Reptiles are a cold-blooded vertebrate animal class that regularly sheds their outer layer of skin, yet are also very similar to birds (in fact, the crocodile is more closely related to a bird than to a lizard). Reptiles are not as old as fish, with the earliest reptile on record emerging 315 million years ago, but they were the dominant class during the Mesozoic age until the extinction of the dinosaurs. Like birds, reptiles are an air-breathing class with many of its species laying eggs instead of giving birth to live young. In fact, out of all the reptiles, the only species to give birth to live young are boas and pythons. Being cold-blooded without the ability to regulate their internal temperature, Reptiles become inactive during the cooler times of the year.