Fossil Reptiles were air-breathing vertebrates covered in special skin made up of scales, bony plates, or a combination of both. They include crocodiles, snakes, lizards, turtles, Mosasaurs, and Plesiosaurs. Their metabolism depends on the temperature of their environment. Unlike birds and mammals, reptiles do not maintain constant internal body temperature. Without fur or feathers for insulation, they cannot stay warm on a cold day, and without sweat glands or the ability to pant, they cannot cool off on a hot one. Instead, they move into the sun or into the shade as needed. During cooler parts of the year, they become inactive. Because of their slow metabolism and heat-seeking behavior, reptiles are cold-blooded.