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. The increased diversity associated with the expansion of each new fauna coincides with an exponential decrease in diversity of the preceding fauna. Major questions remain regarding the cause of these faunal turnovers and also why background extinction rates have declined through time. Here, we use respirometry experiments to investigate whether the parameters controlling temperature-dependent hypoxia responses differ between extant representatives of the Paleozoic and Modern faunas. We performed respirometry experiments on two members of the Paleozoic fauna, brachiopods (T. transversa) and crinoids (F. serratissima), and seven members of the Modern fauna, Atlantic bay scallops (A. irradians), the pink scallop (C. rubida), green, white, and purple urchins (S. droebochiensis, L. pictus, and S. purpuratus), lightning whelk (S. sinistrum), and red abalone (H. rufescens). We can determine the absolute pO₂ tolerance (Pcrit) and the temperature sensitivity of Pcrit of each species by performing these respirometry experiments at five different temperature controls spanning 5-28°C. Body size data of each species was also collected, allowing us to estimate the effect of size on Pcrit. These new experimental data are combined with data from the literature for a preliminary comparison of how these metabolic parameters vary between Paleozoic and Modern fauna. Ultimately, this approach can help us understand how Modern fauna and Paleozoic fauna may respond to ocean warming and deoxygenation and the physiological mechanism driving the turnover events between evolutionary faunas through the Phanerozoic.