Squamata

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Scaled reptiles
Temporal range:
Early Jurassic - Holocene, 199–0Ma[1]
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

black: Range of Squamata
 
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Scaled reptiles
Temporal range:
Early Jurassic - Holocene, 199–0Ma[1]
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

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 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 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 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. Squamate fossils first appear in the middle Jurassic period, but a mitochondrial phylogeny suggests 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 the most problematic. From morphological data, iguanid lizards were thought to have diverged from other squamates very early on, but recent molecular phylogenies, both from mitochondrial and nuclear DNA, do not support this early divergence.[3] Because snakes have faster molecular clocks than other squamates,[3] and few early snake and snake ancestor fossils have been found,[4] resolving the relationship between snakes and other squamate groups is difficult.

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.[5] 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.[6]

Evolution of venom[edit]

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.[7] The fossil record shows the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 Mya to the Late Triassic/Early Jurassic.[7] 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.[8] Previous literature hypothesized that venoms were modifications of salivary or pancreatic proteins,[9] but different toxins have been found to have been recruited from numerous different protein bodies and are as diverse as their functions.[10]

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,[11] which leads to a diversification of toxins that allows the sit-and-wait predators the ability to attack a wide range of prey.[12] The rapid evolution and diversification is thought to be the result of a predator/prey arms race, where both are adapting to counter the other.[13]

Humans and squamates[edit]

Bites and fatalities[edit]

Map showing the global distribution of snakebite morbidity

An estimated 125,000 people a year die from venomous snake bites.[14] In the US alone, more than 8,000 venomous snake bites are reported each year.[15] In addition, large pet constrictors, such as boas and pythons, have been known to kill humans on rare occasions through constriction.[16]

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.[17]

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, the pet trade, 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 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[18] 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.).[18]

List of extant families[edit]

Amphisbaenia
FamilyCommon namesExample speciesExample photo
Amphisbaenidae
Gray, 1865
Tropical worm lizardsDarwin's worm lizard (Amphisbaena darwinii)-
Bipedidae
Taylor, 1951
Bipes worm lizardsMexican mole lizard (Bipes biporus)Bipes biporus.jpg
Rhineuridae
Vanzolini, 1951
North American worm lizardsNorth American worm lizard (Rhineura floridana)Amphisbaenia 1.jpg
Trogonophidae
Gray, 1865
Palearctic worm lizardsCheckerboard worm lizard (Trogonophis wiegmanni)-
Anguidea or Diploglossa
FamilyCommon namesExample speciesExample photo
Anguidae
Oppel, 1811
Glass lizards, alligator lizards and slow wormsSlow worm (Anguis fragilis)Anguidae.jpg
Anniellidae
Gray, 1852
American legless lizardsCalifornia legless lizard (Anniella pulchra)Anniella pulchra.jpg
Xenosauridae
Cope, 1866
Knob-scaled lizardsChinese crocodile lizard (Shinisaurus crocodilurus)Chin-krokodilschwanzechse-01.jpg
Gekkota
FamilyCommon namesExample speciesExample photo
Dibamidae
Boulenger, 1884
Blind lizardsDibamus nicobaricum-
Gekkonidae
Gray, 1825
GeckosThick-tailed gecko (Underwoodisaurus milii)Underwoodisaurus milii.jpg
Pygopodidae
Boulenger, 1884
Legless lizardsBurton's snake lizard (Lialis burtonis)Lialis burtonis.jpg
Iguania
FamilyCommon namesExample speciesExample photo
Agamidae
Spix, 1825
AgamasEastern bearded dragon (Pogona barbata)Bearded dragon04.jpg
Chamaeleonidae
Gray, 1825
ChameleonsVeiled chameleon (Chamaeleo calyptratus)Chamaelio calyptratus.jpg
Corytophanidae
Frost & Etheridge, 1989
Casquehead lizardsPlumed basilisk (Basiliscus plumifrons)Plumedbasiliskcele4 edit.jpg
Crotaphytidae
Frost & Etheridge, 1989
Collared and leopard lizardsCommon collared lizard (Crotaphytus collaris)Collared lizard in Zion National Park.jpg
Hoplocercidae
Frost & Etheridge, 1989
Wood lizards or clubtailsClub-tail iguana (Hoplocercus spinosus)-
IguanidaeIguanasMarine iguana (Amblyrhynchus cristatus)Marineiguana03.jpg
Leiosauridae
Frost et al., 2001
-Darwin's iguana (Diplolaemus darwinii)-
Liolaemidae
Frost & Etheridge, 1989
SwiftsShining tree iguana (Liolaemus nitidus)Atacama lizard1.jpg
Opluridae
Frost & Etheridge, 1989
Madagascan iguanasChalarodon (Chalarodon madagascariensis)-
Phrynosomatidae
Frost & Etheridge, 1989
Earless, spiny, tree, side-blotched and horned lizardsGreater earless lizard (Cophosaurus texanus)Reptile tx usa.jpg
Polychrotidae
Frost & Etheridge, 1989
AnolesCarolina anole (Anolis carolinensis)Anolis carolinensis.jpg
Tropiduridae
Frost & Etheridge, 1989
Neotropical ground lizards(Microlophus peruvianus)Mperuvianus.jpg
Platynota or Varanoidea
FamilyCommon namesExample speciesExample photo
HelodermatidaeGila monstersGila monster (Heloderma suspectum)Gila.monster.arp.jpg
LanthanotidaeEarless monitorEarless monitor (Lanthanotus borneensis)-
VaranidaeMonitor lizardsPerentie (Varanus giganteus)Perentie Lizard Perth Zoo SMC Spet 2005.jpg
Scincomorpha
FamilyCommon NamesExample SpeciesExample Photo
CordylidaeSpinytail lizardsGirdle-tailed lizard (Cordylus warreni)Cordylus breyeri1.jpg
GerrhosauridaePlated lizardsSudan plated lizard (Gerrhosaurus major)Gerrhosaurus major.jpg
GymnophthalmidaeSpectacled lizardsBachia bicolorBachia bicolor.jpg
Lacertidae
Oppel, 1811
Wall or true lizardsOcellated lizard (Lacerta lepida)Perleidechse-20.jpg
Scincidae
Oppel, 1811
SkinksWestern blue-tongued skink (Tiliqua occipitalis)Tiliqua occipitalis.jpg
TeiidaeTegus or whiptailsGold tegu (Tupinambis teguixin)Goldteju Tupinambis teguixin.jpg
XantusiidaeNight lizardsGranite night lizard (Xantusia henshawi)Xantusia henshawi.jpg
Alethinophidia
FamilyCommon namesExample speciesExample photo
Acrochordidae
Bonaparte, 1831[19]
File snakesMarine file snake (Acrochordus granulatus)Wart snake 1.jpg
Aniliidae
Stejneger, 1907[20]
Coral pipe snakesBurrowing false coral (Anilius scytale)
Anomochilidae
Cundall, Wallach and Rossman, 1993.[21]
Dwarf pipe snakesLeonard's pipe snake, (Anomochilus leonardi)
Atractaspididae
Günther, 1858[22]
Mole vipersBibron's burrowing asp (Atractaspis bibroni)
Boidae
Gray, 1825[19]
BoasAmazon tree boa (Corallus hortulanus)Corallushortulanus.GIF
Bolyeriidae
Hoffstetter, 1946
Round Island boasRound Island burrowing boa (Bolyeria multocarinata)
Colubridae
Oppel, 1811[19]
ColubridsGrass snake (Natrix natrix)Natrix natrix (Marek Szczepanek).jpg
Cylindrophiidae
Fitzinger, 1843
Asian pipe snakesRed-tailed pipe snake (Cylindrophis ruffus)Cylindrophis rufus.jpg
Elapidae
Boie, 1827[19]
Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapidsKing cobra (Ophiophagus hannah)Ophiophagus hannah2.jpg
Loxocemidae
Cope, 1861
Mexican burrowing snakesMexican burrowing snake (Loxocemus bicolor)Loxocemus bicolor.jpg
Pythonidae
Fitzinger, 1826
PythonsBall python (Python regius)Ball python lucy.JPG
Tropidophiidae
Brongersma, 1951
Dwarf boasNorthern eyelash boa (Trachyboa boulengeri)
Uropeltidae
Müller, 1832
Shield-tailed snakes, short-tailed snakesCuvier's shieldtail (Uropeltis ceylanica)Silybura shortii.jpg
Viperidae
Oppel, 1811[19]
Vipers, pitvipers, rattlesnakesEuropean asp (Vipera aspis)
Xenopeltidae
Bonaparte, 1845
Sunbeam snakesSunbeam snake (Xenopeltis unicolor)XenopeltisUnicolorRooij.jpg
Scolecophidia
FamilyCommon namesExample speciesExample photo
Anomalepidae
Taylor, 1939[19]
Dawn blind snakesDawn blind snake (Liotyphlops beui)
Leptotyphlopidae
Stejneger, 1892[19]
Slender blind snakesTexas blind snake (Leptotyphlops dulcis)Leptotyphlops dulcis.jpg
Typhlopidae
Merrem, 1820[23]
Blind snakesEuropean blind snake (Typhlops vermicularis)Typhlops vermicularis.jpg

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 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. 
  4. ^ "Lizards & Snakes Alive!". American Museum of Natural History. Retrieved 2007-12-25. 
  5. ^ "Iguana Anatomy". 
  6. ^ Morales, Alex (2006-12-20). "Komodo Dragons, World's Largest Lizards, Have Virgin Births". Bloomberg Television. Retrieved 2008-03-28. 
  7. ^ 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.
  8. ^ 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.
  9. ^ Kochva, E. 1987. The origin of snakes and evolution of the venom apparatus. Toxicon 25:65-106.
  10. ^ 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.
  11. ^ 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.
  12. ^ Calvete, J. J., L. Sanz, Y. Angulo, B. Lomonte, and J. M. Gutierrez. 2009. Venoms, venomics, antivenomics. Febs Letters 583:1736-1743.
  13. ^ 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.
  14. ^ "Snake-bites: appraisal of the global situation". Who.com. Retrieved 2007-12-30. 
  15. ^ "First Aid Snake Bites". University of Maryland Medical Center. Retrieved 2007-12-30. 
  16. ^ "Pet boa constrictor chokes owner". BBC News. 2006-12-18. Retrieved 2007-12-30. 
  17. ^ "Komodo dragon kills boy, 8, in Indonesia". msnbc. Retrieved 2007-12-30. 
  18. ^ 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. 
  19. ^ a b c d e f g Cogger(1991), p.23
  20. ^ "Aniliidae". Integrated Taxonomic Information System. Retrieved 12 December 2007. 
  21. ^ "Anomochilidae". Integrated Taxonomic Information System. Retrieved 13 December 2007. 
  22. ^ "Atractaspididae". Integrated Taxonomic Information System. Retrieved 13 December 2007. 
  23. ^ "Typhlopidae". Integrated Taxonomic Information System. Retrieved 13 December 2007. 

References[edit]

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  • 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
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External links[edit]