ATMOSPHERIC OXYGEN AND VERTEBRATE BODY SIZE

There have been many attempts to correlate changing levels of atmospheric oxygen to a wide variety of evolutionary events from extinctions to insect gigantism to evolutionary novelties. However, most of them are backed only by limited fossil data and even fewer have modern physiological studies to support their arguments. To test the impact of varying oxygen on vertebrate development and evolution, Alligator mississippiensis embryos were reared under nine different oxygen levels. The results showed varying oxygen has a strong effect on growth rate, the timing of developmental events, bone density, phosphate concentration, body part size, and mortality rate. Up to 27% oxygen, there is positive effect of increasing oxygen on these factors, but beyond that level increasing oxygen becomes deleterious to the embryos. The deleterious effect is most likely due to an increase in the production of reactive oxidative species under these increased oxygen levels.

With these data in mind, fossil Reptiliomorph body sizes and cranial lengths were measured and compiled with already published data. The fossils included in the study were from deposits ranging from the Upper Carboniferous to the Upper Permian. Oxygen models predict a rise in atmospheric oxygen from near modern day levels to a maximum of 31% during these time periods. The results of this analysis show a strong correlation between rising oxygen levels and an increase in average body size with a drop when the modeled oxygen values rise above ~28%. Additionally, the effect of oxygen on head/body size ratio in the fossil record matches the results obtained from the modern alligator experiments.

This is one of the first attempts to use both modern physiological and fossil data to test the hypothesis that oxygen has been a driver of evolution over geologic time. More studies on modern organisms are necessary to truly understand the potential impacts of variation in atmospheric oxygen on evolution.