View source for Reptile

{{Short description|Class of animals}}
{{About|the animal class}}
{{pp-pc|small=yes}}
{{automatic taxobox
| name = Reptiles
| fossil_range = {{fossil range|Late Carboniferous|Present|ref=<ref name=M2021>{{cite journal|last1=Marjanović|first1=D.|year=2021|title=The Making of Calibration Sausage Exemplified by Recalibrating the Transcriptomic Timetree of Jawed Vertebrates|journal=Frontiers in Genetics|volume=12|article-number=521693|doi=10.3389/fgene.2021.521693|doi-access=free |pmid=34054911 |pmc=8149952 }}</ref>}}
| image = {{Multiple image
| perrow            = 2/2
| total_width       = 275
| caption_align     = center
| image1            =  TWC Wildlife Centre• Stewart Nimmo • MRD 8910.jpg
| caption1          = [[Tuatara]]
| image2            = SaltwaterCrocodile('Maximo').jpg
| caption2          = [[Saltwater crocodile]]
| image3            = Florida Box Turtle Digon3.jpg
| caption3          = [[Common box turtle]]
| image4            = Rhinechis scalaris cropped.jpg
| caption4          = [[Ladder snake]]
| border            = infobox
}}
| taxon = Reptilia
| authority = [[Josephus Nicolaus Laurenti|Laurenti]], 1768
| subdivision_ranks = Extant groups
| subdivision = * [[Lepidosauria]]
** [[Rhynchocephalia]]  {{small|([[tuatara]])}}
** [[Squamata]] {{small|([[lizard]]s and [[snake]]s)}}
*[[Archelosauria]]
** [[Turtle|Testudines]] {{small|(turtles)}}
** [[Archosaur]]ia
*** [[Crocodilia]]
*** [[Dinosauria]] {{small|(dinosaurs)}}
**** [[Bird|Aves]] {{small|(birds, cladistically included)}}

{{small|See [[#Taxonomy|text]] for extinct groups.}}
| range_map = 
| range_map_caption = Global reptile species distribution
}}

'''Reptiles''', as commonly defined,<!-- We are not employing the cladistic definition in the common definition of reptile --> are a group of [[tetrapod]]s with an [[ectotherm]]ic metabolism and [[Amniotic egg|amniotic development]]. Living traditional reptiles comprise four [[Order (biology)|orders]]: [[Testudines]], [[Crocodilia]], [[Squamata]], and [[Rhynchocephalia]]. About 12,000 living species of reptiles are listed in the [[Reptile Database]].<ref>{{cite web |title=Reptile Database News |url=http://www.reptile-database.org/db-info/news.html |access-date=2022-05-25 |website=reptile-database.org}}</ref> The study of the traditional reptile orders, customarily in combination with the study of modern [[amphibian]]s, is called [[herpetology]].

Reptiles have been subject to several conflicting [[Taxonomy|taxonomic]] definitions.<ref name="modestoanderson2004" /> In [[evolutionary taxonomy]], reptiles are gathered together under the [[Class (biology)|class]] '''Reptilia''' ({{IPAc-en|r|E|p|'|t|I|l|i|ə}} {{respell|rep|TIL|ee|ə}}), which corresponds to common usage. Modern [[Cladistics|cladistic taxonomy]] regards that group as [[Paraphyly|paraphyletic]], since [[Genetics|genetic]] and [[Paleontology|paleontological]] evidence has determined that [[crocodilians]] are more closely related to [[bird]]s (class Aves), members of [[Dinosauria]], than to other living reptiles, and thus birds are nested among reptiles from a phylogenetic perspective. Many cladistic systems therefore redefine Reptilia as a [[clade]] ([[Monophyly|monophyletic]] group) including birds, though the precise definition of this clade varies between authors.<ref name="Gauthier-1994-Prothero-Schoch" /><ref name="modestoanderson2004" /> A similar concept is clade [[Sauropsida]], which refers to all [[amniote]]s more closely related to modern reptiles than to [[mammal]]s.<ref name="Gauthier-1994-Prothero-Schoch" />

The earliest known members of the reptile lineage appeared during the late [[Carboniferous]] period, having evolved from advanced [[Reptiliomorpha|reptiliomorph]] tetrapods which became increasingly adapted to life on dry land.<ref>Reisz, R. R. (1981). ''A diapsid reptile from the Pennsylvanian of Kansas''. Natural History Museum, University of Kansas.</ref> Genetic and fossil data argues that the two largest lineages of reptiles, [[Archosauromorpha]] (crocodilians, birds, and kin) and [[Lepidosauromorpha]] (lizards, and kin), diverged during the [[Permian]] period.<ref>{{cite journal |last1=Ezcurra |first1=M.D. |last2=Scheyer |first2=T.M. |last3=Butler |first3=R.J. |year=2014 |title=The origin and early evolution of Sauria: Reassessing the Permian saurian fossil record and the timing of the crocodile-lizard divergence |journal=PLOS ONE |volume=9 |issue=2 |article-number=e89165 |doi=10.1371/journal.pone.0089165 |doi-access=free |pmc=3937355 |pmid=24586565 |bibcode=2014PLoSO...989165E}}</ref> In addition to the living reptiles, there are many diverse groups that are now [[extinction|extinct]], in some cases due to [[extinction event|mass extinction events]]. In particular, the [[Cretaceous–Paleogene extinction event]] wiped out the [[pterosaur]]s, [[Plesiosauria|plesiosaurs]], and all non-avian [[dinosaurs]] alongside many species of [[Crocodyliformes|crocodyliforms]] and [[Squamata|squamates]] (e.g., [[mosasaur]]s). Modern non-bird reptiles inhabit all the continents except Antarctica.

Reptiles are tetrapod [[vertebrates]], creatures that either have four limbs or, like snakes, are descended from four-limbed ancestors. Unlike [[amphibian]]s, reptiles do not have an aquatic larval stage. Most reptiles are [[oviparous]], although several species of squamates are [[viviparity|viviparous]], as were some extinct aquatic clades<ref name=S12>{{cite journal |last=Sander |first=P. Martin |year=2012 |title=Reproduction in early amniotes |journal=Science |volume=337 |issue=6096 |pages=806–808 |doi=10.1126/science.1224301 |pmid=22904001 |bibcode=2012Sci...337..806S |s2cid=7041966 }}</ref>  – the fetus develops within the mother, using a [[Placentation#In lizards and snakes|(non-mammalian) placenta]] rather than contained in an [[eggshell]]. As amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their [[fetus]]es through various forms of placenta analogous to those of [[mammal]]s, with some providing initial care for their hatchlings. [[neontology|Extant]] reptiles range in size from a tiny gecko, ''[[Sphaerodactylus ariasae]]'', which can grow up to {{convert|17|mm|in|1|abbr=on}} to the [[saltwater crocodile]], ''Crocodylus porosus'', which can reach over {{convert|6|m|ft|1|abbr=on}} in length and weigh over {{convert|1000|kg|lb|abbr=on}}.

==Classification==
{{see also|List of reptiles}}

===Classical taxonomy and research ===
{{See also|Skull roof}}
[[File:Reptile003d.jpg|thumb|right|Reptiles, from ''Nouveau Larousse Illustré'', 1897–1904, notice the inclusion of [[amphibian]]s (below the [[crocodilian]]s)]]

In the 13th century, the category of ''reptile'' was recognized in Europe as consisting of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, lizards, assorted amphibians, and worms", as recorded by [[Vincent of Beauvais|Beauvais]] in his ''Mirror of Nature''.<ref>{{cite book | last = Franklin-Brown | first = Mary | year = 2012 | title = Reading the World: Encyclopedic writing in the scholastic age | publisher = The University of Chicago Press | location = Chicago, IL / London, UK | isbn = 978-0-226-26070-9 |pages=223, 377}}</ref>
In the 18th century, the reptiles were, from the outset of classification, grouped with the [[amphibian]]s. [[Carl Linnaeus|Linnaeus]], working from species-poor [[Sweden]], where the [[common adder]] and [[grass snake]] are often found hunting in water, included all reptiles and amphibians in [[class (biology)|class]] [[Amphibia in the 10th edition of Systema Naturae|{{nobr|"III – Amphibia"}}]] in his ''[[Systema Naturæ]]''.<ref name=Linn1758>{{cite book |last=Linnaeus |first=Carolus |author-link=Carl Linnaeus |year=1758 |title=Systema naturae per regna tria naturae: Secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis |language=la |trans-title=System of Nature through the Three Natural Kingdoms, According to classes, orders, genera, species, with characters, differences, synonyms, places |edition=10th |publisher=Holmiae (Laurentii Salvii) |url=https://www.biodiversitylibrary.org/bibliography/542 |access-date=September 22, 2008}}</ref>
The terms ''reptile'' and ''amphibian'' were largely interchangeable, ''reptile'' (from Latin ''repere'', 'to creep') being preferred by the French.<ref>{{cite encyclopedia |title=Amphibia |encyclopedia=Encyclopædia Britannica |edition=9th |year=1878 |url=http://aleph0.clarku.edu/huxley/UnColl/EnBrit/Amphibia.html}}</ref> [[Josephus Nicolaus Laurenti|J.N. Laurenti]] was the first to formally use the term ''Reptilia'' for an expanded selection of reptiles and amphibians basically similar to that of Linnaeus.<ref>{{cite book |author=Laurenti, J.N. |author-link=Josephus Nicolaus Laurenti |year=1768 |title=Specimen medicum, exhibens synopsin reptilium emendatam cum experimentis circa venena |language=la |trans-title=Medical Specimen: Presenting an improved synopsis of reptiles with experiments on poisons |url=http://gdz.sub.uni-goettingen.de/no_cache/dms/load/img/?IDDOC=281657 |format=facsimile |archive-url=https://web.archive.org/web/20150904013401/http://gdz.sub.uni-goettingen.de/no_cache/dms/load/img/?IDDOC=281657 |archive-date=2015-09-04 }} — shows the mixed composition of ''Reptilia''.</ref> Today, the two groups are still commonly treated under the single heading [[herpetology]].

[[File:MosasaurDiscovery.jpg|thumb|left|"Antediluvian monster", a ''[[Mosasaurus]]'' discovered in a [[Maastricht]] limestone quarry, 1770 (contemporary engraving)]]

It was not until the beginning of the 19th century that it became clear that reptiles and amphibians are, in fact, quite different animals, and [[Pierre André Latreille|P.A. Latreille]] erected the class ''Batracia'' (1825) for the latter, dividing the [[tetrapod]]s into the four familiar classes of reptiles, amphibians, birds, and mammals.<ref>{{cite book |author=Latreielle, P.A. |author-link=Pierre André Latreille |year=1804 |title=Nouveau Dictionnaire à Histoire Naturelle |language=fr |trans-title=New Dictionary of Natural History |page=xxiv}} cited in {{cite book |author=Latreille, P.A. |year=1825 |title=Familles naturelles du règne animal, exposés succinctement et dans un ordre analytique |language=fr |trans-title=Natural families of the animal kingdom, succinctly presented in analytical order}}</ref> The British anatomist [[Thomas Henry Huxley|T.H. Huxley]] made Latreille's definition popular and, together with [[Richard Owen]], expanded Reptilia to include the various fossil "[[antediluvian]] monsters", including [[dinosaur]]s and the mammal-like ([[synapsid]]) ''[[Dicynodon]]'' he helped describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at the [[Royal College of Surgeons of England|Royal College of Surgeons]] in 1863, Huxley grouped the vertebrates into [[mammal]]s, sauroids, and ichthyoids (the latter containing the fishes and amphibians). He subsequently proposed the names of [[Sauropsida]] and [[Ichthyopsida]] for the latter two groups.<ref>{{cite periodical |author=Huxley, T.H. |author-link=Thomas Henry Huxley |year=1863 |title=The structure and classification of the Mammalia |series=Hunterian lectures |periodical=Medical Times and Gazette |url=http://aleph0.clarku.edu/huxley/UnColl/Gazettes/Mamma.html}}</ref> In 1866, [[Ernst Haeckel|Haeckel]] demonstrated that vertebrates could be divided based on their reproductive strategies, and that reptiles, birds, and mammals were united by the [[amniotic egg]].

The terms ''Sauropsida'' ("lizard faces") and ''[[synapsid|Theropsida]]'' ("beast faces") were used again in 1916 by [[Edwin Stephen Goodrich|E.S. Goodrich]] to distinguish between lizards, birds, and their relatives on the one hand (Sauropsida) and [[mammal]]s and their extinct relatives (Theropsida) on the other. Goodrich supported this division by the nature of the hearts and blood vessels in each group, and other features, such as the structure of the forebrain. According to Goodrich, both lineages evolved from an earlier stem group, Protosauria ("first lizards") in which he included some animals today considered [[Reptiliomorpha|reptile-like amphibians]], as well as early reptiles.<ref name=goodrich1916>{{cite journal |last=Goodrich |first=E.S. |title=On the classification of the Reptilia |journal=Proceedings of the Royal Society of London B |volume=89 |pages=261–276 |year=1916 |doi=10.1098/rspb.1916.0012 |doi-access=free |issue=615|bibcode=1916RSPSB..89..261G }}</ref>

In 1956, [[D. M. S. Watson|D.M.S. Watson]] observed that the first two groups diverged very early in reptilian history, so he divided Goodrich's Protosauria between them. He also reinterpreted Sauropsida and Theropsida to exclude birds and mammals, respectively. Thus his Sauropsida included [[Procolophonia]], [[Eosuchia]], [[Millerettid|Millerosauria]], [[Turtle|Chelonia]] (turtles), [[Squamata]] (lizards and snakes), [[Sphenodontia|Rhynchocephalia]], [[Crocodilia]], "[[Thecodontia|thecodonts]]" ([[paraphyly|paraphyletic]] [[basal (phylogenetics)|basal]] [[Archosaur]]ia), non-[[wikt:avian|avian]] [[dinosaur]]s, [[pterosaur]]s, [[Ichthyosauria|ichthyosaur]]s, and [[sauropterygia]]ns.<ref name=watson1956>{{cite journal |last=Watson |first=D.M.S. |year=1957 |title=On millerosaurus and the early history of the sauropsid reptiles |journal=Philosophical Transactions of the Royal Society of London B |volume=240 |issue=673 |pages=325–400 |doi=10.1098/rstb.1957.0003 |doi-access=free |bibcode=1957RSPTB.240..325W}}</ref>

In the late 19th century, a number of definitions of Reptilia were offered. The biological traits listed by [[Richard Lydekker|Lydekker]] in 1896, for example, include a single [[occipital condyle]], a jaw joint formed by the [[Quadrate bone|quadrate]] and [[articular]] bones, and certain characteristics of the [[Vertebral column|vertebrae]].<ref>{{cite book |last=Lydekker |first=Richard |year=1896 |series=The Royal Natural History |title=Reptiles and Fishes |pages=[https://archive.org/details/in.ernet.dli.2015.110219/page/n26 2]–3 |publisher=Frederick Warne & Son |location=London, UK |url=https://archive.org/details/in.ernet.dli.2015.110219 |access-date=March 25, 2016}}</ref> The animals singled out by these formulations, the [[amniote]]s other than the mammals and the birds, are still those considered reptiles today.<ref name="tudge"/>

[[File:Captorhinus aguti p.jpg|thumb|right|The first reptiles had an [[anapsid]] type of [[skull roof]], as seen in the [[Permian]] genus ''[[Captorhinus]]'']]

The synapsid/sauropsid division supplemented another approach, one that split the reptiles into four subclasses based on the number and position of [[temporal fenestra]]e, openings in the sides of the skull behind the eyes. This classification was initiated by [[Henry Fairfield Osborn]] and elaborated and made popular by [[Alfred Romer|Romer]]'s classic ''[[Vertebrate Paleontology (Romer)|Vertebrate Paleontology]]''.<ref>{{cite journal | last1=Osborn | first1=H.F. | author-link=Henry Fairfield Osborn | year=1903 | title=The reptilian subclasses ''Diapsida'' and ''Synapsida'' and [the] early history of ''Diaptosauria'' | journal=Memoirs of the American Museum of Natural History | volume=1 | pages=451–507 }}</ref><ref name=romer1933>{{cite book |last=Romer |first=A.S. |author-link=Alfred Romer |orig-date=1933 |year=1966 |title=Vertebrate Paleontology |edition=3rd |publisher=University of Chicago Press |place=Chicago, IL}}</ref> Those four subclasses were:
* [[Anapsid]]a – no fenestrae – [[Captorhinida|cotylosaurs]] and [[turtle|chelonia]] ([[turtle]]s and relatives){{efn|
This taxonomy does not reflect modern molecular evidence, which places turtles within [[Diapsida]].
}}
* [[Synapsida]] – one low fenestra – [[pelycosaur]]s and [[therapsid]]s (the '[[mammal-like reptiles]]')
* [[Euryapsida]]&nbsp;– one high fenestra (above the postorbital and squamosal)&nbsp;– [[protorosaur]]s (small, early lizard-like reptiles) and the marine [[sauropterygia]]ns and [[Ichthyosauria|ichthyosaurs]], the latter called [[Parapsida]] in Osborn's work.
* [[Diapsid]]a – two fenestrae – most reptiles, including [[lizard]]s, [[snake]]s, [[crocodilian]]s, [[dinosaur]]s and [[pterosaur]]s.

[[File:Varanodon1DB.jpg|thumb|right|Phylogenetic classifications group the traditional "mammal-like reptiles", like this ''[[Varanodon]]'', with other synapsids, not with extant reptiles]]
The composition of Euryapsida was uncertain. [[Ichthyosauria|Ichthyosaurs]] were, at times, considered to have arisen independently of the other euryapsids, and given the older name Parapsida. Parapsida was later discarded as a group for the most part (ichthyosaurs being classified as ''[[incertae sedis]]'' or with Euryapsida). However, four (or three if Euryapsida is merged into Diapsida) subclasses remained more or less universal for non-specialist work throughout the 20th&nbsp;century. It has largely been abandoned by recent researchers: In particular, the anapsid condition has been found to occur so variably among unrelated groups that it is not now considered a useful distinction.<ref>{{cite journal |last1=Tsuji |first1=L.A. |last2=Müller |first2=J. |year=2009 |title=Assembling the History of the Parareptilia: Phylogeny, diversification, and a new definition of the clade |journal=Fossil Record |volume=12 |issue=1 |pages=71–81 |doi=10.1002/mmng.200800011 |bibcode=2009FossR..12...71T |doi-access=free}}</ref>

===Phylogenetics and modern definition===
By the early 21st century, vertebrate paleontologists were beginning to adopt [[phylogenetic]] taxonomy, in which all groups are defined in such a way as to be [[clade|monophyletic]]; that is, groups which include all descendants of a particular ancestor. The reptiles as historically defined are [[paraphyly|paraphyletic]], since they exclude both birds and mammals. These respectively evolved from dinosaurs and from early therapsids, both of which were traditionally called "reptiles".<ref name=Brysse2008>{{cite journal |last=Brysse |first=K. |year=2008 |title=From weird wonders to stem lineages: The second reclassification of the Burgess Shale fauna |journal=Studies in History and Philosophy of Science Part C: Biological and Biomedical Sciences |volume=39 |issue=3 |pages=298–313 |doi=10.1016/j.shpsc.2008.06.004 |pmid=18761282 }}</ref> Birds are more closely related to [[crocodilian]]s than the latter are to the rest of extant reptiles. [[Colin Tudge]] wrote:

<blockquote>Mammals are a [[clade]], and therefore the [[Phylogenetic nomenclature|cladists]] are happy to acknowledge the traditional taxon [[Mammal]]ia; and birds, too, are a clade, universally ascribed to the formal taxon [[bird|Aves]]. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class Reptilia is not a clade. It is just a section of the clade [[Amniota]]: The section that is left after the Mammalia and Aves have been hived off. It cannot be defined by [[synapomorphy|synapomorphies]], as is the proper way. Instead, it is defined by a combination of the features it has and the features it lacks: reptiles are the amniotes that lack fur or feathers. At best, the cladists suggest, we could say that the traditional Reptilia are 'non-avian, non-mammalian amniotes'.<ref name=tudge>{{RefTudgeVariety}}</ref></blockquote>

Despite the early proposals for replacing the paraphyletic Reptilia with a monophyletic [[Sauropsida]], which includes birds, that term was never adopted widely or, when it was, was not applied consistently.<ref name=modestoanderson2004>{{cite journal | last1=Modesto | first1=S.P. | last2=Anderson | first2=J.S. | year=2004 | title=The phylogenetic definition of Reptilia | journal=Systematic Biology | pmid=15545258 | volume=53 | issue=5 | pages=815–821 | doi=10.1080/10635150490503026 |doi-access=free}}</ref>

[[File:Bearded Dragon Skeleton.jpg|thumb|Bearded dragon ([[pogona]]) skeleton on display at the [[Museum of Osteology]]]]
When Sauropsida was used, it often had the same content or even the same definition as Reptilia. In 1988, [[Jacques Gauthier]] proposed a [[cladistics|cladistic]] definition of Reptilia as a monophyletic node-based [[crown group]] containing turtles, lizards and snakes, crocodilians, and birds, their common ancestor and all its descendants. While Gauthier's definition was close to the modern consensus, nonetheless, it became considered inadequate because the actual relationship of turtles to other reptiles was not yet well understood at this time.<ref name=modestoanderson2004/> Major revisions since have included the reassignment of synapsids as non-reptiles, and classification of turtles as diapsids.<ref name=modestoanderson2004/> Gauthier 1994 and Laurin and Reisz 1995's definition of Sauropsida defined the scope of the group as distinct and broader than that of Reptilia, encompassing [[Mesosauridae]] as well as Reptilia ''sensu stricto''.<ref name="Gauthier-1994-Prothero-Schoch" /><ref name="Laurin 95" />

A variety of other definitions were proposed by other scientists in the years following Gauthier's paper. The first such new definition, which attempted to adhere to the standards of the [[PhyloCode]], was published by Modesto and Anderson in 2004.<ref name=modestoanderson2004/> Modesto and Anderson reviewed the many previous definitions and proposed a modified definition, which they intended to retain most traditional content of the group while keeping it stable and monophyletic. They defined Reptilia as all amniotes closer to ''[[Lacerta agilis]]'' and ''[[Crocodylus niloticus]]'' than to ''[[Homo sapiens]]''. This stem-based definition is equivalent to the more common definition of Sauropsida, which Modesto and Anderson synonymized with Reptilia, since the latter is better known and more frequently used. Unlike most previous definitions of Reptilia, however, Modesto and Anderson's definition includes birds, as they are within the clade that includes both lizards and crocodiles.<ref name=modestoanderson2004/>

===Taxonomy===
{{See also|List of reptiles|List of snakes}}
General classification of extinct and living reptiles, focusing on major groups.<ref name=benton2005>{{cite book |last=Benton |first=Michael J. |author-link=Michael J. Benton |year=2005 |title=Vertebrate Palaeontology |edition=3rd |publisher=Blackwell Science |location=Oxford, UK |isbn=978-0-632-05637-8 |url=http://palaeo.gly.bris.ac.uk/benton/vertclass.html |access-date=2015-02-15 |archive-date=2008-10-19 |archive-url=https://web.archive.org/web/20081019121413/http://palaeo.gly.bris.ac.uk/benton/vertclass.html }}</ref><ref name=benton2014>{{cite book |last=Benton |first=Michael J. |author-link=Michael J. Benton |year=2014 |title=Vertebrate Palaeontology |edition=4th |publisher=Blackwell Science |location=Oxford, UK |isbn=978-0-632-05637-8}}</ref><!-- Note to editors: the nested infraclasses are as presented in Benton's 4th edition. Infraclass Diapsida contains Infraclass Neodiapsida, which contains infraclasses Archosauromorpha and Lepidosauromorpha, with the latter containing Infraclass Ichthyosauria within an unnamed Infrasubclass -->
* '''Reptilia'''/'''[[Sauropsida]]'''
**{{extinct}}'''[[Parareptilia]]'''
** '''[[Eureptilia]]'''
***{{extinct}}[[Captorhinidae]]
***'''[[Diapsid]]a''' <!-- note that the multiple nested infraclasses is part of the classification in Benton's 4th edition -->
****{{extinct}}[[Araeoscelidia]]
****'''[[Neodiapsida]]'''
*****{{extinct}}[[Drepanosauromorpha]] (placement uncertain)
*****{{extinct}}[[Younginiformes]] ([[paraphyletic]])
*****{{extinct}}[[Ichthyosauromorpha]] (placement uncertain)
*****{{extinct}}[[Thalattosauria]] (placement uncertain)
*****'''[[Sauria]]'''
******'''[[Lepidosauromorpha]]'''
*******[[Lepidosauriformes]]
********[[Rhynchocephalia]] (tuatara)
********[[Squamata]] (lizards and snakes)
******{{extinct}}[[Choristodera]] (placement uncertain)
******{{extinct}}[[Sauropterygia]] (placement uncertain)
******'''[[Pantestudines]]''' (turtles and kin, placement uncertain)
******'''[[Archosauromorpha]]'''
*******{{extinct}}[[Protorosauria]] (paraphyletic)
*******{{extinct}}[[Rhynchosauria]]
*******{{extinct}}[[Allokotosauria]]
*******[[Archosauriformes]]
********{{extinct}}[[Phytosaur]]ia
********'''[[Archosauria]]'''
*********[[Pseudosuchia]]
**********[[Crocodilia]] (crocodilians)
*********[[Avemetatarsalia]]/[[Ornithodira]]
**********{{extinct}}[[Pterosaur]]ia
**********[[Dinosaur]]ia
***********{{extinct}}[[Ornithischia]]
***********[[Saurischia]] (including birds ('''[[Bird|Aves]]'''))

===Phylogeny===
The [[cladistics#Cladograms|cladogram]] presented here illustrates the "family tree" of reptiles, and follows a simplified version of the relationships found by M.S. Lee, in 2013.<ref name=scaffold2013>{{cite journal | last1=Lee | first1=M.S.Y. | year=2013 | title=Turtle origins: Insights from phylogenetic retrofitting and molecular scaffolds | journal=Journal of Evolutionary Biology | volume=26 | issue=12 | pages=2729–2738 | doi=10.1111/jeb.12268 |doi-access=free | pmid=24256520}}</ref> All [[Molecular phylogenetics|genetic]] studies have supported the hypothesis that turtles are diapsids; some have placed turtles within Archosauromorpha,<ref name=scaffold2013/><ref name="Mannen">{{cite journal |author1=Mannena, Hideyuki |author2=Li, Steven S.-L. |year=1999 |title=Molecular evidence for a clade of turtles |journal=[[Molecular Phylogenetics and Evolution]] |volume=13 |issue=1 |pages=144–148 |doi=10.1006/mpev.1999.0640 |pmid=10508547|bibcode=1999MolPE..13..144M }}</ref><ref name=Zardoya/><ref name=Iwabe/><ref name=Roos/><ref name=Katsu/> though a few have recovered turtles as Lepidosauromorpha instead.<ref>{{cite journal |author1=Lyson, Tyler R. |author2=Sperling, Erik A. |author3=Heimberg, Alysha M. |author4=Gauthier, Jacques A. |author5=King, Benjamin L. |author6=Peterson, Kevin J. |year=2012 |title=MicroRNAs support a turtle + lizard clade |journal=[[Biology Letters]] |volume=8 |issue=1 |pages=104–107 |doi=10.1098/rsbl.2011.0477 |doi-access=free |pmid=21775315 |pmc=3259949 |bibcode=2012BiLet...8..104L }}</ref> The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.<ref name=scaffold2013/>

{{clade| style=font-size:80%;line-height:80%
|label1=[[Amniote|Amniota]]
|1={{clade
   |1=[[Synapsida]] ([[mammal]]s and their extinct relatives) [[File:Rattus norvegicus (white background).png|50px]]
   |label2='''[[Sauropsida|Sauropsida / Reptilia]]''' |sublabel2=(total group)
   |2={{clade
         |label1={{extinct}}[[Parareptilia]]
         |1={{clade
               |1={{extinct}}[[Millerettid]]ae [[File:Milleretta BW flipped.jpg|50px]]
               |label2=<span style="color:white;">unnamed</span>
               |2={{clade
                  |1={{extinct}}''[[Eunotosaurus]]''
                  |label2={{extinct}}[[Ankyramorpha]]
                  |2={{clade
                  |1={{extinct}}[[Lanthanosuchidae]] [[File:Lanthanosuchus NT flipped.jpg|50px]]
                  |label2={{extinct}}[[Procolophonia]]
                    |2={{clade
                        |1={{extinct}}[[Pareiasauromorpha]] [[File:Scutosaurus BW flipped.jpg|50px]]
                        |2={{extinct}}[[Procolophonoidea]] [[File:Sclerosaurus1DB.jpg|50px]]
                        }}
                     }}
                 }}
             }}
      |label2=[[Eureptilia]]
      |2={{clade
         |1={{extinct}}[[Captorhinidae]] [[File:Labidosaurus flipped.jpg|50px]]
         |label2=[[Romeriida]]
         |2={{clade
            |1={{extinct}}''[[Paleothyris]]''
            |label2=[[Diapsid]]a
            |2={{clade
               |1={{extinct}}[[Araeoscelidia]] [[File:Spinoaequalis schultzei reconstruction.jpg|50px]]
               |label2=[[Neodiapsida]]
               |2={{clade
               |1={{extinct}}''[[Claudiosaurus]]''[[File:Claudiosaurus white background.jpg|50px]]
               |2={{clade
                  |1={{extinct}}[[Younginiformes]] [[File:Hovasaurus BW flipped.jpg|50px]]
                  |label2='''Crown Reptilia'''/ |sublabel2=[[Sauria]]
                  |2={{clade
                     |label1=[[Pan-Lepidosauria]]/ |sublabel1=[[Lepidosauromorpha]]
                     |1={{clade
                         |1={{extinct}}[[Kuehneosauridae]] [[File:Icarosaurus white background.jpg|50px]]
                         |label2=[[Lepidosauria]]
                         |2={{clade
                            |1=[[Rhynchocephalia]] ([[tuatara]] and their extinct relatives) [[File:Hatteria white background.jpg|50px]]
                            |2=[[Squamata]] ([[lizard]]s and [[snake]]s) [[File:British reptiles, amphibians, and fresh-water fishes (1920) (Lacerta agilis).jpg|50px]] [[File:Python natalensis Smith 1840 white background.jpg|50px]] }} }}
                     |label2=[[Archelosauria]]/ |sublabel2=[[Archosauromorpha]] ''[[sensu lato|s. l.]]''
                     |2={{clade
                        |label1=[[Pan-Testudines]]/ |sublabel2=[[Pantestudines]]
                        |1={{clade
                          |1={{extinct}}[[Eosauropterygia]] [[File:Dolichorhynchops BW flipped.jpg|50px]]
                          |2={{clade
                            |1={{extinct}}[[Placodontia]] [[File:Psephoderma BW flipped.jpg|50px]]
                            |2={{clade
                              |1={{extinct}}''[[Sinosaurosphargis]]''
                              |2={{clade
                                |1={{extinct}}''[[Odontochelys]]''
                                |label2=[[Testudinata]]
                                |2={{clade
                                  |1={{extinct}}''[[Proganochelys]]''
                                  |2=[[Testudines]] ([[turtle]]s) [[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Centrochelys sulcata).jpg|50px]]
                                   }}
                                 }}
                               }}
                             }}
                           }}
                        |label2=[[Pan-Archosauria]]
                        |2={{clade
                           |1={{extinct}}[[Choristodera]] <span style="{{MirrorH}}">[[File:Hyphalosaurus mmartyniuk wiki.png|60px]]</span>
                           |label2=[[Archosauromorpha]] ''[[sensu stricto|s. s.]]''
                           |2={{clade
                              |1={{extinct}}[[Prolacertiformes]] [[File:Prolacerta broomi.jpg|50px]]
                              |2={{clade
                                   |2=[[Archosauriformes]] ([[crocodilian]]s, [[bird]]s, dinosaurs and extinct relatives) <span style="{{MirrorH}}">[[File:Deinosuchus riograndensis.png|50px]]</span> [[File:Meyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg|30 px]]
                                   |1={{clade
                                      |1={{extinct}}[[Rhynchosauria]] [[File:Hyperodapedon BW2 white background.jpg|50px]]
                                      |2={{extinct}}''[[Trilophosaurus]]''[[File:Trilophosaurus buettneri (flipped).jpg|50px]]
                                      }}
                                  }}
                              }}
                           }}
                         }}
                       }}
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}}

===The position of turtles===
The placement of turtles has historically been highly variable. Classically, turtles were considered to be related to the primitive anapsid reptiles.<ref name="Romer, A 1977"/> Molecular work has usually placed turtles within the diapsids. As of 2013, three turtle genomes have been sequenced.<ref name=Gilbert2013>{{cite journal | last1=Gilbert | first1=S.F. | last2=Corfe | first2=I. | date=May 2013 | title=Turtle origins: picking up speed | journal=Dev. Cell | volume=25 | issue=4 | pages=326–328 | doi=10.1016/j.devcel.2013.05.011 |doi-access=free | pmid=23725759 | url=http://www.cell.com/developmental-cell/pdf/S1534-5807(13)00285-2.pdf}}</ref>{{update inline|date=July 2024}} The results place turtles as a [[sister clade]] to the [[archosaur]]s, the group that includes crocodilians, non-avian dinosaurs, and birds.<ref>{{cite journal |first1=Ylenia |last1=Chiari |first2=Vincent |last2=Cahais |first3=Nicolas |last3=Galtier |first4=Frédéric |last4=Delsuc |year=2012 |title=Phylogenomic analyses support the position of turtles as the sister group of birds and crocodiles (Archosauria) |journal=BMC Biology |volume=10 |issue=65 |page=65 |doi=10.1186/1741-7007-10-65 |pmid = 22839781|pmc=3473239 |doi-access=free}}</ref> However, in their comparative analysis of the timing of [[organogenesis]], Werneburg and Sánchez-Villagra (2009) found support for the hypothesis that turtles belong to a separate clade within [[Sauropsida]], outside the [[sauria]]n clade altogether.<ref name="Werneburg and Sánchez-Villagra 2009">{{cite journal |last1=Werneburg |first1=Ingmar |last2=Sánchez-Villagra |first2=Marcelo R. |date=23 April 2009 |title=Timing of organogenesis support[s] basal position of turtles in the amniote tree of life |journal=BMC Evolutionary Biology |volume=9 |issue=1 |page=82 |id=article&nbsp;82 |doi=10.1186/1471-2148-9-82 |pmid=19389226 |pmc=2679012 |bibcode=2009BMCEE...9...82W |issn=2730-7182 |doi-access=free }}</ref>

==Evolutionary history==
{{Main|Evolution of reptiles}}

===Origin of the reptiles===
[[File:Hylonomus BW.jpg|thumb|An early reptile ''[[Hylonomus]]'']]
[[File:Europasaurus holgeri Scene 2.jpg|thumb|Mesozoic scene showing typical reptilian megafauna: [[dinosaur]]s including ''[[Europasaurus holgeri]]'', [[iguanodont]]s, and ''[[Archaeopteryx lithographica]]'' perched on the foreground tree stump]]

The origin of the reptiles lies about 310–320 million years ago, in the steaming swamps of the late [[Carboniferous]] period, when the first reptiles evolved from advanced [[Reptiliomorpha|reptiliomorphs]].<ref name="Laurin 95">
{{Cite journal |author1=Laurin, M. |author2=Reisz, R.R. |year=1995 |title=A reevaluation of early amniote phylogeny |journal=[[Zoological Journal of the Linnean Society]] |volume=113 |issue=2 |pages=165–223 |doi=10.1111/j.1096-3642.1995.tb00932.x |doi-access=free |url=http://www.iucn-tftsg.org/wp-content/uploads/file/Articles/Laurin_and_Reisz_1995.pdf}}</ref>{{Failed verification|date=May 2023|reason=Couldn't validate "310 million years ago"}}

The oldest known animal that may have been an [[amniote]] is ''[[Casineria]]'' (though it may have been a [[Temnospondyli|temnospondyl]]).<ref>{{cite journal | last1 = Paton | first1 = R.L. | last2 = Smithson | first2 = T.R. | last3 = Clack | first3 = J.A. | year = 1999 | title = An amniote-like skeleton from the Early Carboniferous of Scotland | journal = [[Nature (journal)|Nature]] | volume = 398 | issue = 6727| pages = 508–513 | doi=10.1038/19071| bibcode = 1999Natur.398..508P | s2cid = 204992355 }}</ref><ref>{{cite journal|last1=Monastersky |first1=R |year=1999 |title=Out of the Swamps, How early vertebrates established a foothold – with all 10 toes – on land |url=http://www.sciencenews.org/sn_arc99/5_22_99/bob1.htm |journal=Science News |volume=155 |issue=21 |pages=328–330 |doi=10.2307/4011517 |archive-url=https://web.archive.org/web/20110604220710/http://www.sciencenews.org/sn_arc99/5_22_99/bob1.htm |archive-date=June 4, 2011 |jstor=4011517 |url-access=subscription }}</ref><ref>{{cite thesis |section=Chapter&nbsp;6: Walking with early tetrapods: evolution of the postcranial skeleton and the phylogenetic affinities of the Temnospondyli (Vertebrata: Tetrapoda) |first=Kat |last=Pawley |year=2006 |url=http://hdl.handle.net/1959.9/57256 |title=The postcranial skeleton of temnospondyls (Tetrapoda: temnospondyli) |degree=PhD |publisher=La Trobe University |place=Melbourne, AU |hdl=1959.9/57256}}</ref> A series of footprints from the fossil strata of [[Nova Scotia]] dated to {{val|315|ul=Ma}} show typical reptilian toes and imprints of scales.<ref>{{cite journal | last1 = Falcon-Lang | first1 = H.J. | last2 = Benton | first2 = M.J. | last3 = Stimson | first3 = M. | year = 2007 | title = Ecology of early reptiles inferred from Lower Pennsylvanian trackways | journal = [[Journal of the Geological Society]] | volume = 164 | issue = 6| pages = 1113–1118 | doi=10.1144/0016-76492007-015| citeseerx = 10.1.1.1002.5009 | s2cid = 140568921 }}</ref> These tracks are now attributed to ''[[Hylonomus]]'',<ref>{{cite web|url=http://www.sflorg.com/sciencenews/scn101707_01.html |title=Earliest Evidence For Reptiles |publisher=Sflorg.com |date=2007-10-17 |access-date=March 16, 2010|archive-url=https://web.archive.org/web/20110716044246/http://www.sflorg.com/sciencenews/scn101707_01.html |archive-date=July 16, 2011 }}</ref> historically widely regarded as the oldest known reptile, but whose placement in the group has been recently questioned.<ref>{{Cite journal |last1=Jenkins |first1=Xavier A. |last2=Benson |first2=Roger BJ |last3=Ford |first3=David P. |last4=Browning |first4=Claire |last5=Fernandez |first5=Vincent |last6=Dollman |first6=Kathleen |last7=Gomes |first7=Timothy |last8=Griffiths |first8=Elizabeth |last9=Choiniere |first9=Jonah N. |last10=Peecook |first10=Brandon R. |date=2025 |title=Evolutionary assembly of crown reptile anatomy clarified by late Paleozoic relatives of Neodiapsida |journal=Peer Community Journal |language=fr |volume=5 |article-number=e89 |doi=10.24072/pcjournal.620 |issn=2804-3871|doi-access=free }}</ref> It was a small, lizard-like animal, about {{convert|20|to|30|cm}} long, with numerous sharp teeth indicating an insectivorous diet.<ref name=EoDP>{{cite book |editor=Palmer, D.|year=1999 |title= The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals|publisher= Marshall Editions|location=London|page= 62|isbn= 978-1-84028-152-1}}</ref> Other examples include ''[[Westlothiana]]'' (for the moment considered a [[Reptiliomorpha|reptiliomorph]] rather than a true [[amniote]])<ref>{{cite journal | last1 = Ruta | first1 = M. | last2 = Coates | first2 = M.I. | last3 = Quicke | first3 = D.L.J. | year = 2003 | title = Early tetrapod relationships revisited | url = http://pondside.uchicago.edu/oba/faculty/coates/5.RutCoaQuick2003.pdf | journal = Biological Reviews | volume = 78 | issue = 2 | pages = 251–345 | doi = 10.1017/S1464793102006103 | pmid = 12803423 | s2cid = 31298396 | access-date = 2010-08-19 | archive-date = 2008-05-22 | archive-url = https://web.archive.org/web/20080522124644/http://pondside.uchicago.edu/oba/faculty/coates/5.RutCoaQuick2003.pdf }}</ref> and ''[[Paleothyris]]'', both of similar build and presumably similar habit.

However, [[microsaur]]s have been at times considered true reptiles, so an earlier origin is possible.<ref name="auto">{{Cite journal|title=The First Age of Reptiles? Comparing Reptile and Synapsid Diversity, and the Influence of Lagerstätten, During the Carboniferous and Early Permian|first=Neil|last=Brocklehurst|date=July 31, 2021|journal=Frontiers in Ecology and Evolution|volume=9|article-number=669765 |doi=10.3389/fevo.2021.669765|doi-access=free|bibcode=2021FrEEv...969765B }}</ref>

===Rise of the reptiles===
The earliest amniotes, including stem-reptiles (those amniotes closer to modern reptiles than to mammals), were largely overshadowed by larger stem-tetrapods, such as ''[[Cochleosaurus]]'', and remained a small, inconspicuous part of the fauna until the [[Carboniferous Rainforest Collapse]].<ref name="SahneyBentonFalconLang 2010RainforestCollapse">{{cite journal |author=Sahney |first1=S. |last2=Benton |first2=M.J. |last3=Falcon-Lang |first3=H.J. |year=2010 |title=Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica |journal=Geology |volume=38 |issue=12 |pages=1079–1082 |bibcode=2010Geo....38.1079S |doi=10.1130/G31182.1}}</ref> This sudden collapse affected several large groups. Primitive tetrapods were particularly devastated, while stem-reptiles fared better, being ecologically adapted to the drier conditions that followed. Primitive tetrapods, like modern amphibians, need to return to water to lay eggs; in contrast, amniotes, like modern reptiles&nbsp;– whose eggs possess a shell that allows them to be laid on land&nbsp;– were better adapted to the new conditions. Amniotes acquired new niches at a faster rate than before the collapse and at a much faster rate than primitive tetrapods. They acquired new feeding strategies including herbivory and carnivory, previously only having been insectivores and piscivores.<ref name="SahneyBentonFalconLang 2010RainforestCollapse"/> From this point forward, reptiles dominated communities and had a greater diversity than primitive tetrapods, setting the stage for the Mesozoic (known as the Age of Reptiles).<ref name=Sahney-Benton-Ferry-2010>{{cite journal |author1=Sahney, S. |author2=Benton, M.J. |author3=Ferry, P.A. |year=2010 |title=Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land |journal=Biology Letters |doi=10.1098/rsbl.2009.1024 |doi-access=free |volume=6 |pages=544–547 |issue=4 |pmid=20106856 |pmc=2936204}}</ref> One of the best known early stem-reptiles is ''[[Mesosaurus]]'', a genus from the [[Cisuralian|Early Permian]] that had returned to water, feeding on fish.

A 2021 examination of reptile diversity in the Carboniferous and the Permian suggests a much higher degree of diversity than previously thought, comparable or even exceeding that of synapsids. Thus, the "First Age of Reptiles" was proposed.<ref name="auto"/>

===Anapsids, synapsids, diapsids, and sauropsids===
[[File:Skull comparison.png|thumb|left|upright|A&nbsp;=&nbsp;Anapsid,<br /> B&nbsp;=&nbsp;Synapsid,<br /> C&nbsp;=&nbsp;Diapsid]]

It was traditionally assumed that the first reptiles retained an [[anapsid]] skull inherited from their ancestors.<ref name=Coven>{{cite book |author=Coven, R. |year=2000 |title=History of Life |publisher=[[Blackwell Science]] |place=Oxford, UK |page=[https://books.google.com/books?id=qvyBS4gwPF4C&dq=diapsids+ancestors+synapsids&pg=PA154 154] |via=Google Books}}</ref> This type of skull has a [[skull roof]] with only holes for the nostrils, eyes and a [[pineal eye]].<ref name="Romer, A 1977">{{cite book |author1-link=Alfred Romer |author1=Romer, A.S. |author2=Parsons, T.S. |orig-date=1977 |title=The Vertebrate Body |edition=5th |publisher=Saunders |place=Philadelphia, PA  |year=1985}}</ref> The discoveries of [[synapsid]]-like openings (see below) in the skull roof of the skulls of several members of [[Parareptilia]] (the clade containing most of the amniotes traditionally referred to as "anapsids"), including [[Lanthanosuchoidea|lanthanosuchoids]], [[Millerettidae|millerettids]], [[Bolosauridae|bolosaurids]], some [[Nycteroleteridae|nycteroleterids]], some [[Procolophonoidea|procolophonoids]] and at least some [[mesosaur]]s<ref>{{cite journal |author1=Cisneros, Juan C. |author2=Damiani, Ross |author3=Schultz, Cesar |author4=da&nbsp;Rosa, Átila |author5=Schwanke, Cibele |author6=Neto, Leopoldo W. |author7=Aurélio, Pedro L.P. |year=2004 |title=A procolophonoid reptile with temporal fenestration from the middle Triassic of Brazil |journal=Proceedings of the Royal Society B |volume=271 |issue=1547 |pages=1541–1546 |doi=10.1098/rspb.2004.2748 |pmid=15306328 |pmc=1691751}}</ref><ref name=TsujiMuller2009FR>{{cite journal |author1=Tsuji, Linda A. |author2=Müller, Johannes |name-list-style=amp |year=2009 |title=Assembling the history of the Parareptilia: phylogeny, diversification, and a new definition of the clade |journal=Fossil Record |volume=12 |issue=1 |pages=71–81 |doi=10.1002/mmng.200800011 |bibcode=2009FossR..12...71T |doi-access=free}}</ref><ref name=PineiroetalCRP2012>{{cite journal |author1=Piñeiro, Graciela |author2=Ferigolo, Jorge |author3=Ramos, Alejandro |author4=Laurin, Michel |year=2012 |title=Cranial morphology of the Early Permian mesosaurid ''Mesosaurus tenuidens'' and the evolution of the lower temporal fenestration reassessed |journal=Comptes Rendus Palevol |volume=11 |issue=5 |pages=379–391 |doi=10.1016/j.crpv.2012.02.001 |bibcode=2012CRPal..11..379P }}</ref> made it more ambiguous and it is currently uncertain whether the ancestral amniote had an anapsid-like or synapsid-like skull.<ref name=PineiroetalCRP2012/> These animals are traditionally referred to as "anapsids", and form a [[paraphyly|paraphyletic]] basic stock from which other groups evolved.<ref name=modestoanderson2004/> Very shortly after the first amniotes appeared, a lineage called [[Synapsida]] split off; this group was characterized by a temporal opening in the skull behind each eye giving room for the jaw muscle to move. These are the "mammal-like amniotes", or stem-mammals, that later gave rise to the true [[mammals]].<ref>{{cite journal |last1=van&nbsp;Tuninen |first1=M. |last2=Hadly |first2=E.A. |year=2004 |title=Error in estimation of rate and time inferred from the early amniote fossil record and avian molecular clocks |journal=Journal of Molecular Biology |volume=59 |issue=2 |pages=267–276 |pmid=15486700 |doi=10.1007/s00239-004-2624-9 |bibcode=2004JMolE..59..267V |s2cid=25065918}}</ref> Soon after, another group evolved a similar trait, this time with a double opening behind each eye, earning them the name [[Diapsida]] ("two arches").<ref name=Coven/> The function of the holes in these groups was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite.<ref name="Romer, A 1977"/>

Turtles have been traditionally believed to be surviving parareptiles, on the basis of their anapsid skull structure, which was assumed to be primitive trait.<ref>{{cite book |last=Benton |first=M.J. |orig-date=2000 |title=Vertebrate Paleontology |publisher=Blackwell Science |location=London, UK |isbn=978-0-632-05637-8 |title-link=Vertebrate Paleontology (Benton) |edition=3rd |year=2004}} {{ISBN| 978-0-632-05614-9 }}</ref> The rationale for this classification has been disputed, with some arguing that turtles are diapsids that evolved anapsid skulls, improving their armor.<ref name="Laurin 95"/> Later morphological [[phylogenetics|phylogenetic]] studies with this in mind placed turtles firmly within Diapsida.<ref name=Rieppel>{{cite journal |vauthors=Rieppel O, de Braga M |year=1996 |title=Turtles as diapsid reptiles |journal=[[Nature (journal)|Nature]] |volume=384 |issue= 6608 |pages=453–455 |doi=10.1038/384453a0 |bibcode=1996Natur.384..453R |s2cid=4264378 |url=http://doc.rero.ch/record/16242/files/PAL_E3477.pdf }}</ref> All [[Molecular phylogenetics|molecular]] studies have strongly upheld the placement of turtles within diapsids, most commonly as a sister group to extant [[archosaur]]s.<ref name=Zardoya>{{cite journal |last1=Zardoya |first1=R. |last2=Meyer |first2=A.| year=1998|title=Complete mitochondrial genome suggests diapsid affinities of turtles |journal=[[Proceedings of the National Academy of Sciences USA]] |volume=95 |issue=24 |pages=14226–14231 |doi=10.1073/pnas.95.24.14226 |doi-access=free |pmid=9826682 |pmc=24355 |bibcode=1998PNAS...9514226Z}}</ref><ref name=Iwabe>{{cite journal |author1=Iwabe, N. |author2=Hara, Y. |author3=Kumazawa, Y. |author4=Shibamoto, K. |author5=Saito, Y. |author6=Miyata, T. |author7= Katoh, K. |date = 2004-12-29 |title = Sister group relationship of turtles to the bird-crocodilian clade revealed by nuclear DNA-coded proteins |journal = [[Molecular Biology and Evolution]] |volume = 22 |issue = 4 |pages = 810–813  |doi = 10.1093/molbev/msi075 |doi-access=free |pmid = 15625185 }}</ref><ref name =Roos>{{cite journal |last1 = Roos |first1 = Jonas |last2= Aggarwal |first2=Ramesh K. |last3=Janke |first3=Axel |date = Nov 2007 |title = Extended mitogenomic phylogenetic analyses yield new insight into crocodylian evolution and their survival of the Cretaceous–Tertiary boundary |journal = [[Molecular Phylogenetics and Evolution]] |volume = 45 |issue = 2 |pages = 663–673 |doi = 10.1016/j.ympev.2007.06.018 |pmid = 17719245 |bibcode = 2007MolPE..45..663R }}</ref><ref name = "Katsu">{{Cite journal | last1 = Katsu | first1 = Y. | last2= Braun |first2=E.L. |last3=Guillette |first3=L.J. Jr. |last4=Iguchi |first4=T. | title = From reptilian phylogenomics to reptilian genomes: analyses of c-Jun and DJ-1 proto-oncogenes | journal = Cytogenetic and Genome Research | volume = 127 | issue = 2–4 | pages = 79–93 | date = 2010-03-17 | doi = 10.1159/000297715 | pmid = 20234127| s2cid = 12116018 }}</ref>

===Permian reptiles===
With the close of the [[Carboniferous]], the amniotes became the dominant tetrapod fauna. While primitive, terrestrial [[reptiliomorpha|reptiliomorphs]] still existed, the synapsid amniotes evolved the first truly terrestrial [[megafauna]] (giant animals) in the form of [[pelycosaurs]], such as ''[[Edaphosaurus]]'' and the carnivorous ''[[Dimetrodon]]''. In the mid-Permian period, the climate became drier, resulting in a change of fauna: The pelycosaurs were replaced by the [[therapsids]].<ref name="autogenerated2001">[[Edwin Harris Colbert|Colbert, E.H.]] & Morales, M. (2001): ''[[Evolution of the Vertebrates|Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time]]''. 4th edition. John Wiley & Sons, Inc, New York. {{ISBN|978-0-471-38461-8}}.</ref>

Many stem reptile groups continued to flourished throughout the Permian. These included bipedal [[Bolosauridae|bolosaurids]], semiaquatic [[mesosaur]]s, lumbering [[pareiasaur]]s,<ref name="autogenerated2001"/> and lizard-like [[Millerettidae|millerettids]] and [[neodiapsids]].

In the Late Permian, the modern reptiles, or [[Sauria|crown-group reptiles]], evolved and split into two main lineages: the [[Archosauromorpha]] (forebears of [[turtle]]s, [[crocodilian]]s, and [[dinosaur]]s) and the [[Lepidosauromorpha]] (predecessors of modern [[lizard]]s, [[snake]]s, and [[tuatara]]s). Both groups remained lizard-like and relatively small and inconspicuous during the Permian.

===Mesozoic reptiles===
The close of the Permian saw the greatest mass extinction known (see the [[Permian–Triassic extinction event]]), an event prolonged by the combination of two or more distinct extinction pulses.<ref name="SahneyBenton2008RecoveryFromProfoundExtinction">{{cite journal |author1=Sahney, S. |author2=Benton, M.J.  |name-list-style=amp | year=2008 | title=Recovery from the most profound mass extinction of all time | journal=Proceedings of the Royal Society B | doi=10.1098/rspb.2007.1370 |doi-access=free | volume = 275 | pages = 759–765| pmid=18198148 | issue=1636 | pmc=2596898}}</ref> Most of the earlier parareptile and synapsid megafauna disappeared, being replaced by the true reptiles, particularly [[archosauromorpha|archosauromorphs]]. These were characterized by elongated hind legs and an erect pose, the early forms looking somewhat like long-legged crocodiles. The [[archosaur]]s became the dominant group during the [[Triassic]] period, though it took 30 million years before their diversity was as great as the animals that lived in the Permian.<ref name="SahneyBenton2008RecoveryFromProfoundExtinction"/> Archosaurs developed into the well-known [[dinosaur]]s and [[pterosaur]]s, as well as the ancestors of [[crocodilian]]s. Since reptiles, first [[rauisuchia]]ns and then dinosaurs, dominated the Mesozoic era, the interval is popularly known as the "Age of Reptiles". The dinosaurs also developed smaller forms, including the feather-bearing smaller [[theropoda|theropods]]. In the [[Cretaceous]] period, these gave rise to the first true [[birds]].<ref name=divergence>{{Cite journal | last1 = Lee | first1 = Michael SY | last2 = Cau | first2 = Andrea | last3 = Darren | first3 = Naish | last4 = Gareth J. | first4 = Dyke | year = 2013 | title = Morphological Clocks in Paleontology, and a Mid-Cretaceous Origin of Crown Aves | journal = Systematic Biology | doi = 10.1093/sysbio/syt110 |doi-access=free | pmid=24449041 | volume=63 | issue = 3 | pages=442–449}}</ref>

The [[sister group]] to Archosauromorpha is [[Lepidosauromorpha]], containing [[lizard]]s and [[tuatara]]s, as well as their fossil relatives. Lepidosauromorpha contained at least one major group of the Mesozoic sea reptiles: the [[mosasaurs]], which lived during the [[Cretaceous]] period. The phylogenetic placement of other main groups of fossil sea reptiles – the [[ichthyopterygia]]ns (including [[Ichthyosauria|ichthyosaur]]s) and the [[sauropterygia]]ns, which evolved in the early Triassic – is more controversial. Different authors linked these groups either to lepidosauromorphs<ref name=Gauthier-1994-Prothero-Schoch>{{cite book |author=Gauthier, J.A. |year=1994 |section=The diversification of the amniotes |editor1=Prothero, D.R. |editor2=Schoch, R.M. |title=Major Features of Vertebrate Evolution |journal=Short Courses in Paleontology |volume=7 |pages=129–159 |place=Knoxville, TN |publisher=The Paleontological Society |doi=10.1017/S247526300000129X |chapter-url=https://www.cambridge.org/core/journals/short-courses-in-paleontology/article/abs/diversification-of-the-amniotes/EDBFD2920CC4B45A0BB6299C8B787F90 }}</ref> or to archosauromorphs,<ref>{{Cite journal |author=Merck, John W. |year=1997 |title=A phylogenetic analysis of the euryapsid reptiles |journal=Journal of Vertebrate Paleontology |volume=17 |issue=Supplement to 3 |pages=1–93|doi=10.1080/02724634.1997.10011028}}</ref><ref>{{cite conference |author1=Modesto, Sean |author2=Reisz, Robert |author3=Scott, Diane |year=2011 |title=A neodiapsid reptile from the Lower Permian of Oklahoma |conference=71st Annual Meeting |publisher=Society of Vertebrate Paleontology |series=Program and Abstracts |page=160}}</ref><ref>{{cite web |author=Holtz, T. |title=Fossil tetrapods |series=GEOL&nbsp;331 Vertebrate Paleontology&nbsp;II |type=class handouts |website=geol.umd.edu |publisher=[[University of Maryland]], Department of Geology |url=http://www.geol.umd.edu/~tholtz/G331/lectures/331vertsII.html}}</ref> and ichthyopterygians were also argued to be diapsids that did not belong to the least inclusive clade containing lepidosauromorphs and archosauromorphs.<ref>{{cite journal |author1=Motani, Ryosuke |author2=Minoura, Nachio |author3=Ando, Tatsuro |year=1998 |title=Ichthyosaurian relationships illuminated by new primitive skeletons from Japan |journal=[[Nature (journal)|Nature]] |volume=393 |issue=6682 |pages=255–257 |doi=10.1038/30473 |bibcode=1998Natur.393..255M |s2cid=4416186 }}</ref>

===Cenozoic reptiles===
[[File:Varanus priscus Melbourne Museum.jpg|thumb|''[[Megalania|Varanus priscus]]'' was a giant carnivorous [[goanna]] lizard, perhaps as long as 7&nbsp;metres and weighing up to 1,940&nbsp;kilograms<ref>{{cite book |author=Molnar, Ralph E. |year=2004 |title=Dragons in the Dust: The paleobiology of the giant monitor lizard Megalania |publisher=Indiana University Press |location=Bloomington |isbn=978-0-253-34374-1 }}</ref>]]
[[File:Champsosaurus natator.jpg|thumb|Skeleton of ''[[Champsosaurus]]'', a [[choristodere]], the latest surviving order of extinct reptiles. The last known choristoderes are known from the [[Miocene]], around 11.3 million years ago ]]
The close of the [[Cretaceous]] period saw the demise of the Mesozoic era reptilian megafauna (see the [[Cretaceous–Paleogene extinction event]], also known as K-T&nbsp;extinction event). Of the large [[marine reptile]]s, only [[sea turtle]]s were left; and of the non-marine large reptiles, only the semi-aquatic [[crocodilian]]s and broadly similar [[Choristodera|choristoderes]] survived the extinction, with last members of the latter, the lizard-like ''[[Lazarussuchus]]'', becoming extinct in the [[Miocene]].<ref>{{cite journal |last=Evans |first=Susan E. |author2=Klembara, Jozef |year=2005 |title=A choristoderan reptile (Reptilia: Diapsida) from the Lower Miocene of northwest Bohemia (Czech Republic) |journal=Journal of Vertebrate Paleontology |volume=25 |issue=1 |pages=171–184 |doi=10.1671/0272-4634(2005)025[0171:ACRRDF]2.0.CO;2 |s2cid=84097919 }}</ref> Of the great host of dinosaurs dominating the Mesozoic, only the small beaked [[birds]] survived. This dramatic extinction pattern at the end of the Mesozoic led into the Cenozoic. Mammals and birds filled the empty niches left behind by the reptilian megafauna and, while reptile diversification slowed, bird and mammal diversification took an exponential turn.<ref name=Sahney-Benton-Ferry-2010/> However, reptiles were still important components of the megafauna, particularly in the form of large and giant [[tortoise]]s.<ref name=Hansen>{{cite journal |author1=Hansen, D.M. |author2=Donlan, C.J. |author3=Griffiths, C.J. |author4=Campbell, K.J. |date=April 2010 |title=Ecological history and latent conservation potential: Large and giant tortoises as a model for taxon substitutions |journal=[[Ecography (journal)|Ecography]] |volume=33 |issue=2 |pages=272–284 |doi=10.1111/j.1600-0587.2010.06305.x |bibcode=2010Ecogr..33..272H |doi-access=free}}</ref><ref name="Cione">{{cite journal |last=Cione |first=A.L. |author2=Tonni, E.P. |author3=Soibelzon, L. |year=2003 |title=The broken zig-zag: Late Cenozoic large mammal and tortoise extinction in South America |journal=Rev. Mus. Argentino Cienc. Nat. |series=N.S. |volume=5 |issue=1 |pages=1–19 |doi=10.22179/REVMACN.5.26 |doi-access=free }}</ref>

After the extinction of most archosaur and marine reptile lines by the end of the Cretaceous, reptile diversification continued throughout the Cenozoic. [[Squamata|Squamates]] took a massive hit during the K–Pg event, only recovering ten million years after it,<ref name=LBG12>{{cite journal | last1 = Longrich | first1 = Nicholas R. | last2 = Bhullar | first2 = Bhart-Anjan S. | last3 = Gauthier | first3 = Jacques A. | year = 2012 | title = Mass extinction of lizards and snakes at the Cretaceous–Paleogene boundary | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 52 | pages = 21396–21401 | doi = 10.1073/pnas.1211526110 |doi-access=free| pmid=23236177 | pmc=3535637| bibcode = 2012PNAS..10921396L }}</ref> but they underwent a great radiation event once they recovered, and today squamates make up the majority of living reptiles (> 95%).<ref name="The Reptile Database">{{cite web |title = The Reptile Database |url = http://www.reptile-database.org/ |access-date = February 23, 2016}}</ref><ref>{{cite journal |author1=Reeder, Tod W. |author2=Townsend, Ted M. |author3=Mulcahy, Daniel G. |author4=Noonan, Brice P. |author5=Wood, Perry L. Jr. |author6=Sites, Jack W. Jr. |author7=Wiens, John J. |year=2015 |title=Integrated analyses resolve conflicts over squamate reptile phylogeny and reveal unexpected placements for fossil taxa |journal=[[PLOS One]] |volume=10 |issue=3 |article-number=e0118199 |pmid=25803280 |pmc=4372529 |doi=10.1371/journal.pone.0118199 |doi-access=free |bibcode=2015PLoSO..1018199R }}</ref> Approximately 10,000 extant species of traditional reptiles are known, with birds adding about 10,000 more, almost twice the number of mammals, represented by about 5,700&nbsp;living species (excluding [[List of domesticated animals|domesticated]] species).<ref>{{cite report |section=Numbers of threatened species by major groups of organisms (1996–2012) |title=[[IUCN Red List]] |year=2010 |publisher=[[International Union for Conservation of Nature|IUCN]] |section-url=http://www.iucnredlist.org/documents/summarystatistics/2012_1_RL_Stats_Table_1.pdf |access-date=January 30, 2013 |archive-url=https://web.archive.org/web/20130204111508/http://www.iucnredlist.org/documents/summarystatistics/2012_1_RL_Stats_Table_1.pdf |archive-date=February 4, 2013 }}</ref>

{| class="wikitable"
|+
Species diversity of living reptiles (2013)<ref name=":0">{{Cite journal|last1=Pincheira-Donoso|first1=Daniel|last2=Bauer|first2=Aaron M.|last3=Meiri|first3=Shai|last4=Uetz|first4=Peter|date=2013-03-27|title=Global Taxonomic Diversity of Living Reptiles|journal=PLOS ONE|volume=8|issue=3|article-number=e59741|bibcode=2013PLoSO...859741P|doi=10.1371/journal.pone.0059741|issn=1932-6203|pmc=3609858|pmid=23544091|doi-access=free}}</ref>
!Reptile group
!Described species
!Percent of reptile species
|-
|Squamates
|9193
|96.3%
|-
|''- Lizards''
|''5634''
|''59%''
|-
|''- Snakes''
|''3378''
|''35%''
|-
|''- Amphisbaenians''
|''181''
|''2%''
|-
|Turtles
|327
|3.4%
|-
|Crocodilians
|25
|0.3%
|-
|Rhynchocephalians
|1
|0.01%
|-
|Total
|9546
|100%
|}

==Morphology and physiology{{anchor|Systems}}==

===Circulation===
[[File:wiki varano.jpg|thumb|Thermographic image of [[monitor lizard]]s]]

All [[Lepidosauria|lepidosaurs]] and [[turtle]]s have a three-chambered [[heart]] consisting of two [[atrium (heart)|atria]], one variably partitioned [[ventricle (heart)|ventricle]], and two aortas that lead to the [[systemic circulation]]. The degree of mixing of [[oxygen]]ated and deoxygenated blood in the three-chambered heart varies depending on the species and physiological state. Under different conditions, deoxygenated blood can be shunted back to the body or oxygenated blood can be shunted back to the lungs. This variation in blood flow has been hypothesized to allow more effective thermoregulation and longer diving times for aquatic species, but has not been shown to be a [[fitness (biology)|fitness]] advantage.<ref>{{cite journal |last=Hicks |first=James |year=2002 |title=The Physiological and Evolutionary Significance of Cardiovascular Shunting Patterns in Reptiles |journal=News in Physiological Sciences |volume=17 |issue=6 |pages=241–245 |pmid=12433978|doi=10.1152/nips.01397.2002 |s2cid=20040550 }}</ref>

[[File:Bisected Iguana Heart Image.png|thumb|Juvenile [[iguana]] [[heart]] bisected through the ventricle, bisecting the left and right atrium]]
For example,&nbsp;[[iguana]]&nbsp;hearts, like the majority of the&nbsp;[[Squamata|squamate]]&nbsp;hearts, are composed of three chambers with two aorta and one ventricle, cardiac involuntary muscles.<ref>{{Cite news|url=http://veterinarycalendar.dvm360.com/reptilian-cardiovascular-anatomy-and-physiology-evaluation-and-monitoring-proceedings?id=&pageID=1&sk=&date=|title=Reptilian cardiovascular anatomy and physiology: evaluation and monitoring (Proceedings)|last=DABVP|first=Ryan S. De Voe DVM MSpVM DACZM|work=dvm360.com|access-date=2017-04-22|archive-date=2018-11-06|archive-url=https://web.archive.org/web/20181106205150/http://veterinarycalendar.dvm360.com/reptilian-cardiovascular-anatomy-and-physiology-evaluation-and-monitoring-proceedings?id=&pageID=1&sk=&date=}}</ref> The main structures of the heart are the&nbsp;[[sinus venosus]], the pacemaker, the&nbsp;[[Atrium (heart)|left atrium]], the&nbsp;[[Atrium (heart)|right atrium]], the&nbsp;[[Heart valve|atrioventricular valve]], the cavum venosum, cavum arteriosum, the cavum pulmonale, the muscular ridge, the ventricular ridge,&nbsp;[[pulmonary vein]]s, and paired&nbsp;[[aortic arches]].<ref>{{Cite news|url=http://reptile-parrots.com/forums/showthread.php/825-Iguana-Internal-Body-Parts|title=Iguana Internal Body Parts|work=Reptile & Parrots Forum|access-date=2017-04-22|language=en|archive-date=2017-04-22|archive-url=https://web.archive.org/web/20170422123904/http://reptile-parrots.com/forums/showthread.php/825-Iguana-Internal-Body-Parts}}</ref>

Some squamate species (e.g., pythons and monitor lizards) have three-chambered hearts that become functionally four-chambered hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle during [[cardiac cycle|ventricular diastole]] and completely divides it during [[systole (medicine)|ventricular systole]]. Because of this ridge, some of these [[squamata|squamates]] are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts.<ref>{{cite journal | last=Wang | first=Tobias |author2=Altimiras, Jordi |author3=Klein, Wilfried |author4= Axelsson, Michael | title=Ventricular haemodynamics in Python molurus: separation of pulmonary and systemic pressures | journal=The Journal of Experimental Biology | year=2003 | volume=206 | pages=4242–4245 | doi=10.1242/jeb.00681 | pmid=14581594 | issue=Pt 23| doi-access=free | bibcode=2003JExpB.206.4241W }}</ref>

[[Crocodilia]]ns have an anatomically four-chambered heart, similar to [[bird]]s, but also have two systemic aortas and are therefore capable of bypassing their [[pulmonary circulation]].<ref>{{cite journal |last=Axelsson |first=Michael |author2=Craig E. Franklin |year=1997 |title=From anatomy to angioscopy: 164 years of crocodilian cardiovascular research, recent advances, and speculations |journal=Comparative Biochemistry and Physiology A |volume=188 |issue=1 |pages=51–62 |doi=10.1016/S0300-9629(96)00255-1}}</ref> In turtles, the ventricle is not perfectly divided, so a mix of aerated and nonaerated blood can occur.<ref>{{cite web |last1=Zug |first1=George R. |title=Reptile |url=https://www.britannica.com/animal/reptile. Accessed 30 September 2024. |website=Encyclopedia Britannica |publisher=Britannica |access-date=30 September 2024}}</ref>

===Metabolism===
[[File:Homeothermy-poikilothermy.png|thumb|right|Sustained energy output ([[joule]]s) of a typical reptile versus a similar size mammal as a function of core body temperature. The mammal has a much higher peak output, but can only function over a very narrow range of body temperature.]]

Modern non-avian reptiles exhibit some form of [[Ectotherm|cold-bloodedness]] (i.e. some mix of [[poikilotherm]]y, [[ectotherm]]y, and [[bradymetabolism]]) so that they have limited physiological means of keeping the body temperature constant and often rely on external sources of heat. Due to a less stable core temperature than [[bird]]s and [[mammal]]s, reptilian biochemistry requires [[enzyme]]s capable of maintaining efficiency over a greater range of temperatures than in the case for [[warm-blooded]] animals. The optimum body temperature range varies with species, but is typically below that of warm-blooded animals; for many lizards, it falls in the {{convert|24|–|35|C|F}} range,<ref>Huey, R.B. & Bennett, A.F. (1987):Phylogenetic studies of coadaptation: Preferred temperatures versus optimal performance temperatures of lizards. ''Evolution'' No. 4, vol 5: pp. 1098–1115 [http://faculty.washington.edu/hueyrb/pdfs/Coadaptation_Evolution.pdf PDF] {{Webarchive|url=https://web.archive.org/web/20220911080152/http://faculty.washington.edu/hueyrb/pdfs/Coadaptation_Evolution.pdf |date=2022-09-11 }}</ref> while extreme heat-adapted species, like the American [[desert iguana]] ''Dipsosaurus dorsalis'', can have optimal physiological temperatures in the mammalian range, between {{convert|35|and|40|C|F}}.<ref>Huey, R.B. (1982): Temperature, physiology, and the ecology of reptiles. Side 25–91. In Gans, C. & Pough, F.H. (red), ''Biology of the Reptili'' No. 12, Physiology (C). Academic Press, London.[http://faculty.washington.edu/hueyrb/pdfs/temp_phys_ecol_rep_1982.pdf artikkel] {{Webarchive|url=https://web.archive.org/web/20220419174304/http://faculty.washington.edu/hueyrb/pdfs/temp_phys_ecol_rep_1982.pdf |date=2022-04-19 }}</ref> While the optimum temperature is often encountered when the animal is active, the low basal metabolism makes body temperature drop rapidly when the animal is inactive.

As in all animals, reptilian muscle action produces heat. In large reptiles, like [[leatherback turtle]]s, the low surface-to-volume ratio allows this metabolically produced heat to keep the animals warmer than their environment even though they do not have a [[warm-blooded]] metabolism.<ref>{{cite journal | last1 = Spotila | first1 = J.R. | last2 = Standora | first2 = E.A. | year = 1985 | title = Environmental constraints on the thermal energetics of sea turtles | url = | journal = Copeia | volume = 3 | issue = 3| pages = 694–702 | doi = 10.2307/1444763 | jstor = 1444763 }}</ref> This form of homeothermy is called [[gigantothermy]]; it has been suggested as having been common in large [[dinosaur]]s and other extinct large-bodied reptiles.<ref>Paladino, F.V.; Spotila, J.R. & Dodson, P. (1999): A blueprint for giants: modeling the physiology of large dinosaurs. ''The Complete Dinosaur''. Bloomington, Indiana University Press. pp. 491–504. {{ISBN|978-0-253-21313-6}}.</ref><ref>{{cite journal | last1 = Spotila | first1 = J.R. | last2 = O'Connor | first2 = M.P. | last3 = Dodson | first3 = P. | last4 = Paladino | first4 = F.V. | year = 1991 | title = Hot and cold running dinosaurs: body size, metabolism and migration | journal = Modern Geology | volume = 16 | pages = 203–227 }}</ref>

The benefit of a low resting metabolism is that it requires far less fuel to sustain bodily functions. By using temperature variations in their surroundings, or by remaining cold when they do not need to move, reptiles can save considerable amounts of energy compared to endothermic animals of the same size.<ref>Campbell, N.A. & Reece, J.B. (2006): Outlines & Highlights for Essential Biology. ''Academic Internet Publishers''. 396 pp. {{ISBN|978-0-8053-7473-5}}</ref> A crocodile needs from a tenth to a fifth of the food necessary for a [[lion]] of the same weight and can live half a year without eating.<ref name=Garnett>{{cite journal|last=Garnett|first=S. T.|year= 2009|title=Metabolism and survival of fasting Estuarine crocodiles|journal=Journal of Zoology|number=208|volume=4|pages=493–502|doi=10.1111/j.1469-7998.1986.tb01518.x}}</ref> Lower food requirements and adaptive metabolisms allow reptiles to dominate the animal life in regions where net [[calorie]] availability is too low to sustain large-bodied mammals and birds.

It is generally assumed that reptiles are unable to produce the sustained high energy output necessary for long distance chases or flying.<ref>{{cite book |author1=Willmer, P. |author2=Stone, G. |author3=Johnston, I.A. |year=2000 |title=Environmental Physiology of Animals |publisher=Blackwell Science |place=London, UK  |isbn=978-0-632-03517-5}}</ref> Higher energetic capacity might have been responsible for the evolution of [[warm-blooded]]ness in birds and mammals.<ref>{{cite journal | last1 = Bennett | first1 = A. | last2 = Ruben | first2 = J. | year = 1979 | title = Endothermy and Activity in Vertebrates | journal = [[Science (journal)|Science]] | volume = 206 | issue = 4419 | pages = 649–654 | doi = 10.1126/science.493968 | pmid = 493968 | url = http://compphys.bio.uci.edu/bennett/pubs/30.pdf | bibcode = 1979Sci...206..649B | citeseerx = 10.1.1.551.4016 | access-date = 2017-10-27 | archive-date = 2017-08-11 | archive-url = https://web.archive.org/web/20170811152427/http://compphys.bio.uci.edu/bennett/pubs/30.pdf }}</ref> However, investigation of correlations between active capacity and [[Thermoregulation|thermophysiology]] show a weak relationship.<ref name="CGF1">{{cite journal | author = Farmer, C.G. | year = 2000 | title = Parental care: The key to understanding endothermy and other convergent features in birds and mammals | journal = American Naturalist | volume = 155 | issue = 3 | pages = 326–334 | pmid = 10718729 | s2cid = 17932602 | doi = 10.1086/303323 | bibcode = 2000ANat..155..326F }}</ref> Most extant reptiles are carnivores with a sit-and-wait feeding strategy; whether reptiles are cold blooded due to their ecology <!-- 'or because their metabolism is a result of their ecology' : As this it is just a restatement of 'reptiles are cold blooded due to their ecology', essentially it was saying whether A was because of B or A was because of B, which is stupid! --> is not clear. Energetic studies on some reptiles have shown active capacities equal to or greater than similar sized warm-blooded animals.<ref>{{cite journal | last1 = Hicks | first1 = J.W.  | last2 = Farmer | first2 = C.G. | year = 1999 | title = Gas exchange potential in reptilian lungs: Implications for the dinosaur-avian connection | journal = Respiratory Physiology | volume = 117 | issue = 2–3 | pages = 73–83 | pmid=10563436 | doi = 10.1016/S0034-5687(99)00060-2}}</ref>

===Respiratory system===
[[File:X-ray video of a female American alligator (Alligator mississippiensis) while breathing - pone.0004497.s009.ogv|right|thumb|X-ray [[fluoroscopy]] videos of a female American alligator showing contraction of the lungs while breathing]]
All reptiles breathe using [[lung]]s. Aquatic [[turtle]]s have developed more permeable skin, and some species have modified their [[cloaca]] to increase the area for [[gas exchange]].<ref>{{cite book | last=Orenstein | first=Ronald | title=Turtles, Tortoises & Terrapins: Survivors in Armor | publisher=Firefly Books | year=2001 | isbn=978-1-55209-605-5 | url-access=registration | url=https://archive.org/details/turtlestortoises0000oren }}</ref> Even with these adaptations, breathing is never fully accomplished without lungs. Lung ventilation is accomplished differently in each main reptile group. In [[squamata|squamates]], the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature that is used during locomotion. Because of this [[Carrier's constraint|constraint]], most squamates are forced to hold their breath during intense runs. Some, however, have found a way around it. Varanids, and a few other lizard species, employ [[buccal pumping]] as a complement to their normal "axial breathing". This allows the animals to completely fill their lungs during intense locomotion, and thus remain aerobically active for a long time. [[Tupinambis|Tegu lizards]] are known to possess a proto-[[thoracic diaphragm|diaphragm]], which separates the pulmonary cavity from the visceral cavity. While not actually capable of movement, it does allow for greater lung inflation, by taking the weight of the viscera off the lungs.<ref>{{cite journal | last=Klein | first=Wilfied |author2=Abe, Augusto |author3=Andrade, Denis |author4= Perry, Steven | title=Structure of the posthepatic septum and its influence on visceral topology in the tegu lizard, ''Tupinambis merianae'' (Teidae: Reptilia) | journal=Journal of Morphology | volume=258 | issue=2 | year=2003 | pages=151–157 | doi=10.1002/jmor.10136 | pmid=14518009| bibcode=2003JMorp.258..151K | s2cid=9901649 }}</ref>

[[Crocodilia]]ns actually have a muscular diaphragm that is analogous to the mammalian diaphragm. The difference is that the muscles for the crocodilian diaphragm pull the pubis (part of the pelvis, which is movable in crocodilians) back, which brings the liver down, thus freeing space for the lungs to expand. This type of diaphragmatic setup has been referred to as the "[[Liver|hepatic]] [[piston]]". The [[Bronchus|airways]] form a number of double tubular chambers within each lung. On inhalation and exhalation air moves through the airways in the same direction, thus creating a unidirectional airflow through the lungs. A similar system is found in birds,<ref>{{cite journal|last=Farmer|first=CG|author2=Sanders, K |title=Unidirectional airflow in the lungs of alligators|journal=Science|year=2010|volume=327|issue=5963|pages=338–340|doi=10.1126/science.1180219|pmid=20075253|bibcode=2010Sci...327..338F|s2cid=206522844}}</ref> monitor lizards<ref>{{cite journal | last1 = Schachner | first1 = E.R. | last2 = Cieri | first2 = R.L. | last3 = Butler | first3 = J.P. | last4 = Farmer | first4 = C.G. | year = 2013 | title = Unidirectional pulmonary airflow patterns in the savannah monitor lizard | doi = 10.1038/nature12871 | journal = [[Nature (journal)|Nature]] | pmid = 24336209| volume=506 | issue = 7488 | pages=367–370| bibcode = 2014Natur.506..367S | s2cid = 4456381 | url = http://nrs.harvard.edu/urn-3:HUL.InstRepos:32631102 | url-access = subscription }}</ref> and iguanas.<ref>{{cite journal |author1=Cieri, Robert L. |author2=Craven, Brent A. |author3=Schachner, Emma R. |author4=Farmer, C.G. |year=2014 |title=New insight into the evolution of the vertebrate respiratory system and the discovery of unidirectional airflow in iguana lungs |journal=[[Proceedings of the National Academy of Sciences]] |volume=111 |issue=48 |pages=17218–17223 |pmid=25404314 |pmc=4260542 |doi=10.1073/pnas.1405088111 |bibcode=2014PNAS..11117218C |doi-access=free }}{{open access}}</ref>

Most reptiles lack a [[secondary palate]], meaning that they must hold their breath while swallowing. Crocodilians have evolved a bony secondary palate that allows them to continue breathing while remaining submerged (and protect their brains against damage by struggling prey). Skinks (family [[Skink|Scincidae]]) also have evolved a bony secondary palate, to varying degrees. Snakes took a different approach and extended their trachea instead. Their tracheal extension sticks out like a fleshy straw, and allows these animals to swallow large prey without suffering from asphyxiation.<ref>{{Cite journal|last1=Chiodini|first1=Rodrick J.|last2=Sundberg|first2=John P.|last3=Czikowsky|first3=Joyce A.|date=January 1982|editor-last=Timmins|editor-first=Patricia|title=Gross anatomy of snakes.|url=https://www.researchgate.net/publication/241830127|journal=Veterinary Medicine/Small Animal Clinician|via=ResearchGate}}</ref>

====Turtles and tortoises====
[[File:Tortue de Floride Amiens.jpg|thumb|[[Red-eared slider]] taking a gulp of air]]

How [[turtle]]s breathe has been the subject of much study. To date, only a few species have been studied thoroughly enough to get an idea of how those turtles [[Breathing|breathe]]. The varied results indicate that turtles have found a variety of solutions to this problem.

The difficulty is that most [[turtle shell]]s are rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles, such as the Indian flapshell (''[[Indian flapshell turtle|Lissemys punctata]]''), have a sheet of muscle that envelops the lungs. When it contracts, the turtle can exhale. When at rest, the turtle can retract the limbs into the body cavity and force air out of the lungs. When the turtle protracts its limbs, the pressure inside the lungs is reduced, and the turtle can suck air in. Turtle lungs are attached to the inside of the top of the shell (carapace), with the bottom of the lungs attached (via connective tissue) to the rest of the viscera. By using a series of special muscles (roughly equivalent to a [[thoracic diaphragm|diaphragm]]), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction).<ref>{{cite journal |first1=Tyler R. |last1=Lyson |first2=Emma R. |last2=Schachner |first3=Jennifer |last3=Botha-Brink |first4=Torsten M. |last4=Scheyer |first5=Markus |last5=Lambertz |first6=G.S. |last6=Bever |first7=Bruce S. |last7=Rubidge |first8=Kevin |last8=de Queiroz |year=2014|title=Origin of the unique ventilatory apparatus of turtles |journal=Nature Communications |volume=5 |number=5211 |page=5211 |doi=10.1038/ncomms6211 |doi-access=free |pmid=25376734 |bibcode=2014NatCo...5.5211L |url=http://www.zora.uzh.ch/id/eprint/100716/7/LysonEtAl_NatCommun2014_Vol5_OriginVentilationApparatusTurtles_Supplem_s1.pdf}}{{open access}}</ref>

Breathing during locomotion has been studied in three species, and they show different patterns. Adult female green sea turtles do not breathe as they crutch along their nesting beaches. They hold their breath during terrestrial locomotion and breathe in bouts as they rest. North American box turtles breathe continuously during locomotion, and the ventilation cycle is not coordinated with the limb movements.<ref name=Landberg>{{cite journal | last=Landberg | first=Tobias |author2=Mailhot, Jeffrey |author3=Brainerd, Elizabeth | title=Lung ventilation during treadmill locomotion in a terrestrial turtle, ''Terrapene carolina'' | journal=Journal of Experimental Biology | volume=206 | issue=19 | year=2003 | pages=3391–3404| doi=10.1242/jeb.00553| pmid=12939371| doi-access=free | bibcode=2003JExpB.206.3391L }}</ref> This is because they use their abdominal muscles to breathe during locomotion. The last species to have been studied is the red-eared slider, which also breathes during locomotion, but takes smaller breaths during locomotion than during small pauses between locomotor bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells.<ref name=Landberg/>

===Sound production===
Compared with frogs, birds, and mammals, reptiles are less vocal. Sound production is usually limited to [[wikt:hiss|hissing]], which is produced merely by forcing air though a partly closed [[glottis]] and is not considered to be a true vocalization. The ability to vocalize exists in crocodilians, some lizards and turtles; and typically involves vibrating fold-like structures in the [[larynx]] or glottis. Some [[gecko]]s and turtles possess true [[vocal cord]]s, which have [[elastin]]-rich connective tissue.<ref>{{cite journal |author1=Russell, Anthony P. |author2=Bauer, Aaron M. |year=2020 |title=Vocalization by extant nonavian reptiles: A synthetic overview of phonation and the vocal apparatus |journal=The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology |volume=304 |issue=7 |pages=1478–1528 |doi=10.1002/ar.24553 |pmid=33099849 |s2cid=225069598|doi-access=free }}</ref><ref>{{cite book |author1=Capshaw, Grace |author2=Willis, Katie L. |author3=Han, Dawei |author4=Bierman, Hilary S. |year=2020 |section=Reptile sound production and perception |pages=101–118 |editor1=Rosenfeld, Cheryl S. |editor2=Hoffmann, Frauke |title=Neuroendocrine Regulation of Animal Vocalization |publisher=Academic Press |isbn=978-0-12-815160-0}}</ref>

====Hearing in snakes====
Hearing in humans relies on 3&nbsp;parts of the ear; the outer ear that directs sound waves into the ear canal, the middle ear that transmits incoming sound waves to the inner ear, and the inner ear that helps in hearing and keeping their balance. Unlike humans and other mammals, snakes do not possess an outer ear, a middle ear, and a [[Tympanum (anatomy)|tympanum]] but have an inner ear structure with [[cochlea]]s directly connected to their jawbone.<ref>{{cite journal |last1=Christensen |first1=Christian Bech |last2=Christensen-Dalsgaard |first2=Jakob |last3=Brandt |first3=Christian |last4=Madsen |first4=Peter Teglberg |date=2012-01-15 |title=Hearing with an atympanic ear: Good vibration and poor sound-pressure detection in the royal python,''Python regius'' |journal=Journal of Experimental Biology |volume=215 |issue=2 |pages=331–342 |doi=10.1242/jeb.062539 |pmid=22189777 |s2cid=11909208 |issn=1477-9145|doi-access=free |bibcode=2012JExpB.215..331C }}</ref> They are able to feel the vibrations generated from the sound waves in their jaw as they move on the ground. This is done by the use of [[mechanoreceptor]]s, sensory nerves that run along the body of snakes directing the vibrations along the spinal nerves to the brain. Snakes have a sensitive auditory perception and can tell which direction sound being made is coming from so that they can sense the presence of prey or predator but it is still unclear how sensitive snakes are to sound waves traveling through the air.<ref>{{Cite journal |last=YOUNG |first=BRUCE A. |title=A Review of Sound Production and Hearing in Snakes, with a Discussion of Intraspecific Acoustic Communication in Snakes |date=1997 |journal=Journal of the Pennsylvania Academy of Science |volume=71 |issue=1 |pages=39–46 |jstor=44149431 |issn=1044-6753}}</ref>

===Skin===
[[File:Lacertae skin.jpg|thumb|Skin of a [[sand lizard]], showing [[Squamata|squamate reptiles]] iconic [[scale (zoology)|scales]]]]

Reptilian skin is covered in a horny [[Epidermis (zoology)|epidermis]], making it watertight and enabling reptiles to live on dry land, in contrast to amphibians. Compared to mammalian skin, that of reptiles is rather thin and lacks the thick [[dermis|dermal]] layer that produces [[leather]] in mammals.<ref>Hildebran, M. & Goslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. John Wiley & sons inc, New York. 635 pp. {{ISBN|978-0-471-29505-1}}</ref>
Exposed parts of reptiles are protected by [[reptile scales|scales]] or [[scutes]], sometimes with a bony base ([[osteoderm]]s), forming [[armour (zoology)|armor]]. In [[lepidosaur]]s, such as lizards and snakes, the whole skin is covered in overlapping [[epidermis (zoology)|epidermal]] scales. Such scales were once thought to be typical of the class Reptilia as a whole, but are now known to occur only in lepidosaurs.{{citation needed|date=August 2015}} The scales found in turtles and crocodiles are of [[dermis|dermal]], rather than epidermal, origin and are properly termed scutes.{{citation needed|date=August 2015}} In turtles, the body is hidden inside a hard shell composed of fused scutes.

Lacking a thick dermis, reptilian leather is not as strong as mammalian leather. It is used in leather-wares for decorative purposes for shoes, belts and handbags, particularly crocodile skin.

==== Shedding ====
Reptiles shed their skin through a process called [[ecdysis]] which occurs continuously throughout their lifetime. In particular, younger reptiles tend to shed once every five to six weeks while adults shed three to four times a year.<ref>{{cite book|last1=Paterson|first1=Sue|title=Skin Diseases of Exotic Pets|date=December 17, 2007|publisher=Blackwell Science, Ltd.|isbn=978-0-470-75243-2|pages=74–79}}</ref> Younger reptiles shed more because of their rapid growth rate. Once full size, the frequency of shedding drastically decreases. The process of ecdysis involves forming a new layer of skin under the old one. [[Proteolysis|Proteolytic]] enzymes and [[Lymph|lymphatic fluid]] is secreted between the old and new layers of skin. Consequently, this lifts the old skin from the new one allowing shedding to occur.<ref name="Dermatological Diseases in Lizards">{{cite journal|last1=Hellebuyck|first1=Tom|last2=Pasmans|first2=Frank|last3=Haesbrouck|first3=Freddy|last4=Martel|first4=An|title=Dermatological Diseases in Lizards|journal=The Veterinary Journal|date=July 2012|volume=193|issue=1|pages=38–45|doi=10.1016/j.tvjl.2012.02.001|pmid=22417690}}</ref> Snakes will shed from the head to the tail while lizards shed in a "patchy pattern".<ref name="Dermatological Diseases in Lizards" /> [[Dysecdysis]], a common skin disease in snakes and lizards, will occur when ecdysis, or shedding, fails.<ref name="Veterinary Nursing of Exotic Pets">{{cite book|last1=Girling|first1=Simon|title=Veterinary Nursing of Exotic Pets|date=June 26, 2013|publisher=Blackwell Publishing, Ltd.|isbn=978-1-118-78294-1|edition=2}}</ref> There are numerous reasons why shedding fails and can be related to inadequate humidity and temperature, nutritional deficiencies, dehydration and traumatic injuries.<ref name="Dermatological Diseases in Lizards" /> Nutritional deficiencies decrease proteolytic enzymes while dehydration reduces lymphatic fluids to separate the skin layers. Traumatic injuries on the other hand, form scars that will not allow new scales to form and disrupt the process of ecdysis.<ref name="Veterinary Nursing of Exotic Pets" />

===Excretion===
[[Excretion]] is performed mainly by two small [[kidney]]s. In diapsids, [[uric acid]] is the main [[nitrogen]]ous waste product; turtles, like [[mammal]]s, excrete mainly [[urea]]. Unlike the [[mammalian kidney|kidneys of mammals]] and birds, [[Kidney (vertebrates)#Reptile kidney|reptile kidneys]] are unable to produce liquid urine more concentrated than their body fluid. This is because they lack a specialized structure called a [[loop of Henle]], which is present in the [[nephron]]s of birds and mammals. Because of this, many reptiles use the [[colon (anatomy)|colon]] to aid in the [[reabsorption]] of water. Some are also able to take up water stored in the [[Urinary bladder|bladder]]. Excess salts are also excreted by nasal and lingual [[salt gland]]s in some reptiles.

In all reptiles, the urinogenital ducts and the [[rectum]] both empty into an organ called a [[cloaca]]. In some reptiles, a midventral wall in the cloaca may open into a urinary bladder, but not all. It is present in all turtles and tortoises as well as most lizards, but is lacking in the [[monitor lizard]], the [[legless lizard]]s. It is absent in the snakes, alligators, and crocodiles.<ref>{{cite book |author=Rand, Herbert W. |year=1950 |title=The Chordates |publisher=Balkiston |url=https://archive.org/stream/chordates00rand/#page/276/mode/1up/search/bladder}}</ref>

Many turtles and lizards have proportionally very large bladders. [[Charles Darwin]] noted that the [[Galapagos tortoise]] had a bladder which could store up to 20% of its body weight.<ref>{{cite book |author=Bentley, P.J. |date=14 March 2013 |title=Endocrines and Osmoregulation: A comparative account in vertebrates |publisher=Springer Science & Business Media |isbn=978-3-662-05014-9 |url=https://books.google.com/books?id=U0D3BwAAQBAJ&pg=PA143}}</ref> Such adaptations are the result of environments such as remote islands and deserts where water is very scarce.<ref>{{cite conference |last=Paré|first=Jean |date=11 January 2006 |title=Reptile basics: Clinical anatomy&nbsp;101 |conference=North American Veterinary Conference |volume=20 |pages=1657–1660 |url=http://www.ivis.org/proceedings/navc/2006/SAE/600.pdf?LA=1}}</ref>{{rp|143}} Other desert-dwelling reptiles have large bladders that can store a long-term reservoir of water for up to several months and aid in [[osmoregulation]].<ref>{{Cite journal |last1=Davis |first1=Jon R. |last2=de&nbsp;Nardo |first2=Dale F. |date=2007-04-15 |title=The urinary bladder as a physiological reservoir that moderates dehydration in a large desert lizard, the Gila monster ''Heloderma suspectum'' |journal=Journal of Experimental Biology |language=en |volume=210 |issue=8 |pages=1472–1480 |doi=10.1242/jeb.003061 |issn=0022-0949 |pmid=17401130 |doi-access=free|bibcode=2007JExpB.210.1472D }}</ref>

Turtles have two or more accessory urinary bladders, located lateral to the neck of the urinary bladder and dorsal to the pubis, occupying a significant portion of their body cavity.<ref>{{Cite journal|last1=Wyneken|first1=Jeanette|last2=Witherington|first2=Dawn|date=February 2015|title=Urogenital System|url=http://www.ivis.org/advances/wyneken/16.pdf?LA|journal=Anatomy of Sea Turtles|volume=1|pages=153–165}}</ref> Their bladder is also usually bilobed with a left and right section. The right section is located under the liver, which prevents large stones from remaining in that side while the left section is more likely to have [[Bladder stone (animal)|calculi]].<ref>{{Cite book|title=Reptile Medicine and Surgery|last1=Divers|first1=Stephen J.|last2=Mader|first2=Douglas R.|publisher=Elsevier Health Sciences|year=2005|isbn=978-1-4160-6477-0|location=Amsterdam|pages=481, 597|url=https://books.google.com/books?id=7Ai4BKhi0VUC}}</ref>

===Digestion===
[[File:PikiWiki Israel 37648 Nature and Colors.jpg|thumb|right|A colubrid snake, ''[[Dolichophis jugularis]]'', eating a [[sheltopusik|legless lizard]], ''Pseudopus apodus''. Most reptiles are carnivorous, and many primarily eat other reptiles and small mammals.]]
<!--[[File:Watersnake.JPG|thumb|Watersnake ''[[Malpolon monspessulanus]]'' eating a lizard.]]-->
[[File:PSM V53 D226 Silicious pebbles from the stomach of a plesiosaur.jpg|thumb|right|[[Gastrolith]]s from a [[plesiosaur]]]]

Most reptiles are insectivorous or carnivorous and have simple and comparatively short digestive tracts due to meat being fairly simple to break down and digest. [[Digestion]] is slower than in [[mammals]], reflecting their lower resting [[metabolism]] and their inability to divide and [[masticate]] their food.<ref>{{cite book |author=Karasov, W.H. |year=1986 |chapter=Nutrient requirement and the design and function of guts in fish, reptiles and mammals |editor=Dejours, P. |editor2=Bolis, L. |editor3=Taylor, C.R. |editor4=Weibel, E.R. |title=Comparative Physiology: Life in water and on land |publisher=Liviana Press/Springer Verlag |isbn=978-0-387-96515-4 |pages=181–191 |chapter-url=https://books.google.com/books?id=zuT5z5cPWhcC&dq=reptiles+carnivory+gastric&pg=PA181 |access-date=November 1, 2012}}</ref> Their [[poikilotherm]] metabolism has very low energy requirements, allowing large reptiles like crocodiles and large constrictors to live from a single large meal for months, digesting it slowly.<ref name=Garnett/>

While modern reptiles are predominantly carnivorous, during the early history of reptiles several groups produced some herbivorous [[megafauna]]: in the [[Paleozoic]], the [[pareiasaur]]s; and in the [[Mesozoic]] several lines of [[dinosaur]]s.<ref name=Sahney-Benton-Ferry-2010/> Today, [[turtle]]s are the only predominantly herbivorous reptile group, but several lines of [[Agamidae|agamas]] and [[Iguanidae|iguanas]] have evolved to live wholly or partly on plants.<ref name=herbivory>{{cite book |last=King |first=Gillian |year=1996 |title=Reptiles and Herbivory |edition=1 |publisher=Chapman & Hall |location=London, UK |isbn=978-0-412-46110-1}}</ref>

Herbivorous reptiles face the same problems of mastication as herbivorous mammals but, lacking the complex teeth of mammals, many species swallow rocks and pebbles (so called [[gastrolith]]s) to aid in digestion: The rocks are washed around in the stomach, helping to grind up plant matter.<ref name=herbivory/> Fossil gastroliths have been found associated with both [[ornithopod]]s and [[sauropods]], though whether they actually functioned as a gastric mill in the latter is disputed.<ref>{{cite journal|last=Cerda|first=Ignacio A. |date=1 June 2008|title=Gastroliths in an ornithopod dinosaur|journal=Acta Palaeontologica Polonica |volume=53|issue=2|pages=351–355|doi=10.4202/app.2008.0213|doi-access=free}}</ref><ref>{{cite journal |author1=Wings, O. |author2=Sander, P.M. |title=No gastric mill in sauropod dinosaurs: new evidence from analysis of gastrolith mass and function in ostriches |journal=Proceedings of the Royal Society B: Biological Sciences |date=7 March 2007 |volume=274 |issue=1610 |pages=635–640 |doi=10.1098/rspb.2006.3763 |pmc=2197205  |pmid=17254987}}{{open access}}</ref> [[Salt water crocodile]]s also use gastroliths as [[Sailing ballast|ballast]], stabilizing them in the water or helping them to dive.<ref>{{cite journal|last=Henderson|first=Donald M.|title=Effects of stomach stones on the buoyancy and equilibrium of a floating crocodilian: a computational analysis|journal=Canadian Journal of Zoology|date=1 August 2003|volume=81|issue=8|pages=1346–1357|doi=10.1139/z03-122|bibcode=2003CaJZ...81.1346H }}</ref> A dual function as both stabilizing ballast and digestion aid has been suggested for gastroliths found in [[plesiosaur]]s.<ref>{{cite journal|last=McHenry|first=C.R.|title=Bottom-Feeding Plesiosaurs|journal=Science|date=7 October 2005|volume=310|issue=5745|page=75|doi=10.1126/science.1117241|pmid=16210529|s2cid=28832109}}{{open access}}</ref>

===Nerves===
The reptilian nervous system contains the same basic part of the [[amphibian]] brain, but the reptile [[cerebrum]] and [[cerebellum]] are slightly larger. Most typical sense organs are well developed with certain exceptions, most notably the [[snake]]'s lack of external ears (middle and inner ears are present). There are twelve pairs of [[cranial nerves]].<ref>{{cite web |url=http://www.curator.org/legacyvmnh/weboflife/kingdom/p_chordata/ClassReptilia/reptiles.htm |title=de beste bron van informatie over cultural institution. Deze website is te koop! |publisher=Curator.org |access-date=March 16, 2010 |archive-url=https://web.archive.org/web/20090917002815/http://www.curator.org/legacyvmnh/weboflife/kingdom/p_chordata/ClassReptilia/reptiles.htm |archive-date=September 17, 2009 }}</ref> Due to their short cochlea, reptiles use [[electrical tuning]] to expand their range of audible frequencies.

===Vision===
Most reptiles are [[Diurnality|diurnal]] animals. The vision is typically adapted to daylight conditions, with color vision and more advanced visual [[depth perception]] than in amphibians and most mammals.

Reptiles usually have excellent vision, allowing them to detect shapes and motions at long distances. They often have poor vision in low-light conditions. Birds, crocodiles and turtles have three types of [[Photoreceptor cell|photoreceptor]]: [[rod cell|rods]], single [[Cone cell|cones]] and double cones, which gives them sharp color vision and enables them to see [[ultraviolet]] wavelengths.<ref name=Brames>{{Cite journal |journal=Iguana: Conservation, Natural History, and Husbandry of Reptiles |last=Brames |first=Henry |title=Aspects of Light and Reptile Immunity |url=https://www.academia.edu/6822325 |publisher=International Reptile Conservation Foundation |volume=14 |issue=1 |year=2007 |pages=19–23}}</ref> The lepidosaurs appear to have lost the [[duplex retina]] and only have a single class of receptor that is cone-like or rod-like depending on whether the species is diurnal or nocturnal.<ref>{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S2352154619300257 | doi=10.1016/j.cobeha.2019.10.009 | title=The evolutionary ecology of bird and reptile photoreceptor spectral sensitivities | journal=Current Opinion in Behavioral Sciences | series=Visual perception | date=December 2019 | volume=30 | pages=223–227 | last1=Osorio | first1=Daniel | url-access=subscription }}</ref> In many burrowing species, such as [[blind snake]]s, vision is reduced.

Many [[Lepidosauria|lepidosaurs]] have a photosensory organ on the top of their heads called the [[parietal eye]], which are also called [[third eye]], [[pineal eye]] or [[pineal gland]]. This "eye" does not work the same way as a normal eye does as it has only a rudimentary retina and lens and thus, cannot form images. It is, however, sensitive to changes in light and dark and can detect movement.<ref name=Brames/>

Some snakes have extra sets of visual organs (in the loosest sense of the word) in the form of [[Loreal pit|pits]] sensitive to [[infrared]] radiation (heat). Such heat-sensitive pits are particularly well developed in the [[pit vipers]], but are also found in [[boidae|boas]] and [[pythonidae|pythons]]. These pits allow the snakes to sense the body heat of birds and mammals, enabling pit vipers to hunt rodents in the dark.{{efn|
"The copperhead is a pit viper and, like others pit vipers, it has heat-sensitive pit organs on each side of its head between the eye and the nostril. These pits detect objects that are warmer than the environment and enable copperheads to locate nocturnal, mammalian prey."<ref>{{cite web |title=Northern copperhead |date=25 April 2016 |website=Smithsonian's National Zoo & Conservation Biology Institute |url=https://nationalzoo.si.edu/animals/northern-copperhead |access-date=12 February 2020}}</ref>
}}

Most reptiles, as well as birds, possess a [[nictitating membrane]], a translucent third eyelid which is drawn over the eye from the inner corner. In crocodilians, it protects its eyeball surface while allowing a degree of vision underwater.<ref>{{cite encyclopedia |title=Nictitating membrane |series=Anatomy |encyclopedia=Encyclopædia Britannica |lang=en |url=https://www.britannica.com/science/nictitating-membrane |access-date=2020-02-20}}</ref> However, many squamates, geckos and snakes in particular, lack eyelids, which are replaced by a transparent scale. This is called the [[brille]], spectacle, or eyecap. The brille is usually not visible, except for when the snake molts, and it protects the eyes from dust and dirt.<ref>{{cite news |title=Catalina Island Conservancy |website=www.catalinaconservancy.org |url=https://www.catalinaconservancy.org/index.php?s=news&p=article_149 |access-date=2020-02-20}}</ref>

===Reproduction===
[[File:Crocodile Egg Diagram.svg|thumb|Crocodilian egg diagram<br/>
(1)&nbsp;eggshell, (2)&nbsp;yolk sac, (3)&nbsp;yolk (nutrients), (4)&nbsp;vessels, (5)&nbsp;[[amnion]], (6)&nbsp;[[chorion]], (7)&nbsp;air space, (8)&nbsp;[[allantois]], (9)&nbsp;albumin (egg white), (10)&nbsp;amniotic sac, (11)&nbsp;crocodile embryo, (12)&nbsp;amniotic fluid]]
[[File:Hemidactylus_frenatus_mating,_ventral_view.jpg|thumb|[[Common house gecko]]s mating, ventral view with [[hemipenis]] inserted in the [[cloaca]]]]
[[File:Trachylepis maculilabris mating.jpg|thumb|Most reptiles reproduce sexually, for example this ''Trachylepis maculilabris'' [[skink]]]]
[[File:Tortoise-Hatchling.jpg|thumb|Reptiles have [[amniote|amniotic]] eggs with hard or leathery shells, requiring [[internal fertilization]] when mating.]]

Reptiles generally [[sexual reproduction|reproduce sexually]],<ref>{{cite journal |title=Male reproductive behaviour of ''Naja oxiana'' {{small|(Eichwald, 1831)}} in captivity, with a case of unilateral hemipenile prolapse |year=2018 |journal=Herpetology Notes |url=https://www.researchgate.net/publication/335270872}}</ref> though some are capable of [[asexual reproduction]]. All reproductive activity occurs through the [[cloaca]], the single exit/entrance at the base of the tail where waste is also eliminated. Most reptiles have [[copulatory organ]]s, which are usually retracted or inverted and stored inside the body. In turtles and crocodilians, the male has a single median [[penis]], while squamates, including snakes and lizards, possess a pair of [[hemipenis|hemipenes]], only one of which is typically used in each session. Tuatara, however, lack copulatory organs, and so the male and female simply press their cloacas together as the male discharges sperm.<ref>{{cite book |author=Lutz, Dick |year=2005 |title=Tuatara: A living fossil |place=Salem, OR |publisher=DIMI Press |isbn=978-0-931625-43-5}}</ref>

Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An [[amnion]]&nbsp;(5), [[chorion]]&nbsp;(6), and [[allantois]]&nbsp;(8) are present during [[embryo]]nic life. The eggshell&nbsp;(1) protects the crocodile embryo&nbsp;(11) and keeps it from drying out, but it is flexible to allow gas exchange. The chorion&nbsp;(6) aids in gas exchange between the inside and outside of the egg. It allows carbon dioxide to exit the egg and oxygen gas to enter the egg. The albumin&nbsp;(9) further protects the embryo and serves as a reservoir for water and protein. The allantois&nbsp;(8) is a sac that collects the metabolic waste produced by the embryo. The amniotic sac&nbsp;(10) contains amniotic fluid&nbsp;(12) which protects and cushions the embryo. The amnion&nbsp;(5) aids in osmoregulation and serves as a saltwater reservoir. The yolk sac&nbsp;(2) surrounding the yolk&nbsp;(3) contains protein and fat rich nutrients that are absorbed by the embryo via vessels&nbsp;(4) that allow the embryo to grow and metabolize. The air space&nbsp;(7) provides the embryo with oxygen while it is hatching. This ensures that the embryo will not suffocate while it is hatching. There are no [[larva]]l stages of development. [[Viviparity]] and [[ovoviviparity]] have evolved in squamates and many extinct clades of reptiles. Among squamates, many species, including all boas and most vipers, use this mode of reproduction. The degree of viviparity varies; some species simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the yolk, and yet others lack any yolk and provide all nutrients via a structure similar to the mammalian [[placenta]]. The earliest documented case of viviparity in reptiles is the Early [[Permian]] [[mesosaur]]s,<ref name=PFML12>{{cite journal |author1=Piñeiro, G. |author2=Ferigolo, J. |author3=Meneghel, M. |author4=Laurin, M. |year=2012 |title=The oldest known amniotic embryos suggest viviparity in mesosaurs |journal=Historical Biology |volume=24 |issue=6 |pages=620–630 |s2cid=59475679 |doi=10.1080/08912963.2012.662230 |bibcode=2012HBio...24..620P }}</ref> although some individuals or taxa in that clade may also have been oviparous because a putative isolated egg has also been found. Several groups of Mesozoic marine reptiles also exhibited viviparity, such as [[mosasaur]]s, [[Ichthyosauria|ichthyosaur]]s, and [[Sauropterygia]], a group that includes [[pachypleurosaur]]s and [[Plesiosauria]].<ref name=S12/>

Asexual reproduction has been identified in [[squamata|squamates]] in six families of lizards and one snake. In some species of squamates, a population of females is able to produce a unisexual diploid clone of the mother. This form of asexual reproduction, called [[parthenogenesis]], occurs in several species of [[gecko]], and is particularly widespread in the [[teiidae|teiids]] (especially ''Aspidocelis'') and [[lacertidae|lacertids]] (''[[Lacerta (genus)|Lacerta]]''). In captivity, [[Komodo dragon]]s (Varanidae) have reproduced by [[parthenogenesis]].

Parthenogenetic species are suspected to occur among [[chameleon]]s, [[agamidae|agamids]], [[night lizard|xantusiids]], and [[typhlopidae|typhlopids]].

Some reptiles exhibit [[temperature-dependent sex determination]] (TDSD), in which the incubation temperature determines whether a particular egg hatches as male or female. TDSD is most common in turtles and crocodiles, but also occurs in lizards and tuatara.<ref>{{cite book |title=FireFly Encyclopedia of Reptiles and Amphibians |year=2008 |publisher=Firefly Books Ltd |location=Richmond Hill, Ontario |isbn=978-1-55407-366-5 |pages=117–118 }}</ref> To date, there has been no confirmation of whether TDSD occurs in snakes.<ref>{{cite book |last1=Chadwick |first1=Derek |last2=Goode |first2=Jamie |year=2002 |title=The Genetics and Biology of Sex ... |publisher=John Wiley & Sons |isbn=978-0-470-84346-8 |url=https://books.google.com/books?id=lc5Bg-hmsBYC&q=temperature%20dependent%20sex%20determination%20snake&pg=PA101 |via=Google Books |access-date=March 16, 2010}}</ref>

===Longevity===

Giant [[tortoise]]s are among the longest-lived vertebrate animals (over 100 years by some estimates) and have been used as a model for studying [[longevity]].<ref name="Quesada2019">{{cite journal | last1 = Quesada | first1 = V | last2 = Freitas-Rodríguez | first2 = S | last3 = Miller | first3 = J | last4 = Pérez-Silva | first4 = JG | last5 = Jiang | first5 = ZF | last6 = Tapia | first6 = W | last7 = Santiago-Fernández | first7 = O | last8 = Campos-Iglesias | first8 = D | last9 = Kuderna | first9 = LFK | last10 = Quinzin | first10 = M | last11 = Álvarez | first11 = MG | last12 = Carrero | first12 = D | last13 = Beheregaray | first13 = LB | last14 = Gibbs | first14 = JP | last15 = Chiari | first15 = Y | last16 = Glaberman | first16 = S | last17 = Ciofi | first17 = C | last18 = Araujo-Voces | first18 = M | last19 = Mayoral | first19 = P | last20 = Arango | first20 = JR | last21 = Tamargo-Gómez | first21 = I | last22 = Roiz-Valle | first22 = D | last23 = Pascual-Torner | first23 = M | last24 = Evans | first24 = BR | last25 = Edwards | first25 = DL | last26 = Garrick | first26 = RC | last27 = Russello | first27 = MA | last28 = Poulakakis | first28 = N | last29 = Gaughran | first29 = SJ | last30 = Rueda | first30 = DO | last31 = Bretones | first31 = G | last32 = Marquès-Bonet | first32 = T | last33 = White | first33 = KP | last34 = Caccone | first34 = A | last35 = López-Otín | first35 = C | year = 2019 | title = Giant tortoise genomes provide insights into longevity and age-related disease | journal = Nat Ecol Evol | volume = 3 | issue = 1| pages = 87–95 | doi = 10.1038/s41559-018-0733-x | pmid = 30510174 | pmc = 6314442 }}</ref> DNA analysis of the [[genome]]s of [[Lonesome George]], the iconic last member of ''[[Pinta Island tortoise|Chelonoidis abingdonii]]'', and the [[Aldabra giant tortoise]] ''Aldabrachelys gigantea'' led to the detection of lineage-specific variants affecting [[DNA repair]] genes that might contribute to our understanding of increased lifespan.<ref name = Quesada2019/>

==Cognition==
{{see also|Animal cognition}}

Reptiles are generally considered less intelligent than mammals and birds.<ref name="Romer, A 1977"/> The [[Brain-to-body mass ratio|size of their brain relative to their body]] is much less than that of mammals, the [[encephalization quotient]] being about one tenth of that of mammals,<ref>{{cite web |first=Harry J. |last=Jerison |title=Figure of relative brain size in vertebrates |publisher=Brainmuseum.org |url=http://brainmuseum.org/evolution/paleo/index.html |access-date=March 16, 2010}}</ref> though larger reptiles can show more complex brain development. Larger lizards, like the [[monitor lizard|monitors]], are known to exhibit complex behavior, including cooperation<ref>{{cite book |author1=King, Dennis |author2=Green, Brian |year=1999 |title=Goannas: The biology of varanid lizards |publisher=University of New South Wales Press |isbn=978-0-86840-456-1 |page=43}}</ref> and cognitive abilities allowing them to optimize their [[foraging]] and [[Territory (animal)|territoriality]] over time.<ref>{{cite web |title=Latency in Problem Solving as Evidence for Learning in Varanid and Helodermatid Lizards, with Comments on Foraging Techniques |website=ResearchGate |language=en |url=https://www.researchgate.net/publication/331062010 |access-date=2020-02-20}}</ref> Crocodiles have relatively larger brains and show a fairly complex social structure. The [[Komodo dragon]] is even known to engage in play,<ref name=firefly>{{cite book |editor1=Halliday, Tim  |editor1-link=Tim Halliday |editor2=Adler, Kraig |year=2002 |title=Firefly Encyclopedia of Reptiles and Amphibians |publisher=Firefly Books Ltd |location=Hove |pages=[https://archive.org/details/fireflyencyclope0000unse_p6l7/page/112 112–113], 144, 147, 168–169 |isbn=978-1-55297-613-5 |url=https://archive.org/details/fireflyencyclope0000unse_p6l7/page/112 }}</ref> as are turtles, which are also considered to be social creatures,<ref>{{cite press release |title=Even turtles need recess: Many animals – not just dogs, cats, and monkeys – need a little play time |date=Oct 2010 |website=ScienceDaily |lang=en |url=https://www.sciencedaily.com/releases/2010/10/101019132045.htm |access-date=2020-02-20}}</ref> and sometimes switch between monogamy and promiscuity in their sexual behavior.{{Citation needed|date=May 2017}} One study found that [[wood turtle]]s were better than [[white rat]]s at learning to navigate mazes.<ref>{{cite news |last=Angier |first=Natalie |date=16 December 2006 |title=Ask Science |newspaper=[[The New York Times]] |url=https://www.nytimes.com/2006/12/16/science/15askscience.html?pagewanted=2 |access-date=September 15, 2013}}</ref> Another study found that giant tortoises are capable of learning through [[operant conditioning]], visual discrimination and retained learned behaviors with long-term memory.<ref>{{cite journal |last1=Gutnick |first1=Tamar |last2=Weissenbacher |first2=Anton |last3=Kuba |first3=Michael J. |year=2020 |title=The underestimated giants: operant conditioning, visual discrimination and long-term memory in giant tortoises |journal=Animal Cognition |volume=23 |issue=1 |pages=159–167 |language=en |doi=10.1007/s10071-019-01326-6 |pmid=31720927 |issn=1435-9456 |s2cid=207962281 |url=http://id.nii.ac.jp/1394/00001487/ }}</ref> Sea turtles have been regarded as having simple brains, but their flippers are used for a variety of foraging tasks (holding, bracing, corralling) in common with marine mammals.<ref>{{cite press release |title=Sea turtles use flippers to manipulate food |date=March 2018 |website=ScienceDaily |lang=en |url=https://www.sciencedaily.com/releases/2018/03/180328083421.htm |access-date=2020-02-20}}</ref>

There is evidence that reptiles are [[sentient]] and able to feel emotions including [[anxiety]] and [[pleasure]].<ref>{{cite journal |last1=Lambert |first1=Helen |last2=Carder |first2=Gemma |last3=D'Cruze |first3=Neil |title=Given the Cold Shoulder: A Review of the Scientific Literature for Evidence of Reptile Sentience |journal=Animals |date=17 October 2019 |volume=9 |issue=10 |page=821 |doi=10.3390/ani9100821 |pmid=31627409 |pmc=6827095 |issn=2076-2615 |doi-access=free }}</ref>

==Defense mechanisms==
Many small reptiles, such as snakes and lizards, that live on the ground or in the water are vulnerable to being preyed on by all kinds of carnivorous animals. Thus, [[Conflict avoidance|avoidance]] is the most common form of defense in reptiles.<ref>{{cite encyclopedia |title=Reptile |series=animal |department=Behaviour |encyclopedia=Britannica.com |url=http://www.britannica.com/EBchecked/topic/498684/reptile/38450/Behaviour |access-date=March 16, 2010}}</ref> At the first sign of danger, most snakes and lizards crawl away into the undergrowth, and turtles and crocodiles will plunge into water and sink out of sight.

===Camouflage and warning===
[[File:Phelsuma dubia edit1.jpg|thumb|A camouflaged ''[[Dull day gecko|Phelsuma deubia]]'' on a palm frond]]

Reptiles tend to avoid confrontation through [[camouflage]]. Two major groups of reptile predators are birds and other reptiles, both of which have well-developed color vision. Thus the skins of many reptiles have [[Crypsis|cryptic]] coloration of plain or mottled gray, green, and brown to allow them to blend into the background of their natural environment.<ref>{{cite web |title=Reptile and amphibian defense systems |department=Animal behavior |type=resource |publisher=Teachervision.fen.com |url=http://www.teachervision.fen.com/animal-behavior/resource/8700.html |access-date=March 16, 2010 |archive-date=June 1, 2020 |archive-url=https://web.archive.org/web/20200601010143/https://www.teachervision.com/animal-behavior/reptile-amphibian-defense-systems }}</ref> Aided by the reptiles' capacity for remaining motionless for long periods, the camouflage of many snakes is so effective that people or domestic animals are most typically bitten because they accidentally step on them.<ref>{{cite thesis |last1=Nagel |first1=Salomé Susanna |date=October 2012 |title=Haemostatic function of dogs naturally envenomed by African puffadder (''Bitis arietans'') or snouted cobra (''Naja annulifera'') |degree=MMedVet |department=Companion Animal Clinical Studies |publisher=University of Pretoria |place=South Africa |page=66 |hdl=2263/25851 |url=https://repository.up.ac.za/handle/2263/25851 |access-date=9 Feb 2023}}</ref>

When camouflage fails to protect them, [[blue-tongued skink]]s will try to ward off attackers by displaying their blue tongues, and the [[frill-necked lizard]] will display its brightly colored frill. These same displays are used in territorial disputes and during courtship.<ref name=Cogger>{{cite book |last=Cogger |first=Harold G. |year=1986 |title=Reptiles and Amphibians of Australia |publisher=Reed Books |location=Frenchs Forest, NSW |isbn=978-0-7301-0088-1 |page=238}}</ref> If danger arises so suddenly that flight is useless, crocodiles, turtles, some lizards, and some snakes hiss loudly when confronted by an enemy. [[Rattlesnakes]] rapidly vibrate the tip of the tail, which is composed of a series of nested, hollow beads to ward off approaching danger.

In contrast to the normal drab coloration of most reptiles, the lizards of the genus ''Heloderma'' (the [[Gila monster]] and the [[beaded lizard]]) and many of the [[coral snake]]s have high-contrast warning coloration, warning potential predators they are venomous.<ref>{{cite book |editor=Harris, Tim |display-editors=etal |year=2011 |title=North American Wildlife |publisher=Marshall Cavendish Reference |location=New York, NY |isbn=978-0-76147-938-3 |page=86, picture caption |quote=The bold patterns of the venomous gila monster are an example of warning coloration. |url=https://books.google.com/books?id=fU25LOYnVokC&q=venomous+gila+monster+warning+colors&pg=PA86 |access-date=August 18, 2014}}</ref> A number of non-venomous North American snake species have colorful markings similar to those of the coral snake, an oft cited example of [[Batesian mimicry]].<ref>{{cite journal |author=Brodie, Edmund D., III |year=1993 |title=Differential avoidance of coral snake banded patterns by free-ranging avian predators in Costa Rica |journal=Evolution |volume=47 |issue=1 |pages=227–235 |doi=10.1111/j.1558-5646.1993.tb01212.x |pmid=28568087 |jstor=2410131 |bibcode=1993Evolu..47..227B |s2cid=7159917 |doi-access=free }}</ref><ref>{{cite journal |author1=Brodie, Edmund D., III |author2=Moore, Allen J. |year=1995 |title=Experimental studies of coral snake mimicry: Do snakes mimic millipedes? |journal=Animal Behaviour |volume=49 |issue=2 |pages=534–536 |doi=10.1006/anbe.1995.0072 |bibcode=1995AnBeh..49..534B |s2cid=14576682 }}</ref>

===Alternative defense in snakes===
{{further|Venom|Evolution of snake venom}}

Camouflage does not always fool a predator. When caught out, snake species adopt different defensive tactics and use a complicated set of behaviors when attacked. Some species, like cobras or hognose snakes, first elevate their head and spread out the skin of their neck in an effort to look large and threatening. Failure of this strategy may lead to other measures practiced particularly by cobras, vipers, and closely related species, which use [[snake venom|venom]] to attack. The venom is modified saliva, delivered through fangs from a [[venom gland]].<ref>{{cite book |editor-last=Bauchot |editor-first=Roland |year=1994 |title=Snakes: A natural history |publisher=Sterling Publishing |isbn=978-1-4027-3181-5 |pages=[https://archive.org/details/snakesnaturalhis0000bauc/page/194 194–209] |url=https://archive.org/details/snakesnaturalhis0000bauc |url-access=registration}}</ref><ref>{{cite journal |last1=Casewell |first1=N.R. |last2=Wuster |first2=W. |last3=Vonk |first3=F.J. |last4=Harrison |first4=R.A. |last5=Fry |first5=B.G. |year=2013 |title=Complex cocktails: the evolutionary novelty of venoms |journal=Trends in Ecology & Evolution |volume=28 |issue=4 |pages=219–229 |doi=10.1016/j.tree.2012.10.020 |pmid=23219381 |bibcode=2013TEcoE..28..219C }}</ref> Some non-venomous snakes, such as American [[hognose snakes]] or European [[grass snake]], [[thanatosis|play dead]] when in danger; some, including the grass snake, exude a foul-smelling liquid to deter attackers.<ref>{{cite magazine | first = Susan | last = Milius | date = October 28, 2006 | title = Why play dead? | magazine = Science News | volume = 170 | issue = 18 | pages = 280–281 | doi = 10.2307/4017568 | jstor = 4017568 | s2cid = 85722243 }}</ref><ref>{{cite book |last=Cooke |first=Fred |year=2004 |title=The Encyclopedia of Animals: A complete visual guide |publisher=University of California Press |isbn=978-0-520-24406-1 |page=405 |url=https://books.google.com/books?id=2V1tHqi4hLEC&pg=PA405}}</ref>

==={{anchor|Defense in crocodiles}}Defense in crocodilians===
When a [[crocodilian]] is concerned about its safety, it will gape to expose the teeth and tongue. If this does not work, the crocodilian gets a little more agitated and typically begins to make hissing sounds. After this, the crocodilian will start to change its posture dramatically to make itself look more intimidating. The body is inflated to increase apparent size. If absolutely necessary, it may decide to attack an enemy.
[[File:White-headed dwarf gecko.jpg|thumb|A [[White-headed dwarf gecko]] with shed tail]]
Some species try to bite immediately. Some will use their heads as [[sledgehammer]]s and literally smash an opponent, some will rush or swim toward the threat from a distance, even chasing the opponent onto land or galloping after it.<ref>{{cite web |title=Ferocious Crocs |series=Animal Planet |publisher=Animal.discovery.com |date=2008-09-10 |url=http://animal.discovery.com/convergence/safari/crocs/expert/expert6.html |access-date=March 16, 2010}}</ref> The main weapon in all crocodiles is the bite, which can generate very high bite force. Many species also possess [[Canine tooth|canine]]-like teeth. These are used primarily for seizing prey, but are also used in fighting and display.<ref>{{cite journal |author1=Erickson, Gregory M. |author2=Gignac, Paul M. |author3=Steppan, Scott J. |author4=Lappin, A. Kristopher |author5=Vliet, Kent A. |author6=Brueggen, John D. |author7=Inouye, Brian D. |author8=Kledzik, David |author9=Webb, Grahame J.W. |author10= Claessens, Leon |display-authors=6 |year=2012 |title=Insights into the ecology and evolutionary success of crocodilians revealed through bite-force and tooth-pressure experimentation |journal=PLOS ONE |volume=7 |issue=3 |article-number=e31781 |doi=10.1371/journal.pone.0031781 |doi-access=free |pmid=22431965 |pmc=3303775 |bibcode=2012PLoSO...731781E}}{{open access}}</ref>

===Shedding and regenerating tails===
{{Main|Autotomy}}
[[Geckos]], [[skinks]], and some other lizards that are captured by the tail will shed part of the tail structure through a process called [[autotomy]] and thus be able to flee. The detached tail will continue to thrash, creating a deceptive sense of continued struggle and distracting the predator's attention from the fleeing prey animal. The detached tails of [[leopard gecko]]s can wiggle for up to 20 minutes. The tail grows back in most species, but some, like crested geckos, lose their tails for the rest of their lives.<ref>{{cite magazine |last1=Marshall |first1=Michael |title=Gecko's amputated tail has life of its own |magazine=[[New Scientist]] |department=Zoologger |series=Life |url=https://www.newscientist.com/article/dn21375-zoologger-geckos-amputated-tail-has-life-of-its-own |access-date=August 18, 2014}}</ref> In many species the tails are of a separate and dramatically more intense color than the rest of the body so as to encourage potential predators to strike for the tail first. In the [[Tiliqua rugosa|shingleback skink]] and some species of geckos, the tail is short and broad and resembles the head, so that the predators may attack it rather than the more vulnerable front part.<ref name="pianka">{{Cite book | last1 = Pianka | first1 = Eric R. | last2 = Vitt | first2 = Laurie J. | year = 2003 | title = Lizards: Windows to the evolution of diversity | edition = 1 | series = Organisms and Environments | volume = 5 | publisher = University of California Press | isbn = 978-0-520-23401-7 | url = https://archive.org/details/lizardswindowsto00pian }}</ref>

Reptiles that are capable of shedding their tails can partially [[regeneration (biology)|regenerate]] them over a period of weeks. The new section will however contain cartilage rather than bone, and will never grow to the same length as the original tail. It is often also distinctly discolored compared to the rest of the body and may lack some of the external sculpting features seen in the original tail.<ref>{{cite book |last1=Alibardi |first1=Lorenzo |title=Morphological and Cellular Aspects of Tail and Limb Regeneration in Lizards |chapter=Regeneration in Reptiles and Its Position Among Vertebrates |series=Advances in Anatomy, Embryology and Cell Biology |year=2010 |volume=207 |pages=iii, v-x, 1–109 |publisher=Springer |location=Heidelberg, DE |isbn=978-3-642-03733-7 |pmid=20334040 |doi=10.1007/978-3-642-03733-7_1}}</ref>

== Relations with humans ==
{{further|Human uses of reptiles}}

===In cultures and religions===
{{main|Reptiles in culture}}
[[File:Laelaps-Charles Knight-1897.jpg|thumb|Painting of fighting "''Laelaps''" (now ''[[Dryptosaurus]]'') by [[Charles R. Knight]] (1897)]]
Dinosaurs have been widely depicted in culture since the English palaeontologist [[Richard Owen]] coined the name ''[[dinosaur]]'' in 1842. As soon as 1854, the [[Crystal Palace Dinosaurs]] were on display to the public in south London.<ref name=Torrens>{{cite book |author=Torrens, Hugh |section=Politics and paleontology |title=The Complete Dinosaur |pages=175–190}}</ref><ref name=Glut>{{cite book |last=Glut |first=Donald F. |author-link=Donald F. Glut |author2=Brett-Surman, Michael K. |year=1997 |chapter=Dinosaurs and the media |title=The Complete Dinosaur |publisher=Indiana University Press |pages=[https://archive.org/details/isbn_9780253333490/page/675 675–706] |isbn=978-0-253-33349-0 |chapter-url=https://archive.org/details/isbn_9780253333490/page/675 }}</ref> One dinosaur appeared in literature even earlier, as [[Charles Dickens]] placed a ''[[Megalosaurus]]'' in the first chapter of his novel ''[[Bleak House]]'' in 1852.{{efn|
"Michaelmas term lately over, and the Lord Chancellor sitting in Lincoln's Inn Hall. Implacable November weather. As much mud in the streets, as if the waters had but newly retired from the face of the earth, and it would not be wonderful to meet a ''Megalosaurus'', forty feet long or so, waddling like an elephantine lizard up Holborne Hill."<ref>{{cite book |last=Dickens |first=Charles J.H. |year=1852 |title=Bleak House |section=[[s:Bleak House/Chapter I|Chapter&nbsp;I: In Chancery]] |page=1 |location=London, UK |publisher=Bradbury & Evans |isbn=978-1-85326-082-7 }}</ref>
}}
The dinosaurs featured in books, films, television programs, artwork, and other media have been used for both education and entertainment. The depictions range from the realistic, as in the television [[documentary film|documentaries]] of the 1990s and first decade of the 21st century, to the fantastic, as in the [[monster movie]]s of the 1950s and 1960s.<ref name=Glut/><ref name=Gregory>{{cite book |author=Paul, G.S. |author-link=Gregory S. Paul |editor=Paul, G.S. |year=2000 |section=The art of Charles R. Knight |title=The Scientific American Book of Dinosaurs |publisher=St. Martin's Press |isbn=978-0-312-26226-6 |pages=113–118}}</ref><ref name=Searles>{{cite book |last=Searles |first=Baird |year=1988 |section=Dinosaurs and others |title=Films of Science Fiction and Fantasy |pages=104–116 |publisher=AFI Press |location=New York, NY |isbn=978-0-8109-0922-9}}</ref>

The snake or serpent has played a powerful [[Serpent (symbolism)|symbolic role]] in different cultures. In [[History of Egypt|Egyptian history]], the Nile cobra adorned the crown of the [[pharaoh]]. It was [[snake worship|worshipped]] as one of the gods and was also used for sinister purposes: murder of an adversary and ritual suicide ([[Cleopatra VII of Egypt|Cleopatra]]). In [[Greek mythology]], snakes are associated with deadly antagonists, as a [[chthonic]] symbol, roughly translated as ''earthbound''. The nine-headed [[Lernaean Hydra]] that [[Hercules]] defeated and the three [[Gorgon]] sisters are children of [[Gaia (mythology)|Gaia]], the earth. [[Medusa]] was one of the three Gorgon sisters who [[Perseus]] defeated. Medusa is described as a hideous mortal, with snakes instead of hair and the power to turn men to stone with her gaze. After killing her, Perseus gave her head to [[Athena]] who fixed it to her shield called the [[Aegis]]. The [[Titan (mythology)|Titans]] are depicted in art with their legs replaced by bodies of snakes for the same reason: They are children of Gaia, so they are bound to the earth.<ref name=BF85>{{cite book |last=Bullfinch |first=Thomas |author-link=Thomas Bullfinch |year=2000 |title=Bullfinch's Complete Mythology |edition=reprint |publisher=Chancellor Press |location=London |url=http://etext.library.adelaide.edu.au/b/bulfinch/thomas/ |isbn=978-0-7537-0381-6 |page=85 |archive-url=https://web.archive.org/web/20090209004721/http://etext.library.adelaide.edu.au/b/bulfinch/thomas/ |archive-date=2009-02-09 }}</ref> In Hinduism, [[snake worship|snakes are worshipped]] as gods, with many women pouring milk on snake pits. The cobra is seen on the neck of [[Shiva]], while [[Vishnu]] is depicted often as sleeping on a seven-headed snake or within the coils of a serpent. There are temples in India solely for cobras sometimes called ''Nagraj'' (King of Snakes), and it is believed that snakes are symbols of fertility. In the annual Hindu festival of [[Nag Panchami]], snakes are venerated and prayed to.<ref name=Deane>{{cite book |last=Deane |first=John |author-link=The Worship of the Serpent |year=1833 |title=The Worship of the Serpent |pages=61–64 |publisher=Kessinger Publishing |isbn=978-1-56459-898-1 |url =http://www.sacred-texts.com/etc/wos/index.htm}}</ref> In religious terms, the snake and [[jaguar]] are arguably the most important animals in ancient [[Mesoamerica]]. "In states of ecstasy, lords dance a serpent dance; great descending snakes adorn and support buildings from [[Chichen Itza]] to [[Tenochtitlan]], and the [[Nahuatl]] word ''coatl'' meaning serpent or twin, forms part of primary deities such as [[Mixcoatl]], [[Quetzalcoatl]], and [[Coatlicue]]."<ref>{{cite book |author=Miller, Mary |year=1993 |title=The Gods and Symbols of Ancient Mexico and the Maya |publisher=Thames & Hudson |place=London, UK |isbn=978-0-500-27928-1}}</ref> In Christianity and Judaism, a serpent appears in Genesis to tempt [[Adam and Eve]] with the [[forbidden fruit]] from the [[Tree of Knowledge of Good and Evil]].<ref>{{Bibleverse|Genesis|3:1|NAB}}</ref>

The turtle has a prominent position as a symbol of steadfastness and tranquility in religion, mythology, and folklore from around the world.<ref name=Plotkin>{{cite book |author=Plotkin, Pamela T. |year=2007 |title=Biology and Conservation of Ridley Sea Turtles |publisher=Johns Hopkins University Press |place=Baltimore, MD |isbn=978-0-8018-8611-9}}</ref> A tortoise's longevity is suggested by its long lifespan and its shell, which was thought to protect it from any foe.<ref name=Ball>{{cite book |author=Ball, Catherine |year=2004 |title=Animal Motifs in Asian Art |publisher=Courier Dover Publications |isbn=0-486-43338-2}}</ref> In the [[cosmological myth]]s of several cultures a ''[[World Turtle]]'' carries the world upon its back or supports the heavens.<ref name=Stookey>{{cite book |author=Stookey, Lorena Laura |year=2004 |title=Thematic Guide to World Mythology |publisher=Greenwood Press |isbn=978-0-313-31505-3}}</ref>

===Medicine===
{{See also|Epidemiology of snakebites}}[[File:Rod of Asclepius2.svg|thumb|upright=0.25|The [[Rod of Asclepius]] symbolizes medicine]]
Deaths from [[snakebite]]s are uncommon in many parts of the world, but are still counted in tens of thousands per year in India.<ref name=Sinha>{{cite news |last=Sinha |first=Kounteya |title=No more the land of snake charmers... |newspaper=The Times of India |date=25 July 2006 |url=http://timesofindia.indiatimes.com/articleshow/1803026.cms}}</ref> Snakebite can be treated with [[antivenom]] made from the venom of the snake. To produce antivenom, a mixture of the venoms of different species of snake is injected into the body of a horse in ever-increasing dosages until the horse is immunized. Blood is then extracted; the serum is separated, purified and freeze-dried.<ref>{{cite journal |last=Dubinsky |first=I. |year=1996 |title=Rattlesnake bite in a patient with horse allergy and von&nbsp;Willebrand's disease: Case report |journal=Can. Fam. Physician |volume=42 |pages=2207–2211 |pmc=2146932 |pmid=8939322}}</ref> The [[cytotoxic]] effect of snake venom is being researched as a potential treatment for cancers.<ref>{{cite journal |author1=Vyas, Vivek Kumar |author2=Brahmbahtt, Keyur |author3=Parmar, Ustav |date=February 2012 |title=Therapeutic potential of snake venom in cancer therapy: Current perspective |journal=Asian Pacific Journal of Tropical Medicine |volume=3 |issue=2 |pages=156–162 |doi=10.1016/S2221-1691(13)60042-8 |pmc=3627178 |pmid=23593597}}</ref>

'''Gila monsters''' produce compounds that reduce plasma glucose; one of these substances is now used in the anti-[[diabetes]] drug [[exenatide]] (Byetta), a [[glucagon-like peptide-1]] (GLP-1) receptor agonist like [[Semaglutide|semiglutide]] (Ozempic).<ref name=Casey2013>{{cite magazine |last=Casey |first=Constance |date=26 April 2013 |title=Don't call it a monster |magazine=Slate |url=http://www.slate.com/articles/health_and_science/science/2013/04/gila_monster_revolting_creature_the_large_venomous_lizard_of_the_u_s_southwest.html}}</ref><ref>{{Cite web |date=2012-07-11 |title=Exendin-4: From lizard to laboratory...and beyond |url=https://www.nia.nih.gov/news/exendin-4-lizard-laboratory-and-beyond |archive-url=https://web.archive.org/web/20171005090844/https://www.nia.nih.gov/news/exendin-4-lizard-laboratory-and-beyond |archive-date=October 5, 2017 |access-date=2025-02-22 |website=National Institute on Aging |language=en}}</ref> Another toxin from Gila monster saliva has been studied for use as an anti-[[Alzheimer's]] drug.<ref>{{cite news |title=Alzheimer's research seeks out lizards |date=5 April 2002 |publisher=[[BBC News]] |url=https://news.bbc.co.uk/2/hi/health/1912396.stm}}</ref>

[[Gecko]]s have also been used as '''folk medicine''', especially in China, without any evidence that they have any active compounds.<ref>{{cite journal | last1=Wagner | first1=P. | last2=Dittmann | first2=A. | year=2014 | title=Medicinal use of ''Gekko gecko'' (Squamata: Gekkonidae) has an impact on agamid lizards | journal=Salamandra | volume=50 | issue=3 | pages=185–186 | url=http://www.salamandra-journal.com/index.php?option=com_docman&task=doc_download&gid=375&Itemid=76 | access-date=2014-10-25 | archive-date=2014-10-25 | archive-url=https://web.archive.org/web/20141025183720/http://www.salamandra-journal.com/index.php?option=com_docman&task=doc_download&gid=375&Itemid=76 }}{{open access}}</ref> Turtles have been used in Chinese traditional medicine for thousands of years, with every part of the turtle believed to have medical benefits (again, without scientific evidence). Growing demand for '''turtle meat''' has placed pressure on vulnerable wild populations of turtles.<ref name=MongabayNews-2014-08-08>{{cite news |title=The threat of traditional medicine: China's boom may mean doom for turtles |date=2014-08-08 |website=Mongabay Environmental News |language=en-US |url=https://news.mongabay.com/2014/08/the-threat-of-traditional-medicine-chinas-boom-may-mean-doom-for-turtles/ |access-date=2020-01-16}}</ref>

===Commercial farming===
{{See also|Crocodile farm|Snake farm|Turtle farming}}
Crocodiles are protected in many parts of the world, and are [[crocodile farm|farmed commercially]]. Their hides are tanned and used to make leather goods such as shoes and [[handbag]]s; [[crocodile meat]] is also considered a delicacy.<ref>{{cite news |last=Lyman |first=Rick |date=30 November 1998 |title=Alligator farmer feeds demand for all the parts |department=Anahuac Journal |newspaper=[[The New York Times]] |url=https://www.nytimes.com/1998/11/30/us/anahuac-journal-alligator-farmer-feeds-demand-for-all-the-parts.html |access-date=November 13, 2013}}</ref> The most commonly farmed species are the saltwater and Nile crocodiles. Farming has resulted in an increase in the saltwater crocodile population in [[Australia]], as eggs are usually harvested from the wild, so landowners have an incentive to conserve their habitat. [[Crocodile leather]] is made into wallets, briefcases, purses, handbags, belts, hats, and shoes. [[Crocodile oil]] has been used for various purposes.<ref>{{cite book |first=Elisabeth |last=Janos |year=2004 |title=Country Folk Medicine: Tales of skunk oil, sassafras tea, and other old-time remedies |edition=1 |page=56 |publisher=Lyon's Press |isbn=978-1-59228-178-7 |url={{Google books |plainurl=yes |id=FUaaDBFsYFEC |page=56}} }}</ref>

Snakes are also farmed, primarily in [[East Asia|East]] and [[Southeast Asia]], and their production has become more intensive in the last decade. [[Snake farm]]ing has been troubling for conservation in the past as it can lead to [[overexploitation]] of wild snakes and their natural prey to supply the farms. However, farming snakes can limit the hunting of wild snakes, while reducing the slaughter of higher-order vertebrates like cows. The energy efficiency of snakes is higher than expected for carnivores, due to their ectothermy and low metabolism. Waste protein from the poultry and pig industries is used as feed in snake farms.<ref>{{cite journal |last1=Aust |first1=Patrick W. |last2=Tri |first2=Ngo Van |last3=Natusch |first3=Daniel J.D. |last4=Alexander |first4=Graham J. |date=2017 |title=Asian snake farms: Conservation curse or sustainable enterprise? |journal=Oryx |language=en |volume=51 |issue=3 |pages=498–505 |doi=10.1017/S003060531600034X |doi-access=free |issn=0030-6053}}</ref> Snake farms produce meat, [[Snakeskin|snake skin]], and antivenom.

[[Turtle farming]] is another known but controversial practice. Turtles have been farmed for a variety of reasons, ranging from food to traditional medicine, the pet trade, and scientific conservation. Demand for turtle meat and medicinal products is one of the main threats to turtle conservation in Asia. Though commercial breeding would seem to insulate wild populations, it can stoke the demand for them and increase wild captures.<ref>{{Cite journal |last1=Haitao |first1=Shi |last2=Parham |first2=James F. |last3=Zhiyong |first3=Fan |last4=Meiling |first4=Hong |last5=Feng |first5=Yin |year=2008 |title=Evidence for the massive scale of turtle farming in China |journal=Oryx |language=en |volume=42 |issue=1 |pages=147–150 |doi=10.1017/S0030605308000562 |issn=1365-3008|doi-access=free}}</ref><ref name=MongabayNews-2014-08-08/> Even the potentially appealing concept of raising turtles at a farm to release into the wild is questioned by some veterinarians who have had some experience with farm operations. They caution that this may introduce into the wild populations infectious diseases that occur on the farm, but have not (yet) been occurring in the wild.<ref>{{cite periodical |author=Jacobson, Elliott R. |date=January 1996 |title=Marine turtle farming and health issues |type=guest editorial |periodical=Marine Turtle Newsletter |volume=72 |pages=13–15 |url=http://www.seaturtle.org/mtn/archives/mtn72/mtn72p13.shtml}}</ref><ref>{{cite news |title=This turtle tourist center also raises endangered turtles for meat |date=2017-05-25 |website=National Geographic News |language=en |url=https://www.nationalgeographic.com/news/2017/05/wildlife-watch-cayman-turtle-farm-welfare-controversy/ |archive-url=https://web.archive.org/web/20200116071147/https://www.nationalgeographic.com/news/2017/05/wildlife-watch-cayman-turtle-farm-welfare-controversy/ |archive-date=January 16, 2020 |access-date=2020-01-16}}</ref>

===Reptiles in captivity===
{{further|Herpetoculture}}
A [[herpetarium]] is a [[zoo|zoological exhibition]] space for reptiles and amphibians.

In the Western world, some snakes (especially relatively docile species such as the [[Python regius|ball python]] and [[corn snake]]) are sometimes kept as pets.<ref>{{cite book |last=Ernest |first=Carl |author2=George R. Zug |author3=Molly Dwyer Griffin |title=Snakes in Question: The Smithsonian Answer Book |publisher=Smithsonian Books |year=1996 |page=[https://archive.org/details/snakesinquestion00erns/page/203 203] |isbn=978-1-56098-648-5 |url=https://archive.org/details/snakesinquestion00erns/page/203 }}</ref> Numerous species of lizard are kept as [[pet]]s, including [[Pogona|bearded dragons]],<ref name="Virata">{{cite web|url=http://www.reptilesmagazine.com/Lizards/5-Great-Beginner-Pet-Lizards/|title=5 Great Beginner Pet Lizards|last1=Virata|first1=John B.|publisher=Reptiles Magazine|access-date=28 May 2017}}</ref> [[iguana]]s, [[anole]]s,<ref>{{cite web|url=https://www.thespruce.com/green-anoles-pets-1236900|title=An Introduction to Green Anoles as Pets|last1=McLeod|first1=Lianne|website=The Spruce|access-date=28 May 2017|archive-date=24 March 2018|archive-url=https://web.archive.org/web/20180324224708/https://www.thespruce.com/green-anoles-pets-1236900}}</ref> and [[gecko]]s (such as the popular [[leopard gecko]] and the crested gecko).<ref name="Virata" />

Turtles and tortoises are increasingly popular pets, but keeping them can be challenging due to their particular requirements, such as temperature control, the need for UV light sources, and a varied diet. The long lifespans of turtles and especially tortoises mean they can potentially outlive their owners. Good hygiene and significant maintenance is necessary when keeping reptiles, due to the risks of ''[[Salmonella]]'' and other pathogens.<ref>{{Cite web|url=https://phys.org/news/2018-06-turtles-great-pets-homework.html|title=Turtles can make great pets, but do your homework first|website=phys.org|language=en-us|access-date=2020-01-15}}</ref> Regular hand-washing after handling is an important measure to prevent infection.

==See also==
{{Main|Outline of reptiles}}
* [[Amphibian and reptile tunnel]]
* [[List of reptiles]]
* [[Lists of reptiles by region]]
* [[Reptile Database]]
* {{portal-inline|Reptiles}}

==Notes==
{{notelist}}

==References==
{{reflist}}

==Further reading==
{{refbegin}}
* {{cite book
 |last=Colbert |first=Edwin H. |author-link=Edwin Harris Colbert
 |year=1969
 |title=Evolution of the Vertebrates |edition=2nd
 |publisher=John Wiley and Sons Inc.
 |location=New York, NY
 |isbn=978-0-471-16466-1
 |url=https://archive.org/details/evolutionofverte00colb
}}
* Duellman, William E., Berg, Barbara (1962), ''[https://www.gutenberg.org/ebooks/73551 Type Specimens of Amphibians and Reptiles in the Museum of Natural History, the University of Kansas]''
* {{cite journal
 | author1=Landberg, Tobias
 | author2=Mailhot, Jeffrey
 | author3=Brainerd, Elizabeth
 | year=2003
 | title=Lung ventilation during treadmill locomotion in a terrestrial turtle, ''Terrapene carolina''
 | journal=[[Journal of Experimental Biology]]
 | volume=206 | issue=19 | pages=3391–3404
 | doi=10.1242/jeb.00553 | doi-access=free
 | pmid=12939371
| bibcode=2003JExpB.206.3391L
 }}
* {{cite book
 | author1=Pianka, Eric
 | author2=Vitt, Laurie
 | year=2003
 | title=Lizards: Windows to the evolution of diversity
 | publisher=University of California Press
 | isbn=978-0-520-23401-7
 | pages=[https://archive.org/details/lizardswindowsto00pian/page/116 116–118]
 | url=https://archive.org/details/lizardswindowsto00pian/page/116
}}
* {{cite book
 | author1=Pough, Harvey
 | author2=Janis, Christine
 | author3=Heiser, John
 | year=2005
 | title=Vertebrate Life
 | publisher=Pearson Prentice Hall
 | isbn=978-0-13-145310-4
}}
{{refend}}

==External links==
* {{Wikispecies-inline |Reptilia}}
* {{Wikibooks inline |Dichotomous Key |Reptilia}}
* {{Cite EB1911 |wstitle=Reptiles |short=x}}
* {{cite web |title=Reptile phylogeny |series=Herpetology |website=whozoo.org |url=http://www.whozoo.org/herps/herpphylogeny.html }}
* {{cite book |title=Reptile images |website=biodiversitylibrary.org |date=1833 |url=https://www.biodiversitylibrary.org/item/23742 }}
* {{cite web |title=Sri Lanka wildlife information database |website=wildreach.com |url=http://www.wildreach.com/reptile/ |archive-url=https://web.archive.org/web/20101227082759/http://www.wildreach.com/reptile/ |url-status=usurped |archive-date=December 27, 2010 }}
* {{cite web |title=Biology of the reptilia |website=carlgans.org |url=http://carlgans.org/biology-reptilia-online/ }} — an online full text copy of a 22&nbsp;volume 13,000&nbsp;page summary of the state of reptile research.

{{Chordata}}
{{Early tetrapods|state=autocollapse|A.}}
{{Reptiles}}
{{Taxonbar|from=Q10811}}
{{Authority control}}
{{Portal bar|Reptiles}}

[[Category:Reptiles| ]]
[[Category:Extant Pennsylvanian first appearances]]
[[Category:Paraphyletic groups]]
<!-- [[Category:Animal classes]] moved to the Latin name (Reptilia) redirect -->
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