A reptile is a tetrapod vertebrate typically having scales or scutes for protection, and typically living on land but also known to live in water and air. The first reptiles appeared during the Carboniferous period, between 310–302 million years ago. Reptiles are cold-blooded animals that have skin covered in scales or scutes. They breathe with lungs and lay eggs on land, although some species are viviparous, giving birth to live young.
There are approximately 10,000 extant reptile species, which are distributed among several groups: lizards (about 6,000 species), snakes (about 3,700), turtles and tortoises (about 300), tuataras (2), amphisbaenians or worm lizards (175), crocodilians (25), and the last remaining member of the order Rhynchocephalia, the tuatara from New Zealand.
Reptiles first appeared during the Carboniferous period about 310–302 million years ago; these early reptiles were diapsids of the Araeoscelidia group who share a more recent common ancestor with birds than with other reptiles. The earliest diapsid fossils found so far come from Scotland’s West Kirkton site dating back 315 million years; they may represent basal members of Araeoscelidia such as Eudibamus cursoris.
Thecladists place reptiles within an evolutionary grade called Sauropsida (“lizard faces”), which includes all descendants of a common ancestor that possessed temporal fenestrae—openings low down on either side of the braincase behind each eye—and which excludes birds and mammals whom they place in separate grades called Synapsida (“fused faces”) and Theria (“wild beasts”).
Lizards comprise about 60% of all reptilian diversity with nearly 6000 species described as of 2016. Snakes represent almost 35% of reptilian diversity with about 3700 species as of 2016. Turtles account for only around 2% of reptilian diversity with just over 300 recognized turtle species as of 2016. Crocodilians make up less than 1%of all reptiles described with just 25 living crocodilian species as of 2009 while tuatara comprise an even smaller fractional amount at two known living species as 2015.
Reptiles have been divided traditionally into four subclasses based on skull anatomy: Anapsida (“without arch”),Synapsida(“with arch”), Diapsida(“two arches”), and Lepidosauromorpha which contains two major clades themselves often split into many subgroups; Lepidosauria (“scaly lizards”) including Sphenodon plus Squamata(lizards & snakes) ,and Archosauromorpha containing crocodilians plus extinct relatives including dinosaurs & pterosaurs . However this traditional taxonomy has been largely rejected by modern cladistic analyses which recover numerous paraphyletic assemblages meaning many “subgroups” nest within others .