ing to this idea, then, the amphibians-in-waiting were forced out of these pools and into air. Gradually, those animals that could survive the times of emersion from water had an advantage. These fish still had gills, but the gills themselves allowed some adsorption of oxygen. The problem was that the gills quickly dried out. By evolving ever-tighter and water-resistant pockets around the gills, the transition from gill to lung was under way. But a gill is still an evagination, even if in a pocket. There had to be a complete inversion of this system, for a lung is a series of sacs, whereas a gill is a series of protuberances. It may be that the transitional forms had both gills and primitive lungs.
The transition from aquatic tetrapods such as Ichthyostega or, more probably, Perdepes, involved changes in the wrists, ankles, backbone, and other portions of the axial skeleton that facilitate breathing and locomotion. Rib cages are important to house lungs, while the demands of supporting a heavy body in air, as compared to the near flotation of the same body in water, required extensive changes to the shoulder girdle, pelvic region, and the soft tissues that integrated them. The first forms that had made all of these changes can be thought of as the first terrestrial amphibians and the Perdepes, found in rocks younger than 355 million years in age, may indeed have been the first of all, according to Long and Gordon. But there may be a continuation of the gap after Perdepes. The great radiation of new amphibian species did not occur until 340 million to 330 million years ago. But when it finally took off, it did so in spectacular fashion, and by the end of the Mississippian Period (some 318 million years ago) there were numerous amphibians from localities all over the world.
Let’s reexamine the radiation of amphibian species in the context of atmospheric oxygen levels. A gill is very inefficient when it must act as a lung, and a primitive lung must evolve through many steps before the complex and high surface area surface of internal pockets, all vascularized, with concomitant changes to the circulatory system, is effected. While all these respiratory and circulatory changes are happening, the respiratory system in the stock leading to amphibians would have been less efficient at delivering oxygen than either a gill in water or the complex lung in air that would later be evolved. The high-oxygen peak in the early Devonian would have provided the extra oxy-