One major biological challenge for tetrapods was breathing out of the water. The fish ancestors of tetrapods extracted oxygen from the water using gills. Developing a way to extract oxygen from the air required some key anatomical transformations, such as the development of lungs and a way to pump air in and out of them.
The Dipnoi, or lungfishes, were lobe-finned fishes that appeared in the Early Devonian. Although lungfish were more primitive than the Osteolepiformes, the group of lobe-finned fishes from which tetrapods evolved, it is surmised that lobe-finned fishes had developed respiratory innovations to allow them to breathe air in shallow water habitats. Most fishes have a swim bladder to keep them buoyant in the water. Lungfishes—and probably the osteolepiforms, too—had a primitive lung from which the swim bladder was derived.
Modern tetrapods use two forms of breathing. Costal ventilation is most common; it uses abdominal muscles and the rib cage to expand and contract the lungs, forcing air in and out. This form of respiration is found in all amniotes. Another form of respiration is that of buccal pumping, seen in lungfishes and amphibians such as frogs and salamanders. During buccal pumping, air is sucked into the throat and then squeezed down the throat into the lungs by raising the floor of the mouth.
Fossil evidence suggests that the earliest tetrapods were probably buccal pumpers. Their wide mouths and broad heads would have provided increased volume for moving air in and out of their lungs. They evidently did this without the aid of abdominal muscles and ribs because these features were much reduced in early tetrapods. The high concentration of oxygen in the Devonian and Carboniferous atmosphere would certainly have aided their adaptation to land by fueling their bodies more fully with every breath than is the case with today's land animals.
In time, most land animals drifted to a system of costal ventilation. Clack suggests that this transition, except in the true amphibians and extant lungfishes, probably occurred in three stages as land animals diversified. The first stage would have been buccal pumping only. The second stage would have engaged abdominal muscles only for exhalation. The third stage, achieved by the amniotes, used muscles for breathing in and breathing out.
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