Amphibians evolved from the lobe-finned fishes (Rhipidistia) that inhabited shallow freshwaters during the Devonian period. Several types of amphibian appeared, of which one group, the Anthracosauria, gave rise to the reptiles. There is such a close resemblance between the skulls of the Devonian rhipidistian fishes and those of the earliest amphibians that their relationship is not in doubt.

The Palaeozoic amphibians were of two main types, consisting of the orders Temnospondyli ('divided vertebrae') and Anthracosauria ('coal lizards'). The latter contained forms from which the Reptilia evolved. Modern Amphibia are generally considered to be a single monophyletic group and probably evolved from the Temnospondyli. (Monophyly is the condition in which a group or taxon of related organisms shares a common ancestry.)

Placed in the subclass Lissamphibia ('smooth amphibians'), existing or extant amphibians do not enjoy any close affinity with reptiles. Although they possessed lungs,the ancestors of modern Amphibia must have lost their scales and evolved cutaneous respiration through moist skins. This can be deduced from the fact that they could not have expanded their lungs by means of ribs. Instead, they forced air down their throats with the aid of special muscles and the bones which supported the floor of the mouth, as existing lissamphibians do. When the floor of the mouth of a frog or a salamander is lowered, air is sucked in (Cloudsley-Thompson 1999). The nostrils are then closed and the throat muscles force this air down into the lungs. The muscles are then relaxed, allowing the air to be released. It is highly unlikely that such a method of breathing would have been adequate for active life on land unless supplemented by accessory cutaneous respiration.

No fossils have yet been found of the amphibian group immediately ancestral to reptiles, or of the primitive reptilian stock prior to its initial diversification (Currie and Padian 1977; Farlow and Brett-Surman 1977). The oldest reptilian fossils known are of the ancestral mammal-like reptiles and of Anapsida (Fig. 1). The latter are usually regarded as being the more primitive on account of their small size and unspecialised skeletons. They gave rise to the diapsids (see below) which include the majority of reptilian groups, both fossil and extant.

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