Ward, Peter. "5 The Silurian-Devonian: How an Oxygen Spike Allowed the First Conquest of Land." Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere. Washington, DC: The National Academies Press, 2006.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
the time of the Devonian high-oxygen peak but still in a period of dropping oxygen. This scenario fits the proposal that the times of low, or lowering oxygen, stimulated the most consequential evolutionary changes—the formation of new body plans, which the first tetrapod most assuredly was.
Most of our understanding about the transition from fish to amphibians comes from only a few localities, with the outcrops in Greenland being the most prolific in tetrapod remains. Although the genus Ichythostega is given pride of place in most discussions of animal evolution as being first, actually a different genus, named Ventastega, was first, at about 363 million years ago, followed in several million years by a modest radiation that included Ichythostega,Acanthostega, and Hynerpeton. Are these forms legged fish or fishy amphibians? They are certainly transitional and difficult to categorize. Of these, Ichthyostega is the most renowned. Its bones were first recovered in the 1930s, but they were fragmentary, and it was not until the 1950s that detailed examination led to a reconstruction of the entire skeleton. The animal certainly had well-developed legs, but it also had a fish-like tail. Nevertheless, the legs led to its coronation as the first four-legged land animal. It was only later that further study showed that this inhabitant from so long ago was probably incapable of walking on land. Newer studies of its foot and ankle seemed to suggest that it could not have supported its body without the flotation aid of being immersed in water.
The strata enclosing Ichthyostega and the other primitive tetrapods from Greenland came from a time interval soon after the devastating late Devonian mass extinction, whose cause was most certainly an atmospheric oxygen drop that created widespread anoxia in the seas. The appearance of Ichthyostega and its brethren may have been instigated by this extinction, since evolutionary novelty often follows mass extinction in response to filling empty ecological niches (the traditional view)—and since it was a time of lower oxygen (the view here). And, as postulated in this book, while periods of low oxygen seem to correlate well with times of low organism diversity, just the opposite seems true of the process bringing about radical breakthroughs in body plans: while times of low oxygen may have few spe-