in origination versus extinction is not certain, but these results are consistent with previous findings spanning the Phanerozoic that compared body size selectivity patterns during background intervals to those associated with the Big Five mass extinction events. These results show that climate-associated perturbations change extinction and origination dynamics relative to background intervals, suggesting that climate-associated extinction and origination in the modern and future ocean may differ from geological background.