OXYGEN, TEMPERATURE, AND THE CAMBRIAN RADIATION OF ANIMALS

Changes in atmospheric and marine oxygen levels have long been hypothesized as a likely causal driver of the Cambrian radiation of animals. In the most common formulations of this hypothesis, earlier Proterozoic environments lacked sufficient oxygen for animals (or for specific animal ecologies, such as carnivory) and marine environments then became ‘permissive’ following an oxygen rise in the Ediacaran-Cambrian time interval. However, the maximum oxygen levels required to exclude animals from Proterozoic oceans, and the magnitude of subsequent oxygen increase required to facilitate animal radiation in the Cambrian, remain poorly constrained. Some of the best estimates of these oxygen levels are from space-for-time substitutions of modern animal communities along oxygen gradients in low-oxygen environments—but such environments in the modern (i.e., Oxygen Minimum Zones) are invariably cold and stenothermal. Given that oxygen and temperature interact synergistically in determining an animal’s aerobic capacity, OMZs are useful but imperfect analogues. Here, we use a recently developed ecophysiological framework, the Metabolic Index, to investigate the possible role of oxygen and temperature change in the Cambrian radiation. This framework is calibrated using experimental respirometry and biogeographic data and relates the oxygen supply to an organism to its oxygen demand at a given temperature. The approach will ultimately allow us to map the ‘state space’ of oxygen and temperature change that likely could have facilitated the Cambrian radiation. Preliminary results from this framework demonstrate that there are multiple possible paths of oxygen and/or temperature change that could reflect a transition from a non-permissive to permissive ocean for Cambrian-style animals, and that it is difficult to reconcile the presence of animals with an early Paleozoic Earth system that was both very hot and had lower-than-modern atmospheric oxygen levels.