PHANEROZOIC OXYGEN LEVELS AND THEIR EFFECTS ON MODERN VERTEBRATE DEVELOPMENT

Understanding the environment's effect on vertebrate evolution and development is essential to the study of ecology, paleontology and evolution. Oxygen is the most important atmospheric component for all vertebrate groups. Although much attention has been paid to the rise of oxygen during the Precambrian, little has been paid to its continued evolution through the Phanerozoic. The Berner oxygen curve indicates a marked increase in the partial pressure of oxygen (pO₂) during the Permo-Carboniferous with an upper value of ~31% and a sharp decline at the Permo-Triassic boundary. Large scale changes in pO₂ in the atmosphere would have had wide ranging effects on vertebrate evolution and development.

To better understand these effects in the past, it is crucial to understand the effects of varying pO₂ on the development of modern vertebrate groups. To examine this, I have run multiple experiments raising vertebrates under varying pO₂. The original study was carried out on Alligator mississippiensis. Eggs were raised under five different pO₂ ranging from the control at 21% to 30%. The result of this experiment was a positive correlation between developmental weight and pO₂ up to 27% oxygen and then a negative correlation beyond that point. In addition, higher oxygen levels appear to induce earlier signs of hatching. In the highest level of oxygen, a doubling of the mortality rate was observed. This indicates that while there are positive effects of increased oxygen, there are also negative aspects that begin to dominate at higher pO₂. This is only the first step in understanding the complex interaction between pO₂ and vertebrate development.

To further examine these effects, a second experiment on Alligator mississippiensis was carried out to 1) examine the effects of hypoxia, 2) closely examine the effects of pO₂ near 27% and 3) weight the sampling towards earlier development. In addition, I am running a multi-generational experiment to look at the effects of varying pO₂ on Poecilia reticulata (the common guppy fish) in order to 1)examine the effects of varying pO₂ on the entire development of an organism, 2)examine a stepwise increase in pO₂ on development, and 3) test the plasticity of these changes. Observed trends can then be applied to the fossil record to test for similar ones during hypothesized shifts in pO₂.