In common usage, living reptiles comprise four orders: turtles, crocodilians, lizards & snakes, and tuataras. All these tetrapod vertebrates are unable to regulate their body temperature internally and have amniotic development. In scientific classification, or taxonomy, differing definitions of reptiles arose after genetic and paleontological evidence had determined that crocodilians are more closely related to birds than to other living reptiles. Birds are now considered living dinosaurs and are nestled among reptiles from an evolutionary perspective. Unlike crocodilians, birds can internally regulate their temperature. | 
