Reptilian

Reptilian is an adjective relating to reptiles, a group of vertebrate animals that includes lizards, snakes, turtles, and crocodilians. In biology, the term describes traits typical of these animals, such as keratinized scales, ectothermy, and egg-laying reproduction, though some lineages exhibit live birth.

In taxonomy, Reptilia is a class within the amniotes that traditionally includes several living orders: Testudines (turtles), Crocodylia (crocodilians), Squamata (lizards and snakes), and Rhynchocephalia (tuataras). Reptiles typically have lungs, a shelled amniotic egg in many species, and skin covered by keratinized scales. Most are ectothermic and rely on behavioral means to regulate body temperature.

Reptiles first appeared about 320 million years ago in the late Carboniferous and diversified into a wide range of forms adapted to terrestrial, freshwater, and marine environments. Birds and crocodilians share ancestry with reptiles, with birds evolving from reptilian archosaurs; in modern classifications, birds are usually treated as a separate group descended from reptiles.

In neuroscience, the term reptilian brain refers to the brainstem and basal ganglia components described in the triune brain model proposed by Paul MacLean. The model simplifies brain evolution and does not accurately map the neuroanatomy of living reptiles, though the phrase persists in popular discourse.

Culturally, the word reptilian also appears in fiction and fringe theories, notably claims about shape-shifting reptilian aliens. Such ideas are not supported by credible scientific evidence and are regarded as pseudoscience.