Squamata

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This article is about the Squamata order of reptiles. For the Roman scale armour, see Lorica squamata.
Scaled reptiles
Temporal range:
Early Jurassic - Holocene, 199–0Ma[1]
Blue-toungued skink444.jpg
Eastern blue-tongued lizard
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Lepidosauria
Order: Squamata
Oppel, 1811
Subgroups[2]

Anguimorpha
Gekkota
Iguania
Lacertoidea
Scincomorpha
Serpentes

World.distribution.sauria.1.png
black: Range of Squamata

The Squamata, or the scaled reptiles, are the largest recent order of reptiles, comprising all lizards and snakes. With over 9,000 species, it is also the second-largest order of 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. They are the most variably sized order of reptiles, ranging from the 16 mm (0.63 in) dwarf gecko (Sphaerodactylus ariasae) to the 6.6 m (22 ft) green anaconda (Eunectes murinus) and the now-extinct mosasaurs, which reached lengths of 14 m (46 ft).

Among the other reptiles, squamates are most closely related to tuataras, which superficially resemble lizards.

Evolution[edit]

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,[3] meaning that the lineage leading to squamates must have existed as well. Modern squamates probably originated in the Jurassic,[3] when fossil relatives of geckos and skinks appear, other groups, including iguanians, snakes, 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.[4] At the end of the Cretaceous, squamates suffered a major extinction at the K-T boundary [5] 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 studies of DNA. From morphological data, the iguanians were long thought to be the most ancient branch of the tree [4] however studies of the DNA suggest that the geckos represent the most ancient branch.[6] Iguanians are now united with snakes and anguimorphs in a group called the Toxicofera. DNA also suggests that the various limbless groups- snakes, amphisbaenians, and dibamids- are unrelated, and instead arose independently from lizards.

Reproduction[edit]

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.[7] Only one is used at a time, and some evidence indicates 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 and undergo parthenogenesis.[8]

Evolution of venom[edit]

See also: 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.[9] The fossil record shows the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 Mya to the Late Triassic/Early Jurassic.[9] 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.[10] Previous literature hypothesized that venoms were modifications of salivary or pancreatic proteins,[11] but different toxins have been found to have been recruited from numerous different protein bodies and are as diverse as their functions.[12]

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,[13] which leads to a diversification of toxins that allows the ambush predators the ability to attack a wide range of prey.[14] 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.[15]

Humans and squamates[edit]

Bites and fatalities[edit]

See also: Snakebite
Map showing the global distribution of snakebite morbidity

An estimated 125,000 people a year die from venomous snake bites.[16] In the US alone, more than 8,000 venomous snake bites are reported each year.[17]

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.[18]

Conservation[edit]

Though they survived the most drastic changes[citation needed] in Earth's history, many squamate species are endangered now 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 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, private hobbyists and breeders educate people about the importance of snakes and lizards.

Classification[edit]

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, 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[2] found the following relationships:

Squamata


Dibamidae



Gekkota



unnamed

Scincomorpha


unnamed

Lacertoidea (incl. Amphisbaenia)


Toxicofera

Serpentes




Anguimorpha



Iguania







Some research[19] has suggested that several families may form a hypothetical venom clade, which encompasses a majority (nearly 60%) of squamate species. Named Toxicofera, it would combine the groups Serpentes (snakes), Iguania (agamids, chameleons, iguanids, etc.), and Anguimorpha (monitor lizards, Gila monster, glass lizards, etc.).[19]

List of extant families[edit]

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) Bipes biporus.jpg
Blanidae Mediterranean worm lizards Mediterranean worm lizard (Blanus cinereus) -
Cadeidae
Vidal & Hedges, 2008[20]
Cuban worm lizards Cadea blanoides -
Rhineuridae
Vanzolini, 1951
North American worm lizards North American worm lizard (Rhineura floridana) Amphisbaenia 1.jpg
Trogonophidae
Gray, 1865
Palearctic worm lizards Checkerboard worm lizard (Trogonophis wiegmanni) -
Gekkota (incl. Dibamia)
Family Common names Example species Example photo
Dibamidae
Boulenger, 1884
Blind lizards Dibamus nicobaricum -
Gekkonidae
Gray, 1825 (paraphyletic)
Geckos Thick-tailed gecko (Underwoodisaurus milii) Underwoodisaurus milii.jpg
Pygopodidae
Boulenger, 1884
Legless lizards Burton's snake lizard (Lialis burtonis) Lialis burtonis.jpg
Iguania
Family Common names Example species Example photo
Agamidae
Spix, 1825
Agamas Eastern bearded dragon (Pogona barbata) Bearded dragon04.jpg
Chamaeleonidae
Gray, 1825
Chameleons Veiled chameleon (Chamaeleo calyptratus) Chamaelio calyptratus.jpg
Corytophanidae
Frost & Etheridge, 1989
Casquehead lizards Plumed basilisk (Basiliscus plumifrons) Plumedbasiliskcele4 edit.jpg
Crotaphytidae
Frost & Etheridge, 1989
Collared and leopard lizards Common collared lizard (Crotaphytus collaris) Collared lizard in Zion National Park.jpg
Hoplocercidae
Frost & Etheridge, 1989
Wood lizards or clubtails Club-tail iguana (Hoplocercus spinosus) -
Iguanidae Iguanas Marine iguana (Amblyrhynchus cristatus) Marineiguana03.jpg
Leiosauridae
Frost et al., 2001
- Darwin's iguana (Diplolaemus darwinii) -
Liolaemidae
Frost & Etheridge, 1989
Swifts Shining tree iguana (Liolaemus nitidus) Atacama lizard1.jpg
Opluridae
Frost & Etheridge, 1989
Madagascan iguanas Chalarodon (Chalarodon madagascariensis) -
Phrynosomatidae
Frost & Etheridge, 1989
Earless, spiny, tree, side-blotched and horned lizards Greater earless lizard (Cophosaurus texanus) Reptile tx usa.jpg
Polychrotidae
Frost & Etheridge, 1989 (+ Dactyloidae)
Anoles Carolina anole (Anolis carolinensis) Anolis carolinensis.jpg
Tropiduridae
Frost & Etheridge, 1989
Neotropical ground lizards (Microlophus peruvianus) Mperuvianus.jpg
Lacertoidea (excl. Amphisbaenia)
Family Common Names Example Species Example Photo
Gymnophthalmidae Spectacled lizards Bachia bicolor Bachia bicolor.jpg
Lacertidae
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
Neoanguimorpha
Family Common names Example species Example photo
Anguidae
Oppel, 1811
Glass lizards, alligator lizards and slow worms Slow worm (Anguis fragilis) Anguidae.jpg
Anniellidae
Gray, 1852
American legless lizards California legless lizard (Anniella pulchra) Anniella pulchra.jpg
Helodermatidae Gila monsters Gila monster (Heloderma suspectum) Gila.monster.arp.jpg
Xenosauridae
Cope, 1866
Knob-scaled lizards Mexican knob-scaled lizard (Xenosaurus grandis) Xenosaurus grandis.jpg
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) Chin-krokodilschwanzechse-01.jpg
Varanidae Monitor lizards Perentie (Varanus giganteus) Perentie Lizard Perth Zoo SMC Spet 2005.jpg
Scincoidea
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
Scincidae
Oppel, 1811
Skinks Western blue-tongued skink (Tiliqua occipitalis) Tiliqua occipitalis.jpg
Xantusiidae Night lizards Granite night lizard (Xantusia henshawi) Xantusia henshawi.jpg
Alethinophidia
Family Common names Example species Example photo
Acrochordidae
Bonaparte, 1831[21]
File snakes Marine file snake (Acrochordus granulatus) Wart snake 1.jpg
Aniliidae
Stejneger, 1907[22]
Coral pipe snakes Burrowing false coral (Anilius scytale)
Anomochilidae
Cundall, Wallach and Rossman, 1993.[23]
Dwarf pipe snakes Leonard's pipe snake, (Anomochilus leonardi)
Boidae
Gray, 1825[21] (incl. Calabariidae)
Boas Amazon tree boa (Corallus hortulanus) Corallushortulanus.GIF
Bolyeriidae
Hoffstetter, 1946
Round Island boas Round Island burrowing boa (Bolyeria multocarinata)
Colubridae
Oppel, 1811[21] sensu lato (incl. Dipsadidae, Natricidae, Pseudoxenodontidae)
Colubrids Grass snake (Natrix natrix) Natrix natrix (Marek Szczepanek).jpg
Cylindrophiidae
Fitzinger, 1843
Asian pipe snakes Red-tailed pipe snake (Cylindrophis ruffus) Cylindrophis rufus.jpg
Elapidae
Boie, 1827[21]
Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids King cobra (Ophiophagus hannah) Ophiophagus hannah2.jpg
Homalopsidae
Bonaparte, 1845
-
Lamprophiidae
Fitzinger, 1843[24]
Bibron's burrowing asp (Atractaspis bibroni)
Loxocemidae
Cope, 1861
Mexican burrowing snakes Mexican burrowing snake (Loxocemus bicolor) Loxocemus bicolor.jpg
Pareatidae
Romer, 1956
-
Pythonidae
Fitzinger, 1826
Pythons Ball python (Python regius) Ball python lucy.JPG
Tropidophiidae
Brongersma, 1951
Dwarf boas Northern eyelash boa (Trachyboa boulengeri)
Uropeltidae
Müller, 1832
Shield-tailed snakes, short-tailed snakes Cuvier's shieldtail (Uropeltis ceylanica) Silybura shortii.jpg
Viperidae
Oppel, 1811[21]
Vipers, pitvipers, rattlesnakes European asp (Vipera aspis)
Xenodermatidae
Fitzinger, 1826
-
Xenopeltidae
Gray, 1849
Sunbeam snakes Sunbeam snake (Xenopeltis unicolor) XenopeltisUnicolorRooij.jpg
Scolecophidia (incl. Anomalepidae)
Family Common names Example species Example photo
Anomalepidae
Taylor, 1939[21]
Dawn blind snakes Dawn blind snake (Liotyphlops beui)
Gerrhopilidae
Vidal et al., 2010[20]
-
Leptotyphlopidae
Stejneger, 1892[21]
Slender blind snakes Texas blind snake (Leptotyphlops dulcis) Leptotyphlops dulcis.jpg
Typhlopidae
Merrem, 1820[25]
Blind snakes European blind snake (Typhlops vermicularis) Typhlops vermicularis.jpg
Xenotyphlopidae
Vidal et al., 2010[20]
Xenotyphlops grandidieri -

Notes[edit]

  1. ^ a b 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.  edit
  2. ^ a b 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.
  3. ^ a b Jones, M.E., Anderson, C.L., Hipsley, C.A., Müller, J., Evans, S.E., Schoch, R.R., 2013. Integration of molecules and new fossils supports a Triassic origin for Lepidosauria (lizards, snakes, and tuatara). BMC evolutionary biology 13, 208.
  4. ^ a b Gauthier, J., Kearney, M., Maisano, J.A., Rieppel, O., Behlke, A., 2012. Assembling the squamate tree of life: perspectives from the phenotype and the fossil record. Bulletin Yale Peabody Museum 53, 3-308.
  5. ^ Longrich, N.R., Bhullar, A.-B.S., Gauthier, J., 2012. Mass extinction of lizards and snakes at the Cretaceous-Paleogene boundary. Proceedings of the National Academy of Sciences 109, 21396--21401.
  6. ^ Pyron, R.A., Burbrink, F.T., Wiens, J.J., 2013. A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes. BMC evolutionary biology 13, 93.
  7. ^ "Iguana Anatomy". 
  8. ^ Morales, Alex (2006-12-20). "Komodo Dragons, World's Largest Lizards, Have Virgin Births". Bloomberg Television. Retrieved 2008-03-28. 
  9. ^ 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.
  10. ^ 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.
  11. ^ Kochva, E. 1987. The origin of snakes and evolution of the venom apparatus. Toxicon 25:65-106.
  12. ^ 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.
  13. ^ 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.
  14. ^ Calvete, J. J., L. Sanz, Y. Angulo, B. Lomonte, and J. M. Gutierrez. 2009. Venoms, venomics, antivenomics. Febs Letters 583:1736-1743.
  15. ^ 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.
  16. ^ "Snake-bites: appraisal of the global situation". Who.com. Retrieved 2007-12-30. 
  17. ^ "First Aid Snake Bites". University of Maryland Medical Center. Retrieved 2007-12-30. 
  18. ^ "Komodo dragon kills boy, 8, in Indonesia". msnbc. Retrieved 2007-12-30. 
  19. ^ 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. 
  20. ^ a b c [1]
  21. ^ a b c d e f g Cogger(1991), p.23
  22. ^ "Aniliidae". Integrated Taxonomic Information System. Retrieved 12 December 2007. 
  23. ^ "Anomochilidae". Integrated Taxonomic Information System. Retrieved 13 December 2007. 
  24. ^ "Atractaspididae". Integrated Taxonomic Information System. Retrieved 13 December 2007. 
  25. ^ "Typhlopidae". Integrated Taxonomic Information System. Retrieved 13 December 2007. 

References[edit]

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External links[edit]