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, gharials, caimans and alligators): 23 species
* Sphenodontia (tuataras 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.