# Squamata

Squamata (/skwæˈmeɪtə/, Latin squamatus, 'scaly, having scales') is the largest order of reptiles, comprising lizards and snakes. With over 11,991 species, it is also the second-largest order of extant (living) vertebrates, after the perciform fish. Squamates 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 widely 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.

## Evolution

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.
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. However, a 2021 study found the genus to be a lepidosaur of uncertain position, in a polytomy with Squamata and Rhynchocephalia.
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. A 2023 study found that Cryptovaranoides most likely represents an archosauromorph with no apparent squamate affinities, though the original describers maintained their original conclusion that this taxon represents a squamate. 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.
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. 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. Squamates suffered a mass extinction at the Cretaceous–Paleogene (K–Pg) boundary, which wiped out polyglyphanodontians, mosasaurs, and many other distinct lineages.
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. 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.

## Reproduction

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. 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. Some species, such as the Komodo dragon, can reproduce asexually through parthenogenesis.
Studies have been conducted on how sexual selection manifests itself in snakes and lizards. Snakes use a variety of tactics in acquiring mates. 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.

### Facultative parthenogenesis

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. 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. 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. 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.

## 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. 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.
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. Previous literature hypothesized that venoms were modifications of salivary or pancreatic proteins, but different toxins have been found to have been recruited from numerous different protein bodies and are as diverse as their functions.
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, which leads to a diversification of toxins that allows the ambush predators the ability to attack a wide range of prey. 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.

## Humans and squamates
### Bites and fatalities

An estimated 125,000 people a year die from venomous snake bites. 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.
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. The Gila monster and beaded lizards of North and Central America are venomous, but not deadly to humans.

### Conservation

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

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, 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 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.

## List of extant families

The over 10,900 extant squamates are divided into 67 families.
| Amphisbaenia |
| Family | Common names | Example species | Example photo |
| Amphisbaenidae Gray, 1865 | Tropical worm lizards | Darwin's worm lizard (Amphisbaena darwinii) | |
| Bipedidae Taylor, 1951 | Bipes worm lizards | Mexican mole lizard (Bipes biporus) | |
| Blanidae Kearney, 2003 | Mediterranean worm lizards | Mediterranean worm lizard (Blanus cinereus) | |
| Cadeidae Vidal & Hedges, 2007 | Cuban worm lizards | Cadea blanoides | |
| Rhineuridae Vanzolini, 1951 | North American worm lizards | North American worm lizard (Rhineura floridana) | |
| Trogonophidae Gray, 1865 | Palearctic worm lizards | Checkerboard worm lizard (Trogonophis wiegmanni) | |
| Gekkota (geckos, incl. Dibamia) |
| Family | Common names | Example species | Example photo |
| Carphodactylidae Kluge, 1967 | Southern padless geckos | Thick-tailed gecko (Underwoodisaurus milii) | |
| Dibamidae Boulenger, 1884 | Blind lizards | Dibamus nicobaricum | |
| Diplodactylidae Underwood, 1954 | Australasian geckos | Golden-tailed gecko (Strophurus taenicauda) | |
| Eublepharidae Boulenger, 1883 | Eyelid geckos | Common leopard gecko (Eublepharis macularius) | |
| Gekkonidae Gray, 1825 | Geckos | Madagascar giant day gecko (Phelsuma grandis) | |
| Phyllodactylidae Gamble et al., 2008 | Leaf finger geckos | Moorish gecko (Tarentola mauritanica) | |
| Pygopodidae Boulenger, 1884 | Flap-footed lizards | Burton's snake lizard (Lialis burtonis) | |
| Sphaerodactylidae Underwood, 1954 | Round finger geckos | Fantastic least gecko (Sphaerodactylus fantasticus) | |
| Iguania |
| Family | Common names | Example species | Example photo |
| Agamidae Gray, 1827 | Agamas | Eastern bearded dragon (Pogona barbata) | |
| Chamaeleonidae Rafinesque, 1815 | Chameleons | Veiled chameleon (Chamaeleo calyptratus) | |
| Corytophanidae Fitzinger, 1843 | Casquehead lizards | Plumed basilisk (Basiliscus plumifrons) | |
| Crotaphytidae H.M. Smith & Brodie, 1982 | Collared and leopard lizards | Common collared lizard (Crotaphytus collaris) | |
| Dactyloidae Fitzinger, 1843 | Anoles | Carolina anole (Anolis carolinensis) | |
| Hoplocercidae Frost & Etheridge, 1989 | Wood lizards or clubtails | Enyalioides binzayedi | |
| Iguanidae Oppel, 1811 | Iguanas | Marine iguana (Amblyrhynchus cristatus) | |
| Leiocephalidae Frost & Etheridge, 1989 | Curly-tailed lizards | Hispaniolan masked curly-tailed lizard (Leiocephalus personatus) | |
| Leiosauridae Frost et al., 2001 | Leiosaurid lizards | Enyalius bilineatus | |
| Liolaemidae Frost & Etheridge, 1989 | Tree iguanas, snow swifts | Shining tree iguana (Liolaemus nitidus) | |
| Opluridae Titus & Frost, 1996 | Malagasy iguanas | Chalarodon madagascariensis | |
| Phrynosomatidae Fitzinger, 1843 | Earless, spiny, tree, side-blotched and horned lizards | Greater earless lizard (Cophosaurus texanus) | |
| Polychrotidae Frost & Etheridge, 1989 | Bush anoles | Brazilian bush anole (Polychrus acutirostris) | |
| Tropiduridae Bell, 1843 | Neotropical ground lizards | Microlophus peruvianus | |
| Lacertoidea (excl. Amphisbaenia) |
| Family | Common Names | Example Species | Example Photo |
| Alopoglossidae Goicoechea, Frost, De la Riva, Pellegrino, Sites Jr., Rodrigues, & Padial, 2016 | Alopoglossid lizards | Alopoglossus vallensis | |
| Gymnophthalmidae Fitzinger, 1826 | Spectacled lizards | Bachia bicolor | |
| Lacertidae Oppel, 1811 | Wall lizards | Ocellated lizard (Lacerta lepida) | |
| Teiidae Gray, 1827 | Tegus and whiptails | Gold tegu (Tupinambis teguixin) | |
| Anguimorpha |
| Family | Common names | Example species | Example photo |
| Anguidae Gray, 1825 | Glass lizards, alligator lizards and slowworms | Slowworm (Anguis fragilis) | |
| Anniellidae Boulenger, 1885 | American legless lizards | California legless lizard (Anniella pulchra) | |
| Diploglossidae Bocourt, 1873 | galliwasps, legless lizards | Jamaican giant galliwasp (Celestus occiduus) | - |
| Helodermatidae Gray, 1837 | Beaded lizards | Gila monster (Heloderma suspectum) | - |
| Lanthanotidae Steindachner, 1877 | Earless monitor | Earless monitor (Lanthanotus borneensis) | |
| Shinisauridae Ahl, 1930 | Chinese crocodile lizard | Chinese crocodile lizard (Shinisaurus crocodilurus) | |
| Varanidae Merrem, 1820 | Monitor lizards | Perentie (Varanus giganteus) | |
| Xenosauridae Cope, 1866 | Knob-scaled lizards | Mexican knob-scaled lizard (Xenosaurus grandis) | |
| Scincoidea |
| Family | Common Names | Example Species | Example Photo |
| Cordylidae Fitzinger, 1826 | Girdled lizards | Girdle-tailed lizard (Cordylus warreni) | |
| Gerrhosauridae Fitzinger, 1843 | Plated lizards | Sudan plated lizard (Gerrhosaurus major) | |
| Scincidae Oppel, 1811 | Skinks | Western blue-tongued skink (Tiliqua occipitalis) | |
| Xantusiidae Baird, 1858 | Night lizards | Granite night lizard (Xantusia henshawi) | |
| Alethinophidia |
| Family | Common names | Example species | Example photo |
| Acrochordidae Bonaparte, 1831 | File snakes | Marine file snake (Acrochordus granulatus) | |
| Aniliidae Stejneger, 1907 | Coral pipe snakes | Burrowing false coral (Anilius scytale) | |
| Anomochilidae Cundall, Wallach and Rossman, 1993. | Dwarf pipe snakes | Leonard's pipe snake, (Anomochilus leonardi) | |
| Boidae Gray, 1825 (incl. Calabariidae) | Boas | Amazon tree boa (Corallus hortulanus) | |
| Bolyeriidae Hoffstetter, 1946 | Round Island boas | Round Island burrowing boa (Bolyeria multocarinata) | |
| Colubridae Oppel, 1811 sensu lato (incl. Dipsadidae, Natricidae, Pseudoxenodontidae) | Colubrids | Grass snake (Natrix natrix) | |
| Cylindrophiidae Fitzinger, 1843 | Asian pipe snakes | Red-tailed pipe snake (Cylindrophis ruffus) | |
| Elapidae Boie, 1827 | Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids | King cobra (Ophiophagus hannah) | |
| Homalopsidae Bonaparte, 1845 | Indo-Australian water snakes, mudsnakes, bockadams | New Guinea bockadam (Cerberus rynchops) | |
| Lamprophiidae Fitzinger, 1843 | Lamprophiid snakes | Bibron's burrowing asp (Atractaspis bibroni) | |
| Loxocemidae Cope, 1861 | Mexican burrowing snakes | Mexican burrowing snake (Loxocemus bicolor) | |
| Pareidae Romer, 1956 | Pareid snakes | Perrotet's mountain snake (Xylophis perroteti) | |
| Pythonidae Fitzinger, 1826 | Pythons | Ball python (Python regius) | |
| Tropidophiidae Brongersma, 1951 | Dwarf boas | Northern eyelash boa (Trachyboa boulengeri) | |
| Uropeltidae Müller, 1832 | Shield-tailed snakes, short-tailed snakes | Cuvier's shieldtail (Uropeltis ceylanica) | |
| Viperidae Oppel, 1811 | Vipers, pitvipers, rattlesnakes | European asp (Vipera aspis) | |
| Xenodermidae Fitzinger, 1826 | Odd-scaled snakes and relatives | Khase earth snake (Stoliczkia khasiensis) | |
| Xenopeltidae Gray, 1849 | Sunbeam snakes | Sunbeam snake (Xenopeltis unicolor) | |
| Scolecophidia (incl. Anomalepidae) |
| Family | Common names | Example species | Example photo |
| Anomalepidae Taylor, 1939 | Dawn blind snakes | Dawn blind snake (Liotyphlops beui) | |
| Gerrhopilidae Vidal et al., 2010 | Indo-Malayan blindsnakes | Andaman worm snake (Gerrhopilus andamanensis) | – |
| Leptotyphlopidae Stejneger, 1892 | Slender blind snakes | Texas blind snake (Leptotyphlops dulcis) | |
| Typhlopidae Merrem, 1820 | Blind snakes | European blind snake (Typhlops vermicularis) | |
| Xenotyphlopidae Vidal et al., 2010 | Malagasy blind snakes | Xenotyphlops grandidieri | – |