Amphibians are vertebrates that usually have an aquatic juvenile and terrestrial adult stage. Modern amphibians include animals such as frogs, salamanders, and caecilians. Frogs have an aquatic juvenile form (the tadpole) that swims with fins and a tail and breathes with gills, while the adult frog has no tail, has legs, and breathes with lungs. Most newts and salamanders, which are long and tailed, also have aquatic juvenile and terrestrial adult forms, although in some, such as the axolotl, aquatic juveniles become sexually mature. Caecilians, often mistaken for snakes, are legless and live in burrows. The term amphibian refers to the fact that most of them live (bio-) both (amphi-) on land and in water. Genetic analyses suggest that all amphibians share a common evolutionary ancestor that lived on the earth during the Devonian period. Amphibians were the first tetrapods, or animals that walked upon four (tetra-) feet (-pod).

The common evolutionary ancestor of all modern amphibians has not been identified, and it certainly was not the only amphibian alive at the time. Nine genera of Devonian amphibians have been found, spanning a 20-million-year period. Many of these fish-amphibian animals lived at the same time, and scientists cannot determine with certainty which if any of them was the ancestor of all modern amphibians. There is no doubt of the evolutionary transition from fish to amphibian, as it was occurring simultaneously in many different lineages.

In April 2006, paleontologist Neil Shubin announced the discovery of a new transitional form between fishes and amphibians. It was named Tiktaalik roseae from a word in the language of the Nunavat, the Canadian First Nations community that owns the fossil. This animal had a skull, neck, ribs, elbows, and wrists that resemble those of later amphibians, but had fishlike fins.

By the early Carboniferous period, there were many different amphibian lineages. All have become extinct except for two: the branch that led to modern amphibians, and the branch that led to reptiles . . .

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