THE EFFECT OF VARYING PO2 ON VERTEBRATE EVOLUTION

Oxygen is an important atmospheric component that influences the metabolism of terrestrial aerobic organisms. 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 from the Late Carboniferous through the Permian with an upper value of 31% and a sharp decline at the Permo-Triassic boundary. Large scale changes in pO₂ would have effects on bone histology, growth rates, and carbon isotopic ratios of vertebrates.

To examine the effects of varying pO₂ on vertebrate evolution, Alligator mississippiensis eggs were collected and allowed to develop under different pO₂. Forty eggs were placed into each of five separate tanks, with one tank per pO₂ [21%(control), 23%, 25%, 27%, and 30%]. All other variables were held constant both within and between tanks. Growth series were plotted for the organisms in each of the concentrations of oxygen, including both length and weight data. Thin sections were made of the long bones from randomly selected embryos to examine histological changes. The timing of ossifications and cartilage formations were compared vs. pO₂.

With the data gathered from these experiments, a correlation between pO₂ and morphological and geochemical characters was used to establish a paleo-proxy for oxygen. This provides a possible paleontological test for the Berner curve, since histological changes in bones, increased growth rates, and lowered carbon isotopic ratios are expected to correlate to pO₂ in fossils as well. Examples of these changes in the fossil record, specifically in temnospondyls, the hypothesized ancestors of modern amphibians, were used to start to examine these same effects and to begin to better understand the ramifications of the oxygen spike proposed by Berner.