Amphibians are a group of tetrapod vertebrates that include modern-day frogs and toads, caecilians, and newts and salamanders. The first amphibians evolved from lobe-finned fishes approximately 370 million years ago during the Devonian Period. They were the first vertebrates to make the move from life in water to life on land. Despite their early colonization of terrestrial habitats, most lineages of amphibians have never fully severed their ties with aquatic habitats. In this article, we'll take a look at six groups of amphibians, their characteristics and the organisms that belong to each group.
Newts and salamanders diverged from other amphibians during the Permian Period (286 to 248 million years ago). Newts and salamanders are slender-bodied amphibians that have a long tail and four legs. Newts spend most of their life on land and return to water to breed. Salamanders, in contrast, spend their entire lives in water. Newts and salamanders are classified into about ten families, some of which include mole salamanders, giant salamanders, Asiatic salamanders, lungless salamanders, sirens, and mudpuppies.
Frogs and toads belong to the largest of the three groups of amphibians. There are more than 4,000 species of frogs and toads alive today. The earliest known frog-like ancestor is Gerobatrachus, a toothed amphibian that lived about 290 million years ago. Another early frog was Triadobatrachus, an extinct genus of amphibian that dates back 250 million years. Modern adult frogs and toads have four legs but do not have tails. There are about 25 families of frogs including such groups as gold frogs, true toads, ghost frogs, Old World tree frogs, African tree frogs, spadefoot toads, and many others.
Caecilians are the least-known group of amphibians. Caecilians have no limbs and only a very short tail. They have a superficial resemblance to snakes, worms, or eels but are not closely related to any of these animals. The evolutionary history of caecilians remains obscure and few fossils of this group of amphibians have been discovered. Some scientists suggest that caecilians arose from a group of tetrapods known as the Lepospondyli.