Vertebrates made a significant evolutionary move in the Devonian when the first tetrapods stepped on to the land. Dramatic new discoveries have filled in many details of the transition from fish to tetrapod. The basal tetrapods then radiated extensively during the Carboniferous and Early Permian, some as small semi-aquatic forms, but many as larger forms that fed on fishes and other tetrapods, and that could, in some cases, live fully terrestrial lives.

The basal tetrapods are often termed 'amphibians' in common with the living forms, frogs, salamanders and newts. The name amphibian ('both life') refers to the fact that the modern forms—frogs, newts and salamanders—live both in the water and on land, and it is assumed that many of the fossil forms had similar double lifestyles. The Class Amphibia used to include all the Palaeozoic basal tetrapods, and so was paraphyletic, because it excludes many descendant groups, the reptiles, birds and mammals. The term Amphibia can be redefined to include only the modern groups, which arose in the Triassic and radiated thereafter.

In this chapter, the major anatomical and physiological changes that were necessary when a lobefin fish became a tetrapod are reviewed, and the evolution and biology of the extinct and living forms are described.

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