Tetrapoda

a superclass of the subphylum Vertebrata. Tetrapods are predominantly terrestrial animals. However, they include amphibians, which did not lose their ties with water; the groups Ichthyosauria, Plesiosauria, Mosasauridae, Cetácea, and Pinnipedia, which returned to an aquatic way of life; and Pterosauria, birds, and bats, which adapted to life in the air.

In contrast to fish, which move by means of fins, tetrapods have two pairs of extremities that became adapted to locomotion on dry land. Another example of adaptation to life on land was the replacement of gill respiration by pulmonary respiration, which is preserved even in tetrapods that have returned to aquatic life. The body, naked in amphibians, the earliest tetrapods, became covered with horny scales, feathers, and fur in the higher vertebrates—reptiles, birds, and mammals. Other changes include the disconnection of the shoulder girdle from the skull, the separation of the cervical and sacral sections of the spinal column, and the development, in mammals, of the middle ear and later the outer ear to supplement the inner ear. The excretory and circulatory systems of tetrapods improved; in higher reptiles (mammal-like reptiles and Archosauria), birds, and mammals, arterial and venous blood circulated separately, and warm-bloodedness developed. The central nervous system also developed progressively.

Remains of the oldest tetrapods, of the genus Ichthyostega, have been discovered in Upper Devonian deposits.