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Any of various cold-blooded vertebrates of the class Reptilia, having skin covered with scales or horny plates, breathing air with lungs, and usually having a three-chambered heart. Unlike amphibians, whose eggs are fertilized outside the female body, reptiles reproduce by eggs that are fertilized inside the female. Though once varied, widespread, and numerous, reptilian lineages, including the pterosaurs, ichthyosaurs, plesiosaurs, and dinosaurs, have mostly become extinct (though birds are living descendants of dinosaurs). The earliest reptiles were the cotylosaurs (or stem reptiles) of the late Mississippian or early Pennsylvanian Period, from which mammals evolved. Modern reptiles include crocodiles, snakes, turtles, and lizards.