Amphibians are a class of cold-blooded vertebrates, mostly four-limbed. They inhabit a wide variety of habitats in freshwater, on or under the ground, or in trees. Typically starting their lives as aquatic larvae with gills, they generally undergo metamorphosis into adults with air-breathing lungs. They use their skins as a secondary respiratory surface; some small terrestrial salamanders and frogs lack lungs and rely entirely on their skins. The earliest amphibians evolved in the Devonian Period from fish with lungs and bony-limbed fins. The three modern orders of amphibians are Anura (the frogs and toads), Caudata (the salamanders), and Gymnophiona (the caecilians). The number of known species is approximately 7,000, of which nearly 90% are frogs. The smallest living amphibian is a frog from New Guinea with a length of just 7.7 mm (0.3 in). The largest is the 1.8 m (5 ft 11 in) Chinese giant salamander, but this is dwarfed by the extinct 9 m (30 ft) Prionosuchus from Brazil. With their complex reproductive needs and permeable skins, amphibians are often indicators of ecological disturbance, and in recent decades their populations have declined around the globe. (Full article...)
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