Together with lizards, amphisbaenids, snakes, crocodilians, and the Tuatara, turtles constitute the vertebrate class Reptilia. Reptiles are ectotherms that, in part, have evolved walking limbs and a dry, scaly skin. They evolved from the amphibians during the Pennsylvanian Period, toward the close of the Paleozoic Era. During the following Mesozoic Era the reptiles underwent rapid adaptive radiation and became the dominant animals on earth.
Reptiles, along with the amphibians, represent a transitional group in vertebrate evolution, occurring between the aquatic fishes and the terrestrial birds and mammals. Reptiles were the first vertebrates adapted to life in dry places. While it is true that many amphibians spend much time on land, their eggs must be laid in water or damp places. Reptiles were able to lay a specialized egg, which has a calcareous or parchmentlike shell that retards moisture loss. Hence, reptiles could take advantage of the great expanses of dry land not previously available to their ancestors. Their eggs also have embryonic membranes (amnion, chorion, and allantois) that are not found in amphibian eggs, as well as a yolk sac that contains nutrients. The amnion forms a fluid-filled compartment surrounding the embryo, thus bringing the aquatic environment within the egg.
Other characteristics also evolved that contributed to the freeing of reptiles from the water requirements of their amphibian predecessors. The scaly skin has few surface glands, and very little moisture is lost cutaneously. The well-developed lungs provided ample oxygen for the heart, now a three-chambered organ in most reptiles, but four-chambered in the crocodilians. The partitioning of the ventricle into two chambers was more efficient, produced higher blood pressure, and led indirectly to the development of more efficient kidneys. Other major developments include the appearance of claws on the toes, a palate separating the oral and nasal passages, and the evolution of a male copulatory organ (absent in the Tuatara) which facilitates internal fertilization.