Reptile

Reptiles Temporal range:Pennsylvanian–present,312–0 mya
Clockwise from above left:Green sea turtle(Chelonia mydas),Tuatara(Sphenodon punctatus),Nile crocodile(Crocodylus niloticus), andSinai agama(Pseudotrapelus sinaitus)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Sauropsida
Class:	Reptilia Laurenti,1768
Living groups
Lepidosauria(lepidosaurs) Rhynchocephalia(tuataraand relatives) Squamata(lizards and snakes) Testudines(turtles) Archosauria(archosaurs) Crocodilia(crocodilians) Aves(birds)(included incladisticanalyses, but excluded in traditional taxonomy) Seetextfor extinct groups.
Global reptile distribution (excluding birds)

Reptileis the common name for one of the main groups of landvertebrates.It is not used so much by biologists, who use more accurate terms.

The name "reptile" comes fromLatinand means "one who creeps". All living reptile species arecold blooded,have scalyskin,and laycleidoic eggs.They excreteuric acid(instead ofurea), and have acloaca.A cloaca is a shared opening for theanus,urinary tract and reproductive ducts. Reptiles also share an arrangement of theheartand major blood vessels which is different from that of mammals.Birds have all of these features.

Many important groups of reptiles are nowextinct,for example themosasaurs.We used to say thedinosaurswere extinct, but they survive in the form of their feathered descendants (birds). Ancient reptiles that do survive include theturtles,thecrocodilesand theTuatara,the lone survivor of its group. The great majority of present-day reptiles aresnakesandlizards.

The study of living reptiles is calledherpetology.

Birds in relation to reptiles

Some reptiles are more closely related tobirdsthan they are to other reptiles. Crocodiles are more closely related to birds than they are to lizards.Theropoddinosaursare even more closely related, because birds evolved from them.

Cladisticwriters prefer to put the birds (over 10,000 species) with what people usually call reptiles.(seeSauropsida)

Taxonomy

Reptilia is anevolutionary graderather than aclade.The main reason is that the term 'reptile' does not include birds, the descendents oftheropoddinosaurs.Another reason is that the word 'reptile' is misleading because many extinct types were very different from living reptiles.

So instead of Reptilia as a taxonomic class, today many experts use ClassSauropsida(which includes all reptiles and birds, living and extinct). ClassSynapsidaincludes mammals and all their forebears.Reptileis still the usualinformalterm to describe livingsnakesandlizards.Mammals are a genuine clade, and so Mammalia is still the taxonomic term.

Since reptiles are notmonophyletic,reclassifying them is one of the key aims of researchers.Some taxonomists, such as Benton,make Sauropsida and Synapsida class-leveltaxa.The two groups split in theCarboniferous,from stem-groupAmniotes(the earlytetrapods,which laidcleidoic eggs).

Eye feature

Amembraneforms an innereyelidin reptiles and birds. Whitish or translucent, it can be drawn across the eye to protect it from dust and keep it moist. It is called thenictitating membrane.

Size

Reptiles can live in large and small sizes. Their land sizes can be both bigger and smaller than mammals.Titanosaurswere the largest land reptiles, and the smallest land reptile is achameleon13.5mm long.

Living reptiles

• Reptiles (living groups only)
  • OrderCrocodilia(crocodiles,gavials,caimans,andalligators): 23 species
  • OrderSphenodontia(TuatarafromNew Zealand): 2 species
  • OrderSquamata(lizards,snakes,andamphisbaenids( "worm-lizards" ): about 7,000 species.
  • OrderTestudines(turtles,tortoises,andterrapins): approximately 300 species

• 
  Crocodile
• 
  Tuatara
• 
  Turtle

• 
  Chameleons,a lizard
  
• 
  Snake
• 
  Komodo dragon,sleeping. Largest livinglizard