Amphibians are cold-blooded tetrapod animals that spend time on land and in water. They lack scales and require water to lay eggs, which hatch into tadpoles and undergo metamorphosis into adult frogs. There are only 6,200 living species, with many extinct amphibians. They are less successful than reptiles and mammals. The first amphibian was Acanthostega, which lived 365 million years ago and resembled a salamander. For 25 million years, amphibians were the only tetrapods until the evolution of reptiles.
An amphibian is any non-amniotic (no shelled egg) tetrapod animal that is cold-blooded and 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 geological past, having been overtaken by reptiles and mammals.
Amphibians are animals that enter and exit the water. Lacking scales like reptiles, they are more prone to drying out, so most species require frequent dipping to stay moist. With the exception of a few species of frogs, these animals depend on pools of fresh water in which to lay their eggs. These eggs sit in the water, some entering into symbiotic relationships with unicellular algae. After a few days, these eggs hatch into tadpoles, the larval form of frogs, which swim around in the water, eating detritus. Through a process called metamorphosis, these tadpoles transform into adult frogs.
These animals are most closely related to mammals which still have a larval stage. Other tetrapods go through their larval stage in the egg or uterus 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 into 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, although there were several early tetrapods that lived around the same time. Acanthostega resembled a salamander, with eight toes on each limb. The limbs are thought to have initially evolved to overcome root-choked swamps, and eventually became strong enough to be used for forays onto land. Because of 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 terrestrial vertebrates, until the evolution of amniotes (reptiles) with animals such as Casineria, which lived 340 million years ago.
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