Reptiles are air-breathing, cold-blooded vertebrates that have scaly bodies rather than hair or feathers; most reptile species are egg-laying, though certain “squamates” — lizards, snakes and worm-lizards — give birth to live young. The earliest reptile is usually said to have been Hylonomus (a so-called "forest mouse"), which lived about 315 million years ago and resembled contemporary lizards. "Reptile" is an ambiguous category: It usually refers to lizards, snakes, turtles, alligators and crocodiles, but to be genetically consistent should also include birds, since crocodilians are more closely related to birds than to lizards, snakes or turtles. Turtles are so genetically distinct that many scientists recommend treating them as their own class (Chelonia) on an equal footing with birds, mammals, amphibians, fish and reptiles. Some scientists would also elevate crocodilians to the class level.