Reptiles, as most commonly defined, are the animals in the class Reptilia /rɛpˈtɪliə/, a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). The class comprises turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara, and their extinct relatives. In the traditional Linnaean classification system, birds are considered a separate class to reptiles. However, crocodilians and turtles are more closely related to birds than they are to other living reptiles, and so modern cladistic classification systems include birds within Reptilia, redefining the term as a clade. Other cladistic definitions abandon the term reptile altogether in favor of the clade Sauropsida, which refers to all animals more closely related to modern r