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.