Reptiles are air-breathing, cold-blooded vertebrates that have skin covered in scales as opposed to hair or feathers. They are tetrapods (having or having descended from vertebrates with four limbs) and amniotes, whose embryos are surrounded by an amniotic membrane. Modern reptiles inhabit every continent with the exception of Antarctica, and are represented by four living orders:
Crocodilia (crocodiles, gavials, caimans and alligators): 23 species
Sphenodontia (tuatara from New Zealand): 2 species
Squamata (lizards, snakes and amphisbaenids ("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, either through ovoviviparity (egg retention), or viviparity (offspring born without use 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 the newly-discovered Jaragua Sphaero, at 1.6 cm (0.6 in), to the Saltwater Crocodile, at up to at least 7 m (23 ft).