Squamata

It has been suggested that Lizard be merged into this article. (Discuss) Proposed since November 2023.

Squamates Temporal range: Early Jurassic–Present PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	Lepidosauria
Order:	Squamata Oppel, 1811
Subgroups[1]
Dibamidae Gekkota (Geckos) Scincomorpha Laterata Toxicofera Anguimorpha Iguania Ophidia (Snakes) † Mosasauria

Squamata (

Among other reptiles, squamates are most closely related to the tuatara, the last surviving member of the once diverse Rhynchocephalia, with both groups being placed in the clade Lepidosauria.

Evolution

Squamates are a

A study in 2018 found that Megachirella, an extinct genus of lepidosaurs that lived about 240 million years ago during the Middle Triassic, was a stem-squamate, making it the oldest known squamate. The phylogenetic analysis was conducted by performing high-resolution microfocus X-ray computed tomography (micro-CT) scans on the fossil specimen of Megachirella to gather detailed data about its anatomy. These data were then compared with a phylogenetic dataset combining the morphological and molecular data of 129 extant and extinct reptilian taxa. The comparison revealed Megachirella had certain features that are unique to squamates. The study also found that geckos are the earliest crown group squamates, not iguanians.^[5][6]

In 2022, the extinct genus Cryptovaranoides was described from the Late Triassic of England as a highly derived squamate belonging to the group Anguimorpha, which contains many extant lineages such as monitor lizards, beaded lizards and anguids. The presence of an essentially modern crown group squamate so far back in time was unexpected, as their diversification was previously thought to have occurred during the Jurassic and Cretaceous.^[7] However, a 2023 study found that Cryptovaranoides most likely represents an archosauromorph with no apparent squamate affinities.^[8] The oldest unambigious fossils of Squamata date to the Middle Jurassic of the Northern Hemisphere,^[9] with the first appearance of many modern groups, including snakes, during this period.^[10]

Scientists believe

The male members of the group Squamata have

Studies have been conducted on how sexual selection manifests itself in snakes and lizards. Snakes use a variety of tactics in acquiring mates.^{[19][dubious – discuss]} Ritual combat between males for the females with which they want to mate includes topping, a behavior exhibited by most viperids, in which one male twists around the vertically elevated fore body of his opponent and forcing it downward. Neck biting commonly occurs while the snakes are entwined.^[20]

Facultative parthenogenesis

Parthenogenesis is a natural form of reproduction in which the growth and development of embryos occur without fertilization.

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.^[22] The WW females are likely produced by terminal automixis.

Inbreeding avoidance

When female sand lizards mate with two or more males, sperm competition within the female's reproductive tract may occur. Active selection of sperm by females appears to occur in a manner that enhances female fitness.^[23] 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.^[23] This preference may enhance the fitness of progeny by reducing inbreeding depression.

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

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.[26] Previous literature hypothesized that venoms were modifications of salivary or pancreatic proteins,^[27] but different toxins have been found to have been recruited from numerous different protein bodies and are as diverse as their functions.^[28]

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,^[29] which leads to a diversification of toxins that allows the ambush predators the ability to attack a wide range of prey.^[30] 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.^[31]

Humans and squamates

Bites and fatalities

An estimated 125,000 people a year die from venomous snake bites.^[32] In the US alone, more than 8,000 venomous snake bites are reported each year, but only one in 50 million people (five or six fatalities per year in the USA) will die from venomous snake bites.^[33][34]

Lizard bites, unlike venomous snake bites, are usually 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.

Conservation

Though they survived the

Classification and phylogeny

Historically, the order Squamata has been divided into three suborders:

• Lacertilia
  , the lizards
• Serpentes, the snakes (see also Ophidia
  )
• Amphisbaenia, the worm lizards

Of these, the lizards form a

One example of a modern classification of the squamates is shown below.[1]^[37]

Squamata

Dibamia Dibamidae Bifurcata Gekkota Pygopodomorpha Diplodactylidae Underwood 1954 Pygopodidae Boulenger 1884 Carphodactylidae Gekkomorpha Eublepharidae Gekkonoidea Sphaerodactylidae Underwood 1954 Phyllodactylidae Gekkonidae Unidentata Scinciformata Scincomorpha Scincidae Cordylomorpha Xantusiidae Gerrhosauridae Cordylidae Episquamata Laterata Teiformata Gymnophthalmidae Merrem 1820 Teiidae Gray 1827 Lacertibaenia Lacertiformata Lacertidae Amphisbaenia Rhineuridae Vanzolini 1951 Bipedidae Taylor 1951 Blanidae Kearney & Stuart 2004 Cadeidae Vidal & Hedges 2008 Trogonophidae Gray 1865 Amphisbaenidae Gray 1865 Toxicofera Anguimorpha Paleoanguimorpha Shinisauria Shinisauridae Ahl 1930 sensu Conrad 2006 Varanoidea Lanthanotidae Varanidae Neoanguimorpha Helodermatoidea Helodermatidae Gray 1837 Xenosauroidea Xenosauridae Anguioidea Diploglossidae Anniellidae Anguidae Gray 1825 Iguania Acrodonta Chamaeleonidae Agamidae Gray 1827 Pleurodonta Leiocephalidae Iguanidae Hoplocercidae Frost & Etheridge 1989 Crotaphytidae Corytophanidae Tropiduridae Phrynosomatidae Dactyloidae Polychrotidae Liolaemidae Leiosauridae Opluridae Serpentes Scolecophidia Leptotyphlopidae Stejneger 1892 Gerrhopilidae Vidal et al. 2010 Xenotyphlopidae Vidal et al. 2010 Typhlopidae Merrem 1820 Anomalepididae Alethinophidia Amerophidia Aniliidae Tropidophiidae Brongersma 1951 Afrophidia Booidea Uropeltidae Anomochilidae Cylindrophiidae Xenopeltidae Bonaparte 1845 Loxocemidae Pythonidae Fitzinger 1826 Boidae Xenophidiidae Bolyeriidae Hoffstetter 1946 Caenophidia Acrochordidae Bonaparte 1831 Colubroides Xenodermidae Pareidae Viperidae Proteroglypha Homalopsidae Colubridae Lamprophiidae Elapidae

List of extant families

The over 10,900 extant squamates are divided into 67 families.

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)
Blanidae Kearney, 2003	Mediterranean worm lizards	Mediterranean worm lizard ( Blanus cinereus )
Cadeidae Vidal & Hedges, 2007[38]	Cuban worm lizards	Cadea blanoides
Rhineuridae Vanzolini, 1951	North American worm lizards	North American worm lizard (Rhineura floridana)
Trogonophidae Gray, 1865	Palearctic worm lizards	Checkerboard worm lizard (Trogonophis wiegmanni)
Gekkota (geckos, incl. Dibamia)
Family	Common names	Example species	Example photo
Carphodactylidae Kluge, 1967	Southern padless geckos	Thick-tailed gecko (Underwoodisaurus milii)
Dibamidae Boulenger, 1884	Blind lizards	Dibamus nicobaricum
Diplodactylidae Underwood, 1954	Australasian geckos	Golden-tailed gecko (Strophurus taenicauda)
Eublepharidae Boulenger, 1883	Eyelid geckos	Common leopard gecko (Eublepharis macularius)
Gekkonidae Gray, 1825	Geckos	Madagascar giant day gecko (Phelsuma grandis)
Phyllodactylidae Gamble et al., 2008	Leaf finger geckos	Moorish gecko (Tarentola mauritanica)
Pygopodidae Boulenger, 1884	Flap-footed lizards	Burton's snake lizard (Lialis burtonis)
Sphaerodactylidae Underwood, 1954	Round finger geckos	Fantastic least gecko (Sphaerodactylus fantasticus)
Iguania
Family	Common names	Example species	Example photo
Agamidae Gray, 1827	Agamas	Eastern bearded dragon (Pogona barbata)
Chamaeleonidae Rafinesque , 1815	Chameleons	Veiled chameleon (Chamaeleo calyptratus)
Corytophanidae Fitzinger, 1843	Casquehead lizards	Plumed basilisk (Basiliscus plumifrons)
Crotaphytidae H.M. Smith & Brodie, 1982	Collared and leopard lizards	Common collared lizard (Crotaphytus collaris)
Dactyloidae Fitzinger, 1843	Anoles	Carolina anole (Anolis carolinensis)
Hoplocercidae Frost & Etheridge, 1989	Wood lizards or clubtails	Enyalioides binzayedi
Iguanidae Oppel, 1811	Iguanas	Marine iguana (Amblyrhynchus cristatus)
Leiocephalidae Frost & Etheridge, 1989	Curly-tailed lizards	Hispaniolan masked curly-tailed lizard (Leiocephalus personatus)
Leiosauridae Frost et al., 2001	Leiosaurid lizards	Enyalius bilineatus
Liolaemidae Frost & Etheridge, 1989	Tree iguanas, snow swifts	Shining tree iguana (Liolaemus nitidus)
Opluridae Titus & Frost, 1996	Malagasy iguanas	Chalarodon madagascariensis
Phrynosomatidae Fitzinger, 1843	Earless, spiny, tree, side-blotched and horned lizards	Greater earless lizard (Cophosaurus texanus)
Polychrotidae Frost & Etheridge, 1989	Bush anoles	Brazilian bush anole (Polychrus acutirostris)
Tropiduridae Bell, 1843	Neotropical ground lizards	Microlophus peruvianus
Lacertoidea (excl. Amphisbaenia)
Family	Common Names	Example Species	Example Photo
Alopoglossidae Goicoechea, Frost, De la Riva, Pellegrino, Sites Jr., Rodrigues, & Padial, 2016	Alopoglossid lizards	Alopoglossus vallensis
Gymnophthalmidae Fitzinger, 1826	Spectacled lizards	Bachia bicolor
Lacertidae Oppel, 1811	Wall lizards	Ocellated lizard (Lacerta lepida)
Teiidae Gray, 1827	Tegus and whiptails	Gold tegu (Tupinambis teguixin)
Anguimorpha
Family	Common names	Example species	Example photo
Anguidae Gray, 1825	Glass lizards, alligator lizards and slowworms	Slowworm ( Anguis fragilis )
Anniellidae Boulenger , 1885	American legless lizards	California legless lizard (Anniella pulchra)
Diploglossidae Bocourt, 1873	galliwasps, legless lizards	Jamaican giant galliwasp (Celestus occiduus)	-
Helodermatidae Gray, 1837	Beaded lizards	Gila monster (Heloderma suspectum)	-
Lanthanotidae Steindachner , 1877	Earless monitor	Earless monitor (Lanthanotus borneensis)
Shinisauridae Ahl , 1930	Chinese crocodile lizard	Chinese crocodile lizard (Shinisaurus crocodilurus)
Varanidae Merrem, 1820	Monitor lizards	Perentie (Varanus giganteus)
Xenosauridae Cope, 1866	Knob-scaled lizards	Mexican knob-scaled lizard (Xenosaurus grandis)
Scincoidea
Family	Common Names	Example Species	Example Photo
Cordylidae Fitzinger, 1826	Girdled lizards	Girdle-tailed lizard (Cordylus warreni)
Gerrhosauridae Fitzinger, 1843	Plated lizards	Sudan plated lizard (Gerrhosaurus major)
Scincidae Oppel , 1811	Skinks	Western blue-tongued skink (Tiliqua occipitalis)
Xantusiidae Baird , 1858	Night lizards	Granite night lizard (Xantusia henshawi)
Alethinophidia
Family	Common names	Example species	Example photo
Acrochordidae Bonaparte, 1831[39]	File snakes	Marine file snake (Acrochordus granulatus)
Stejneger, 1907[40]	Coral pipe snakes	Burrowing false coral (Anilius scytale)
Rossman, 1993.[41]	Dwarf pipe snakes	Leonard's pipe snake, (Anomochilus leonardi)
Calabariidae )	Boas	Amazon tree boa (Corallus hortulanus)
Bolyeriidae Hoffstetter, 1946	Round Island boas	Round Island burrowing boa (Bolyeria multocarinata)
Pseudoxenodontidae )	Colubrids	Grass snake (Natrix natrix)
Cylindrophiidae Fitzinger , 1843	Asian pipe snakes	Red-tailed pipe snake (Cylindrophis ruffus)
Elapidae Boie, 1827[39]	Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids	King cobra (Ophiophagus hannah)
Homalopsidae Bonaparte, 1845	Indo-Australian water snakes, mudsnakes, bockadams	New Guinea bockadam (Cerberus rynchops)
Lamprophiidae Fitzinger, 1843[42]	Lamprophiid snakes	Bibron's burrowing asp (Atractaspis bibroni)
Loxocemidae Cope , 1861	Mexican burrowing snakes	Mexican burrowing snake (Loxocemus bicolor)
Pareidae Romer, 1956	Pareid snakes	Perrotet's mountain snake (Xylophis perroteti)
Pythonidae Fitzinger, 1826	Pythons	Ball python (Python regius)
Brongersma , 1951	Dwarf boas	Northern eyelash boa (Trachyboa boulengeri)
Uropeltidae Müller, 1832	Shield-tailed snakes, short-tailed snakes	Cuvier's shieldtail (Uropeltis ceylanica)
Viperidae Oppel, 1811[39]	Vipers, pitvipers, rattlesnakes	European asp (Vipera aspis)
Xenodermidae Fitzinger, 1826	Odd-scaled snakes and relatives	Khase earth snake (Stoliczkia khasiensis)
Xenopeltidae Gray , 1849	Sunbeam snakes	Sunbeam snake (Xenopeltis unicolor)
Scolecophidia (incl. Anomalepidae)
Family	Common names	Example species	Example photo
Anomalepidae Taylor, 1939[39]	Dawn blind snakes	Dawn blind snake (Liotyphlops beui)
Gerrhopilidae Vidal et al., 2010[38]	Indo-Malayan blindsnakes	Andaman worm snake (Gerrhopilus andamanensis)	–
Stejneger, 1892[39]	Slender blind snakes	Texas blind snake (Leptotyphlops dulcis)
Typhlopidae Merrem, 1820[43]	Blind snakes	European blind snake (Typhlops vermicularis)
Xenotyphlopidae Vidal et al., 2010[38]	Malagasy blind snakes	Xenotyphlops grandidieri	–

References

Further reading

• Bebler, John L.; King, F. Wayne (1979). The Audubon Society Field Guide to Reptiles and Amphibians of North America. New York:
  ISBN 978-0-394-50824-5
  .
• Capula, Massimo; Behler, John L. (1989). Simon & Schuster's Guide to Reptiles and Amphibians of the World. New York:
  ISBN 978-0-671-69098-4
  .
• ISBN 978-0-8317-2786-4
  .
• ISBN 978-0-395-58389-0
  .
• 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)" (PDF). Biological Reviews, Cambridge. 78 (4): 513–551.
  S2CID 4845536
  .
• Evans, SE (2008). "The skull of lizards and tuatara". In Gans, C; Gaunt, A S; Adler, K (eds.). Biology of the Reptilia. Vol. 20, Morphology H: the skull of Lepidosauria. Ithaca, New York: Society for the Study of Amphibians and Reptiles. pp. 1–344.
• Evans, SE; Jones, MEH (2010). "The Origin, Early History and Diversification of Lepidosauromorph Reptiles". In Bandyopadhyay, S. (ed.). New Aspects of Mesozoic Biodiversity. Lecture Notes in Earth Sciences. Vol. 132. pp. 27–44.
  ISBN 978-3-642-10310-0
  .
• Freiberg, Marcos; Walls, Jerry (1984). The World of Venomous Animals.
  ISBN 978-0-87666-567-1
  .
• Gibbons, J. Whitfield; Gibbons, Whit (1983). Their Blood Runs Cold: Adventures With Reptiles and Amphibians.
  ISBN 978-0-8173-0135-4
  .
• McDiarmid, RW; Campbell, JA; Touré, T (1999). Snake Species of the World: A Taxonomic and Geographic Reference. Vol. 1. Herpetologists' League. p. 511.
  ISBN 978-1-893777-00-2
  .
• Mehrtens, John (1987). Living Snakes of the World in Color. New York: Sterling.
  ISBN 978-0-8069-6461-4
  .
• Rosenfeld, Arthur (1989). Exotic Pets. New York:
  ISBN 978-0-671-47654-0
  .

External links

Wikispecies has information related to Squamata.

Wikimedia Commons has media related to Squamata.

• Palaeos.com: Squamata
• "Squamata". National Center for Biotechnology Information (NCBI).
• "Squamata". Integrated Taxonomic Information System.