Scaled reptiles
Temporal range: Late Triassic-recent
Eastern blue-tongued lizard
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Oppel, 1811

see text

black: range of Squamata

Squamata, or the scaled reptiles, is the largest recent order of reptiles, including lizards and snakes. 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 braincase. 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-millimetre (0.63 in) Jaragua Sphaero (Sphaerodactylus ariasae) to the 8-metre (26 ft) Green Anaconda (Eunectes murinus) and the now-extinct mosasaurs, which reached lengths of 14 metres (46 ft).



Squamates are a monophyletic group that is a sister group to the tuatara. The squamates and tuatara together are a sister group to crocodiles and birds, the extant archosaurs. Squamate fossils first appear in the late Triassic, but a mitochondrial phylogeny suggests that they evolved in the late Permian. The evolutionary relationships within the squamates are not yet completely worked out, with the relationship of snakes to other groups being most problematic. From morphological data, Iguanid lizards have been thought to have diverged from other squamates very early, but recent molecular phylogenies, both from mitochondrial and nuclear DNA, do not support this early divergence.[1] Because snakes have a faster molecular clock than other squamates,[1] and there are few early snake and snake ancestor fossils,[2] it is difficult to resolve the relationship between snakes and other squamate groups.


Trachylepis maculilabris, skinks mating

The male members of the group Squamata have a hemipenis. Hemipenes are usually held inverted, within the body, and are everted for reproduction via erectile tissue like that in the human penis.[3] Only one is used at a time, and some evidence indicates males alternate use between copulations. The hemipenis itself has a variety of shapes, depending on species. Often the hemipenis 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 which seals as the erectile tissue expands. This is also the only reptile group in which can be found both viviparous and ovoviviparous species, as well as the usual oviparous reptiles. Some species, like the Komodo dragon, can actually reproduce asexually and undergo parthenogenesis.[4]

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 families Caenophidia, Anguimorpha, and Iguania and has been shown that it evolved a single time along these lineages before the three families diverged because all lineages share nine common toxins.[5] The fossil record does show that the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 MYA to the Late Triassic/Early Jurassic.[5]

It has been shown that snake venom 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.[6] Previous literature hypothesized that venom were modifications of salivary or pancreatic proteins,[7] but it has been discovered that different toxins have been recruited from numerous different protein bodies and are diverse as the functions themselves.[8]

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,[9] which leads to a diversification of toxins that allows the sit-and-wait predators the ability to attack a wide range of prey.[10] It has been hypothesized that the rapid evolution and diversification is the result of a prey/predator arms race where both are adapting to counter the other.[11]

Humans and squamates

Bites and fatalities

Map showing global distribution of snakebite morbidity.

It is estimated that 125,000 people a year die from venomous snake bites.[12] In the US alone, more than 8,000 venomous snake bites are reported each year.[13] In addition, large pet constrictors, like boas and pythons, have been known to kill humans through constriction on rare occasions.[14]

Lizard bites, unlike venomous snake bites, are not fatal. The Komodo dragon has been known to kill people due to its size.[15] The two known venomous species of lizard, the Gila monster and Mexican beaded lizard have never caused a human death by envenomation.


Even though they survived the worst changes in Earth's history, today many squamate species are endangered due to habitat loss, hunting and poaching, the pet trade, alien species being introduced to their habitat (which puts native creatures at risk through competition, disease, and predation), and many other unnecessary reasons. Because of this, some are in fact extinct with Africa having the most extinct species of squamates. However, breeding programs and wildlife parks are trying to save many endangered reptiles from extinction. Many zoos & breeders educate people about the importance of snakes and lizards.


Desert iguana from Amboy Crater, Mojave Desert, California.

Classically, the order is divided into three suborders:

  • Lacertilia, the lizards;
  • Serpentes, the snakes;
  • Amphisbaenia, the worm lizards.

Of these, the lizards form a paraphyletic group (since "lizards" excludes the sub-clade of snakes). In newer classifications the name Sauria is used for reptiles and birds in general, and the Squamata are divided differently:

The relationships between these suborders is not yet certain, though recent research[16] suggests that several families may form a hypothetical venom clade which encompasses a majority (nearly 60%) of squamate species. Named Toxicofera, it combines the following groups from traditional classification[16]:

  • Suborder Serpentes (snakes)
  • Suborder Iguania (agamids, chameleons, iguanids, etc.)
  • Infraorder Anguimorpha, consisting of:
    • Family Varanidae (monitor lizards, including the Komodo dragon)
    • Family Anguidae (alligator lizards, glass lizards, etc.)
    • Family Helodermatidae (Gila monster and Mexican beaded lizard)

List of extant families

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) Bipes biporus.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) -
Anguidea or Diploglossa
Family Common Names Example Species Example Photo
Oppel, 1811
Glass lizards, alligator lizards & slow worms Slow worm (Anguis fragilis) Anguidae.jpg
Gray, 1852
American legless lizards California legless lizard (Anniella pulchra) Anniella pulchra.jpg
Cope, 1866
Knob-scaled lizards Chinese crocodile lizard (Shinisaurus crocodilurus) Chin-krokodilschwanzechse-01.jpg
Family Common Names Example Species Example Photo
Boulenger, 1884
Blind lizards Dibamus nicobaricum -
Gray, 1825
Geckos Thick-tailed gecko (Underwoodisaurus milii) Underwoodisaurus milii.jpg
Boulenger, 1884
Legless lizards Burton's snake lizard (Lialis burtonis) Lialis burtonis.jpg
Family Common Names Example Species Example 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 clubtails Club-tail iguana (Hoplocercus spinosus) -
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) -
Frost & Etheridge, 1989
Earless, spiny, tree, side-blotched and horned lizards Greater earless lizard (Cophosaurus texanus) Reptile tx usa.jpg
Frost & Etheridge, 1989
Anoles Carolina anole (Anolis carolinensis) Anolis carolinensis.jpg
Frost & Etheridge, 1989
Neotropical ground lizards (Microlophus peruvianus) Mperuvianus.jpg
Platynota or Varanoidea
Family Common Names Example Species Example Photo
Helodermatidae Gila monsters Gila monster (Heloderma suspectum) Gila.monster.arp.jpg
Lanthanotidae Earless monitor Earless monitor (Lanthanotus borneensis) -
Varanidae Monitor lizards Perentie (Varanus giganteus) Perentie Lizard Perth Zoo SMC Spet 2005.jpg
Family Common Names Example Species Example Photo
Cordylidae Spinytail lizards Girdle-tailed lizard (Cordylus warreni) Cordylus breyeri1.jpg
Gerrhosauridae Plated lizards Sudan plated lizard (Gerrhosaurus major) Gerrhosaurus major.jpg
Gymnophthalmidae Spectacled lizards Bachia bicolor Bachia bicolor.jpg
Oppel, 1811
Wall or true lizards Ocellated lizard (Lacerta lepida) Perleidechse-20.jpg
Oppel, 1811
Skinks Western blue-tongued skink (Tiliqua occipitalis) Tiliqua occipitalis.jpg
Teiidae Tegus or whiptails Blue tegu (Tupinambis teguixin) Goldteju Tupinambis teguixin.jpg
Xantusiidae Night lizards Granite night lizard (Xantusia henshawi) Xantusia henshawi.jpg
Family Common Names Example Species Example Photo
Bonaparte, 1831[17]
File snakes Marine file snake (Acrochordus granulatus) Wart snake 1.jpg
Stejneger, 1907[18]
Coral pipe snakes Burrowing false coral (Anilius scytale)
Cundall, Wallach and Rossman, 1993.[19]
Dwarf pipe snakes Leonard's pipe snake, (Anomochilus leonardi)
Günther, 1858[20]
Mole vipers Bibron's burrowing asp (Atractaspis bibroni)
Gray, 1825[17]
Boas Amazon tree boa (Corallus hortulanus) Corallushortulanus.GIF
Hoffstetter, 1946
Round Island boas Round Island burrowing boa (Bolyeria multocarinata)
Oppel, 1811[17]
Colubrids Grass snake (Natrix natrix) Natrix natrix (Marek Szczepanek).jpg
Fitzinger, 1843
Asian pipe snakes Red-tailed pipe snake (Cylindrophis ruffus) Cylindrophis rufus1.jpg
Boie, 1827[17]
Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids King cobra (Ophiophagus hannah) Ophiophagus hannah2.jpg
Cope, 1861
Mexican burrowing snakes Mexican burrowing snake (Loxocemus bicolor) Loxocemus bicolor.jpg
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[17]
Vipers, pitvipers, rattlesnakes European asp (Vipera aspis) Vipera-aspis-aspis-1.jpg
Bonaparte, 1845
Sunbeam snakes Sunbeam snake (Xenopeltis unicolor) XenopeltisUnicolorRooij.jpg
Family Common Names Example Species Example Photo
Taylor, 1939[17]
Dawn blind snakes Dawn blind snake (Liotyphlops beui)
Stejneger, 1892[17]
Slender blind snakes Texas blind snake (Leptotyphlops dulcis) Leptotyphlops dulcis.jpg
Merrem, 1820[21]
Blind snakes European blind snake (Typhlops vermicularis) Typhlops vermicularis.jpg


  1. ^ a b Kumazawa, Yoshinori (2007). "Mitochondrial genomes from major lizard families suggest their phylogenetic relationships and ancient radiations". Gene 388 (1-2): 19–26. doi:10.1016/j.gene.2006.09.026. PMID 17118581. 
  2. ^ "Lizards & Snakes Alive!". American Museum of Natural History. http://www.amnh.org/exhibitions/lizards/snakes/world.php. Retrieved 2007-12-25. 
  3. ^ "Iguana Anatomy". http://www.greenigsociety.org/anatomy.htm. 
  4. ^ Morales, Alex. "Komodo Dragons, World's Largest Lizards, Have Virgin Births". Bloomberg Television. http://www.bloomberg.com/apps/news?pid=20601082&sid=apLYpeppu8ag&refer=canada. Retrieved 2008-03-28. 
  5. ^ a b Fry, B. G., N. Vidal, J. A. Norman, F. J. Vonk, H. Scheib, S. F. R. Ramjan, S. Kuruppu. 2006. Early evolution of the venom system in lizards and snakes. Nature 439:584-588.
  6. ^ Fry, B. G., N. Vidal, L. van der Weerd, E. Kochva, and C. Renjifo. 2009. Evolution and diversification of the toxicofera reptile venom system.Journal of Proteomics 72:127-136.
  7. ^ Kochva, E. 1987. The origin of snakes and evolution of the venom apparatus. Toxicon 25:65-106.
  8. ^ 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 15:403-420.
  9. ^ Fry, B. G., H. Scheib, L. van der Weerd, B. Young, J. McNaughtan, S. F. R. Ramjan, N. Vidal. 2008. Evolution of an arsenal. Molecular & Cellular Proteomics 7:215-246.
  10. ^ Calvete, J. J., L. Sanz, Y. Angulo, B. Lomonte, and J. M. Gutierrez. 2009. Venoms, venomics, antivenomics. Febs Letters 583:1736-1743.
  11. ^ Barlow, A., C. E. Pook, R. A. Harrison, and W. Wuster. 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 276:2443-2449.
  12. ^ "Snake-bites: appraisal of the global situation". Who.com. http://www.who.int/bloodproducts/publications/en/bulletin_1998_76(5)_515-524.pdf. Retrieved 2007-12-30. 
  13. ^ "First Aid Snake Bites". University of Maryland Medical Center. http://www.umm.edu/non_trauma/snake.htm. Retrieved 2007-12-30. 
  14. ^ "Pet boa constrictor chokes owner". BBC News. 2006-12-18. http://news.bbc.co.uk/2/hi/americas/6191305.stm. Retrieved 2007-12-30. 
  15. ^ "Komodo dragon kills boy, 8, in Indonesia". msnbc. http://www.msnbc.msn.com/id/19026658/. Retrieved 2007-12-30. 
  16. ^ a b Fry, B. et al. (February 2006). "Early evolution of the venom system in lizards and snakes" (PDF). Nature 439 (7076): 584–588. doi:10.1038/nature04328. PMID 16292255. http://www.nature.com/nature/journal/v439/n7076/abs/nature04328.html. 
  17. ^ a b c d e f g Cogger(1991), p.23
  18. ^ "Aniliidae". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=209611. Retrieved 12 December 2007. 
  19. ^ "Anomochilidae". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=563894. Retrieved 13 December 2007. 
  20. ^ "Atractaspididae". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=563895. Retrieved 13 December 2007. 
  21. ^ "Typhlopidae". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=174338. Retrieved 13 December 2007. 


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Look at other dictionaries:

  • Squamata — Squa*ma ta (skw[.a]*m[=a] t[.a]), n. pl. [NL., fr. L. squamatus scaly.] (Zo[ o]l.) A division of edentates having the body covered with large, imbricated horny scales. It includes the pangolins. [1913 Webster] …   The Collaborative International Dictionary of English

  • Squamata — es el orden reciente más largo de reptiles e incluye los siguientes subórdenes: ● Sauria (Lagartos) ● Amphisbaenia ● Ophidia (Serpientes) …   Enciclopedia Universal

  • Squamata —   [zu lateinisch squama »Schuppe«], die Schuppenkriechtiere …   Universal-Lexikon

  • Squamata — Squamata, Schuppenkriechtiere, ca. 5500 meist landbewohnende Arten umfassende, zu den Lepidosauria gehörende Ordnung der ⇒ Reptilia. Unterer Jochbogen am Schädel rückgebildet; Extremitäten bei manchen Gruppen rückgebildet oder fehlend; Haut mit… …   Deutsch wörterbuch der biologie

  • Squamata — Squamates …   Wikipédia en Français

  • Squamata —   Escamosos Lagartos, según Haeckel …   Wikipedia Español

  • Squamata — Schuppenkriechtiere Männliche Zauneidechse (Lacerta agilis), eines der in Deutschland häufigeren Schuppenkriechtiere Systematik …   Deutsch Wikipedia

  • Squamata — ? Чешуйчатые Хамелеон Джексона (Chamaeleo jacksonii) Научная классификация Царство: Животные Т …   Википедия

  • Squamata — noun diapsid reptiles: snakes and lizards • Syn: ↑order Squamata • Hypernyms: ↑animal order • Member Holonyms: ↑Lepidosauria, ↑subclass Lepidosauria • Member Meronyms …   Useful english dictionary

  • squamata — squa·ma·ta …   English syllables

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