flourished in high-oxygen conditions, the other in low-oxygen. With this perspective, changing oxygen conditions led to ecological replacements of one group by another. For example, a large pattern of diversification throughout the Triassic (a time of dropping oxygen to low levels) was the result of high-oxygen terrestrial vertebrates, such as the mammal-like reptiles (the group that was the direct ancestor of we mammals, an event taking place in the Triassic Period of the Mesozoic Era) being replaced by low-oxygen organisms, most importantly the saurischian dinosaurs, which were the first-ever dinosaurs.
Let’s summarize this section. While it cannot yet be demonstrated that it was the change in oxygen that actually stimulated the evolutionary change (correlation does not imply causation), our understanding of the importance of respiration to all animals leads to the conjecture that the change in oxygen values was indeed the major stimulus. This can be formalized as follows:
Hypothesis 2.1: Reduced levels of oxygen stimulate higher rates of disparity (the diversity of body plans) than do high levels of oxygen.
Hypothesis 2.2: The diversity of animals is correlated with oxygen levels. The highest diversities are present during times of relatively high-oxygen content.
For terrestrial vertebrates, oxygen levels of less than about 15 percent seem to promote the formation of new taxa, stimulated largely by the anatomical and perhaps physiological needs of organisms in lower-oxygen environments. Another aspect of the influence of low-oxygen levels on animal disparity and diversity is that times of lowered oxygen also produced increased partitioning of land surface by what Ray Huey and I, in a 2005 paper in Science, have named “altitudinal compression.” Our hypothesis suggests that during times in Earth’s history of lowered-oxygen values, even modest elevations would have become effective barriers to gene flow and thus would have stimulated new species formation by isolating populations. Thus, there would have been more endemism (animals found in small geographic areas) during the low-oxygen times.