Editing Snake (section)

== Taxonomy ==

{{See also|List of snake genera}}
All modern snakes are grouped within the [[Class (biology)|suborder]] '''Serpentes''' in [[Linnean taxonomy]], part of the [[class (biology)|order]] [[Squamata]], though their precise placement within squamates remains controversial.<ref name="ITIS">{{ITIS|id=174118|taxon=Serpentes|access-date=4 April 2017}}</ref>

The two [[infraorder]]s of Serpentes are [[Alethinophidia]] and [[Scolecophidia]].<ref name="ITIS"/> This separation is based on [[comparative anatomy|morphological]] characteristics and [[mitochondrial DNA]] sequence similarity. Alethinophidia is sometimes split into [[Henophidia]] and [[Caenophidia]], with the latter consisting of "colubroid" snakes ([[colubrids]], [[Viperidae|vipers]], [[elapids]], [[Hydrophiinae|hydrophiids]], and [[Atractaspidinae|atractaspids]]) and acrochordids, while the other alethinophidian families comprise Henophidia.<ref name="Pou92">{{cite book |vauthors=Pough FH |orig-date=1992 |title=Herpetology: Third Edition |publisher=Pearson [[Prentice Hall]] |year=2002 |isbn=978-0-13-100849-6}}</ref> While not extant today, the [[Madtsoiidae]], a family of giant, primitive, python-like snakes, lived until 50,000 years ago in Australia, represented by genera such as ''[[Wonambi]]''.<ref name="Vasuki">{{Cite journal |last1=Datta |first1=Debajit |last2=Bajpai |first2=Sunil |date=2024-04-18 |title=Largest known madtsoiid snake from warm Eocene period of India suggests intercontinental Gondwana dispersal |journal=[[Scientific Reports]] |language=en |volume=14 |issue=1 |pages=8054 |doi=10.1038/s41598-024-58377-0 |issn=2045-2322|doi-access=free |pmid=38637509 |bibcode=2024NatSR..14.8054D |pmc=11549349 }}</ref>

Recent molecular studies support the [[monophyly]] of the [[clades]] of modern snakes, scolecophidians, typhlopids + anomalepidids, alethinophidians, core alethinophidians, uropeltids (''Cylindrophis'', ''Anomochilus'', uropeltines), macrostomatans, booids, boids, pythonids and caenophidians.<ref name=Lee>{{cite journal |first1=Michael S. Y. |last1=Lee |first2=Andrew F. |last2=Hugall |first3=Robin |last3=Lawson |first4=John D. |last4=Scanlon |name-list-style=vanc |title=Phylogeny of snakes (Serpentes): combining morphological and molecular data in likelihood, Bayesian and parsimony analyses |journal=[[Systematics and Biodiversity]] |volume=5 |issue=4 |pages=371–389 |doi=10.1017/S1477200007002290 |year=2007 |bibcode=2007SyBio...5..371L |hdl=2440/44258 |s2cid=85912034 |hdl-access=free}}</ref>

=== Families ===
{|class="wikitable"
|-
| colspan="100%" style="text-align:center; background:#bbf;"|Infraorder '''[[Alethinophidia]]''' 25 families
|-
! style="background:#f0f0f0;"|Family<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Taxon author<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Genera<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Species<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Common name
! style="background:#f0f0f0;"|Geographic range<ref name="McD99">McDiarmid RW, Campbell JA, Touré T. 1999. Snake Species of the World: A Taxonomic and Geographic Reference, vol. 1. Herpetologists' League. 511 pp. {{ISBN|1-893777-00-6}} (series). {{ISBN|1-893777-01-4}} (volume).</ref>
|-
|[[Acrochordidae]]
|[[Charles Lucien Bonaparte|Bonaparte]], 1831
| style="text-align:center;"|1
| style="text-align:center;"|3
|Wart snakes
|style="width:40%"|Western India and Sri Lanka through tropical Southeast Asia to the Philippines, south through the Indonesian/Malaysian island group to Timor, east through New Guinea to the northern coast of Australia to [[Mussau Island]], the [[Bismarck Archipelago]] and [[Guadalcanal Island]] in the Solomon Islands.
|-
|[[Aniliidae]]
|[[Leonhard Hess Stejneger|Stejneger]], 1907
| style="text-align:center;"|1
| style="text-align:center;"|1
|False coral snake
|Tropical South America.
|-
|[[Anomochilidae]]
|Cundall, Wallach, 1993
| style="text-align:center;"|1
| style="text-align:center;"|3
|Dwarf pipe snakes
|West Malaysia and on the Indonesian island of [[Sumatra]].
|-
|[[Atractaspididae]]
|[[Albert Günther|Günther]], 1858
| style="text-align:center;"|12
| style="text-align:center;"|72
|Burrowing asps
|Africa and the Middle East
|-
|[[Boidae]]
|[[John Edward Gray|Gray]], 1825
| style="text-align:center;"|14
| style="text-align:center;"|61
|Boas
|Northern, Central and South America, the Caribbean, southeastern Europe and Asia Minor, Northern, Central and East Africa, Madagascar and [[Reunion Island]], the Arabian Peninsula, Central and southwestern Asia, India and Sri Lanka, the [[Maluku Islands|Moluccas]] and New Guinea through to Melanesia and Samoa.
|-
|[[Bolyeriidae]]
|[[Robert Hoffstetter|Hoffstetter]], 1946
| style="text-align:center;"|2
| style="text-align:center;"|2
|Splitjaw snakes
|[[Mauritius]].
|-
|[[Colubridae]]
|[[Nicolaus Michael Oppel|Oppel]], 1811
| style="text-align:center;"|258<ref name="NRDB-Cs"/>
| style="text-align:center;"|2055<ref name="NRDB-Cs"/>
|Typical snakes
|Widespread on all continents, except Antarctica.<ref name="Spa04">{{cite book |last1=Spawls |first1=S. |last2=Howell |first2=K. |last3=Drewes |first3=R. |last4=Ashe |first4=J. |date=2004 |title=A Field Guide To The Reptiles Of East Africa |location=London |publisher=A & C Black Publishers Ltd |isbn=0-7136-6817-2}}</ref>
|-
|[[Cyclocoridae]]
|[[Jeffrey L.Weinell|Weinell]] & [[Rafe M.Brown|Brown]], 2017
| style="text-align:center;"|5
| style="text-align:center;"|8
|Cyclocorids
|The Philippines
|-
|[[Cylindrophiidae]]
|[[Leopold Fitzinger|Fitzinger]], 1843
| style="text-align:center;"|1
| style="text-align:center;"|14
|Asian pipe snakes
|Sri Lanka east through Myanmar, Thailand, Cambodia, Vietnam and the Malay Archipelago to as far east as [[Aru Islands Regency|Aru Islands]] off the southwestern coast of New Guinea. Also found in southern China (Fujian, Hong Kong and on Hainan Island) and in Laos.
|-
|[[Elapidae]]
|[[Friedrich Boie|Boie]], 1827
| style="text-align:center;"|55
| style="text-align:center;"|389
|Elapids
|On land, worldwide in tropical and subtropical regions, except in Europe. Sea snakes occur in the Indian Ocean and the Pacific.<ref name="NRDB-E">{{NRDB family |page=elapidae.php |family=Elapidae |date=3 December |year=2008}}</ref>
|-
|[[Homalopsidae]]
|[[Charles Lucien Bonaparte|Bonaparte]], 1845
| style="text-align:center;"|28
| style="text-align:center;"|53
|Homalopsids
|Southeastern Asia and northern Australia.
|-
|[[Lamprophiidae]]
|[[Leopold Fitzinger|Fitzinger]], 1843
| style="text-align:center;"|16
| style="text-align:center;"|89
|Lamprophiids (formerly included Atracaspididae, Psammophiidae, and several other families)
|Africa (including the Seychelles)
|-
|[[Loxocemidae]]
|[[Edward Drinker Cope|Cope]], 1861
| style="text-align:center;"|1
| style="text-align:center;"|1
|Mexican burrowing snake
|Along the Pacific versant from Mexico south to Costa Rica.
|-
|[[Micrelapidae]]
|[[Sunandan Das|Das]] ''et al''., 2023
| style="text-align:center;"|1
| style="text-align:center;"|4
|Two-headed snakes
|Eastern Africa and the Levant
|-
|[[Pareidae]]
|Romer, 1956
| style="text-align:center;"|3
| style="text-align:center;"|20
|Snail-eating snakes
|Southeast Asia and islands on the Sunda Shelf (Sumatra, Borneo, Java, and their surrounding smaller islands).
|-
|[[Prosymnidae]]
|Kelly, Barker, Villet & Broadley, 2009
| style="text-align:center;"|1
| style="text-align:center;"|16
|Shovel-snout snakes
|Subsaharan Africa
|-
|[[Psammodynastidae]]
|Das ''et al''., 2024
| style="text-align:center;"|1
| style="text-align:center;"|2
|Mock vipers
|Tropical Asia
|-
|[[Psammophiidae]]
|[[Monique Bourgeois|Bourgeois]], 1968
| style="text-align:center;"|8
| style="text-align:center;"|55
|Psammophiids
|Africa (including Madagascar), Asia and southern Europe
|-
|[[Pseudaspididae]]
|[[Edward Drinker Cope|Cope]], 1893
| style="text-align:center;"|2
| style="text-align:center;"|2
| Pseudaspidids
|Subsaharan Africa
|-
|[[Pseudoxyrhophiidae]]
|[[Herndon G. Dowling|Dowling]], 1975
| style="text-align:center;"|22
| style="text-align:center;"|89
| Pseudoxyrhophiids
|Mostly Madagascar and the Comoros; 5 species in subsaharan Africa, 1 in Socotra
|-
|[[Pythonidae]]
|[[Leopold Fitzinger|Fitzinger]], 1826
| style="text-align:center;"|8
| style="text-align:center;"|40
|Pythons
|Subsaharan Africa, India, Myanmar, southern China, Southeast Asia and from the Philippines southeast through Indonesia to New Guinea and Australia.
|-
|[[Tropidophiidae]]
|[[L.D. Brongersma|Brongersma]], 1951
| style="text-align:center;"|2
| style="text-align:center;"|34
|Dwarf boas
|West Indies; also Panama and northwestern South America, as well as in northwestern and southeastern Brazil.
|-
|[[Uropeltidae]]
|[[Johannes Peter Müller|Müller]], 1832
| style="text-align:center;"|8
| style="text-align:center;"|55
|Shield-tailed snakes
|Southern India and Sri Lanka.
|-
|[[Viperidae]]
|[[Nicolaus Michael Oppel|Oppel]], 1811
| style="text-align:center;"|35
| style="text-align:center;"|341
|Vipers
|The Americas, Africa, and Eurasia east to [[Wallace Line|Wallace's Line]].
|-
|[[Xenodermidae]]
|[[Edward Drinker Cope|Cope]], 1900
| style="text-align:center;"|6
| style="text-align:center;"|18
|Dragon and odd-scaled snakes
|East Asia, Southern and southeastern Asia, and islands on the Sunda Shelf (Sumatra, Borneo, Java, and their surrounding smaller islands).
|-
|[[Xenopeltidae]]
|[[Charles Lucien Bonaparte|Bonaparte]], 1845
| style="text-align:center;"|1
| style="text-align:center;"|2
|Sunbeam snakes
|Southeast Asia from the [[Andaman Islands|Andaman]] and [[Nicobar Islands]], east through Myanmar to southern China, Thailand, Laos, Cambodia, Vietnam, the Malay Peninsula and the East Indies to [[Sulawesi]], as well as the Philippines.
|-
|[[Xenophidiidae]]
|Wallach & Günther, 1998
| style="text-align:center;"|1
| style="text-align:center;"|2
|Spine-jawed snakes
|Borneo and peninsular Malaysia.
|}
<br />
{|class="wikitable"
|-
| colspan="100%" style="text-align:center; background:#bbf;"|Infraorder '''[[Scolecophidia]]''' 5 families
|-
! style="background:#f0f0f0;"|Family<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Taxon author<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Genera<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Species<ref name="NRDB-Cs"/>
! style="background:#f0f0f0;"|Common name
! style="background:#f0f0f0;"|Geographic range<ref name="McD99"/>
|-
|[[Anomalepidae]]
|[[Edward Harrison Taylor|Taylor]], 1939
| style="text-align:center;"|4
| style="text-align:center;"|18
|Primitive blind snakes
|style="width:40%"|From southern Central America to northwestern South America. Disjunct populations in northeastern and southeastern South America.
|-
|[[Gerrhopilidae]]
|style="width:15%"|Vidal, Wynn, Donnellan and Hedges 2010
| style="text-align:center;"|2
| style="text-align:center;"|18
|Indo-Malayan blindsnakes
|Southern and southeastern Asia, including Sri Lanka, the Philippines, and New Guinea.
|-
|[[Leptotyphlopidae]]
|[[Leonhard Hess Stejneger|Stejneger]], 1892
| style="text-align:center;"|13
| style="text-align:center;"|139
|Slender blind snakes
|Africa, western Asia from Turkey to northwestern India, on [[Socotra Island]], from the southwestern United States south through Mexico and Central to South America, though not in the high [[Andes]]. In Pacific South America they occur as far south as southern coastal Peru, and on the Atlantic side as far as Uruguay and Argentina. In the Caribbean they are found on the Bahamas, [[Hispaniola]] and the [[Lesser Antilles]].
|-
|[[Typhlopidae]]
|[[Blasius Merrem|Merrem]], 1820
| style="text-align:center;"|18
| style="text-align:center;"|266
|Typical blind snakes
|Most tropical and many subtropical regions around the world, particularly in Africa, Madagascar, Asia, islands in the Pacific, tropical America and in southeastern Europe.
|-
|[[Xenotyphlopidae]]
|Vidal, Vences, Branch and Hedges 2010
| style="text-align:center;"|1
| style="text-align:center;"|1
|Round-nosed blindsnake
|Northern Madagascar.
|}

=== Legless lizards ===
{{Main|Legless lizard}}
While snakes are limbless reptiles, evolved from (and grouped with) lizards, there are many other species of lizards that have lost their limbs independently but which superficially look similar to snakes. These include the [[slowworm]], [[glass snake]], and [[amphisbaenia]]ns.<ref name="Southwest">{{cite book |last3=Browne-Cooper |first3=Robert |last1=Bush |first1=Brian |last2=Maryan |first2=Brad |last4=Robinson |first4=David |title=Reptiles and Frogs in the Bush: Southwestern Australia |year=2007 |publisher=[[University of Western Australia Press]] |isbn=978-1-920694-74-6 |pages=243, 244 }}</ref>

=== Evolution ===
{{Unsolved|biology|Did snakes evolve from burrowing lizards or aquatic lizards?}}
{{Cladogram|title=A family level phylogenetic overview of modern snakes.
|clades={{clade|style=font-size:88.888888%;line-height:100%
 |label1=Snakes
 |1={{clade
     |label1=[[Scolecophidia]]
     |1={{clade
         |1=[[Leptotyphlopidae]]
         |label2=
         |2={{clade
             |1=[[Gerrhopilidae]]
             |2={{clade
                |1=[[Typhlopidae]]
                |2=[[Xenophidiidae]]
                }}
            }}
        }}
     |label2=
     |2={{clade
      |label1=
      |1=[[Anomalepididae]]
      |label2=[[Alethinophidia]]
         |2={{clade
            |1={{clade
            |label1=
            |1={{clade
                 |1=[[Aniliidae]]
                 |2=[[Tropidophiidae]]
               }}
         |label2=
                 |2={{clade
                   |1={{clade
                      |1={{clade
                         |1={{clade
                            |1=[[Xenopeltidae]]}}
                            |2={{clade
                               |1=[[Loxocemidae]]
                               |2=[[Pythonidae]]}}
                           
                       }}
                      |2={{clade
                          |label1=
                             |1=[[Boidae]]
                          |label2=
                             |2={{clade
                                |1=[[Bolyeridae]]
                                |2=[[Xenophidiidae]]
                                }}
                          }}
                      }}
                 
                      |2={{clade
                          |label1=
                             |1=[[Uropeltidae]]
                          |label2=
                             |2={{clade
                                |1=[[Anomochilidae]]
                                |2=[[Cylindrophiidae]]
                                }}
                                }}
               }}
         }}
         |2={{clade
                                |1={{clade
                                   |1=[[Acrochordidae]]}}
                                |2={{clade
                                   |1={{clade
                                   |1=[[Xenodermidae]]}}
                                   |2={{clade
                                      |1={{clade
                                        |1=[[Pareidae]]}}
                                |2={{clade|1={{clade
                                |1=[[Viperidae]]}}
                                |2={{clade|1={{clade
                                |1=[[Homalopsidae]]}}
                                |2={{clade|1={{clade
                                |1=[[Colubridae]]}}
                                |2={{clade
                                |1=[[Lamprophiidae]]
                                |2=[[Elapidae]]}}}}}}}}}}}}
                                }}
      }}
     }}
     }}
|caption=Note: the tree only indicates relationships, not evolutionary branching times. This is not a definitive tree. It is the best that could be done as of 2024. See original paper for a discussion of difficulties.<ref name="ScienceArticle">{{cite journal |last1=Title |first1=Pascal O. |last2=Singhal |first2=Sonal |last3=Grundler |first3=Michael C. |last4=Costa |first4=Gabriel C. |last5=Pyron |first5=R. Alexander |last6=Colston |first6=Timothy J. |last7=Grundler |first7=Maggie R. |last8=Prates |first8=Ivan |last9=Stepanova |first9=Natasha |last10=Jones |first10=Marc E. H. |last11=Cavalcanti |first11=Lucas B. Q. |last12=Colli |first12=Guarino R. |last13=Di-Poï |first13=Nicolas |last14=Donnellan |first14=Stephen C. |last15=Moritz |first15=Craig |last16=Mesquita |first16=Daniel O. |last17=Pianka |first17=Eric R. |last18=Smith |first18=Stephen A. |last19=Vitt |first19=Laurie J. |last20=Rabosky |first20=Daniel L. |title=The macroevolutionary singularity of snakes |journal=Science |date=23 February 2024 |volume=383 |issue=6685 |pages=918–923 |doi=10.1126/science.adh2449 |pmid=38386744 |bibcode=2024Sci...383..918T |url=https://www.science.org |access-date=22 March 2024|url-access=subscription }}</ref>
}}
}}
The fossil record of snakes is relatively poor because snake [[skeleton]]s are typically small and fragile making [[fossil]]ization uncommon. Fossils readily identifiable as snakes (though often retaining hind limbs) first appear in the fossil record during the [[Cretaceous]] period.<ref>{{cite book |last=Durand |first=J.F. |date=2004 |title=The origin of snakes |series=Geoscience Africa |volume=Abstract |publisher=[[University of the Witwatersrand]] |location=Johannesburg, South Africa |page=187}}</ref> The earliest known true snake fossils (members of the crown group Serpentes) come from the marine [[Simoliophiidae|simoliophiid]]s, the oldest of which is the [[Late Cretaceous]] ([[Cenomanian]] age) ''[[Haasiophis terrasanctus]]'' from the [[West Bank]],<ref name=snake_origin_ecology>{{cite journal |vauthors=Hsiang AY, Field DJ, Webster TH, Behlke AD, Davis MB, Racicot RA, Gauthier JA |title=The origin of snakes: revealing the ecology, behavior, and evolutionary history of early snakes using genomics, phenomics, and the fossil record |journal=BMC Evolutionary Biology |volume=15 |pages=87 |date = May 2015 |issue=1 |pmid=25989795 |pmc=4438441 |doi=10.1186/s12862-015-0358-5 |bibcode=2015BMCEE..15...87H |doi-access=free }}</ref> dated to between 112 and 94 million years old.<ref name=timetree>{{cite book |last1=Vidal |first1=N. |last2=Rage |first2=J.-C. |last3=Couloux |first3=A. |last4=Hedges |first4=S.B. |date=2009 |chapter=Snakes (Serpentes) |pages=390–397 |editor1-last=Hedges |editor1-first=S. B. |editor2-last=Kumar |editor2-first=S. |title=The Timetree of Life |publisher=[[Oxford University Press]]}}</ref>

Based on genomic analysis it is certain that snakes descend from [[lizard]]s.<ref name="ScienceArticle"/> This conclusion is also supported by [[comparative anatomy]], and the fossil record.<ref name="Meh87">{{cite book |last=Mehrtens |first=J. M. |date=1987 |title=Living Snakes of the World in Color |location=New York |publisher=Sterling Publishers |isbn=0-8069-6460-X}}</ref>{{Rp|11}}<ref name="Sanchez">{{cite web |last=Sanchez |first=Alejandro |name-list-style=vanc |title=Diapsids III: Snakes |work=Father Sanchez's Web Site of West Indian Natural History |url=http://www.kingsnake.com/westindian/metazoa12.html |access-date=November 26, 2007|url-status=live |archive-url=https://web.archive.org/web/20071127163408/http://www.kingsnake.com/westindian/metazoa12.html |archive-date=November 27, 2007}}</ref><ref name="ScienceArticle"/>

[[Pythonidae|Python]]s and [[Boidae|boa]]s—primitive groups among modern snakes—have vestigial hind limbs: tiny, clawed digits known as [[anal spur]]s, which are used to grasp during mating.<ref name="Meh87"/>{{Rp|11}}<ref name="legs"/> The families [[Leptotyphlopidae]] and [[Typhlopidae]] also possess remnants of the pelvic girdle, appearing as horny projections when visible.

Front limbs are nonexistent in all known snakes. This is caused by the evolution of their [[Hox gene]]s, controlling limb [[morphogenesis]]. The axial skeleton of the snakes' common ancestor, like most other tetrapods, had regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic), and caudal (tail) vertebrae. Early in snake evolution, the Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant. As a result, the vertebrae anterior to the hindlimb buds (when present) all have the same thoracic-like identity (except from the [[Atlas (anatomy)|atlas]], [[Axis (anatomy)|axis]], and 1–3 neck vertebrae). In other words, most of a snake's skeleton is an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. Neck, lumbar and pelvic vertebrae are very reduced in number (only 2–10 lumbar and pelvic vertebrae are present), while only a short tail remains of the caudal vertebrae. However, the tail is still long enough to be of important use in many species, and is modified in some aquatic and tree-dwelling species.{{Citation needed|date=November 2024}}

Many modern snake groups originated during the [[Paleocene]], alongside the [[adaptive radiation]] of mammals following the extinction of (non-avian) [[dinosaur]]s. The expansion of grasslands in North America also led to an explosive radiation among snakes.<ref name="Fossil Snakes of North America">{{cite book |last1=Holman |first1=J. Alan |name-list-style=vanc |title=Fossil Snakes of North America |date=2000 |publisher=[[Indiana University Press]] |location=Bloomington, IN |isbn=978-0253337214 |pages=284–323 |edition=First}}</ref> Previously, snakes were a minor component of the North American fauna, but during the Miocene, the number of species and their prevalence increased dramatically with the first appearances of [[viper]]s and [[elapid]]s in North America and the significant diversification of [[Colubridae]] (including the origin of many modern genera such as ''[[Nerodia]]'', ''[[Lampropeltis]]'', ''[[Pituophis]]'', and ''[[Pantherophis]]'').<ref name="Fossil Snakes of North America"/>

=== Fossils ===
There is fossil evidence to suggest that snakes may have evolved from burrowing lizards during the [[Cretaceous|Cretaceous Period]].<ref>{{cite journal |last1=Yi |first1=Hongyu |last2=Norell |first2=Mark A. |title=The burrowing origin of modern snakes |journal=[[Science Advances]] |year=2015 |volume=1 |issue=10 |pages=e1500743 |doi=10.1126/sciadv.1500743 |pmid=26702436 |pmc=4681343 |bibcode=2015SciA....1E0743Y |s2cid=8912706}}</ref><ref name="EB">{{Cite book |last=Mc Dowell |first=Samuel |title=Evolutionary Biology |chapter=The Evolution of the Tongue of Snakes, and its Bearing on Snake Origins |name-list-style=vanc |year=1972 |volume=6 |pages=191–273 |doi=10.1007/978-1-4684-9063-3_8 |isbn =978-1-4684-9065-7}}</ref> An early fossil snake relative, ''[[Najash rionegrina]]'', was a two-legged burrowing animal with a [[sacrum]], and was fully [[Terrestrial animal|terrestrial]].<ref name=najash>{{cite journal |vauthors=Apesteguía S, Zaher H |title=A Cretaceous terrestrial snake with robust hindlimbs and a sacrum |journal=[[Nature (journal)|Nature]] |volume=440 |issue=7087 |pages=1037–40 |date=April 2006 |pmid=16625194 |doi=10.1038/nature04413 |url=http://www.nature.com/nature/journal/v440/n7087/edsumm/e060420-11.html |url-status=live |archive-url=https://web.archive.org/web/20071218045427/http://www.nature.com/nature/journal/v440/n7087/edsumm/e060420-11.html |bibcode=2006Natur.440.1037A |s2cid=4417196 |archive-date=December 18, 2007|url-access=subscription }}</ref> ''Najash'', which lived 95 million years ago, also had a skull with several features typical for lizards, but had evolved some of the mobile skull joints that define the flexible skull in most modern snakes. The species did not show any resemblances to the modern burrowing blind snakes, which have often been seen as the most primitive group of extant forms.<ref>{{Cite web |url=https://theconversation.com/extraordinary-skull-fossil-reveals-secrets-of-snake-evolution-127307 |title=Extraordinary skull fossil reveals secrets of snake evolution |date=20 November 2019 |access-date=24 February 2024 |archive-date=24 February 2024 |archive-url=https://web.archive.org/web/20240224202530/https://theconversation.com/extraordinary-skull-fossil-reveals-secrets-of-snake-evolution-127307 |url-status=live }}</ref> One [[extant taxon|extant]] analog of these putative ancestors is the earless monitor ''[[Lanthanotidae|Lanthanotus]]'' of [[Borneo]] (though it is also [[Aquatic animal|semiaquatic]]).<ref name="M2">{{Cite journal |last=Mertens |first=Robert |name-list-style=vanc |title=Lanthanotus: an important lizard in evolution |journal=Sarawak Museum Journal |year=1961 |volume=10 |pages=320–322}}</ref> [[wikt:subterranean|Subterranean]] species evolved bodies streamlined for burrowing, and eventually lost their limbs.<ref name="M2"/> According to this hypothesis, features such as the [[Transparency (optics)|transparent]], fused eyelids ([[brille]]) and loss of external ears evolved to cope with [[fossorial]] difficulties, such as scratched [[cornea]]s and dirt in the ears.<ref name="EB"/><ref name="M2"/> Some primitive snakes are known to have possessed hindlimbs, but their pelvic bones lacked a direct connection to the vertebrae. These include fossil species like ''[[Haasiophis]]'', ''[[Pachyrhachis]]'' and ''[[Eupodophis]]'', which are slightly older than ''[[Najash]]''.<ref name="legs">{{cite web |title=New Fossil Snake With Legs |work=UNEP WCMC Database |publisher=[[American Association for the Advancement of Science]] |location=Washington, D.C. |url=http://www.wildlifenews.co.uk/articles2000/march/march2500a.htm |access-date=November 29, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20071225225126/http://www.wildlifenews.co.uk/articles2000/march/march2500a.htm |archive-date=December 25, 2007}}
</ref>

This hypothesis was strengthened in 2015 by the discovery of a 113-million-year-old fossil of a four-legged snake in Brazil that has been named ''[[Tetrapodophis|Tetrapodophis amplectus]]''. It has many snake-like features, is adapted for burrowing and its stomach indicates that it was preying on other animals.<ref>{{cite web |title=Four-legged snake ancestor 'dug burrows' |url=https://www.bbc.co.uk/news/science-environment-33621491 |first=Webb |last=Jonathan |name-list-style=vanc |date=July 24, 2014 |publisher=[[BBC|BBC Science & Environment]] |access-date=Jul 24, 2015 |url-status=live|archive-url=https://web.archive.org/web/20150726051600/http://www.bbc.co.uk/news/science-environment-33621491 |archive-date=July 26, 2015}}</ref> It is currently uncertain if ''Tetrapodophis'' is a snake or another species, in the [[Squamata|squamate]] order, as a snake-like body has independently evolved at least 26 times. ''Tetrapodophis'' does not have distinctive snake features in its spine and skull.<ref>{{cite web |title=A Fossil Snake With Four Legs |website=[[National Geographic]] |url=http://phenomena.nationalgeographic.com/2015/07/23/a-fossil-snake-with-four-legs/ |access-date=July 24, 2015 |first=Ed |last=Yong |name-list-style=vanc |url-status=dead |archive-url=https://web.archive.org/web/20150723221358/http://phenomena.nationalgeographic.com/2015/07/23/a-fossil-snake-with-four-legs/ |archive-date=July 23, 2015 |date=July 23, 2015}}</ref><ref>{{cite journal |vauthors=Martill DM, Tischlinger H, Longrich NR |title=EVOLUTION. A four-legged snake from the Early Cretaceous of Gondwana |journal=[[Science (journal)|Science]] |volume=349 |issue=6246 |pages=416–9 |date=July 2015 |pmid=26206932 |doi=10.1126/science.aaa9208 |url=https://researchportal.port.ac.uk/portal/en/publications/a-fourlegged-snake-from-the-early-cretaceous-of-gondwana(b6583c89-da01-4b9a-9f1e-dc142b9331ff).html |bibcode=2015Sci...349..416M |s2cid=25822461 |access-date=20 March 2020 |archive-date=2 August 2021 |archive-url=https://web.archive.org/web/20210802182407/https://researchportal.port.ac.uk/portal/en/publications/a-fourlegged-snake-from-the-early-cretaceous-of-gondwana(b6583c89-da01-4b9a-9f1e-dc142b9331ff).html |url-status=live }}</ref> A study in 2021 places the animal in a group of extinct marine lizards from the Cretaceous period known as [[Dolichosaurus|dolichosaur]]s and not directly related to snakes.<ref>{{Cite web |title=Famous Discovery of Four-Legged Snake Fossil Turns Out to Have a Twist in The Tale |url=https://www.msn.com/en-us/news/technology/famous-discovery-of-four-legged-snake-fossil-turns-out-to-have-a-twist-in-the-tale/ar-AAQQeEM?ocid=msedgdhp&pc=U531#comments |access-date=November 18, 2021 |website=www.msn.com |language=en-US |archive-date=18 November 2021 |archive-url=https://web.archive.org/web/20211118231312/https://www.msn.com/en-us/news/technology/famous-discovery-of-four-legged-snake-fossil-turns-out-to-have-a-twist-in-the-tale/ar-AAQQeEM?ocid=msedgdhp&pc=U531#comments |url-status=live }}</ref>

An alternative hypothesis, based on [[Morphology (biology)|morphology]], suggests the ancestors of snakes were related to [[mosasaur]]s—extinct [[aquatic animal|aquatic]] reptiles from the [[Cretaceous]]—forming the clade [[Pythonomorpha]].<ref name="Sanchez"/> According to this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), and the external ears were lost through disuse in an aquatic environment. This ultimately led to an animal similar to today's [[sea snake]]s. In the Late [[Cretaceous]], snakes recolonized land, and continued to diversify into today's snakes. Fossilized snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis; particularly so, as they are older than the terrestrial ''Najash rionegrina''. Similar skull structure, reduced or absent limbs, and other anatomical features found in both mosasaurs and snakes lead to a positive [[Cladistics|cladistical]] correlation, although some of these features are shared with varanids.{{citation needed|date=June 2016}}

Genetic studies in recent years have indicated snakes are not as closely related to monitor lizards as was once believed—and therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, more evidence links mosasaurs to snakes than to varanids. Fragmented remains found from the [[Jurassic]] and Early Cretaceous indicate deeper fossil records for these groups, which may potentially refute either hypothesis.<ref>{{cite journal |vauthors=Vidal N, Hedges SB |title=Molecular evidence for a terrestrial origin of snakes |journal=[[Proceedings. Biological Sciences]] |volume=271 |issue=Suppl 4 |pages=S226-9 |date=May 2004 |pmid=15252991 |pmc=1810015 |doi=10.1098/rsbl.2003.0151}}</ref><ref>{{cite journal |vauthors=Caldwell MW, Nydam RL, Palci A, Apesteguía S |title=The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution |journal=[[Nature Communications]] |volume=6 |issue=1 |pages=5996 |date=January 2015 |pmid=25625704 |doi=10.1038/ncomms6996 |bibcode=2015NatCo...6.5996C |doi-access=free|hdl=11336/37995 |hdl-access=free }}</ref>

<gallery>
File:Tetrapodophis amplectus 3483.jpg|''[[Tetrapodophis]]''
File:Eupodophis at Royal Belgian Institute of Natural Sciences, Brussels.jpg|''[[Eupodophis descouensi]]''
File:Eupodophis descouensi Holotype.jpg|''[[Eupodophis descouensi]]''
File:Eupodophis descouensi Holotype hind leg.jpg|''[[Eupodophis]] descouensi'' hind leg
</gallery>

=== Genetic basis of snake evolution ===
{{Main|Limb development}}
Both fossils and [[Phylogenetics|phylogenetic]] studies demonstrate that snakes evolved from [[lizard]]s, hence the question became which genetic changes led to limb loss in the snake ancestor. Limb loss is actually very common in extant reptiles and has happened dozens of times within [[skink]]s, [[Anguidae|anguids]], and other lizards.<ref>{{cite journal |last1=Bergmann |first1=Philip J. |last2=Morinaga |first2=Gen |title=The convergent evolution of snake-like forms by divergent evolutionary pathways in squamate reptiles |journal=Evolution |date=March 2019 |volume=73 |issue=3 |pages=481–496 |doi=10.1111/evo.13651 |pmid=30460998 }}</ref>

In 2016, two studies reported that limb loss in snakes is associated with DNA mutations in the Zone of Polarizing Activity Regulatory Sequence (ZRS), a regulatory region of the [[sonic hedgehog]] gene which is critically required for limb development. More advanced snakes have no remnants of limbs, but basal snakes such as pythons and boas do have traces of highly reduced, vestigial hind limbs. Python embryos even have fully developed hind limb buds, but their later development is stopped by the DNA mutations in the ZRS.<ref>{{Cite news |url=https://www.theatlantic.com/science/archive/2016/10/what-a-legless-mouse-tells-us-about-snake-evolution/504779/ |title=What a Legless Mouse Tells Us About Snake Evolution |newspaper=[[The Atlantic]] |access-date=October 25, 2016 |url-status=live |archive-url=https://web.archive.org/web/20161024222452/http://www.theatlantic.com/science/archive/2016/10/what-a-legless-mouse-tells-us-about-snake-evolution/504779/ |archive-date=October 24, 2016}}</ref><ref>{{Cite news |url=http://www.livescience.com/56573-mutation-caused-snakes-to-lose-legs.html |title=Snakes Used to Have Legs and Arms … Until These Mutations Happened |newspaper=Live Science |access-date=October 22, 2016 |url-status=live |archive-url=https://web.archive.org/web/20161022150634/http://www.livescience.com/56573-mutation-caused-snakes-to-lose-legs.html |archive-date=October 22, 2016}}</ref><ref>{{cite journal |vauthors=Leal F, Cohn MJ |title=Loss and Re-emergence of Legs in Snakes by Modular Evolution of Sonic hedgehog and HOXD Enhancers |journal=[[Current Biology]] |volume=26 |issue=21 |pages=2966–2973 |date=November 2016 |pmid=27773569 |doi=10.1016/j.cub.2016.09.020 |doi-access=free|bibcode=2016CBio...26.2966L }}</ref><ref>{{cite journal |vauthors=Kvon EZ, Kamneva OK, Melo US, Barozzi I, Osterwalder M, Mannion BJ, Tissières V, Pickle CS, Plajzer-Frick I, Lee EA, Kato M, Garvin TH, Akiyama JA, Afzal V, Lopez-Rios J, Rubin EM, Dickel DE, Pennacchio LA, Visel A |display-authors=6 |title=Progressive Loss of Function in a Limb Enhancer during Snake Evolution |journal=[[Cell (journal)|Cell]] |volume=167 |issue=3 |pages=633–642.e11 |date=October 2016 |pmid=27768887 |pmc=5484524 |doi=10.1016/j.cell.2016.09.028 |url=}}</ref>