BURNING CALORIES, BURNING OCEAN: THE EFFECTS OF METABOLIC RATE ON SELECTIVITY DURING HYPERTHERMAL-DRIVEN EXTINCTIONS

Climate change is a major driving force behind extinction events in Earth’s history. High temperatures and associated hypoxia during episodes of rapid climate warming, dubbed ‘hyperthermals’, are expected to have had drastic and far-reaching consequences for marine faunas. ‘Multi-stressor global change’ associated with hyperthermal conditions—the combination of warming, deoxygenation, carbon dioxide saturation, and ocean acidification— creates conditions that are unsustainable for marine life. Understanding how climate change has affected organisms and biotas in the past and will continue to affect them into the future, is thus an important consideration moving forward. Metabolic rate and organismal activity levels have been suggested to play a role in determining survivability because as ambient temperatures rise, the metabolic rates and consequently oxygen demands of organisms increase while at the same time oxygen solubility is decreasing. Previous analyses using the activity quotient have indicated that higher activity levels are selected against during major climate-driven extinction events. Here, we directly estimate the metabolic rates of taxa in the fossil record using a general model of metabolic rate based on temperature and biomass, originally derived by Gillooly et al. (2001), in which metabolic rate scales with body mass and ambient temperature. Using datasets of body size from Heim et al. (2015) and Payne and Heim (2020) and paleotemperature estimates from tectonic and climate models, in combination with metabolic rate scaling coefficients from Vladimirova et al. (2003) for bivalves and Vladimirova (2001) for gastropods, we estimate basal metabolic rates for organisms in the fossil record and use these estimates to compare extinction selectivity during major post-Paleozoic hyperthermal-driven mass extinctions to baseline selectivity, we calculate relative hyperthermal vulnerability (RHV), which allows for direct comparisons between organisms with very different rates of turnover. Bivalves and gastropods with higher metabolic rates appear to preferentially survive climate-driven extinctions. Future work will explore the implications of this trend, whether it is followed by other marine invertebrate groups, and whether it aligns with expectations of extinction selectivity based on oxygen demands in a warming and hypoxic ocean.