Amphibians, members of the class Amphibia, whose living forms include frogs, toads, salamanders, newtsand caecilians, are ectothermic tetrapod vertebrates whose non-amniote eggs are not surrounded by membranes. Most amphibians lay their eggs in water, with their larvae underging metamorphosis from a juvenile form with gills to an adult air-breathing form with lungs. Some, however, like the Common Coquí frog develop directly into the adult form, while others like Mudpuppies and olms are paedomorphs that retain the juvenile gilled water-breathing form throughout life. Adult amphibians also use their skin for respiration, with some small terrestrial salamanders even lacking lungs.
The earliest amphibians evolved in the Devonian Period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became dominant during theCarboniferous and Permian periods, but were later displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia.
The three modern orders of amphibians are the Anura (frogs and toads), Caudata (salamanders and newts) and Gymnophiona (caecilians, limbless amphibians that resemble large earthworms with jaws). The total number of known amphibian species is approximately 7,000. They are superficially similar to reptiles, but reptiles, along with mammals and birds, are amniotes, having impervious membranes that surround the egg. With their often complex reproductive needs and permeable skins, amphibians are often ecological indicators, and in recent decades there has been a dramatic decline in amphibian populations of many species around the globe. The smallest vertebrate in the world is the New Guinea frog, Paedophryne amauensis. The largest amphibian is the Chinese Giant Salamander, Andrias davidianus. The study of amphibians is called batrachology while the study of both reptiles and amphibians is called herpetology.