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Temporal range: Early Jurassic–Present
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Lepidosauria
Order: Squamata
Oppel, 1811
Subgroups [1]

Squamata ( /skwæˈmtə/ , Latin squamatus, 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 11,500 species, [2] it is also the second-largest order of extant (living) vertebrates, after the perciform fish. Members of the order are distinguished by their skins, which bear horny scales or shields, and must periodically engage in molting. They also possess movable quadrate bones, making possible movement of the upper jaw relative to the neurocranium. This is particularly visible in snakes, which are able to open their mouths very wide to accommodate comparatively large prey. Squamates are the most variably sized living reptiles, ranging from the 16 mm (0.63 in) dwarf gecko (Sphaerodactylus ariasae) to the 6.5 m (21 ft) reticulated python (Malayopython reticulatus). The now-extinct mosasaurs reached lengths over 14 m (46 ft).


Among other reptiles, squamates are most closely related to the tuatara, the last surviving member of the once diverse Rhynchocephalia, with both groups being placed in the clade Lepidosauria.


The holotype of Slavoia darevskii, a fossil squamate Slavoia darevskii.jpg
The holotype 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 subclass 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. [3] [4]

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. [5] [6]

In 2022, the extinct genus Cryptovaranoides was described from the Late Triassic 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. [7] However, a 2023 study found that Cryptovaranoides most likely represents an archosauromorph with no apparent squamate affinities. [8] The oldest unambigious fossils of Squamata date to the Middle Jurassic of the Northern Hemisphere. [9] with the first appearance of many modern groups, including snakes, during this period. [10]

Scientists believe crown group squamates probably originated in the Early Jurassic based on the fossil record, [3] with the oldest unambigious fossils of squamates dating to the Middle Jurassic. [11] Squamate morphological and ecological diversity substantially increased over the course of the Cretaceous, [10] 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. [12] Squamates suffered a mass extinction at the Cretaceous–Paleogene (K–PG) boundary, which wiped out polyglyphanodontians, mosasaurs, and many other distinct lineages. [13]

The relationships of squamates is 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, [12] but genetic data suggest that geckoes are the earliest crown group squamates. [14] 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.


Trachylepis maculilabris skinks mating Trachylepis maculilabris mating.jpg
Trachylepis maculilabris skinks 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. [15] 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. Some species, such as the Komodo dragon, can reproduce asexually through parthenogenesis. [16]

The Japanese striped snake has been studied in sexual selection. Elaphe quadrivirgata.JPG
The Japanese striped snake has been studied in sexual selection.

Studies have been conducted on how sexual selection manifests itself in snakes and lizards. Snakes use a variety of tactics in acquiring mates. [17] [ dubious ] 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. [18]

Facultative parthenogenesis

The effects of central fusion and terminal fusion on heterozygosity Central fusion and terminal fusion automixis.svg
The effects of central fusion and terminal fusion on heterozygosity

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. [19] 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. [19]

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. [20] The WW females are likely produced by terminal automixis.

Inbreeding avoidance

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. [21] 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. [21] This preference may enhance the fitness of progeny by reducing inbreeding depression.

Evolution of 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. [22] 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, [22] but the only good fossil evidence is from the Middle Jurassic. [23]

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. [24] Previous literature hypothesized that venoms were modifications of salivary or pancreatic proteins, [25] but different toxins have been found to have been recruited from numerous different protein bodies and are as diverse as their functions. [26]

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, [27] which leads to a diversification of toxins that allows the ambush predators the ability to attack a wide range of prey. [28] 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. [29]

Humans and squamates

Bites and fatalities

Map showing the global distribution of venomous snakebites Number of snake envenomings.svg
Map showing the global distribution of venomous snakebites

cAn estimated 125,000 people a year die from venomous snake bites. [30] 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. [31] [32]

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. [33] 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.

Classification and phylogeny

Desert iguana from Amboy Crater, Mojave Desert, California DesertIguana031611.jpg
Desert iguana from Amboy Crater, Mojave Desert, California

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

Of these, the lizards form a paraphyletic group, [34] since "lizards" excludes the subclades of snakes and amphisbaenians. Studies of squamate relationships using molecular biology have found several distinct lineages, though the specific details of their interrelationships vary from one study to the next. One example of a modern classification of the squamates is [1] [35]




Diplodactylidae Underwood 1954 Hoplodactylus pomarii white background.jpg

Pygopodidae Boulenger 1884 The zoology of the voyage of the H.M.S. Erebus and Terror (Lialis burtonis).jpg





Sphaerodactylidae Underwood 1954

Phyllodactylidae Phyllodactylus gerrhopygus 1847 - white background.jpg



Scincidae Natural history of Victoria (Egernia cunninghami).jpg



Gerrhosauridae Gerrhosaurus ocellatus flipped.jpg

Cordylidae Illustrations of the zoology of South Africa (Smaug giganteus).jpg


Gymnophthalmidae Merrem 1820 PZSL1851PlateReptilia06 Cercosaura ocellata.png

Teiidae Gray 1827 Bilder-Atlas zur wissenschaftlich-popularen Naturgeschichte der Wirbelthiere (Tupinambis teguixin).jpg


Lacertidae Brockhaus' Konversations-Lexikon (1892) (Lacerta agilis).jpg


Rhineuridae Vanzolini 1951

Bipedidae Taylor 1951 Bilder-Atlas zur wissenschaftlich-popularen Naturgeschichte der Wirbelthiere (Bipes canaliculatus).jpg

Blanidae Kearney & Stuart 2004 Blanus cinereus flipped.jpg

Cadeidae Vidal & Hedges 2008

Trogonophidae Gray 1865

Amphisbaenidae Gray 1865 Amphisbaena microcephalum 1847 - white background.jpg


Shinisauridae Ahl 1930 sensu Conrad 2006



Varanidae Zoology of Egypt (1898) (Varanus exanthematicus).png


Helodermatidae Gray 1837 Gila monster ncd 2012 white background.jpg






Anguidae Gray 1825


Chamaeleonidae Zoology of Egypt (1898) (Chamaeleo calyptratus).jpg

Agamidae Gray 1827 Haeckel Lacertilia (Chlamydosaurus kingii).jpg



Iguanidae Stamps of Germany (Berlin) 1977, Cyclura cornuta.jpg

Hoplocercidae Frost & Etheridge 1989











Leptotyphlopidae Stejneger 1892 Epictia tenella 1847 -white background.jpg

Gerrhopilidae Vidal et al. 2010

Xenotyphlopidae Vidal et al. 2010

Typhlopidae Merrem 1820 Typhlops vermicularis3 white background.jpg




Tropidophiidae Brongersma 1951


Uropeltidae Uropeltis ceylanica (2) flipped.jpg


Cylindrophiidae Cylind resplendens Wagler white background.JPG

Xenopeltidae Bonaparte 1845


Pythonidae Fitzinger 1826 Python natalensis Smith 1840 white background.jpg

Boidae Boa constrictor - 1800-1839 - Print - Iconographia Zoologica - (white background).jpg


Bolyeriidae Hoffstetter 1946


Acrochordidae Bonaparte 1831




Viperidae Illustrations of the zoology of South Africa (Bitis caudalis).jpg



Colubridae Xenochrophis piscator 1 Hardwicke white background.jpg


Elapidae Bilder-Atlas zur wissenschaftlich-popularen Naturgeschichte der Wirbelthiere (Naja naja).jpg

All recent molecular studies [22] suggest that several groups form a venom clade, which encompasses a majority (nearly 60%) of squamate species. Named Toxicofera, it combines the groups Serpentes (snakes), Iguania (agamids, chameleons, iguanids, etc.), and Anguimorpha (monitor lizards, Gila monster, glass lizards, etc.). [22]

List of extant families

The over 10,900 extant squamates are divided into 60 families.

FamilyCommon namesExample speciesExample photo
Gray, 1865
Tropical worm lizardsDarwin's worm lizard ( Amphisbaena darwinii ) Amphisbaenidae - Amphisbaena darwinii.JPG
Taylor, 1951
Bipes worm lizards Mexican mole lizard (Bipes biporus) Bipes biporus.jpg
Kearney, 2003
Mediterranean worm lizardsMediterranean worm lizard ( Blanus cinereus ) Culebra Ciega - panoramio.jpg
Vidal & Hedges, 2007 [36]
Cuban worm lizards Cadea blanoides Cadea palirostrata Dickerson 1916.jpg
Vanzolini, 1951
North American worm lizards North American worm lizard (Rhineura floridana) Amphisbaenia 1.jpg
Gray, 1865
Palearctic worm lizards Checkerboard worm lizard (Trogonophis wiegmanni) Trogonophis wiegmanni imported from iNaturalist photo 24355639 on 14 January 2020.jpg
Gekkota (geckos, incl. Dibamia)
FamilyCommon namesExample speciesExample photo
Kluge, 1967
Southern padless geckos Thick-tailed gecko (Underwoodisaurus milii) Thick-tailed Gecko (Underwoodisaurus milii) (8636512143).jpg
Boulenger, 1884
Blind lizards Dibamus nicobaricum Anelytropsis.jpg
Underwood, 1954
Australasian geckos Golden-tailed gecko (Strophurus taenicauda) Golden Tailed Gecko.jpg
Boulenger, 1883
Eyelid geckos Common leopard gecko (Eublepharis macularius) Eublepharis macularius1.jpg
Gray, 1825
Geckos Madagascar giant day gecko (Phelsuma grandis) Madagascar giant day gecko (Phelsuma grandis) Nosy Komba.jpg
Gamble et al., 2008
Leaf finger geckos Moorish gecko (Tarentola mauritanica) Konstantinos Kalaentzis Tarentola mauritanica (A1).jpg
Boulenger, 1884
Flap-footed lizards Burton's snake lizard (Lialis burtonis) Lialis burtonis.jpg
Underwood, 1954
Round finger geckos Fantastic least gecko (Sphaerodactylus fantasticus) Sphaerodactylus fantasticus fantasticus (51113243252).jpg
FamilyCommon namesExample speciesExample photo
Gray, 1827
Agamas Eastern bearded dragon (Pogona barbata) Bearded dragon04.jpg
Rafinesque, 1815
Chameleons Veiled chameleon (Chamaeleo calyptratus) Chamaelio calyptratus.jpg
Fitzinger, 1843
Casquehead lizards Plumed basilisk (Basiliscus plumifrons) Plumedbasiliskcele4 edit.jpg
H.M. Smith & Brodie, 1982
Collared and leopard lizards Common collared lizard (Crotaphytus collaris) Collared lizard in Zion National Park.jpg
Fitzinger, 1843
Anoles Carolina anole (Anolis carolinensis) Anolis carolinensis.jpg
Frost & Etheridge, 1989
Wood lizards or clubtails Enyalioides binzayedi Holotype of Enyalioides binzayedi - ZooKeys-277-069-g007-top.jpg
Oppel, 1811
Iguanas Marine iguana (Amblyrhynchus cristatus) Marineiguana03.jpg
Frost & Etheridge, 1989
Curly-tailed lizards Hispaniolan masked curly-tailed lizard (Leiocephalus personatus) Leiocephalus-personatus-maskenleguan.jpg
Frost et al., 2001
Leiosaurid lizards Enyalius bilineatus Enyalius bilineatus no Parque Estadual de Caparao por Lucas Rosado (08).jpg
Frost & Etheridge, 1989
Tree iguanas, snow swifts Shining tree iguana (Liolaemus nitidus) Atacama lizard1.jpg
Titus & Frost, 1996
Malagasy iguanas Chalarodon madagascariensis Chalarodon madagascariensis male.jpg
Fitzinger, 1843
Earless, spiny, tree, side-blotched and horned lizards Greater earless lizard (Cophosaurus texanus) Reptile tx usa.jpg
Frost & Etheridge, 1989
Bush anoles Brazilian bush anole (Polychrus acutirostris) Polychrus acutirostris.JPG
Bell, 1843
Neotropical ground lizards Microlophus peruvianus Mperuvianus.jpg
Lacertoidea (excl. Amphisbaenia)
FamilyCommon NamesExample SpeciesExample Photo
Goicoechea, Frost, De la Riva, Pellegrino, Sites Jr., Rodrigues, & Padial, 2016
Alopoglossid lizards Alopoglossus vallensis Ptychoglossus vallensis.jpg
Fitzinger, 1826
Spectacled lizards Bachia bicolor Bachia bicolor.jpg
Oppel, 1811
Wall lizards Ocellated lizard (Lacerta lepida) Perleidechse-20.jpg
Gray, 1827
Tegus and whiptails Gold tegu (Tupinambis teguixin) Goldteju Tupinambis teguixin.jpg
FamilyCommon namesExample speciesExample photo
Gray, 1825
Glass lizards, alligator lizards and slowwormsSlowworm ( Anguis fragilis ) Anguidae.jpg
Boulenger, 1885
American legless lizards California legless lizard (Anniella pulchra) Anniella pulchra.jpg
Bocourt, 1873
galliwasps, legless lizards Jamaican giant galliwasp (Celestus occiduus) Celestus occiduus museum specimen.jpeg -
Gray, 1837
Beaded lizards Gila monster (Heloderma suspectum) Gila.monster.arp.jpg -
Steindachner, 1877
Earless monitor Earless monitor (Lanthanotus borneensis) Real Lanthanotus borneensis.jpg
Ahl, 1930
Chinese crocodile lizard Chinese crocodile lizard (Shinisaurus crocodilurus) Chin-krokodilschwanzechse-01.jpg
Merrem, 1820
Monitor lizards Perentie (Varanus giganteus) Perentie Lizard Perth Zoo SMC Spet 2005.jpg
Cope, 1866
Knob-scaled lizards Mexican knob-scaled lizard (Xenosaurus grandis) Xenosaurus grandis.jpg
FamilyCommon NamesExample SpeciesExample Photo
Fitzinger, 1826
Girdled lizards Girdle-tailed lizard (Cordylus warreni) Cordylus breyeri1.jpg
Fitzinger, 1843
Plated lizards Sudan plated lizard (Gerrhosaurus major) Gerrhosaurus major.jpg
Oppel, 1811
Skinks Western blue-tongued skink (Tiliqua occipitalis) Tiliqua occipitalis.jpg
Baird, 1858
Night lizards Granite night lizard (Xantusia henshawi) Xantusia henshawi.jpg
FamilyCommon namesExample speciesExample photo
Bonaparte, 1831 [37]
File snakes Marine file snake (Acrochordus granulatus) Wart snake 1.jpg
Stejneger, 1907 [38]
Coral pipe snakes Burrowing false coral (Anilius scytale) False Coral Snake (Anilius scytale) close-up (13929278050).jpg
Cundall, Wallach and Rossman, 1993. [39]
Dwarf pipe snakes Leonard's pipe snake, (Anomochilus leonardi) Anomochilus weberi.jpg
Gray, 1825 [37] (incl. Calabariidae)
Boas Amazon tree boa (Corallus hortulanus) Corallushortulanus.png
Hoffstetter, 1946
Round Island boas Round Island burrowing boa (Bolyeria multocarinata) Round Island Boa.jpeg
Oppel, 1811 [37] sensu lato (incl. Dipsadidae, Natricidae, Pseudoxenodontidae)
Colubrids Grass snake (Natrix natrix) Natrix natrix (Marek Szczepanek).jpg
Fitzinger, 1843
Asian pipe snakes Red-tailed pipe snake (Cylindrophis ruffus) Cylindrophis rufus.jpg
Boie, 1827 [37]
Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids King cobra (Ophiophagus hannah) Ophiophagus hannah2.jpg
Bonaparte, 1845
Indo-Australian water snakes, mudsnakes, bockadams New Guinea bockadam (Cerberus rynchops) HerpetonTentaculatumFord.jpg
Fitzinger, 1843 [40]
Lamprophiid snakes Bibron's burrowing asp (Atractaspis bibroni) Lamprophis fuliginosus02.jpg
Cope, 1861
Mexican burrowing snakes Mexican burrowing snake (Loxocemus bicolor) Loxocemus bicolor.jpg
Romer, 1956
Pareid snakes Perrotet's mountain snake (Xylophis perroteti) Xylophis sp. Munnar.jpg
Fitzinger, 1826
Pythons Ball python (Python regius) Ball python lucy.JPG
Brongersma, 1951
Dwarf boas Northern eyelash boa (Trachyboa boulengeri) Cuban Giant Trope (Tropidophis melanurus) (8577519420).jpg
Müller, 1832
Shield-tailed snakes, short-tailed snakes Cuvier's shieldtail (Uropeltis ceylanica) Silybura shortii.jpg
Oppel, 1811 [37]
Vipers, pitvipers, rattlesnakes European asp (Vipera aspis) Vipera aspis aspis, Lorraine, France.jpg
Fitzinger, 1826
Odd-scaled snakes and relatives Khase earth snake (Stoliczkia khasiensis) Achalinus formosanus formosanus full body shot.jpg
Gray, 1849
Sunbeam snakes Sunbeam snake (Xenopeltis unicolor) XenopeltisUnicolorRooij.jpg
Scolecophidia (incl. Anomalepidae)
FamilyCommon namesExample speciesExample photo
Taylor, 1939 [37]
Dawn blind snakes Dawn blind snake (Liotyphlops beui) Liotyphlops beui.jpg
Vidal et al., 2010 [36]
Indo-Malayan blindsnakes Andaman worm snake (Gerrhopilus andamanensis)
Stejneger, 1892 [37]
Slender blind snakes Texas blind snake (Leptotyphlops dulcis) Leptotyphlops dulcis.jpg
Merrem, 1820 [41]
Blind snakes European blind snake (Typhlops vermicularis) Typhlops vermicularis.jpg
Vidal et al., 2010 [36]
Malagasy blind snakes Xenotyphlops grandidieri

Related Research Articles

<span class="mw-page-title-main">Lizard</span> Informal group of reptiles

Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia, and some lizards are more closely related to these two excluded groups than they are to other lizards. Lizards range in size from chameleons and geckos a few centimeters long to the 3-meter-long Komodo dragon.

<span class="mw-page-title-main">Snake</span> Limbless, scaly, elongate reptile

Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes. Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads. To accommodate their narrow bodies, snakes' paired organs appear one in front of the other instead of side by side, and most have only one functional lung. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca. Lizards have independently evolved elongate bodies without limbs or with greatly reduced limbs at least twenty-five times via convergent evolution, leading to many lineages of legless lizards. These resemble snakes, but several common groups of legless lizards have eyelids and external ears, which snakes lack, although this rule is not universal.

<span class="mw-page-title-main">Colubridae</span> Family of snakes

Colubridae is a family of snakes. With 249 genera, it is the largest snake family. The earliest species of the family date back to the Oligocene epoch. Colubrid snakes are found on every continent except Antarctica.

<span class="mw-page-title-main">Lepidosauria</span> Superorder of reptiles

The Lepidosauria is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata includes snakes, lizards, and amphisbaenians. Squamata contains over 9,000 species, making it by far the most species-rich and diverse order of non-avian reptiles in the present day. Rhynchocephalia was a formerly widespread and diverse group of reptiles in the Mesozoic Era. However, it is represented by only one living species: the tuatara, a superficially lizard-like reptile native to New Zealand.

<span class="mw-page-title-main">Ophidia</span> Group of squamate reptiles

Ophidia is a group of squamate reptiles including modern snakes and reptiles more closely related to snakes than to other living groups of lizards.

<span class="mw-page-title-main">Rhynchocephalia</span> Order of reptiles

Rhynchocephalia is an order of lizard-like reptiles that includes only one living species, the tuatara of New Zealand. Despite its current lack of diversity, during the Mesozoic rhynchocephalians were a speciose group with high morphological and ecological diversity. The oldest record of the group is dated to the Middle Triassic around 238 to 240 million years ago, and they had achieved a worldwide distribution by the Early Jurassic. Most rhynchocephalians belong to the group Sphenodontia ('wedge-teeth'). Their closest living relatives are lizards and snakes in the order Squamata, with the two orders being grouped together in the superorder Lepidosauria.

<span class="mw-page-title-main">Lepidosauromorpha</span> Clade of reptiles

Lepidosauromorpha is a group of reptiles comprising all diapsids closer to lizards than to archosaurs. The only living sub-group is the Lepidosauria, which contains two subdivisions, Squamata, which contains lizards and snakes, and Rhynchocephalia, the only extant species of which is the tuatara.

<span class="mw-page-title-main">Toxicofera</span> Proposed clade of scaled reptiles

Toxicofera is a proposed clade of scaled reptiles (squamates) that includes the Serpentes (snakes), Anguimorpha and Iguania. Toxicofera contains about 4,600 species, of extant Squamata. It encompasses all venomous reptile species, as well as numerous related non-venomous species. There is little morphological evidence to support this grouping; however, it has been recovered by all molecular analyses as of 2012.

<span class="mw-page-title-main">Iguanomorpha</span> Infraorder of lizards

Iguania is an infraorder of squamate reptiles that includes iguanas, chameleons, agamids, and New World lizards like anoles and phrynosomatids. Using morphological features as a guide to evolutionary relationships, the Iguania are believed to form the sister group to the remainder of the Squamata, which comprise nearly 11,000 named species, roughly 2000 of which are iguanians. However, molecular information has placed Iguania well within the Squamata as sister taxa to the Anguimorpha and closely related to snakes. The order has been under debate and revisions after being classified by Charles Lewis Camp in 1923 due to difficulties finding adequate synapomorphic morphological characteristics. Most Iguanias are arboreal but there are several terrestrial groups. They usually have primitive fleshy, non-prehensile tongues, although the tongue is highly modified in chameleons. The group has a fossil record that extends back to the Early Jurassic. Today they are scattered occurring in Madagascar, the Fiji and Friendly Islands and Western Hemisphere.

<span class="mw-page-title-main">Varanoidea</span> Superfamily of reptiles

Varanoidea is a superfamily of lizards, including the well-known family Varanidae. Also included in the Varanoidea are the Lanthanotidae, and the extinct Palaeovaranidae.

<span class="mw-page-title-main">Anguimorpha</span> Order of lizards

The Anguimorpha is a suborder of squamates. The group was named by Fürbringer in 1900 to include all autarchoglossans closer to Varanus and Anguis than Scincus. These lizards, along with iguanians and snakes, constitute the proposed "venom clade" Toxicofera of all venomous reptiles.

<span class="mw-page-title-main">Evolution of reptiles</span> Origin and diversification of reptiles through geologic time

Reptiles arose about 320 million years ago during the Carboniferous period. Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, and possess ectothermic metabolisms. So defined, the group is paraphyletic, excluding endothermic animals like birds that are descended from early traditionally-defined reptiles. A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their synapsid ancestors. So defined, Reptilia is identical to Sauropsida.

<i>Huehuecuetzpalli</i> Extinct genus of lizards

Huehuecuetzpalli mixtecus is an extinct lizard from the Early Cretaceous Tlayúa Formation in Tepexi de Rodríguez, Central Mexico. Although it is not the oldest known lizard, Huehuecuetzpalli may be amongst the most basal members of Squamata, making it an important taxon in understanding the origins of squamates.

<span class="mw-page-title-main">Polyglyphanodontia</span> Extinct clade of lizards

Polyglyphanodontia, also known as the Borioteiioidea, is an extinct clade of lizards from the Cretaceous that includes around a dozen genera. Polyglyphanodontians were the dominant group of lizards in North America and Asia during the Late Cretaceous. Most polyglyphanodontians are Late Cretaceous in age, though the oldest one, Kuwajimalla kagaensis, is known from the Early Cretaceous Kuwajima Formation (Japan). Early Cretaceous South American taxon Tijubina, and possibly also Olindalacerta, might also fall within Polyglyphanodontia or be closely allied to the group, but if so, they would be two of only four Gondwanan examples of an otherwise Laurasian clade. They produced a remarkable range of forms. Chamopsiids, including Chamops, were characterized by large, blunt, crushing teeth, and were most likely omnivores. Polyglyphanodon, from the Maastrichtian of Utah, was another herbivore, but its teeth formed a series of transverse blades, similar to those of Trilophosaurus. Peneteius had remarkable, multicusped teeth, similar to those of mammals. The polyglyphanodontids first appear in the latter part of the Early Cretaceous in North America, and became extinct during the Cretaceous-Paleogene extinction event. Polyglyphanodontians closely resembled the teiid lizards, and purported teiid lizards from the Late Cretaceous appear to be polyglyphanodontians. The only species known to have survived the Cretaceous was Chamops, which survived until the very early Ypresian.

<span class="mw-page-title-main">Lacertoidea</span> Superfamily of lizards

The Lacertoidea is a group of squamate reptiles that includes the Lacertidae, Teiidae, Gymnophthalmidae, and Amphisbaenia. The finding from molecular phylogenetic studies that the burrowing Amphisbaenia were nested in a clade with the lizard forms led Vidal & Hedges (2005) to propose a new name for the group based on shared morphogical characters, Laterata, "referring to the presence of tile-like scales that form the rings in Amphisbaenia, and are also present ventrally in Lacertiformata and Teiformata".

<span class="mw-page-title-main">Acrodonta (lizard)</span> Subclade of lizards

Acrodonta are a subclade of iguanian squamates consisting almost entirely of Old World taxa. Extant representation include the families Chamaeleonidae (chameleons) and Agamidae, with at least over 500 species described. A fossil genus, Gueragama, was found in Brazil, making it the only known American representative of the group.

<span class="mw-page-title-main">Paleoanguimorpha</span> Clade of lizards

Paleoanguimorpha is a clade of anguimorphs comprising Shinisauria and Goannasauria. Morphological studies in the past also classified helodermatids and pythonomorphs with the varanoids in the clade Platynota, while the Chinese crocodile lizard was classified as a xenosaurid. Current molecular work finds no support in these groupings and instead has found the helodermatids more related to Diploglossa in the sister clade Neoanguimorpha, while the Chinese crocodile lizard is the closet living relative to varanoids. Pythonomorphs represented by snakes today are not closely related to varanoids and are instead a sister lineage to Anguimorpha and Iguania in the clade Toxicofera.

Paleontology or palaeontology is the study of prehistoric life forms on Earth through the examination of plant and animal fossils. This includes the study of body fossils, tracks (ichnites), burrows, cast-off parts, fossilised feces (coprolites), palynomorphs and chemical residues. Because humans have encountered fossils for millennia, paleontology has a long history both before and after becoming formalized as a science. This article records significant discoveries and events related to paleontology that occurred or were published in the year 2018.

This list of fossil reptiles described in 2021 is a list of new taxa of fossil reptiles that were described during the year 2021, as well as other significant discoveries and events related to reptile paleontology that occurred in 2021.

This list of fossil reptiles described in 2023 is a list of new taxa of fossil reptiles that were described during the year 2023, as well as other significant discoveries and events related to reptile paleontology that occurred in 2023.


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Further reading