View source for Ophidia

{{Short description|Group of squamate reptiles}}
{{Automatic taxobox
| name = Ophidians
| fossil_range = <br/>[[Aptian]] – Recent, <br/>{{Fossil range|125|0|earliest=170}}Questionable [[Middle Jurassic]] records<ref name=oldest_snakes/>
| image = Coast Garter Snake.jpg
| image_caption = Coast garter snake,<br /> ''[[Garter Snake|Thamnophis elegans terrestris]]''
| taxon = Ophidia
| authority = Latreille, 1804
| subdivision_ranks = Subgroups
| subdivision = * {{extinct}}''[[Coniophis]]''
* {{extinct}}''[[Eupodophis]]''?
* {{extinct}}''[[Najash]]''
* {{extinct}}[[Lapparentophiidae]]
* {{extinct}}[[Madtsoiidae]]?
* {{extinct}}[[Parviraptoridae]]?
* [[Serpentes]]
}}
__FORCETOC__
'''Ophidia''' {{IPAc-en|ou|'|f|I|d|i|@}} (also known as '''Pan-Serpentes'''<ref name=snake_origin_ecology>{{Cite journal |doi=10.1186/s12862-015-0358-5 |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 |page=87 |year=2015 |last1=Hsiang |first1=A. Y. |last2=Field |first2=D. J. |last3=Webster |first3=T. H. |last4=Behlke |first4=A. D. |last5=Davis |first5=M. B. |last6=Racicot |first6=R. A. |last7=Gauthier |first7=J. A. |pmid=25989795 |pmc=4438441 |doi-access=free }}</ref>) is a group of [[Squamata|squamate]] reptiles including modern [[snake]]s and reptiles more closely related to snakes than to other living groups of lizards.

Ophidia was defined as the "most recent common ancestor of ''Pachyrhachis'' and Serpentes (modern snakes), and all its descendants" by Lee and Caldwell (1998: 1551).<ref>{{cite journal |last1=Lee |first1=M. S. Y. |title=Anatomy and relationships of Pachyrhachis problematicus, a primitive snake with hindlimbs |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=29 October 1998 |volume=353 |issue=1375 |pages=1521–1552 |doi=10.1098/rstb.1998.0308 |pmc=1692386}}</ref>

The clade name Ophidia derives from the [[Ancient Greek]] word {{wikt-lang|grc|ὀφίδιον}} ({{grc-transl|ὀφίδιον}}), meaning "small snake".<ref>{{Cite book |title=Abrégé du dictionnaire grec français |last=Bailly |first=Anatole |author-link=Anatole Bailly |date=1981-01-01 |publisher=Hachette |isbn=978-2-01-003528-9 |location=Paris |oclc=461974285}}</ref><ref>{{Cite web |url=http://www.tabularium.be/bailly/ |title=Greek-french dictionary online |last=Bailly |first=Anatole |author-link=Anatole Bailly |website=www.tabularium.be |access-date=January 7, 2019 |archive-date=March 18, 2022 |archive-url=https://web.archive.org/web/20220318000653/http://www.tabularium.be/bailly/ }}</ref>

== Evolution ==
Modern snakes are thought to have evolved from either burrowing or aquatic lizards during the mid-[[Cretaceous]] period, and the earliest known fossils date to around 112 [[Mega-annum|Ma]] ago. However, the relationship between modern snake and more primitive snake ancestors, many of which retained hind limbs, is less clear. While many of these "stem-snakes" are known from Mesozoic fossils, some of them may be descendants of the earliest true snakes rather than more primitive lineages. Below is a [[cladogram]] modified from a study by Wilson ''et al.'' (2010), which found many stem-snakes of other studies to be true snakes instead.<ref name=Wetal10>{{cite journal |last1=Wilson |first1=J.A. |last2=Mohabey |first2=D.M. |last3=Peters |first3=S.E. |last4=Head |first4=J.J. |year=2010 |title=Predation upon Hatchling Dinosaurs by a New Snake from the Late Cretaceous of India |article-number=e1000322 |journal=[[PLOS Biology]] |pmid=20209142 |volume=8 |pmc=2830453 |issue=3 |doi=10.1371/journal.pbio.1000322 |editor1-last=Benton |editor1-first=Michael J. |editor1-link=Michael Benton |doi-access=free }}</ref>

{{clade| style=font-size:100%;line-height:85%
|label1=[[Serpentes]] (modern snakes)
|1={{clade
   |1=[[Scolecophidia]]
   |2=''[[Najash|Najash rionegrina]]''
   |3={{clade
      |1=''[[Dinilysia|Dinilysia patagonica]]''
      |label2=[[Alethinophidia]]
      |2={{clade
         |1={{clade
            |1=''[[Anilius]]''
            |2={{clade
               |1=''[[Cylindrophis]]''
               |2={{clade
                  |1=[[Uropeltinae]]
                  |2=''[[Anomochilus]]''}} }} }}
         |2={{clade
            |1=[[Madtsoiidae]]
            |2={{clade
               |1=''[[Loxocemus]]''
               |2=[[Macrostomata]]
 }} }} }} }} }} }}

Below is different phylogenetic overview of ophidians, following the study by Caldwell ''et al.'' 2015.<ref name=oldest_snakes>{{Cite journal |doi=10.1038/ncomms6996 |title=The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution |journal=[[Nature Communications]] |volume=6 |page=5996 |year=2015 |last1=Caldwell |first1=M. W. |last2=Nydam |first2=R. L. |last3=Palci |first3=A. |last4=Apesteguía |first4=S. N. |author4-link=:es:Sebastián Apesteguía |pmid=25625704 |bibcode=2015NatCo...6.5996C |doi-access=free|hdl=11336/37995 |hdl-access=free }}</ref>
{{clade|style=font-size:100%;line-height:100%
 |label1='''Ophidia'''
  |1={{clade
  |1=''[[Portugalophis]]''
   |2=''[[Parviraptor]]''
   |label3=
   |3={{clade
    |1=''[[Eophis]]''
      |2={{clade
        |1=Unnamed sp.
        |2=''[[Diablophis]]''
        }}
     }}
    |label4=
    |4={{clade
      |1=''[[Dinilysia]]''
       |2={{clade
           |1=[[Madtsoiidae]]
           |2={{clade
               |1=''[[Najash]]''
               |2={{clade
                 |1=''[[Coniophis]]''
                 |2=[[Serpentes]] (modern snakes)
   }} }} }} }} }} }}

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>Durand, J.F. (2004). "The origin of snakes". Geoscience Africa 2004. Abstract Volume, University of the Witwatersrand, Johannesburg, South Africa, pp. 187.</ref> The earliest known snake fossils come from sites in Utah and Algeria, represented by the genera ''[[Coniophis]]'' and ''[[Lapparentophis]]'', respectively. These fossil sites have been tentatively dated to the [[Albian]] or [[Cenomanian]] age of the late Cretaceous, between 112 and 94 [[Megaannum|Ma]] ago. However, an even greater age has been suggested for one of the Algerian sites, which may be as old as the [[Aptian]], 125 to 112 Ma ago.<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> Caldwell ''et al.'' 2015 described three genera of stem-snakes from Jurassic and considered that ''[[Parviraptor]]'' as stem-snake as well.<ref name="oldest_snakes" /> However, later review considers that affinity of those fossils as stem-snakes are doubtful, which makes the earliest unequivocal records date around Aptian-Cenomanian.<ref>{{Cite book |last=Evans |first=Susan E. |title=The Origin and Early Evolutionary History of Snakes |date=2022-08-11 |publisher=Cambridge University Press |isbn=978-1-108-93889-1 |editor-last=Gower |editor-first=David J. |edition=1 |pages=7–25 |chapter=The Origin and Early Diversification of Squamates |doi=10.1017/9781108938891.004 |access-date=2023-03-28 |editor2-last=Zaher |editor2-first=Hussam |chapter-url=https://www.cambridge.org/core/product/identifier/9781108938891%23CN-bp-2/type/book_part |doi-access=free}}</ref> In 2025, Benson and colleagues described a new parviraptorid, ''[[Breugnathair]]'', known from relatively complete remains. While these authors did not rule out snake affinities for parviraptorids, they proposed other possible placements for the clade, including as stem squamates (in which case they would have convergently evolved their snake-like anatomy) or as early [[Toxicofera|toxicoferans]] outside of Ophidia.<ref>{{Cite journal |last=Benson |first=Roger B. J. |last2=Walsh |first2=Stig A. |last3=Griffiths |first3=Elizabeth F. |last4=Kulik |first4=Zoe T. |last5=Botha |first5=Jennifer |last6=Fernandez |first6=Vincent |last7=Head |first7=Jason J. |last8=Evans |first8=Susan E. |date=2025-10-01 |title=Mosaic anatomy in an early fossil squamate |url=https://www.nature.com/articles/s41586-025-09566-y |journal=Nature |language=en |pages=1–7 |doi=10.1038/s41586-025-09566-y |issn=1476-4687|doi-access=free }}</ref> 

Based on [[comparative anatomy]], there is consensus that snakes descended from [[lizard]]s.<ref name="Meh87">{{cite book |last=Mehrtens |first=JM |date=1987 |title=Living Snakes of the World in Color |location=New York |publisher=Sterling Publishers |page=480 |isbn=0-8069-6460-X}}</ref>{{Rp|11}}<ref name="Sanchez">{{cite web |last=Sanchez |first=Alejandro |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=2007-11-26}}</ref> [[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 [[Leptotyphlopidae]] and [[Typhlopidae]] groups also possess remnants of the pelvic girdle, sometimes appearing as horny projections when visible.

Front limbs are nonexistent in all known snakes. This is caused by the evolution of [[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.

Modern snakes greatly diversified during the [[Paleocene]].  This occurred alongside the [[adaptive radiation]] of mammals, following the extinction of (non-avian) [[dinosaur]]s. The [[colubrids]], one of the more common snake groups, became particularly diverse due to preying on [[rodents]], an especially successful mammal group.

=== Origins ===
The origin of snakes remains an unresolved issue.  There are two main hypotheses competing for acceptance.

==== Burrowing lizard hypothesis ====
There is fossil evidence to suggest that snakes may have evolved from burrowing lizards, such as the [[monitor lizard|varanids]] (or a similar group) during the [[Cretaceous|Cretaceous Period]].<ref name="EB">{{Cite book |last=Mc Dowell |first=Samuel |chapter=The Evolution of the Tongue of Snakes, and its Bearing on Snake Origins |title=Evolutionary Biology |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, ''[[Najash rionegrina]]'', was a two-legged burrowing animal with a [[sacrum]], and was fully [[Terrestrial animal|terrestrial]].<ref name=najash>{{cite journal |last1=Apesteguía |first1=Sebastián |author1-link=:es:Sebastián Apesteguía |last2=Zaher |first2=Hussam |date=April 2006 |title=A Cretaceous terrestrial snake with robust hindlimbs and a sacrum |journal=[[Nature (journal)|Nature]] |volume=440 |issue=7087 |pages=1037–1040 |doi=10.1038/nature04413 |url=http://www.nature.com/nature/journal/v440/n7087/edsumm/e060420-11.html |access-date=2007-11-29 |pmid=16625194 |bibcode=2006Natur.440.1037A |s2cid=4417196|url-access=subscription }}</ref> One [[extant taxon|extant]] analog of these putative ancestors is the earless monitor ''[[Lanthanotidae|Lanthanotus]]'' of [[Borneo]] (though it also is [[semiaquatic]]).<ref name="M2">{{Cite journal |last=Mertens |first=Robert |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=2007-11-29 |archive-url=https://web.archive.org/web/20071225225126/http://www.wildlifenews.co.uk/articles2000/march/march2500a.htm |archive-date=2007-12-25}}</ref>
[[File:Naturkundemuseum Berlin - Archaeophis proavus Massalongo - Monte Bolca.jpg|thumb|left|Fossil of ''Archaeophis proavus''.]]

==== Aquatic lizard hypothesis ====
Many researchers believe that [[snakes]] share a common marine ancestry with mosasaurs, a suggestion advanced in 1869 by [[Edward Drinker Cope]], who coined the term [[Pythonomorpha]] to unite them. The idea lay dormant for more than a century, to be revived in the 1990s.<ref name=palaeos>{{cite web |url=http://palaeos.com/vertebrates/squamata/pythonomorpha.html |title=Palaeos Vertebrates Squamata: Pythonomorpha |work=palaeos.com |year=2012 |access-date=25 November 2014}}</ref><ref>{{cite journal |url=http://oceansofkansas.com/Greatrep.html |title=Mosasaurs: Last of the Great Marine Reptiles |journal=Prehistoric Times |first=M. J. |last=Everhart  |issue=44 |pages=29–31 |year=2000 |access-date=25 November 2014}}</ref> The [[mosasaur]]s were [[aquatic animal|aquatic]] [[varanid]]<ref name=LCKC10>{{cite journal |last1=Lindgren |first1=J. |last2=Caldwell |first2=M.W. |last3=Konishi |first3=T. |last4=Chiappe |first4=L.M. |year=2010 |editor1-last=Farke |editor1-first=Andrew Allen |title=Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil Mosasaur |journal=[[PLOS One]] |pmid=20711249 |volume=5 |issue=8 |pmc=2918493 |article-number=e11998 |doi=10.1371/journal.pone.0011998|bibcode=2010PLoSO...511998L |doi-access=free}}</ref> lizards from the [[Cretaceous]]—which in turn are derived from [[Aigialosauridae|Aigialosaurids]].<ref name="Sanchez"/><ref name="Wilf2015">{{cite journal |last1=Wilf |first1=P. |last2=Reeder |first2=T. W. |last3=Townsend |first3=T. M. |last4=Mulcahy |first4=D. G. |last5=Noonan |first5=B. P. |last6=Wood |first6=P. L. |last7=Sites |first7=J. W. |last8=Wiens |first8=J. J. |title=Integrated Analyses Resolve Conflicts over Squamate Reptile Phylogeny and Reveal Unexpected Placements for Fossil Taxa |journal=[[PLOS One]] |volume=10 |issue=3 |year=2015 |article-number=e0118199 |doi=10.1371/journal.pone.0118199 |pmid=25803280 |pmc=4372529 |bibcode=2015PLoSO..1018199R |doi-access=free}}</ref> 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 led to a positive [[Cladistics|cladistical]] correlation, and some of these features are shared with varanids.{{clarify|date=June 2019}}

The [[cladogram]] below, based on Palci and Caldwell (2010), suggests that snakes are more closely related to [[Dolichosauridae|Dolichosaurids]] than [[mosasaur]]s proper.<ref name="Palci2010">{{cite journal |author1=Alessandro Palci |author2=Michael W. Caldwell |year=2010a |title=Redescription of ''Acteosaurus tommasinii'' von Meyer, 1860, and a discussion of evolutionary trends within the clade Ophidiomorpha |journal=[[Journal of Vertebrate Paleontology]] |volume=30 |issue=1 |pages=94–108 |doi=10.1080/02724630903409139 |s2cid=85217455|doi-access=free }}</ref><ref>{{cite journal |doi=10.1080/02724631003762963 |title=A new species of marine ophidiomorph lizard, Adriosaurus skrbinensis, from the Upper Cretaceous of Slovenia |year=2010 |last1=Caldwell |first1=Michael W. |last2=Palci |first2=Alessandro |journal=[[Journal of Vertebrate Paleontology]] |volume=30 |issue=3 |pages=747–755 |s2cid=85570665}}</ref> placing the Dolichosauridae as a sister group to modern snakes and their closest ancestors.

{{clade|{{clade
   |1=[[Anguidae]]   
   |2={{clade
      |1={{clade
         |1=''[[Xenosaurus]]''
         |2=''[[Shinisaurus]]'' }}
      |2={{clade
         |1=''[[Heloderma]]''
         |2={{clade
            |1={{clade
               |1=''[[Lanthanotus]]''
               |2=''[[Varanus]]'' }}
            |label2=[[Pythonomorpha]]
            |2={{clade
               |1={{clade
                  |1=[[Aigialosauridae]]
                  |2=[[Mosasauridae]] }}
               |label2=[[Ophidiomorpha]]
               |2={{clade
                  |label1=Dolichosauridae
                  |1={{clade
                     |1=''[[Coniasaurus]]''
                     |2=''[[Dolichosaurus]]'' }}
                  |2={{clade
                     |1=''[[Pontosaurus]]''
                     |2={{clade
                        |1=''[[Aphanizocnemus]]''
                        |2={{clade
                           |1=''[[Adriosaurus]]''
                           |2={{clade
                              |1=''[[Acteosaurus]]''
                              |2=Ophidia }} }} }} }} }} }} }} }} }} }}|style=font-size:100%;line-height:85%|label1='''[[Anguimorpha]]'''}}

== References ==
{{Reflist}}

{{Taxonbar|from=Q16986904}}

[[Category:Ophidia| ]]
[[Category:Toxicofera]]
[[Category:Aalenian first appearances]]
[[Category:Extant Middle Jurassic first appearances]]