Amphibian

Amphibians (class Amphibia, from Amphi- meaning "on both sides" and -bios meaning "life"), are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic (or cold-blooded) tetrapods. Most Amphibians undergo metamorphosis from a juvenile water-breathing form to an adult air-breathing form, but some are paedomorphs that retain the juvenile water-breathing form throughout life. Mudpuppies, for example, retain juvenile gills in adulthood. The three modern orders of amphibians are Anura (frogs and toads), Caudata (salamanders and newts), and Gymnophiona (caecilians, limbless amphibians that resemble snakes), and in total they number approximately 6,500 species. Many amphibians lay their eggs in water. Amphibians are superficially similar to reptiles, but reptiles are amniotes, along with mammals and birds. The study of amphibians is called batrachology. Amphibians are ecological indicators, and in recent decades there has been a dramatic decline in amphibian populations around the globe. Many species are now threatened or extinct. The earliest amphibians evolved in the Devonian period from lobe-finned fish that used their strong, bony fins to venture onto dry land. They were the top predators in the Carboniferous and Permian periods, but they later faced competition from their descendants, the reptiles, and many lineages were wiped out during the Permian–Triassic extinction. One group, the metoposaurs, remained important predators during the Triassic, but as the world became drier during the Early Jurassic they died out, leaving a handful of relict temnospondyls like Koolasuchus and the modern orders of Lissamphibia.

Etymology
Amphibian is derived from the Ancient Greek term ἀμφίβιος amphíbios, which means both kinds of life, amphi meaning “both” and bio meaning life. The term was initially used for all kinds of combined natures. Eventually it was used to refer to animals that live both in the water and on land.

Evolutionary history
The first major groups of amphibians developed in the Devonian period from lobe-finned fish similar to the modern coelacanth and lungfish, which had evolved multi-jointed leg-like fins that enabled them to crawl along the sea bottom. Some fish had developed primitive lungs to help them breath air when the stagnant pools of the Devonian swamps were lacking in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances required it. Eventually, their bony fins would evolve into limbs and they would become the ancestors to all tetrapods, including amphibians, reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric tetrapodomorph fish still spent most of their time in the water. Amphibians evolved adaptations which allowed them to stay out for longer periods. However, they never developed the ability to live their entire lives on land, having a fully aquatic tadpole stage and still needing to return to water to lay their shell-less eggs.

The first true amphibians appeared in the Carboniferous Period, by which time they were already moving up the food chain and occupying the ecological position currently claimed by such animals as crocodiles. Amphibians were once the top land predators, sometimes reaching several meters in length, preying on the large insects on land and many types of fish in the water. During the Triassic Period, the better-adapted reptiles began to compete with amphibians, leading to the reduction of their size and importance in the biosphere. Lissamphibia, which includes all modern amphibians and is the only surviving lineage of amphibians left, could have branched off from the extinct groups Temnospondyli and Lepospondyli anytime between the mid-Permian to the early Triassic, but the relative scarcity of fossil evidence does not permit an exact date.