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 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
Subgroups[2]

The order Squamata, or the scaled reptiles, are the largest recent order of reptiles, comprising all lizards and snakes. With over 9,000 species,[3] 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 5.21 m (17.1 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 strongly 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,[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 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.[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 parthenogenesis[edit]

The effects of central fusion and terminal fusion on heterozygosity

Parthenogenesis is a natural form of reproduction in which 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 avoidance[edit]

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 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.[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 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.[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.

Conservation[edit]

Though they survived the Cretaceous–Paleogene extinction event, 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 other anthropogenic causes. Because of this, some squamates species have recently become 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. Zoos, private hobbyists and breeders help 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,[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[2][27] found the following relationships:

Squamata
Dibamia

Dibamidae


Bifurcata
Gekkota
Pygopodomorpha

Diplodactylidae Underwood 1954Hoplodactylus pomarii white background.jpg




Pygopodidae Boulenger 1884The zoology of the voyage of the H.M.S. Erebus and Terror (Lialis burtonis).jpg



Carphodactylidae




Gekkomorpha

Eublepharidae


Gekkonoidea

Sphaerodactylidae Underwood 1954




Phyllodactylidae Phyllodactylus gerrhopygus 1847 - white background.jpg



Gekkonidae






Unidentata
Scinciformata
Scincomorpha

ScincidaeBilder-Atlas zur wissenschaftlich-populären Naturgeschichte der Wirbelthiere (Plate (24)) Tribolonotus novaeguineae.jpg


Cordylomorpha

Xantusiidae




GerrhosauridaeGerrhosaurus ocellatus flipped.jpg



CordylidaeIllustrations of the zoology of South Africa (Smaug giganteus).jpg





Episquamata
Laterata
Teiformata

Gymnophthalmidae Merrem 1820PZSL1851PlateReptilia06 Cercosaura ocellata.png



Teiidae Gray 1827Bilder-Atlas zur wissenschaftlich-populären Naturgeschichte der Wirbelthiere (Tupinambis teguixin).jpg



Lacertibaenia
Lacertiformata

Lacertidae Brockhaus' Konversations-Lexikon (1892) (Lacerta agilis).jpg


Amphisbaenia

Rhineuridae Vanzolini 1951




Bipedidae Taylor 1951Bilder-Atlas zur wissenschaftlich-populären Naturgeschichte der Wirbelthiere (Bipes canaliculatus).jpg





Blanidae Kearney & Stuart 2004Blanus cinereus flipped.jpg



Cadeidae Vidal & Hedges 2008





Trogonophiidae Gray 1865



Amphisbaenidae Gray 1865Amphisbaena microcephalum 1847 - white background.jpg








Toxicofera

Anguimorpha
Palaeoanguimorpha
Shinisauria

Shinisauridae Ahl 1930 sensu Conrad 2006


Varanoidea

Lanthanotidae



VaranidaeZoology of Egypt (1898) (Varanus griseus).png




Neoanguimorpha
Helodermatoidea

Helodermatidae Gray 1837Gila monster ncd 2012 white background.jpg



Xenosauroidea

Xenosauridae


Anguioidea

Diploglossidae




Anniellidae



Anguidae Gray 1825







Iguania
Acrodonta

ChamaeleonidaeZoology of Egypt (1898) (Chamaeleo calyptratus).jpg



Agamidae Gray 1827Haeckel Lacertilia (Chlamydosaurus kingii).jpg



Pleurodonta

Leiocephalidae




IguanidaeStamps of Germany (Berlin) 1977, Cyclura cornuta.jpg





Hoplocercidae Frost & Etheridge 1989




Crotaphytidae



Corytophanidae






Tropiduridae





Phrynosomatidae




Dactyloidae



Polychrotidae






Liolaemidae




Leiosauridae



Opluridae











Serpentes
Scolecophidia

Leptotyphlopidae Stejneger 1892Epictia tenella 1847 -white background.jpg




Gerrhopilidae Vidal et al. 2010




Xenotyphlopidae Vidal et al. 2010



Typhlopidae Merrem 1820Typhlops vermicularis3 white background.jpg







Anomalepididae


Alethinophidia
Amerophidia

Aniliidae



Tropidophiidae Brongersma 1951



Afrophidia
Booidea


UropeltidaeUropeltis ceylanica (2) flipped.jpg




Anomochilidae



CylindrophiidaeCylind resplendens Wagler white background.JPG







Xenopeltidae Bonaparte 1845




Loxocemidae



Pythonidae Fitzinger 1826Python natalensis Smith 1840 white background.jpg






BoidaeBoa Iconographia Zoologica white background.tif




Xenophidiidae



Bolyeriidae Hoffstetter 1946






Caenophidia

Acrochordidae Bonaparte 1831




Xenodermatidae


Colubroidea

Pareatidae




ViperidaeOur 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


Proteroglypha

Homalopsidae




ColubridaeXenochrophis piscator 1 Hardwicke white background.jpg




Lamprophiidae



ElapidaeBilder-Atlas zur wissenschaftlich-populären Naturgeschichte der Wirbelthiere (Naja naja).jpg


















All recent molecular studies[28] 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.).[28]

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[29]
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[30]
File snakes Marine file snake (Acrochordus granulatus) Wart snake 1.jpg
Aniliidae
Stejneger, 1907[31]
Coral pipe snakes Burrowing false coral (Anilius scytale)
Anomochilidae
Cundall, Wallach and Rossman, 1993.[32]
Dwarf pipe snakes Leonard's pipe snake, (Anomochilus leonardi)
Boidae
Gray, 1825[30] (incl. Calabariidae)
Boas Amazon tree boa (Corallus hortulanus) Corallushortulanus.png
Bolyeriidae
Hoffstetter, 1946
Round Island boas Round Island burrowing boa (Bolyeria multocarinata)
Colubridae
Oppel, 1811[30] 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[30]
Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids King cobra (Ophiophagus hannah) Ophiophagus hannah2.jpg
Homalopsidae
Bonaparte, 1845
Lamprophiidae
Fitzinger, 1843[33]
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[30]
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[30]
Dawn blind snakes Dawn blind snake (Liotyphlops beui)
Gerrhopilidae
Vidal et al., 2010[29]
Leptotyphlopidae
Stejneger, 1892[30]
Slender blind snakes Texas blind snake (Leptotyphlops dulcis) Leptotyphlops dulcis.jpg
Typhlopidae
Merrem, 1820[34]
Blind snakes European blind snake (Typhlops vermicularis) Typhlops vermicularis.jpg
Xenotyphlopidae
Vidal et al., 2010[29]
Xenotyphlops grandidieri

References[edit]

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Further reading[edit]

  • 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). Ithica, 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 links[edit]