Amphibians (class Amphibia), such as frogs, toads, salamanders, newts, and caecilians, are ectothermic or cold-blooded animals that metamorphose from a juvenile, water-breathing form to an adult, air-breathing form. Typically, amphibians have four limbs. Unlike other land animals (amniotes), amphibians lay eggs in water, as their fish ancestors did. Amphibians are superficially similar to reptiles.

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. Scientists do not agree on the cause, but it is widely believed to be a direct result of the amount of water pollution emitted from industrial factories and other similar sources.

Amphibians evolved in the Devonian Period and were top predators in the Carboniferous and Permian Periods, but 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 "amphíbios" which means both kinds of life. Amphi referring to "both" and bio referring to 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 fish similar to the modern coelacanth and lungfish where the fins had evolved into legs. These amphibians were around one to five meters long. The land was safe as the giant fish and sharks in the ocean could not come onto land. However, there were two problems with living out their entire lives on land. Primarily, the food that these amphibians consumed was in the water, but also at this point the skin on most of these amphibians was not water-tight.

In the Carboniferous Period, the amphibians moved up in the food chain and began to occupy the ecological position currently occupied by crocodiles. These amphibians were notable for eating the mega insects on land and many types of fishes in the water. During the Triassic Period, the amphibians started competing with proto-crocodiles, which led to their drop in size in the temperate zones, or leaving for the poles. Amphibians were able to hibernate during the winter, whereas crocodiles could not, allowing the amphibians in higher latitudes protection from the reptiles.

Taxonomy

Traditionally, amphibians have included all tetrapods that are not amniotes. They are divided into three subclasses, of which two are only known as extinct subclasses:

• Subclass Labyrinthodontia (diverse Paleozoic and early Mesozoic group)
• Subclass Lepospondyli (small Paleozoic group)
• Subclass Lissamphibia (frogs, toads, salamanders, newts, etc.)