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The Squamata, or the scaled reptiles, are the largest recent order of reptiles, comprising all lizards and snakes. With over 10,000 species,[3] it is also the second-largest order of extant vertebrates, after the perciform fish, and roughly equal in number to the Saurischia (one of the two major groups of dinosaurs). 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. They are 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 14 m (46 ft).

Scaled reptiles
Temporal range:
Early JurassicPresent, 199–0 Ma[1]
Blue-toungued skink444.jpg
Eastern blue-tongued lizard
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Lepidosauria
Order: Squamata
Oppel, 1811

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



Slavoia darevskii, a fossil squamate

Squamates are a monophyletic sister group to the tuatara. The squamates and tuatara together are a sister group to crocodiles and birds, the extant archosaurs. Fossils of the squamate sister group, the Rhynchocephalia, appear in the Early Triassic,[4] meaning that the lineage leading to squamates must have existed as well. Modern squamates probably originated in the mid Jurassic,[4] when fossil relatives of geckos and skinks and snakes[5] appear; other groups, including iguanians and varanoids, first appear in the Cretaceous period. Also appearing in the Cretaceous are the polyglyphanodonts, a lizard group of uncertain affinities, and the mosasaurs, a group of predatory, marine lizards that grew to enormous sizes.[6] At the end of the Cretaceous, squamates suffered a major extinction at the K-T boundary,[7] which wiped out polyglyphanodonts, mosasaurs, and a number of other groups.

The relationships of squamates have been debated. 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 DNA. From morphological data, the iguanians were long thought to be the most ancient branch of the tree;[6] however, studies of the DNA suggest that the geckos represent the most ancient branch.[8] Iguanians are now united with snakes and anguimorphs in a group called Toxicofera. DNA also suggests that the various limbless groups—snakes, amphisbaenians, and dibamids—are unrelated, and instead arose independently from lizards.


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.[9] 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 actually reproduce asexually through parthenogenesis.[10]

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.[11][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.[12]

Facultative parthenogenesisEdit

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.[13] 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.[13]

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

Inbreeding avoidanceEdit

When female sand lizards mate with two or more males, sperm competition within the females reproductive tract may occur. Active selection of sperm by females appears to occur in a manner that enhances female fitness.[15] 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.[15] This preference may enhance the fitness of progeny by reducing inbreeding depression.

Evolution of venomEdit

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.[16] The fossil record shows the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 Mya to the Late Triassic/Early Jurassic.[16] 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.[17] Previous literature hypothesized that venoms were modifications of salivary or pancreatic proteins,[18] but different toxins have been found to have been recruited from numerous different protein bodies and are as diverse as their functions.[19]

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,[20] which leads to a diversification of toxins that allows the ambush predators the ability to attack a wide range of prey.[21] 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.[22]

Humans and squamatesEdit

Bites and fatalitiesEdit

Map showing the global distribution of venomous snakebites

An estimated 125,000 people a year die from venomous snake bites.[23] In the US alone, more than 8,000 venomous snake bites are reported each year.[24]

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.[25] 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 phylogenyEdit

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,[26] 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][27]




Diplodactylidae Underwood 1954 

Pygopodidae Boulenger 1884 





Sphaerodactylidae Underwood 1954










Gymnophthalmidae Merrem 1820 

Teiidae Gray 1827 




Rhineuridae Vanzolini 1951

Bipedidae Taylor 1951 

Blanidae Kearney & Stuart 2004 

Cadeidae Vidal & Hedges 2008

Trogonophiidae Gray 1865

Amphisbaenidae Gray 1865 



Shinisauridae Ahl 1930 sensu Conrad 2006





Helodermatidae Gray 1837 






Anguidae Gray 1825



Agamidae Gray 1827 




Hoplocercidae Frost & Etheridge 1989











Leptotyphlopidae Stejneger 1892 

Gerrhopilidae Vidal et al. 2010

Xenotyphlopidae Vidal et al. 2010

Typhlopidae Merrem 1820 




Tropidophiidae Brongersma 1951





Xenopeltidae Bonaparte 1845


Pythonidae Fitzinger 1826 



Bolyeriidae Hoffstetter 1946


Acrochordidae Bonaparte 1831










All recent molecular studies[16] 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.).[16]

List of extant familiesEdit

Family Common names Example species Example photo
Gray, 1865
Tropical worm lizards Darwin's worm lizard (Amphisbaena darwinii)
Taylor, 1951
Bipes worm lizards Mexican mole lizard (Bipes biporus)  
Blanidae Mediterranean worm lizards Mediterranean worm lizard (Blanus cinereus)
Vidal & Hedges, 2008[28]
Cuban worm lizards Cadea blanoides
Vanzolini, 1951
North American worm lizards North American worm lizard (Rhineura floridana)  
Gray, 1865
Palearctic worm lizards Checkerboard worm lizard (Trogonophis wiegmanni)
Gekkota (incl. Dibamia)
Family Common names Example species Example photo
Boulenger, 1884
Blind lizards Dibamus nicobaricum
Gray, 1825 (paraphyletic)
Geckos Thick-tailed gecko (Underwoodisaurus milii)  
Boulenger, 1884
Legless lizards Burton's snake lizard (Lialis burtonis)  
Family Common names Example species Example photo
Spix, 1825
Agamas Eastern bearded dragon (Pogona barbata)  
Gray, 1825
Chameleons Veiled chameleon (Chamaeleo calyptratus)  
Frost & Etheridge, 1989
Casquehead lizards Plumed basilisk (Basiliscus plumifrons)  
Frost & Etheridge, 1989
Collared and leopard lizards Common collared lizard (Crotaphytus collaris)  
Frost & Etheridge, 1989
Wood lizards or clubtails Club-tail iguana (Hoplocercus spinosus)
Iguanidae Iguanas Marine iguana (Amblyrhynchus cristatus)  
Frost et al., 2001
Darwin's iguana (Diplolaemus darwinii)
Frost & Etheridge, 1989
Swifts Shining tree iguana (Liolaemus nitidus)  
Frost & Etheridge, 1989
Madagascan iguanas Chalarodon (Chalarodon madagascariensis)
Frost & Etheridge, 1989
Earless, spiny, tree, side-blotched and horned lizards Greater earless lizard (Cophosaurus texanus)  
Frost & Etheridge, 1989 (+ Dactyloidae)
Anoles Carolina anole (Anolis carolinensis)  
Frost & Etheridge, 1989
Neotropical ground lizards (Microlophus peruvianus)  
Lacertoidea (excl. Amphisbaenia)
Family Common Names Example Species Example Photo
Gymnophthalmidae Spectacled lizards Bachia bicolor  
Oppel, 1811
Wall or true lizards Ocellated lizard (Lacerta lepida)  
Teiidae Tegus or whiptails Gold tegu (Tupinambis teguixin)  
Family Common names Example species Example photo
Oppel, 1811
Glass lizards, alligator lizards and slowworms Slowworm (Anguis fragilis)  
Gray, 1852
American legless lizards California legless lizard (Anniella pulchra)  
Helodermatidae Gila monsters Gila monster (Heloderma suspectum)  
Cope, 1866
Knob-scaled lizards Mexican knob-scaled lizard (Xenosaurus grandis)
Paleoanguimorpha or Varanoidea
Family Common names Example species Example photo
Lanthanotidae Earless monitor Earless monitor (Lanthanotus borneensis)  
Shinisauridae Chinese crocodile lizard Chinese crocodile lizard (Shinisaurus crocodilurus)  
Varanidae Monitor lizards Perentie (Varanus giganteus)  
Family Common Names Example Species Example Photo
Cordylidae Spinytail lizards Girdle-tailed lizard (Cordylus warreni)  
Gerrhosauridae Plated lizards Sudan plated lizard (Gerrhosaurus major)  
Oppel, 1811
Skinks Western blue-tongued skink (Tiliqua occipitalis)  
Xantusiidae Night lizards Granite night lizard (Xantusia henshawi)  
Family Common names Example species Example photo
Bonaparte, 1831[29]
File snakes Marine file snake (Acrochordus granulatus)  
Stejneger, 1907[30]
Coral pipe snakes Burrowing false coral (Anilius scytale)
Cundall, Wallach and Rossman, 1993.[31]
Dwarf pipe snakes Leonard's pipe snake, (Anomochilus leonardi)
Gray, 1825[29] (incl. Calabariidae)
Boas Amazon tree boa (Corallus hortulanus)  
Hoffstetter, 1946
Round Island boas Round Island burrowing boa (Bolyeria multocarinata)
Oppel, 1811[29] sensu lato (incl. Dipsadidae, Natricidae, Pseudoxenodontidae)
Colubrids Grass snake (Natrix natrix)  
Fitzinger, 1843
Asian pipe snakes Red-tailed pipe snake (Cylindrophis ruffus)  
Boie, 1827[29]
Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids King cobra (Ophiophagus hannah)  
Bonaparte, 1845
Fitzinger, 1843[32]
Bibron's burrowing asp (Atractaspis bibroni)
Cope, 1861
Mexican burrowing snakes Mexican burrowing snake (Loxocemus bicolor)  
Romer, 1956
Fitzinger, 1826
Pythons Ball python (Python regius)  
Brongersma, 1951
Dwarf boas Northern eyelash boa (Trachyboa boulengeri)
Müller, 1832
Shield-tailed snakes, short-tailed snakes Cuvier's shieldtail (Uropeltis ceylanica)  
Oppel, 1811[29]
Vipers, pitvipers, rattlesnakes European asp (Vipera aspis)
Fitzinger, 1826
Gray, 1849
Sunbeam snakes Sunbeam snake (Xenopeltis unicolor)  
Scolecophidia (incl. Anomalepidae)
Family Common names Example species Example photo
Taylor, 1939[29]
Dawn blind snakes Dawn blind snake (Liotyphlops beui)
Vidal et al., 2010[28]
Stejneger, 1892[29]
Slender blind snakes Texas blind snake (Leptotyphlops dulcis)  
Merrem, 1820[33]
Blind snakes European blind snake (Typhlops vermicularis)  
Vidal et al., 2010[28]
Xenotyphlops grandidieri


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Further readingEdit

  • Bebler, John L.; King, F. Wayne (1979). The Audubon Society Field Guide to Reptiles and Amphibians of North America. New York: Alfred A. Knopf. p. 581. ISBN 0-394-50824-6. 
  • Capula, Massimo; Behler (1989). Simon & Schuster's Guide to Reptiles and Amphibians of the World. New York: Simon & Schuster. ISBN 0-671-69098-1. 
  • Cogger, Harold; Zweifel, Richard (1992). Reptiles & Amphibians. Sydney: Weldon Owen. ISBN 0-8317-2786-1. 
  • Conant, Roger; Collins, Joseph (1991). A Field Guide to Reptiles and Amphibians Eastern/Central North America. Boston, Massachusetts: Houghton Mifflin Company. ISBN 0-395-58389-6. 
  • Ditmars, Raymond L (1933). Reptiles of the World: The Crocodilians, Lizards, Snakes, Turtles and Tortoises of the Eastern and Western Hemispheres. New York: Macmillan. p. 321. 
  • Evans, SE (2003). "At the feet of the dinosaurs: the origin, evolution and early diversification of squamate reptiles (Lepidosauria: Diapsida)". Biological Reviews, Cambridge. 78: 513–551. doi:10.1017/S1464793103006134. PMID 14700390. 
  • Evans SE. 2008. The skull of lizards and tuatara. In Biology of the Reptilia, Vol.20, Morphology H: the skull of Lepidosauria, Gans C, Gaunt A S, Adler K. (eds). Ithaca, New York, Society for the study of Amphibians and Reptiles. pp1–344. Weblink to purchase
  • Evans, SE; Jones, MEH (2010). "The origin, early history and diversification of lepidosauromorph reptiles. In Bandyopadhyay S. (ed.), New Aspects of Mesozoic Biodiversity". 27 Lecture Notes in Earth Sciences. 132: 27–44. doi:10.1007/978-3-642-10311-7_2. 
  • Freiberg, Dr. Marcos; Walls, Jerry (1984). The World of Venomous Animals. New Jersey: TFH Publications. ISBN 0-87666-567-9. 
  • Gibbons, J. Whitfield; Gibbons, Whit (1983). Their Blood Runs Cold: Adventures With Reptiles and Amphibians. Alabama: University of Alabama Press. p. 164. ISBN 978-0-8173-0135-4. 
  • McDiarmid, RW; Campbell, JA; Touré, T (1999). Snake Species of the World: A Taxonomic and Geographic Reference. 1. Herpetologists' League. p. 511. ISBN 1-893777-00-6. 
  • Mehrtens, John (1987). Living Snakes of the World in Color. New York: Sterling. ISBN 0-8069-6461-8. 
  • Rosenfeld, Arthur (1989). Exotic Pets. New York: Simon & Schuster. p. 293. ISBN 0-671-47654-8. 

External linksEdit