The Devonian period, some 350 million years ago, some of the crossopterygian fishes came out onto the land. It is very likely that these fishes were of a type represented by the genus Eustenopteron. In evolutionary terms, this was one of the boldest steps in history: a venturing of early vertebrates into a completely new environment to which they were only partially adapted. Once this step had been made, however, it was not long before these advanced, air-breathing fishes became transformed into primitive Amphibia.
From available fossil evidence it has been calculated that the earliest amphibians belonged to a group known as chthyostegids, creatures that had characteristics of both the crossopterygian fishes and the later, more advanced amphibians. A typical chthyostegid, a member of the genus Ichthyostega, had a skull about 15 cm (6 in) in lenght. Although similar in many respects to the skull of crossoptergian fish ancestor, there were certain important changes between that of the fish and that of the amphibian. In the fish, for example, the part of the skull in front of the eyes was comparatively short, while the portion behind the eyes was comparatively long. In Ichthyostega a paradoxical situation prevailed in which the portion of the skull behind the eyes was relatively shorter than that of the fishes and the portion in front relatively longer. In amphibians, the eyes tended to be oriented more toward the top of the skull than in the fishes. Although Ichthyostega had developed strong pectoral and pelvic girdles that carried completely developed limbs and feet, the fin rays of the fish tail were retained! From this early fish/amphibian we can follow the evolution of the later amphibians as they radiated into different lines.
In changing from a totally aquatic experience to a new terrestrial life, various problems had to be resolved. While a fish normally obtains its oxygen from water by means of gills, the early amphibians had to further develop and perfect the lungs that they had inherited from their crossopterygian ancestors, although in the larval stage (as is still the case with modern amphibians) they continued to respire by means of gills. Another problem that land-dwelling animals had to deal with was the possibility of desiccation or drying up.
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