COMPARING THE HYPOXIA TOLERANCE AND TEMPERATURE SENSITIVITIES OF PALEOZOIC AND MODERN MARINE FAUNA

In a classic analysis of the Phanerozoic fossil record, Sepkoski (1980) determined that all the metazoan classes can be categorized into three great evolutionary faunas: 1) the shelly and trilobite-dominated Cambrian fauna, 2) the brachiopod-dominated Paleozoic fauna, and 3) the mollusc-dominated Modern fauna. Major questions remain regarding the physiological drivers underpinning these macroevolutionary faunal turnover events. Here, we use respirometry experiments to investigate whether tolerance to temperature-dependent hypoxia differs between extant representatives of the Paleozoic and Modern faunas. Specifically, we can determine the absolute pO2 tolerance (Pcrit) and the temperature sensitivity of Pcrit of each species by performing respirometry experiments at five different temperature controls spanning 5-28°C. These data suggest that extant representatives of the Paleozoic fauna may have a greater tolerance for hypoxia, but are also more vulnerable to changes in temperature than extant representatives of the Modern. We then tested these results using an independent biogeographic dataset that was derived from mapping the oxygen-temperature conditions of species’ range data as described in the field guide “Beneath Pacific Tides.” We found similar patterns of temperature-dependent hypoxia tolerance as with the direct respirometry experiments. The next step is to use a Community Earth System Model to better understand how these physiological differences between the Paleozoic and Modern faunas may have contributed to differences in extinction selectivity during the end-Permian mass extinction. Ultimately, this study can help us better understand how various organisms may fare with respect to modern ocean warming and deoxygenation.