The tetrapods are the most advanced chordates and are typified by the presence of lungs, a three- or four-chambered heart, and paired appendages. Included here are the classes Amphibia (frogs, toads, and salamanders), Reptilia (lizards, snakes, turtles, and the extinct dinosaurs), Aves (birds), and Mammalia (including the mammals, such as men, dogs, whales, etc.).
CLASS AMPHIBIA.—
The amphibians were the earliest developed four-legged animals and are represented by such living forms as the toads, frogs, and salamanders. Amphibians are cold-blooded animals that primarily breathe by lungs and spend most of their life on land, but during their early stages of development they live in the water where they breathe by means of gills.
The amphibians apparently developed from the crossopterygian fishes during late [Devonian] time and were relatively abundant in the [Pennsylvanian], [Permian], and [Triassic].
Amphibian remains in Texas are confined largely to lower [Paleozoic] and upper [Mesozoic] rocks. Numerous interesting and important discoveries of [fossil] amphibians have been made in north and west Texas where their remains ([Pl. 40]) have been collected in association with early types of reptiles. The areas where [Permian] red beds are exposed in Archer and Baylor counties and where [Triassic] red beds are exposed from Big Spring north along the edge of the High Plains have furnished most of these specimens.
CLASS REPTILIA.—
The reptiles have become adapted to permanent life on land and need not rely on an aquatic environment. They are cold-blooded and are normally characterized by a scaly skin. Reptiles have been much more abundant in the past than they are today, and they assumed many different shapes and sizes in the geologic past. Modern classifications recognize a large number of reptilian groups, but only the more important of these are briefly reviewed here.
Cotylosaurs.—
These were a group of primitive reptiles which, although retaining some amphibian characteristics, became adapted to an exclusively land-dwelling existence. The cotylosaurs lived during the [Pennsylvanian] and [Permian] and apparently became extinct sometime during the late Permian. Cotylosaurs ([Pl. 40]) are well known from the Permian of north Texas.