VANDEN BROOKS, J.M.; Yale University: The development of Alligator mississippiensis under varying pO₂

Oxygen is the most important environmental component for all terrestrial aerobic organisms. Traditionally, the partial pressure of oxygen (pO₂) was thought to have varied little since its original rise during the Precambrian. Contrary to this, the Berner oxygen curve indicates a marked increase in pO₂ during the Permo-Carboniferous with an upper value of ~31% and a sharp decline at the Permo-Triassic boundary to below modern day levels. Such large scale changes in pO₂ would have had wide ranging effects on vertebrate evolution and development. To begin to examine the effects of varying pO₂ on vertebrate development and evolution, 750 Alligator mississippiensis eggs were raised under nine different pO₂ levels ranging from 16% to 35%. Temperature, humidity, and carbon dioxide were held constant. The results of this experiment demonstrate that pO₂ has an effect on growth rate, the timing of developmental events, bone density, phosphate concentration within the bones, and mortality rate in developing Alligator mississippiensis embryos. In addition, a threshold value of 27% oxygen was determined, beyond which increasing pO₂ has a deleterious effect on development. Up to this value, increasing pO₂ has a positive effect. This data was then applied to the fossil record. Some of the same trends seen in the Alligator experiment were observed in the fossil taxa that existed during the time of the hypothesized oxygen spike. This is the first step in understanding the complex interactions between pO₂ and vertebrate evolution and development.