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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
patterns completely new compared to their Paleozoic ancestors—and why not? As shown above, this kind of animal was the preeminent, low-oxygen adaptation among all invertebrates. A new kind of coral, the scleractinians, began to build reefs, and many land reptiles returned to the sea. But it is on land that the most sweeping changes in terms of body plan replacements—and body plan experimentation—took place. Never before and never since has the world seen such a diverse group of different anatomies on land. Some were familiar Permian types: the therapsids that survived the Permian extinction diversified and competed with archosaurs for dominance of the land early in the Triassic, but this ascendance was short lived. Many kinds of reptiles were locked in a competitive struggle with them and with each other for land dominance. From mammal-like reptiles to lizards, earliest mammals to true dinosaurs, the Triassic was a huge experiment in animal design.
Why was this? The conventional answer is that the Permian extinction removed so many of the dominant land animals that it opened the way for more innovation than at any other nonextinction time, perhaps any other mass extinction time as well. It was the most devastating of the mass extinctions. Perhaps, as well, it was simply that many terrestrial animal body plans finally came to an evolutionary point of really working efficiently rather than the sprawling posture of the early reptiles and amphibians. Even as late as the end of the Permian and into the Triassic, groups as mature as the dicynodonts and cynodonts were still trying to attain the most efficient kind of upright posture—rather than the less efficient, splayed-leg orientation of the land reptiles—with all of the ramifications and penalties in respiration that this entailed. But perhaps there is more than this. Body plans were being stimulated into creation by intense selective pressures, and dominant among these was the need to access sufficient oxygen to feed, breed, and compete in a low-oxygen world. There is an old adage about nothing sharpening the mind faster than imminent death. The same might be said about evolutionary forces when faced with the most pressing of all selective pressures—attaining the oxygen necessary for the high levels of animal activity that had been evolutionarily attained in the high-oxygen world of the Permian, when nothing was