The amphibians are vertebrate animals, and represent the descendents of the first tetrapods to emerge from an aquatic lifestyle. The amphibians arose in the early upper Devonian Period (395 million years ago) from the lobe-finned fish (see my article "The Fish, Part III: The Osteichthyans" for more information).
The amphibians differ from the reptiles, birds, and mammals, called amniotes, in the construction of their eggs. Amphibians have an egg with little yolk as food for the embryo, and have no protective shell, necessitating that it be laid in water to prevent it from drying out. In contrast, the amniote egg has a food supply, a store of liquid, and an area for waste storage. It also has, in the case of reptiles and birds, a hard protective shell that allows respiration while preventing dessication. Mammals have an amniote egg that has been modified somewhat to allow it to mature inside the mother.
Amphibians also differ from the amniote animals in that amniotes emerge from their eggs fully formed, resembling their parents, while young amphibians pass through a tadpole stage. Some amphibians have developed the ability to produce non-amniote eggs that can be laid on land, while others have developed a form of viviparity (the ability to give birth to live young).
Modern amphibians all have moist, permeable skin, necessitating that they stay close to water. Their permeable skin makes them particularly susceptible to pollutants. This makes them good indicators of pollution levels; when biologists notice a decline in the amphibian population in an area, it can usually be attributed to an increase in pollutants.
Modern amphibians also have simple lungs, reduced ribs that cannot be used to expand their lungs. They have a simplified heart with three chambers that allows oxygen-rich and oxygen-poor blood to mix, but their metabolic needs are not demanding so this does not seem to be a problem. They also have skulls in which the number and size of the bones are reduced, and reduced teeth.
There are three groups of extant (still living) amphibians, called the Lissamphibia. The frogs and toads are in the group Anura, meaning "without a tail". The Anurans arose in the lower Triassic Period (230 million years ago), and the first true frogs appeared in the Jurassic Period (195 to 140 million years ago). All of the fossil frogs, with the exception of one family, can be identified to families that are still living, indicating that their body plan and lifestyle is one that works.
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