Amphibians are animals that live both on land and in water. The members of the class Amphibia are believed to have evolved from the lobe-finned fishes about 370 million years ago, taking advantage of the higher concentration of oxygen in air than in water. A fossil called Tiktaalik, discovered in 2006, was an amazing model of the intermediate form between early tetrapods and their fish ancestors. Tiktaalik had qualities of a fish (fins, gills), but unlike a fish, also possessed a full set of ribs, a neck, and shoulders. The bones of the front fin also had the basic pattern (humerus, radius, ulna, wrist bones) of all limbed animals.
Today’s amphibians are represented by frogs, toads, and salamanders. Amphibians live on land and breathe air to meet their oxygen demands. Amphibians are also able to exchange gases through their lungs, their skin, and the inner lining of their mouths. Gas exchange is enhanced by an efficient circulatory system.
Amphibians remain in moist environments or water to avoid dehydration. Amphibians also lay their eggs in water because the eggs would quickly dry out on land. Sperm cells are released into the water, where they fertilize the egg mass. The early-stage tadpoles lead an aquatic existence and later emerge onto land as adult amphibians.