A period of severe drought prevailed during the Devonian period. This was precisely the time when the crossopterygian fishes were around, and their ability to use atmospheric air favored them. The lung was probably developed by bony fishes because they were haunting poorly oxygenated fresh waters. The dryness may have forced the crossopterygians to crawl on their paired fins from one pond to the other. One supposes that these heavy animals were preadapted, in that they used to walk already at the bottom of the streams on their stumps. The development of fingers also had probably initiated in water, as a means of encroachment on the rocky bottom of swift rivers.
The emergence of the amphibians from the crossopterygians, depending on the existence of a lung, water and salt retention, a muzzle, crawling fins, adequate oxygen transport and delivery, must be considered an extremely rare event, although all the amphibians were required to do was simply to swim on land.
Two options could be taken by the amphibians first venturing on dry land. One was to keep the skin moist with mucus so that it was able to continue fulfilling a respiratory function. The respiratory function in some of these amphibians is then sometimes solely fulfilled through the skin, as can be observed today with certain salamanders. Their lungs have remained rudimentary organs.
Figure 5.17. Ichthyostega is one of the most primitive stegocephale amphibians known. It is still heavily built.
Other amphibians opted for a complete isolation of their interior medium. Their skin became dry and keratinous. In this case, the respiratory function of the skin dwindled to zero. These inherited the earth by giving birth to all the rest of the tetrapodes. In fact, the morphological aspect of these amphibians (Ichtyostega, fig. 5.17) is not much different from that of very heavily built reptiles.
Amphibians may be considered a convergent phase of evolution, which led to the divergent reptilian one.