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Temporal range:
Early JurassicPresent, 199–0  Ma [1]
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Lepidosauria
Order: Squamata
Oppel, 1811
Subgroups [2]

Squamata (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 10,000 species, [3] 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. They also possess movable quadrate bones, making it possible to move 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. Squamata is the most variably sized order of reptiles, ranging from the 16 mm (0.63 in) dwarf gecko (Sphaerodactylus ariasae) to the 5.21 m (17.1 ft) green anaconda (Eunectes murinus) and the now-extinct mosasaurs, which reached lengths of over 14 m (46 ft).


Among other reptiles, squamates are most closely related to the tuatara, which superficially resembles lizards.


Slavoia darevskii, a fossil squamate Slavoia darevskii.jpg
Slavoia darevskii, a fossil squamate

Squamates are a monophyletic sister group to the rhynchocephalians, members of the order Rhynchocephalia. The only surviving member of Rhynchocephalia is the tuatara. Squamata and Rhynchocephalia form the subclass Lepidosauria, which is the sister group to 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. [4] Scientists believe crown group squamates probably originated in the Early Jurassic based on the fossil record. [4] The first fossils of geckos, skinks and snakes appear in the Middle Jurassic. [5] Other groups like iguanians and varanoids appeared in the Cretaceous. Polyglyphanodontians, a distinct clade of lizards, and mosasaurs, a group of predatory marine lizards that grew to enormous sizes, also appeared in the Cretaceous. [6] Squamates suffered a mass extinction at the Cretaceous–Paleogene (K–PG) boundary, which wiped out polyglyphanodontians, mosasaurs and many other distinct lineages. [7]

The relationships of squamates is debatable. Although many of the groups originally recognized on the basis of morphology are still accepted, our 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, [6] however, genetic data suggests that geckoes are the earliest crown group squamates. [8] Iguanians are now united with snakes and anguimorphs in a clade called Toxicofera . Genetic data also suggests that the various limbless groups; snakes, amphisbaenians and dibamids, are unrelated, and instead arose independently from lizards.

A study in 2018 found that Megachirella , an extinct genus of lepidosaur 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. This data was 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. [9] [10]


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 human penis. [11] 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. [12]

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

There have been studies on how sexual selection manifests itself in snakes and lizards. Snakes use a variety of tactics in acquiring mates. [13] [ dubious ] Ritual combat between males for the females they want to mate with includes topping, a behavior exhibited by most viperids, in which one male will twist around the vertically elevated fore body of its opponent and forcing it downward. It is common for neck biting to occur while the snakes are entwined. [14]

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 (cotton mouth snake) can reproduce by facultative parthenogenesis. That is, they are capable of switching from a sexual mode of reproduction to an asexual mode. [15] 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. [15]

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. [16] 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. [17] 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. [17] 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. [18] The fossil record shows the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 Mya to the Late Triassic/Early Jurassic. [18] But the only good fossil evidence is from the Jurassic. [1]

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. [19] Previous literature hypothesized that venoms were modifications of salivary or pancreatic proteins, [20] but different toxins have been found to have been recruited from numerous different protein bodies and are as diverse as their functions. [21]

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, [22] which leads to a diversification of toxins that allows the ambush predators the ability to attack a wide range of prey. [23] 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. [24]

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

An estimated 125,000 people a year die from venomous snake bites. [25] In the US alone, more than 8,000 venomous snake bites are reported each year, but only 1 in 50 million people (5-6 fatalities per year in the USA) will die from venomous snake bites. [26] [27]

Lizard bites, unlike venomous snake bites, are 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. [28] 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. However, breeding programs and wildlife parks 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, [29] 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 [2] [30]




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 Bilder-Atlas zur wissenschaftlich-popularen Naturgeschichte der Wirbelthiere (Plate (24)) Tribolonotus novaeguineae.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 griseus).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 Iconographia Zoologica white background.tif


Bolyeriidae Hoffstetter 1946


Acrochordidae Bonaparte 1831




Viperidae Our reptiles and batrachians; a plain and easy account of the lizards, snakes, newts, toads, frogs and tortoises indigenous to Great Britain (1893) (Vipera berus).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 [18] 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.). [18]

List of extant families

The over 10,000 extant squamates are divided into 58 families.

FamilyCommon namesExample speciesExample photo
Gray, 1865
Tropical worm lizardsDarwin's worm lizard ( Amphisbaena darwinii )
Taylor, 1951
Bipes worm lizards Mexican mole lizard (Bipes biporus) Bipes biporus.jpg
Blanidae Mediterranean worm lizardsMediterranean worm lizard ( Blanus cinereus ) Culebra Ciega - panoramio.jpg
Vidal & Hedges, 2008 [31]
Cuban worm lizards Cadea blanoides
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)
Gekkota (incl. Dibamia)
FamilyCommon namesExample speciesExample photo
Boulenger, 1884
Blind lizards Dibamus nicobaricum
Gray, 1825 (paraphyletic)
GeckosThick-tailed gecko ( Underwoodisaurus milii ) Underwoodisaurus milii.jpg
Boulenger, 1884
Legless lizardsBurton's snake lizard ( Lialis burtonis ) Lialis burtonis.jpg
FamilyCommon namesExample speciesExample photo
Spix, 1825
Agamas Eastern bearded dragon (Pogona barbata) Bearded dragon04.jpg
Gray, 1825
Chameleons Veiled chameleon (Chamaeleo calyptratus) Chamaelio calyptratus.jpg
Frost & Etheridge, 1989
Casquehead lizards Plumed basilisk (Basiliscus plumifrons) Plumedbasiliskcele4 edit.jpg
Frost & Etheridge, 1989
Collared and leopard lizards Common collared lizard (Crotaphytus collaris) Collared lizard in Zion National Park.jpg
Frost & Etheridge, 1989
Wood lizards or clubtailsEnyalioides binzayedi Holotype of Enyalioides binzayedi - ZooKeys-277-069-g007-top.jpg
Iguanidae Iguanas Marine iguana (Amblyrhynchus cristatus) Marineiguana03.jpg
Frost et al., 2001
Darwin's iguana (Diplolaemus darwinii)
Frost & Etheridge, 1989
Swifts Shining tree iguana (Liolaemus nitidus) Atacama lizard1.jpg
Frost & Etheridge, 1989
Madagascan iguanas Chalarodon (Chalarodon madagascariensis) Chalarodon madagascariensis male.jpg
Frost & Etheridge, 1989
Earless, spiny, tree, side-blotched and horned lizards Greater earless lizard (Cophosaurus texanus) Reptile tx usa.jpg
Frost & Etheridge, 1989 (+ Dactyloidae)
Anoles Carolina anole (Anolis carolinensis) Anolis carolinensis.jpg
Frost & Etheridge, 1989
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
Alopoglossus vallensis Ptychoglossus vallensis.jpg
Fitzinger, 1826
Spectacled lizardsBachia bicolor Bachia bicolor.jpg
Oppel, 1811
Wall or true lizards Ocellated lizard (Lacerta lepida) Perleidechse-20.jpg
Teiidae Tegus or whiptails Gold tegu (Tupinambis teguixin) Goldteju Tupinambis teguixin.jpg
FamilyCommon namesExample speciesExample photo
Oppel, 1811
Glass lizards, alligator lizards and slowwormsSlowworm ( Anguis fragilis ) Anguidae.jpg
Gray, 1852
American legless lizards California legless lizard (Anniella pulchra) Anniella pulchra.jpg
Helodermatidae Gila monsters Gila monster (Heloderma suspectum) Gila.monster.arp.jpg
Cope, 1866
Knob-scaled lizards Mexican knob-scaled lizard (Xenosaurus grandis)
Paleoanguimorpha or Varanoidea
FamilyCommon namesExample speciesExample photo
Lanthanotidae Earless monitor Earless monitor (Lanthanotus borneensis) Real Lanthanotus borneensis.jpg
Shinisauridae Chinese crocodile lizard Chinese crocodile lizard (Shinisaurus crocodilurus) Chin-krokodilschwanzechse-01.jpg
Varanidae Monitor lizards Perentie (Varanus giganteus) Perentie Lizard Perth Zoo SMC Spet 2005.jpg
FamilyCommon NamesExample SpeciesExample Photo
Cordylidae Spinytail lizards Girdle-tailed lizard (Cordylus warreni) Cordylus breyeri1.jpg
Gerrhosauridae Plated lizards Sudan plated lizard (Gerrhosaurus major) Gerrhosaurus major.jpg
Oppel, 1811
Skinks Western blue-tongued skink (Tiliqua occipitalis) Tiliqua occipitalis.jpg
Xantusiidae Night lizards Granite night lizard (Xantusia henshawi) Xantusia henshawi.jpg
FamilyCommon namesExample speciesExample photo
Bonaparte, 1831 [32]
File snakes Marine file snake (Acrochordus granulatus) Wart snake 1.jpg
Stejneger, 1907 [33]
Coral pipe snakes Burrowing false coral (Anilius scytale) False Coral Snake (Anilius scytale) close-up (13929278050).jpg
Cundall, Wallach and Rossman, 1993. [34]
Dwarf pipe snakes Leonard's pipe snake, (Anomochilus leonardi)
Gray, 1825 [32] (incl. Calabariidae)
Boas Amazon tree boa (Corallus hortulanus) Corallushortulanus.png
Hoffstetter, 1946
Round Island boas Round Island burrowing boa (Bolyeria multocarinata)
Oppel, 1811 [32] 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 [32]
Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids King cobra (Ophiophagus hannah) Ophiophagus hannah2.jpg
Bonaparte, 1845
Fitzinger, 1843 [35]
Bibron's burrowing asp (Atractaspis bibroni)
Cope, 1861
Mexican burrowing snakes Mexican burrowing snake (Loxocemus bicolor) Loxocemus bicolor.jpg
Romer, 1956
Fitzinger, 1826
Pythons Ball python (Python regius) Ball python lucy.JPG
Brongersma, 1951
Dwarf boas Northern eyelash boa (Trachyboa boulengeri)
Müller, 1832
Shield-tailed snakes, short-tailed snakes Cuvier's shieldtail (Uropeltis ceylanica) Silybura shortii.jpg
Oppel, 1811 [32]
Vipers, pitvipers, rattlesnakes European asp (Vipera aspis)
Fitzinger, 1826
Gray, 1849
Sunbeam snakes Sunbeam snake (Xenopeltis unicolor) XenopeltisUnicolorRooij.jpg
Scolecophidia (incl. Anomalepidae)
FamilyCommon namesExample speciesExample photo
Taylor, 1939 [32]
Dawn blind snakes Dawn blind snake (Liotyphlops beui)
Vidal et al., 2010 [31]
Stejneger, 1892 [32]
Slender blind snakes Texas blind snake (Leptotyphlops dulcis) Leptotyphlops dulcis.jpg
Merrem, 1820 [36]
Blind snakes European blind snake (Typhlops vermicularis) Typhlops vermicularis.jpg
Vidal et al., 2010 [31]
Xenotyphlops grandidieri

Related Research Articles

Komodo dragon Species of lizard

The Komodo dragon, also known as the Komodo monitor, is a species of lizard found in the Indonesian islands of Komodo, Rinca, Flores, and Gili Motang. A member of the monitor lizard family Varanidae, it is the largest extant species of lizard, growing to a maximum length of 3 metres (10 ft) in rare cases and weighing up to approximately 70 kilograms (150 lb).

Lizard Suborder of reptiles

Lizards are a widespread group of squamate reptiles, with over 6,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic as it excludes the snakes and Amphisbaenia; 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.

Snake Limbless, scaly, elongate reptile

Snakes are elongated, legless, 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 with their highly mobile jaws. 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 evolved elongate bodies without limbs or with greatly reduced limbs about twenty-five times independently via convergent evolution, leading to many lineages of legless lizards. Legless lizards resemble snakes, but several common groups of legless lizards have eyelids and external ears, which snakes lack, although this rule is not universal.

Venom Form of toxin secreted by an animal for the purpose of causing harm to another

Venom is a secretion containing one or more toxins produced by an animal. Venom has evolved in a wide variety of animals, both predators and prey, and both vertebrates and invertebrates.

Lepidosauria Superorder of reptiles

The Lepidosauria are reptiles with overlapping scales. This subclass includes Squamata and Rhynchocephalia. It is a monophyletic group and therefore contains all descendants of a common ancestor. Squamata includes snakes, lizards, and amphisbaenia. Rhynchocephalia was a widespread and diverse group 220-100 million years ago; however, it is now represented only by the genus Sphenodon, which contains a single species of tuatara, native to New Zealand. Lepidosauria is the sister taxon to Archosauria, which includes Aves and Crocodilia. Lizards and snakes are the most speciose group of lepidosaurs and, combined, contain over 9,000 species. There are many noticeable distinguishing morphological differences between lizards, tuataras, and snakes.


Dibamidae or blind skinks is a family of lizards characterized by their elongated cylindrical body and an apparent lack of limbs. Female dibamids are entirely limbless and the males retain small flap-like hind limbs, which they use to grip their partner during mating. They have a rigidly fused skull, lack pterygoid teeth and external ears. Their eyes are greatly reduced, and covered with a scale.

Monitor lizard Genus of reptiles

Monitor lizards are large lizards in the genus Varanus. They are native to Africa, Asia, and Oceania, but are now found also in the Americas as an invasive species. About 80 species are recognized.

Megalania Species of reptile

Megalania refers to an extinct giant goanna or monitor lizard, recognised as either Megalania prisca or Varanus priscus, part of the megafaunal assemblage that inhabited southern Australia during the Pleistocene. The youngest fossil remains date to around 50,000 years ago. The first indigenous settlers of Australia might have encountered them and been a factor in their extinction.

Ophidia Group of squamate reptiles

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

Toxicofera 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.

Venomous snakes are species of the suborder Serpentes that are capable of producing venom, which they use for killing prey, for defense, and to assist with digestion of their prey. The venom is typically delivered by injection using hollow or grooved fangs, although some venomous snakes lack well-developed fangs. Common venomous snakes include the families Elapidae, Viperidae, Atractaspididae, and some of the Colubridae. The toxicity of venom is mainly indicated by murine LD50, while multiple factors are considered to judge the potential danger to humans. Other important factors for risk assessment include the likelihood that a snake will bite, the quantity of venom delivered with the bite, the efficiency of the delivery mechanism, and the location of a bite on the body of the victim. Snake venom may have both neurotoxic and hemotoxic properties.


Anguimorphs of the infraorder Anguimorpha include the anguids. The infraorder 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.

<i>Iguana</i> Reptile genus of herbivorous lizards

Iguana is a genus of herbivorous lizards that are native to tropical areas of Mexico, Central America, South America, and the Caribbean. The genus was first described in 1768 by Austrian naturalist Josephus Nicolaus Laurenti in his book Specimen Medicum, Exhibens Synopsin Reptilium Emendatam cum Experimentis circa Venena. There were formerly considered to be only two species in the genus; the green iguana, which is widespread throughout its range and a popular pet, and the Lesser Antillean iguana, which is native to the Lesser Antilles; however, genetic analysis indicates that the green iguana may comprise a species complex of multiple species, some of which have been recently described.


Polyglyphanodontia 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 three 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. Macrocephalosaurus, from the Gobi Desert, was a specialized herbivore; it grew to roughly a meter long and had multicusped, leaf-shaped teeth like those of modern iguanas. 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 Lacertoidea is a group of lizards that includes the Lacertidae, Teiidae, Gymnophthalmidae, and the burrowing Amphisbaenia.

Parthenogenesis is a mode of asexual reproduction in which offspring are produced by females without the genetic contribution of a male. Among all the sexual vertebrates, the only examples of true parthenogenesis, in which all-female populations reproduce without the involvement of males, are found in squamate reptiles. There are about 50 species of lizard and 1 species of snake that reproduce solely through parthenogenesis. It is unknown how many sexually reproducing species are also capable of parthenogenesis in the absence of males, but recent research has revealed that this ability is widespread among squamates.

Evolution of snake venom The origin and diversification of snake venom through geologic time

Venom in snakes and some lizards is a form of saliva that has been modified into venom over its evolutionary history. In snakes, venom has evolved to kill or subdue prey, as well as to perform other diet-related functions. The evolution of venom is thought to be responsible for the enormous expansion of snakes across the globe.

Acrodonta (lizard)

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.

Magnuviator is a genus of extinct iguanomorph lizard from the Late Cretaceous of Montana, US. It contains one species, M. ovimonsensis, described in 2017 by DeMar et al. from two specimens that were discovered in the Egg Mountain nesting site. Magnuviator is closest related to the Asian Saichangurvel and Temujinia, which form the group Temujiniidae. Unlike other members of the Iguanomorpha, however, Magnuviator bears a distinct articulating notch on its tibia for the ankle bones, which has traditionally been considered a characteristic of non-iguanomorph lizards. The morphology of its teeth suggests that its diet would have mainly consisted of wasps, like the modern phyrnosomatid iguanians Callisaurus and Urosaurus, although it also shows some adaptations to herbivory.


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.


  1. 1 2 Hutchinson, M. N.; Skinner, A.; Lee, M. S. Y. (2012). "Tikiguania and the antiquity of squamate reptiles (lizards and snakes)". Biology Letters. 8 (4): 665–669. doi:10.1098/rsbl.2011.1216. PMC   3391445 . PMID   22279152.
  2. 1 2 Wiens, J. J.; Hutter, C. R.; Mulcahy, D. G.; Noonan, B. P.; Townsend, T. M.; Sites, J. W.; Reeder, T. W. (2012). "Resolving the phylogeny of lizards and snakes (Squamata) with extensive sampling of genes and species". Biology Letters. 8 (6): 1043–1046. doi:10.1098/rsbl.2012.0703. PMC   3497141 . PMID   22993238.
  3. http://www.reptile-database.org/db-info/SpeciesStat.html
  4. 1 2 Jones, Marc E.; Anderson, Cajsa Lipsa; Hipsley, Christy A.; Müller, Johannes; Evans, Susan E.; Schoch, Rainer R. (25 September 2013). "Integration of molecules and new fossils supports a Triassic origin for Lepidosauria (lizards, snakes, and tuatara)". BMC Evolutionary Biology. 13: 208. doi:10.1186/1471-2148-13-208. PMC   4016551 . PMID   24063680.
  5. Caldwell, Michael W.; Nydam, Randall L.; Alessandro, Palci; Apesteguía, Sebástian (27 January 2015). "The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution". Nature Communications. 6: 5996. Bibcode:2015NatCo...6.5996C. doi: 10.1038/ncomms6996 . ISSN   2041-1723. PMID   25625704.
  6. 1 2 Gauthier, Jacques; Kearney, Maureen; Maisano, Jessica Anderson; Rieppel, Olivier; Behlke, Adam D. B. (April 2012). "Assembling the squamate tree of life: perspectives from the phenotype and the fossil record". Bulletin of the Peabody Museum of Natural History. 53: 3–308. doi:10.3374/014.053.0101. S2CID   86355757.
  7. Longrich, Nicholas R.; Bhullar, Bhart-Anjan S.; Gauthier, Jacques (10 December 2012). "Mass extinction of lizards and snakes at the Cretaceous-Paleogene boundary". Proceedings of the National Academy of Sciences. 109 (52): 21396–21401. Bibcode:2012PNAS..10921396L. doi:10.1073/pnas.1211526110. PMC   3535637 . PMID   23236177.
  8. Pyron, R. Alexander; Burbrink, Frank T.; Wiens, John J. (29 April 2013). "A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes". BMC Evolutionary Biology. 13: 93. doi:10.1186/1471-2148-13-93. PMC   3682911 . PMID   23627680.
  9. Simōes, Tiago R.; Caldwell, Michael W.; Talanda, Mateusz; Bernardi, Massimo; Palci, Alessandro; Vernygora, Oksana; Bernardini, Federico; Mancini, Lucia; Nydam, Randall L. (30 May 2018). "The origin of squamates revealed by a Middle Triassic lizard from the Italian Alps". Nature . 557 (7707): 706–709. Bibcode:2018Natur.557..706S. doi:10.1038/s41586-018-0093-3. PMID   29849156. S2CID   44108416.
  10. Weisberger, Mindy (30 May 2018). "This 240-Million-Year-Old Reptile Is the 'Mother of All Lizards'". Live Science . Purch Group . Retrieved 2 June 2018.
  11. "Iguana Anatomy".
  12. Morales, Alex (20 December 2006). "Komodo Dragons, World's Largest Lizards, Have Virgin Births". Bloomberg Television . Retrieved 28 March 2008.
  13. Shine, Richard; Langkilde, Tracy; Mason, Robert T (2004). "Courtship tactics in garter snakes: How do a male's morphology and behaviour influence his mating success?". Animal Behaviour. 67 (3): 477–83. doi:10.1016/j.anbehav.2003.05.007.
  14. Blouin-Demers, Gabriel; Gibbs, H. Lisle; Weatherhead, Patrick J. (2005). "Genetic evidence for sexual selection in black ratsnakes, Elaphe obsoleta". Animal Behaviour. 69 (1): 225–34. doi:10.1016/j.anbehav.2004.03.012.
  15. 1 2 Booth W, Smith CF, Eskridge PH, Hoss SK, Mendelson JR, Schuett GW (2012). "Facultative parthenogenesis discovered in wild vertebrates". Biol. Lett. 8 (6): 983–5. doi:10.1098/rsbl.2012.0666. PMC   3497136 . PMID   22977071.
  16. Booth W, Million L, Reynolds RG, Burghardt GM, Vargo EL, Schal C, Tzika AC, Schuett GW (2011). "Consecutive virgin births in the new world boid snake, the Colombian rainbow Boa, Epicrates maurus". J. Hered. 102 (6): 759–63. doi: 10.1093/jhered/esr080 . PMID   21868391.
  17. 1 2 Olsson M, Shine R, Madsen T, Gullberg A, Tegelström H (1997). "Sperm choice by females". Trends Ecol. Evol. 12 (11): 445–6. doi:10.1016/s0169-5347(97)85751-5. PMID   21238151.
  18. 1 2 3 4 Fry, Brian G.; Vidal, Nicolas; Norman, Janette A.; Vonk, Freek J.; Scheib, Holger; Ramjan, S.F. Ryan; et al. (February 2006). "Early evolution of the venom system in lizards and snakes". Nature. 439 (7076): 584–588. doi:10.1038/nature04328. PMID   16292255.
  19. Fry, B. G.; Vidal, N.; Kochva, E.; Renjifo, C. (2009). "Evolution and diversification of the toxicofera reptile venom system". Journal of Proteomics. 72 (2): 127–136. doi:10.1016/j.jprot.2009.01.009. PMID   19457354.
  20. Kochva, E (1987). "The origin of snakes and evolution of the venom apparatus". Toxicon. 25 (1): 65–106. doi:10.1016/0041-0101(87)90150-4. PMID   3564066.
  21. Fry, B. G. (2005). "From genome to "Venome": Molecular origin and evolution of the snake venom proteome inferred from phylogenetic analysis of toxin sequences and related body proteins". Genome Research. 15 (3): 403–420. doi:10.1101/gr.3228405. PMC   551567 . PMID   15741511.
  22. Fry, B. G.; Scheib, H.; Young, B.; McNaughtan, J.; Ramjan, S. F. R.; Vidal, N. (2008). "Evolution of an arsenal". Molecular & Cellular Proteomics. 7 (2): 215–246. doi: 10.1074/mcp.m700094-mcp200 . PMID   17855442.
  23. Calvete, J. J.; Sanz, L.; Angulo, Y.; Lomonte, B.; Gutierrez, J. M. (2009). "Venoms, venomics, antivenomics". FEBS Letters. 583 (11): 1736–1743. doi:10.1016/j.febslet.2009.03.029. PMID   19303875.
  24. Barlow, A.; Pook, C. E.; Harrison, R. A.; Wuster, W. (2009). "Coevolution of diet and prey-specific venom activity supports the role of selection in snake venom evolution". Proceedings of the Royal Society B: Biological Sciences. 276 (1666): 2443–2449. doi:10.1098/rspb.2009.0048. PMC   2690460 . PMID   19364745.
  25. "Snake-bites: appraisal of the global situation" (PDF). Who.com. Retrieved 30 December 2007.
  26. Venomous Snake FAQs http://ufwildlife.ifas.ufl.edu/venomous_snake_faqs.shtml . Retrieved 17 September 2019.Missing or empty |title= (help)
  27. "First Aid Snake Bites". University of Maryland Medical Center. Retrieved 30 December 2007.
  28. "Komodo dragon kills boy, 8, in Indonesia". NBC News. Retrieved 30 December 2007.
  29. Reeder, Tod W.; Townsend, Ted M.; Mulcahy, Daniel G.; Noonan, Brice P.; Wood, Perry L.; Sites, Jack W.; Wiens, John J. (2015). "Integrated Analyses Resolve Conflicts over Squamate Reptile Phylogeny and Reveal Unexpected Placements for Fossil Taxa". PLOS ONE. 10 (3): e0118199. doi:10.1371/journal.pone.0118199. PMC   4372529 . PMID   25803280.
  30. Zheng, Yuchi; Wiens, John J. (2016). "Combining phylogenomic and supermatrix approaches, and a time-calibrated phylogeny for squamate reptiles (lizards and snakes) based on 52 genes and 4162 species". Molecular Phylogenetics and Evolution. 94 (Part B): 537–547. doi:10.1016/j.ympev.2015.10.009. PMID   26475614.
  31. 1 2 3 S. Blair Hedges. "Families described". Hedges Lab | Evolutionary Biology.
  32. 1 2 3 4 5 6 7 Cogger(1991), p.23
  33. "Aniliidae". Integrated Taxonomic Information System . Retrieved 12 December 2007.
  34. "Anomochilidae". Integrated Taxonomic Information System . Retrieved 13 December 2007.
  35. "Atractaspididae". Integrated Taxonomic Information System . Retrieved 13 December 2007.
  36. "Typhlopidae". Integrated Taxonomic Information System . Retrieved 13 December 2007.

Further reading