squamata

File:Slavoia darevskii.jpg of Slavoia darevskii, a fossil squamate]]

Squamates are a monophyletic sister group to the rhynchocephalians, members of the order Rhynchocephalia. The only surviving member of the Rhynchocephalia is the tuatara. Squamata and Rhynchocephalia form the superorder Lepidosauria, which is the sister group to the Archosauria, the clade that contains crocodiles and birds, and their extinct relatives. Fossils of rhynchocephalians first appear in the Early Triassic, meaning that the lineage leading to squamates must have also existed at the time.{{Cite journal |last1=Jones |first1=Marc E. |last2=Anderson |first2=Cajsa Lipsa |last3=Hipsley |first3=Christy A. |last4=Müller |first4=Johannes |last5=Evans |first5=Susan E. |last6=Schoch |first6=Rainer R. |title=Integration of molecules and new fossils supports a Triassic origin for Lepidosauria (lizards, snakes, and tuatara) |journal=BMC Evolutionary Biology |date=25 September 2013 |volume=13 |issue=1 |page=208 |doi=10.1186/1471-2148-13-208 |pmid=24063680 |pmc=4016551 |doi-access=free |bibcode=2013BMCEE..13..208J }}{{cite journal |doi=10.7554/eLife.66511 |title=The Jurassic rise of squamates as supported by lepidosaur disparity and evolutionary rates |year=2022 |last1=Bolet |first1=Arnau |last2=Stubbs |first2=Thomas L. |last3=Herrera-Flores |first3=Jorge A. |last4=Benton |first4=Michael J. |journal=eLife |volume=11 |pmid=35502582 |pmc=9064307 |doi-access=free }}

A study in 2018 found that Megachirella, an extinct genus of lepidosaurs that lived about 240 million years ago during the Middle Triassic, was a stem-squamate, making it the oldest known squamate. The phylogenetic analysis was conducted by performing high-resolution microfocus X-ray computed tomography (micro-CT) scans on the fossil specimen of Megachirella to gather detailed data about its anatomy. These data were then compared with a phylogenetic dataset combining the morphological and molecular data of 129 extant and extinct reptilian taxa. The comparison revealed Megachirella had certain features that are unique to squamates. The study also found that geckos are the earliest crown group squamates, not iguanians.{{Cite journal |last1=Simōes |first1=Tiago R. |last2=Caldwell |first2=Michael W. |last3=Talanda |first3=Mateusz |last4=Bernardi |first4=Massimo |last5=Palci |first5=Alessandro |last6=Vernygora |first6=Oksana |last7=Bernardini |first7=Federico |last8=Mancini |first8=Lucia |last9=Nydam |first9=Randall L. |date=30 May 2018 |title=The origin of squamates revealed by a Middle Triassic lizard from the Italian Alps |journal=Nature |volume=557 |issue=7707 |pages=706–709 |bibcode=2018Natur.557..706S |doi=10.1038/s41586-018-0093-3 |pmid=29849156 |s2cid=44108416}}{{Cite web |last=Weisberger |first=Mindy |date=30 May 2018 |title=This 240-Million-Year-Old Reptile Is the 'Mother of All Lizards' |url=https://www.livescience.com/62693-mother-of-lizards-fossil.html |access-date=2 June 2018 |work=Live Science |publisher=Purch Group |archive-date=21 June 2019 |archive-url=https://web.archive.org/web/20190621104947/https://amp.livescience.com/62693-mother-of-lizards-fossil.html |url-status=live }} However, a 2021 study found the genus to be a lepidosaur of uncertain position, in a polytomy with Squamata and Rhynchocephalia.{{Cite journal |last=Ford |first=David P. |last2=Evans |first2=Susan E. |last3=Choiniere |first3=Jonah N. |last4=Fernandez |first4=Vincent |last5=Benson |first5=Roger B. J. |date=2021-08-25 |title=A reassessment of the enigmatic diapsid Paliguana whitei and the early history of Lepidosauromorpha |url=https://royalsocietypublishing.org/doi/10.1098/rspb.2021.1084 |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=288 |issue=1957 |pages=20211084 |doi=10.1098/rspb.2021.1084 |issn=0962-8452 |pmc=8385343 |pmid=34428965}}

In 2022, the extinct genus Cryptovaranoides was described from the Late Triassic (Rhaetian age) of England as a highly derived squamate belonging to the group Anguimorpha, which contains many extant lineages such as monitor lizards, beaded lizards and anguids. The presence of an essentially modern crown group squamate so far back in time was unexpected, as their diversification was previously thought to have occurred during the Jurassic and Cretaceous.{{Cite journal |last1=Whiteside |first1=David I. |last2=Chambi-Trowell |first2=Sofía A. V. |last3=Benton |first3=Michael J. |author3-link=Michael Benton |date=2022-12-02 |title=A Triassic crown squamate |journal=Science Advances |language=en |volume=8 |issue=48 |pages=eabq8274 |bibcode=2022SciA....8.8274W |doi=10.1126/sciadv.abq8274 |issn=2375-2548 |pmid=36459546 |pmc=10936055 |s2cid=254180027 |hdl-access=free |hdl=1983/a3c7a019-cfe6-4eb3-9ac0-d50c61c5319e }} A 2023 study found that Cryptovaranoides most likely represents an archosauromorph with no apparent squamate affinities,{{Cite journal |last1=Brownstein |first1=Chase D. |last2=Simões |first2=Tiago R. |last3=Caldwell |first3=Michael W. |last4=Lee |first4=Michael S. Y. |last5=Meyer |first5=Dalton L. |last6=Scarpetta |first6=Simon G. |date=October 2023 |title=The affinities of the Late Triassic Cryptovaranoides and the age of crown squamates |journal=Royal Society Open Science |language=en |volume=10 |issue=10 |doi=10.1098/rsos.230968 |pmid=37830017 |pmc=10565374 |issn=2054-5703 |s2cid=263802572}} though the original describers maintained their original conclusion that this taxon represents a squamate.{{Cite journal|last1=Whiteside |first1=D. I. |last2=Chambi-Trowell |first2=S. A. V. |last3=Benton |first3=M. J. |year=2024 |title=Late Triassic †Cryptovaranoides microlanius is a squamate, not an archosauromorph |journal=Royal Society Open Science |volume=11 |issue=11 |at=231874 |doi=10.1098/rsos.231874 |doi-access=free |pmc=11597406 }} The oldest unambiguous fossils of Squamata date to the Bathonian age of the Middle Jurassic of the Northern Hemisphere, with the first appearance of many modern groups, including snakes, during this period.{{Cite journal |last1=Herrera-Flores |first1=Jorge A. |last2=Stubbs |first2=Thomas L. |last3=Benton |first3=Michael J. |date=March 2021 |title=Ecomorphological diversification of squamates in the Cretaceous |journal=Royal Society Open Science |language=en |volume=8 |issue=3 |pages=rsos.201961, 201961 |doi=10.1098/rsos.201961 |issn=2054-5703 |pmc=8074880 |pmid=33959350|bibcode=2021RSOS....801961H }}

Scientists believe crown group squamates probably originated in the Early Jurassic based on the fossil record, with the oldest unambiguous fossils of squamates dating to the Middle Jurassic.{{Cite journal |last1=Tałanda |first1=Mateusz |last2=Fernandez |first2=Vincent |last3=Panciroli |first3=Elsa |last4=Evans |first4=Susan E. |last5=Benson |first5=Roger J. |date=2022-10-26 |title=Synchrotron tomography of a stem lizard elucidates early squamate anatomy |url=https://www.nature.com/articles/s41586-022-05332-6 |journal=Nature |language=en |volume=611 |issue=7934 |pages=99–104 |doi=10.1038/s41586-022-05332-6 |issn=0028-0836 |pmid=36289329 |bibcode=2022Natur.611...99T |s2cid=253160713 |access-date=13 October 2023 |archive-date=28 December 2023 |archive-url=https://web.archive.org/web/20231228173131/https://www.nature.com/articles/s41586-022-05332-6 |url-status=live }} Squamate morphological and ecological diversity substantially increased over the course of the Cretaceous, including the appeance of groups like iguanians and varanoids, and true snakes. Polyglyphanodontia, an extinct clade of lizards, and mosasaurs, a group of predatory marine lizards that grew to enormous sizes, also appeared in the Cretaceous.{{Cite journal |last1=Gauthier |first1=Jacques |last2=Kearney |first2=Maureen |last3=Maisano |first3=Jessica Anderson |last4=Rieppel |first4=Olivier |last5=Behlke |first5=Adam D. B. |s2cid=86355757 |title=Assembling the squamate tree of life: perspectives from the phenotype and the fossil record |journal=Bulletin of the Peabody Museum of Natural History |date=April 2012 |volume=53 |pages=3–308 |doi=10.3374/014.053.0101}} Squamates suffered a mass extinction at the Cretaceous–Paleogene (K–Pg) boundary, which wiped out polyglyphanodontians, mosasaurs, and many other distinct lineages.{{cite journal |last1=Longrich |first1=Nicholas R. |last2=Bhullar |first2=Bhart-Anjan S. |last3=Gauthier |first3=Jacques |author3-link=Jacques Gauthier |title=Mass extinction of lizards and snakes at the Cretaceous-Paleogene boundary |journal=Proceedings of the National Academy of Sciences |date=10 December 2012 |volume=109 |issue=52 |pages=21396–21401 |doi=10.1073/pnas.1211526110 |pmid=23236177 |pmc=3535637 |bibcode=2012PNAS..10921396L |doi-access=free}}

The relationships of squamates are debatable. Although many of the groups originally recognized on the basis of morphology are still accepted, understanding of their relationships to each other has changed radically as a result of studying their genomes. Iguanians were long thought to be the earliest crown group squamates based on morphological data, but genetic data suggest that geckos are the earliest crown group squamates.{{Cite journal |last1=Pyron |first1=R. Alexander |last2=Burbrink |first2=Frank T. |last3=Wiens |first3=John J. |title=A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes |journal=BMC Evolutionary Biology |date=29 April 2013 |volume=13 |issue=1 |page=93 |doi=10.1186/1471-2148-13-93 |pmid=23627680 |pmc=3682911 |doi-access=free |bibcode=2013BMCEE..13...93P }} Iguanians are now united with snakes and anguimorphs in a clade called Toxicofera. Genetic data also suggest that the various limbless groups – snakes, amphisbaenians, and dibamids – are unrelated, and instead arose independently from lizards.

{{See also|Sexual selection in scaled reptiles}}

File:Trachylepis maculilabris mating.jpg mating]]

The male members of the group Squamata have hemipenes, which are usually held inverted within their bodies, and are everted for reproduction via erectile tissue like that in the mammalian penis.{{cite web |url=http://www.greenigsociety.org/anatomy.htm |title=Iguana Anatomy |access-date=28 September 2008 |archive-date=16 March 2010 |archive-url=https://web.archive.org/web/20100316160245/http://www.greenigsociety.org/anatomy.htm |url-status=live }} Only one is used at a time, and some evidence indicates that males alternate use between copulations. The hemipenis has a variety of shapes, depending on the species. Often it bears spines or hooks, to anchor the male within the female. Some species even have forked hemipenes (each hemipenis has two tips). Due to being everted and inverted, hemipenes do not have a completely enclosed channel for the conduction of sperm, but rather a seminal groove that seals as the erectile tissue expands. This is also the only reptile group in which both viviparous and ovoviviparous species are found, as well as the usual oviparous reptiles. The eggs in oviparous species have a parchment-like shell. The only exception is found in blind lizards and three families of geckos (Gekkonidae, Phyllodactylidae and Sphaerodactylidae), where many lay rigid and calcified eggs.{{Cite journal|title=A comparative study of eggshells of Gekkota with morphological, chemical compositional and crystallographic approaches and its evolutionary implications - PMC|date=2018 |pmc=6014675 |last1=Choi |first1=S. |last2=Han |first2=S. |last3=Kim |first3=N. H. |last4=Lee |first4=Y. N. |journal=PLOS ONE |volume=13 |issue=6 |pages=e0199496 |doi=10.1371/journal.pone.0199496 |doi-access=free |pmid=29933400 |bibcode=2018PLoSO..1399496C }}{{Cite web|url=https://www.faculty.biol.vt.edu/andrews/PDF%20files-new/2015AndrewsRSEggs.pdf|title=Rigid Shells Enhance Survival of Gekkotan Eggs}} Some species, such as the Komodo dragon, can reproduce asexually through parthenogenesis.{{cite news |last=Morales |first=Alex |publisher=Bloomberg Television |url=https://www.bloomberg.com/apps/news?pid=20601082&sid=apLYpeppu8ag&refer=canada |title=Komodo Dragons, World's Largest Lizards, Have Virgin Births |access-date=2008-03-28 |date=20 December 2006 |archive-date=8 October 2007 |archive-url=https://web.archive.org/web/20071008112514/http://www.bloomberg.com/apps/news?pid=20601082 |url-status=live }}

File:Elaphe quadrivirgata.JPG

Studies have been conducted on how sexual selection manifests itself in snakes and lizards. Snakes use a variety of tactics in acquiring mates.{{cite journal |doi=10.1016/j.anbehav.2003.05.007 |title=Courtship tactics in garter snakes: How do a male's morphology and behaviour influence his mating success? |year=2004 |last1=Shine |first1=Richard |last2=Langkilde |first2=Tracy |last3=Mason |first3=Robert T |journal=Animal Behaviour |volume=67 |issue=3 |pages=477–83 |s2cid=4830666}}{{dubious|reason=See talk for 'Sexual Selection'|date=December 2015}} Ritual combat between males for the females with which they want to mate includes topping, a behavior exhibited by most viperids, in which one male twists around the vertically elevated fore body of his opponent and forcing it downward. Neck biting commonly occurs while the snakes are entwined.{{cite journal |doi=10.1016/j.anbehav.2004.03.012 |title=Genetic evidence for sexual selection in black ratsnakes, Elaphe obsoleta |year=2005 |last1=Blouin-Demers |first1=Gabriel |last2=Gibbs |first2=H. Lisle |last3=Weatherhead |first3=Patrick J. |journal=Animal Behaviour |volume=69 |issue=1 |pages=225–34 |s2cid=3907523}}

File:Central fusion and terminal fusion automixis.svg

Parthenogenesis is a natural form of reproduction in which the growth and development of embryos occur without fertilization. Agkistrodon contortrix (copperhead snake) and Agkistrodon piscivorus (cottonmouth snake) can reproduce by facultative parthenogenesis; they are capable of switching from a sexual mode of reproduction to an asexual mode.{{cite journal |vauthors=Booth W, Smith CF, Eskridge PH, Hoss SK, Mendelson JR, Schuett GW |title=Facultative parthenogenesis discovered in wild vertebrates |journal=Biology Letters |volume=8 |issue=6 |pages=983–5 |year=2012 |pmid=22977071 |pmc=3497136 |doi=10.1098/rsbl.2012.0666}} The type of parthenogenesis that likely occurs is automixis with terminal fusion (see figure), a process in which two terminal products from the same meiosis fuse to form a diploid zygote. This process leads to genome-wide homozygosity, expression of deleterious recessive alleles, and often to developmental abnormalities. Both captive-born and wild-born A. contortrix and A. piscivorus appear to be capable of this form of parthenogenesis.

Reproduction in squamate reptiles is ordinarily sexual, with males having a ZZ pair of sex-determining chromosomes, and females a ZW pair. However, the Colombian rainbow boa, Epicrates maurus, can also reproduce by facultative parthenogenesis, resulting in production of WW female progeny.{{cite journal |author6-link=Coby Schal |vauthors=Booth W, Million L, Reynolds RG, Burghardt GM, Vargo EL, Schal C, Tzika AC, Schuett GW |title=Consecutive virgin births in the new world boid snake, the Colombian rainbow Boa, Epicrates maurus |journal=Journal of Heredity |volume=102 |issue=6 |pages=759–63 |year=2011 |pmid=21868391 |doi=10.1093/jhered/esr080 |doi-access=free}} The WW females are likely produced by terminal automixis.

When female sand lizards mate with two or more males, sperm competition within the female's reproductive tract may occur. Active selection of sperm by females appears to occur in a manner that enhances female fitness.{{cite journal |vauthors=Olsson M, Shine R, Madsen T, Gullberg A, Tegelström H |title=Sperm choice by females |journal=Trends in Ecology & Evolution |volume=12 |issue=11 |pages=445–6 |year=1997 |pmid=21238151 |doi=10.1016/s0169-5347(97)85751-5|bibcode=1997TEcoE..12..445O }} On the basis of this selective process, the sperm of males that are more distantly related to the female are preferentially used for fertilization, rather than the sperm of close relatives. This preference may enhance the fitness of progeny by reducing inbreeding depression.

{{Main|Evolution of snake venom}}

{{See also|Venom}}

Recent research suggests that the evolutionary origin of venom may exist deep in the squamate phylogeny, with 60% of squamates placed in this hypothetical group called Toxicofera. Venom has been known in the clades Caenophidia, Anguimorpha, and Iguania, and has been shown to have evolved a single time along these lineages before the three groups diverged, because all lineages share nine common toxins. The fossil record shows the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 million years ago (Mya) to the Late Triassic/Early Jurassic, but the only good fossil evidence is from the Middle Jurassic.{{Cite journal |last1=Hutchinson |first1=M. N. |last2=Skinner |first2=A. |last3=Lee |first3=M. S. Y. |doi=10.1098/rsbl.2011.1216 |title=Tikiguania and the antiquity of squamate reptiles (lizards and snakes) |journal=Biology Letters |volume=8 |issue=4 |pages=665–669 |year=2012 |pmid=22279152 |pmc=3391445}}

Snake venom has been shown to have evolved via a process by which a gene encoding for a normal body protein, typically one involved in key regulatory processes or bioactivity, is duplicated, and the copy is selectively expressed in the venom gland.{{cite journal |last1=Fry |first1=B. G. |last2=Vidal |first2=N. |last3=Kochva |first3=E. |last4=Renjifo |first4=C. |year=2009 |title=Evolution and diversification of the toxicofera reptile venom system |journal=Journal of Proteomics |volume=72 |issue=2 |pages=127–136 |doi=10.1016/j.jprot.2009.01.009 |pmid=19457354}} Previous literature hypothesized that venoms were modifications of salivary or pancreatic proteins,{{cite journal |last1=Kochva |first1=E |year=1987 |title=The origin of snakes and evolution of the venom apparatus |journal=Toxicon |volume=25 |issue=1 |pages=65–106 |doi=10.1016/0041-0101(87)90150-4 |pmid=3564066|bibcode=1987Txcn...25...65K }} but different toxins have been found to have been recruited from numerous different protein bodies and are as diverse as their functions.{{cite journal |last1=Fry |first1=B. G. |year=2005 |title=From genome to "Venome": Molecular origin and evolution of the snake venom proteome inferred from phylogenetic analysis of toxin sequences and related body proteins |journal=Genome Research |volume=15 |issue=3 |pages=403–420 |doi=10.1101/gr.3228405 |pmid=15741511 |pmc=551567}}

Natural selection has driven the origination and diversification of the toxins to counter the defenses of their prey. Once toxins have been recruited into the venom proteome, they form large, multigene families and evolve via the birth-and-death model of protein evolution,{{cite journal |last1=Fry |first1=B. G. |last2=Scheib |first2=H. |last3=Young |first3=B. |last4=McNaughtan |first4=J. |last5=Ramjan |first5=S. F. R. |last6=Vidal |first6=N. |year=2008 |title=Evolution of an arsenal |journal=Molecular & Cellular Proteomics |volume=7 |issue=2 |pages=215–246 |doi=10.1074/mcp.m700094-mcp200 |pmid=17855442 |doi-access=free}} which leads to a diversification of toxins that allows the ambush predators the ability to attack a wide range of prey.{{cite journal |last1=Calvete |first1=J. J. |last2=Sanz |first2=L. |last3=Angulo |first3=Y. |last4=Lomonte |first4=B. |last5=Gutierrez |first5=J. M. |year=2009 |title=Venoms, venomics, antivenomics |journal=FEBS Letters |volume=583 |issue=11 |pages=1736–1743 |doi=10.1016/j.febslet.2009.03.029 |pmid=19303875 |s2cid=904161 |doi-access=free|bibcode=2009FEBSL.583.1736C }} The rapid evolution and diversification is thought to be the result of a predator–prey evolutionary arms race, where both are adapting to counter the other.{{cite journal |last1=Barlow |first1=A. |last2=Pook |first2=C. E. |last3=Harrison |first3=R. A. |last4=Wuster |first4=W. |year=2009 |title=Coevolution of diet and prey-specific venom activity supports the role of selection in snake venom evolution |journal=Proceedings of the Royal Society B: Biological Sciences |volume=276 |issue=1666 |pages=2443–2449 |doi=10.1098/rspb.2009.0048 |pmid=19364745 |pmc=2690460}}

{{See also|Snakebite}}

File:Number of snake envenomings (2007).svg

An estimated 125,000 people a year die from venomous snake bites.{{cite web |title=Snake-bites: appraisal of the global situation |publisher=World Health Organization |url=https://www.who.int/bloodproducts/publications/en/bulletin_1998_76(5)_515-524.pdf |access-date=2007-12-30 |archive-date=27 February 2021 |archive-url=https://web.archive.org/web/20210227041036/http://www.who.int/bloodproducts/publications/en/bulletin_1998_76(5)_515-524.pdf |url-status=live }} In the US alone, more than 8,000 venomous snake bites are reported each year, but only one in 50 million people (five or six fatalities per year in the USA) will die from venomous snake bites.{{cite web |url=http://ufwildlife.ifas.ufl.edu/venomous_snake_faqs.shtml |title=Venomous Snake FAQs |publisher=University of Florida |access-date=17 September 2019 |archive-date=7 December 2020 |archive-url=https://web.archive.org/web/20201207064318/http://ufwildlife.ifas.ufl.edu/venomous_snake_faqs.shtml |url-status=live }}{{cite web |title=First Aid Snake Bites |publisher=University of Maryland Medical Center |url=http://www.umm.edu/non_trauma/snake.htm |access-date=2007-12-30 |archive-date=11 October 2007 |archive-url=https://web.archive.org/web/20071011065938/http://www.umm.edu/non_trauma/snake.htm |url-status=dead }}

Lizard bites, unlike venomous snake bites, are usually not fatal. The Komodo dragon has been known to kill people due to its size, and recent studies show it may have a passive envenomation system. Recent studies also show that the close relatives of the Komodo, the monitor lizards, all have a similar envenomation system, but the toxicity of the bites is relatively low to humans.{{cite web| title = Komodo dragon kills boy, 8, in Indonesia| date = 4 June 2007| publisher = NBC News| url = https://www.nbcnews.com/id/wbna19026658| access-date = 2007-12-30| archive-date = 6 September 2017| archive-url = https://web.archive.org/web/20170906224720/http://www.nbcnews.com/id/19026658/| url-status = live}} The Gila monster and beaded lizards of North and Central America are venomous, but not deadly to humans.

Though they survived the Cretaceous–Paleogene extinction event, many squamate species are now endangered due to habitat loss, hunting and poaching, illegal wildlife trading, alien species being introduced to their habitats (which puts native creatures at risk through competition, disease, and predation), and other anthropogenic causes. Because of this, some squamate species have recently become extinct, with Africa having the most extinct species. Breeding programs and wildlife parks, though, are trying to save many endangered reptiles from extinction. Zoos, private hobbyists, and breeders help educate people about the importance of snakes and lizards.

Image:DesertIguana031611.jpg

Historically, the order Squamata has been divided into three suborders:

• Lacertilia, the lizards
• Serpentes, the snakes (see also Ophidia)
• Amphisbaenia, the worm lizards

Of these, the lizards form a paraphyletic group,{{cite journal |last1=Reeder |first1=Tod W. |last2=Townsend |first2=Ted M. |last3=Mulcahy |first3=Daniel G. |last4=Noonan |first4=Brice P. |last5=Wood |first5=Perry L. |last6=Sites |first6=Jack W. |last7=Wiens |first7=John J. |title=Integrated Analyses Resolve Conflicts over Squamate Reptile Phylogeny and Reveal Unexpected Placements for Fossil Taxa |journal=PLOS One |date=2015 |volume=10 |issue=3 |pages=e0118199 |doi=10.1371/journal.pone.0118199 |pmid=25803280 |pmc=4372529|bibcode=2015PLoSO..1018199R |doi-access=free }} since the "lizards" are found in several distinct lineages, with snakes and amphisbaenians recovered as monophyletic groups nested within. Although studies of squamate relationships using molecular biology have found different relationships between some squamata lineages, all recent molecular studies{{cite journal |date=February 2006 |title=Early evolution of the venom system in lizards and snakes |journal=Nature |volume=439 |issue=7076 |pages=584–588 |doi=10.1038/nature04328 |pmid=16292255 |last1=Fry |first1=Brian G. |last2=Vidal |first2=Nicolas |last3=Norman |first3=Janette A. |last4=Vonk |first4=Freek J. |last5=Scheib |first5=Holger |last6=Ramjan |first6=S.F. Ryan |last7=Kuruppu |first7=Sanjaya |last8=Fung |first8=Kim |last9=Hedges |first9=S. Blair |last10=Richardson |first10=Michael K. |last11=Hodgson |first11=Wayne. C. |last12=Ignjatovic |first12=Vera |last13=Summerhayes |first13=Robyn |last14=Kochva |first14=Elazar |bibcode=2006Natur.439..584F |s2cid=4386245 |display-authors=6 }} suggest that the venomous groups are united in a venom clade. Named Toxicofera, it encompasses a majority (nearly 60%) of squamate species and includes Serpentes (snakes), Iguania (agamids, chameleons, iguanids, etc.), and Anguimorpha (monitor lizards, Gila monster, glass lizards, etc.).

One example of a modern classification of the squamates is shown below.{{cite journal |last1=Zheng |first1=Yuchi |author1-link=Yuchi Zheng |last2=Wiens |first2=John J. |title=Combining phylogenomic and supermatrix approaches, and a time-calibrated phylogeny for squamate reptiles (lizards and snakes) based on 52 genes and 4162 species |journal=Molecular Phylogenetics and Evolution |date=2016 |volume=94 |issue=Part B |pages=537–547 |doi=10.1016/j.ympev.2015.10.009 |pmid=26475614|bibcode=2016MolPE..94..537Z }}

{{clade |style=font-size:85%;line-height:80% |overflow=yes

|label1=Squamata

|1={{clade

|label1=Dibamia

|1=Dibamidae

|label2=Bifurcata

|2={{clade

|label1=Gekkota

|1={{clade

|label1=Pygopodomorpha

|1={{clade

|1=Diplodactylidae Underwood 195470 px

|2={{clade

|1=Pygopodidae Boulenger 188470 px

|2=Carphodactylidae

}}

}}

|label2=Gekkomorpha

|2={{clade

|1=Eublepharidae

|label2=Gekkonoidea

|2={{clade

|1=Sphaerodactylidae Underwood 1954

|2={{clade

|1=Phyllodactylidae 70 px

|2=Gekkonidae

}}

}}

}}

}}

|label2=Unidentata

|2={{clade

|label1=Scinciformata

|1={{clade

|label1=Scincomorpha

|1=Scincidae 70 px

|label2=Cordylomorpha

|2={{clade

|1=Xantusiidae

|2={{clade

|1=Gerrhosauridae 70 px

|2=Cordylidae 70 px

}}

}}

}}

|label2=Episquamata

|2={{clade

|label1=Laterata

|1={{clade

|label1=Teiformata

|1={{clade

|1=Gymnophthalmidae Merrem 182070 px

|2=Teiidae Gray 182790 px

}}

|label2=Lacertibaenia

|2={{clade

|label1=Lacertiformata

|1=Lacertidae 70 px

|label2=Amphisbaenia

|2={{clade

|1=Rhineuridae Vanzolini 1951

|2={{clade

|1=Bipedidae Taylor 195170 px

|2={{clade

|1={{clade

|1=Blanidae Kearney & Stuart 200470 px

|2=Cadeidae Vidal & Hedges 2008

}}

|2={{clade

|1=Trogonophidae Gray 1865

|2=Amphisbaenidae Gray 186570 px

}}

}}

}}

}}

}}

}}

|label2=Toxicofera

|2={{clade

|1={{clade

|label1=Anguimorpha

|1={{clade

|label1=Paleoanguimorpha

|1={{clade

|label1=Shinisauria

|1=Shinisauridae Ahl 1930 sensu Conrad 2006

|label2=Varanoidea

|2={{clade

|1=Lanthanotidae

|2=Varanidae 90 px

}}

}}

|label2=Neoanguimorpha

|2={{clade

|label1=Helodermatoidea

|1=Helodermatidae Gray 183770 px

|2={{clade

|label1=Xenosauroidea

|1=Xenosauridae

|label2=Anguioidea

|2={{clade

|1=Diploglossidae

|2={{clade

|1=Anniellidae

|2=Anguidae Gray 1825

}}

}}

}}

}}

}}

|label2=Iguania

|2={{clade

|label1=Acrodonta

|1={{clade

|1=Chamaeleonidae 70 px

|2=Agamidae Gray 182770 px

}}

|label2=Pleurodonta

|2={{clade

|1=Leiocephalidae

|2={{clade

|1=Iguanidae 70 px

|2={{clade

|1={{clade

|1=Hoplocercidae Frost & Etheridge 1989

|2={{clade

|1=Crotaphytidae

|2=Corytophanidae

}}

}}

|2={{clade

|1=Tropiduridae

|2={{clade

|1={{clade

|1=Phrynosomatidae

|2={{clade

|1=Dactyloidae

|2=Polychrotidae

}}

}}

|2={{clade

|1=Liolaemidae

|2={{clade

|1=Leiosauridae

|2=Opluridae

}}

}}

}}

}}

}}

}}

}}

}}

}}

|label2=Serpentes

|2={{clade

|label1=Scolecophidia

|1={{clade

|1=Leptotyphlopidae Stejneger 189270 px

|2={{clade

|1=Gerrhopilidae Vidal et al. 2010

|2={{clade

|1=Xenotyphlopidae Vidal et al. 2010

|2=Typhlopidae Merrem 182070 px

}}

}}

}}

|2={{clade

|1=Anomalepididae

|label2=Alethinophidia

|2={{clade

|label1=Amerophidia

|1={{clade

|1=Aniliidae

|2=Tropidophiidae Brongersma 1951

}}

|label2=Afrophidia

|2={{clade

|label1=Booidea

|1={{clade

|1={{clade

|1=Uropeltidae 70 px

|2={{clade

|1=Anomochilidae

|2=Cylindrophiidae 70 px

}}

}}

|2={{clade

|1={{clade

|1=Xenopeltidae Bonaparte 1845

|2={{clade

|1=Loxocemidae

|2=Pythonidae Fitzinger 1826120 px

}}

}}

|2={{clade

|1=Boidae 100 px

|2={{clade

|1=Xenophidiidae

|2=Bolyeriidae Hoffstetter 1946

}}

}}

}}

}}

|label2=Caenophidia

|2={{clade

|1=Acrochordidae Bonaparte 1831

|label2=Colubroides

|2={{clade

|1=Xenodermidae

|2={{clade

|1=Pareidae

|2={{clade

|1=Viperidae 70 px

|label2=Proteroglypha

|2={{clade

|1=Homalopsidae

|2={{clade

|1=Colubridae|image1=70 px

|2={{clade

|1=Lamprophiidae

|2=Elapidae|image2=70 px

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

The over 10,900 extant squamates are divided into 68 families.

{{Reflist|30em}}

{{Wikispecies}}

{{Commons category}}

• [https://web.archive.org/web/20081006054822/http://www.palaeos.com/Vertebrates/Units/240Squamata/240.000.html Palaeos.com: Squamata]
• {{NCBI taxid|8509}}
• {{ITIS|ID=173861|taxon=Squamata}}

{{Lepidosauromorpha|state=collapsed}}

{{Extinct squamates|state=collapsed}}

{{Squamata families}}

{{Reptiles}}

{{Chordata}}

{{Taxonbar|from=Q122422}}

{{Authority control}}

Category:Early Jurassic reptiles

Category:Taxa named by Nicolaus Michael Oppel

Category:Extant Late Triassic first appearances