Reptiles are animals in the (Linnaean) class Reptilia. They are characterized by breathing air, laying tough-shelled amniotic eggs, and having skin covered in scales and/or scutes. Reptiles are classically viewed as having a "cold-blooded" metabolism. They are tetrapods (either having four limbs or being descended from four-limbed ancestors). Modern reptiles inhabit every continent with the exception of Antarctica, and four living orders are currently recognized:
- Crocodilia (crocodiles, gavials, caimans, and alligators): 23 species
- Sphenodontia (tuataras from New Zealand): 2 species
- Squamata (lizards, snakes, and worm lizards): approximately 7,900 species
- Testudines (turtles and tortoises): approximately 300 species
The majority of reptile species are oviparous (egg-laying), although certain species of squamates are capable of giving live birth. This is achieved by either ovoviviparity (egg retention) or viviparity (birth of offspring without the development of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, that grows to only 1.6 cm (0.6 in) to the saltwater crocodile, Crocodylus porosus, that may reach 6 m in length and weigh over 1,000 kg. The science dealing with reptiles is called herpetology.