An amphibian is any non-amniotic (lacking eggs with a shell), cold-blooded, tetrapod animal that spends at least part of its time on land. Living examples include frogs, toads, salamanders, newts, and caecilians. There are only about 6,200 living species described by science, but there are many extinct amphibians in the fossil record. Of the three subclasses of amphibians (Labyrinthodonti, Lepospondyli, Lissamphibia), only one, Lissamphibia, is extant. Amphibians are much less successful today than in the geologic past, having been outcompeted by reptiles and mammals.
Amphibians are animals that move in and out of the water. Lacking scales like reptiles, they are more prone to drying out, and so most species require frequent dips to stay moist. Except for a few frog species, these animals are dependent on pools of fresh water to lay their eggs in. These eggs sit in the water, with some entering into symbiotic relationships with unicellular algae. After a few days, these eggs hatch into tadpoles, the larval form of frogs, which swim through the water, eating detritus. Through a process called metamorphosis, these tadpoles transform into adult frogs.
These animals are most closely related to mammals that still have a larval stage. Other tetrapods go through their larval stage in the egg or womb and emerge as small versions of the adult form. This can be seen in some tropical frogs, which lay their eggs on the forest floor, and hatch in a miniature adult form.
The first amphibian was also the first tetrapod. An animal that lived about 365 million years ago, Acanthostega, is usually cited, though there were several early tetrapods that lived around the same time. Acanthostega resembled a salamander, with eight digits on each limb. It is thought that limbs initially evolved to get through root-choked swamps, and eventually became strong enough to use for forays on land. For their combination of fish and tetrapod characteristics, some of the earliest land animals have been referred to as fishapods. For about 25 million years, amphibians were the only tetrapods and land vertebrates, until the evolution of amniotes (reptiles) with animals like Casineria, which lived 340 million years ago.
