The salamanders are closest to the basic amphibian stock in form and in method of locomotion. Although supported by limbs, they move with a wriggling motion similar to that of a fish. The frogs and toads are specialized for jumping, with long, muscular hind legs, while the tropical caecilians are burrowing forms that have lost all but vestigial traces of their limbs. The reptiles , which evolved from amphibians during the Carboniferous period, were the first vertebrate group to become entirely independent of water. This was made possible by the development of a scaly, water-resistant skin and of the terrestrial, or amniote, type of egg found in all higher land vertebrates.
Reptilian eggs have porous shells and large amounts of yolk.
Fertilization is internal. Reptiles, including such forms as turtles and sea snakes that have returned to an aquatic life, are air-breathing at all stages, and nearly all lay their eggs on land. Gill passages appear, as in birds and mammals, only in the embryo. During the Mesozoic era, reptiles were exceedingly diverse and numerous. The reptilian dinosaurs included the largest terrestrial animals that have ever lived, as well as many smaller forms.