Scaled reptiles Temporal range: Middle Jurassic-recent, 165–0 Ma PreЄ Є O S D C P T J K Pg N
Eastern blue-tongued lizard
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Superclass:	Tetrapoda
Class:	Reptilia
Subclass:	Diapsida
Superorder:	Lepidosauria
Order:	Squamata Oppel, 1811
Suborders
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) dwarf gecko (Sphaerodactylus ariasae) to the 6.6-metre (22 ft) green anaconda (Eunectes murinus) and the now-extinct mosasaurs, which reached lengths of 14 metres (46 ft).

Evolution[link]

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 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.

Reproduction[link]

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 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 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, such as the komodo dragon, can actually reproduce asexually and undergo parthenogenesis.^[4]

Evolution of venom[link]

Recent research suggests 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 to have evolved a single time along these lineages before the three families diverged, because all lineages share nine common toxins.^[5] The fossil record shows the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 Mya to the Late Triassic/Early Jurassic.^[5]

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.^[6] Previous literature hypothesized venoms 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[link]

Bites and fatalities[link]

An estimated 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, such as 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, and recent studies show it may have a passive envenomation system. Recent studies also show 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.^[15]

Conservation[link]

Though they survived the worst changes 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[link]

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 subclade of snakes). In newer classifications, the name Sauria is used for reptiles and birds in general, and the Squamata are divided differently:

• Suborder Iguania – (the agamids, chameleons, iguanids and other New World lizards)
• Suborder Scleroglossa
  • Infraorder Anguimorpha – (the monitors, Gila monster, alligator lizards, galliwasps, slow worms and others)
  • Infraorder Amphisbaenia – worm lizards
  • Infraorder Gekkota – (the geckos)
  • Infraorder Scincomorpha – (skinks, whiptail lizards and common European lizards)
  • Infraorder Serpentes – (the snakes)

The relationships between these suborders is not yet certain, though recent research^[16] suggests 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[link]

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)
Rhineuridae Vanzolini, 1951	North American worm lizards	North American worm lizard (Rhineura floridana)
Trogonophidae Gray, 1865	Palearctic worm lizards	Checkerboard worm lizard (Trogonophis wiegmanni)	-
Anguidea or Diploglossa
Family	Common Names	Example Species	Example Photo
Anguidae Oppel, 1811	Glass lizards, alligator lizards and slow worms	Slow worm (Anguis fragilis)
Anniellidae Gray, 1852	American legless lizards	California legless lizard (Anniella pulchra)
Xenosauridae Cope, 1866	Knob-scaled lizards	Chinese crocodile lizard (Shinisaurus crocodilurus)
Gekkota
Family	Common Names	Example Species	Example Photo
Dibamidae Boulenger, 1884	Blind lizards	Dibamus nicobaricum	-
Gekkonidae Gray, 1825	Geckos	Thick-tailed gecko (Underwoodisaurus milii)
Pygopodidae Boulenger, 1884	Legless lizards	Burton's snake lizard (Lialis burtonis)
Iguania
Family	Common Names	Example Species	Example Photo
Agamidae Spix, 1825	Agamas	Eastern bearded dragon (Pogona barbata)
Chamaeleonidae Gray, 1825	Chameleons	Veiled chameleon (Chamaeleo calyptratus)
Corytophanidae Frost & Etheridge, 1989	Casquehead lizards	Plumed basilisk (Basiliscus plumifrons)
Crotaphytidae Frost & Etheridge, 1989	Collared and leopard lizards	Common collared lizard (Crotaphytus collaris)
Hoplocercidae Frost & Etheridge, 1989	Wood lizards or clubtails	Club-tail iguana (Hoplocercus spinosus)	-
Iguanidae	Iguanas	Marine iguana (Amblyrhynchus cristatus)
Leiosauridae Frost et al., 2001	-	Darwin's iguana (Diplolaemus darwinii)	-
Liolaemidae Frost & Etheridge, 1989	Swifts	Shining tree iguana (Liolaemus nitidus)
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)
Polychrotidae Frost & Etheridge, 1989	Anoles	Carolina anole (Anolis carolinensis)
Tropiduridae Frost & Etheridge, 1989	Neotropical ground lizards	(Microlophus peruvianus)
Platynota or Varanoidea
Family	Common Names	Example Species	Example Photo
Helodermatidae	Gila monsters	Gila monster (Heloderma suspectum)
Lanthanotidae	Earless monitor	Earless monitor (Lanthanotus borneensis)	-
Varanidae	Monitor lizards	Perentie (Varanus giganteus)
Scincomorpha
Family	Common Names	Example Species	Example Photo
Cordylidae	Spinytail lizards	Girdle-tailed lizard (Cordylus warreni)
Gerrhosauridae	Plated lizards	Sudan plated lizard (Gerrhosaurus major)
Gymnophthalmidae	Spectacled lizards	Bachia bicolor
Lacertidae Oppel, 1811	Wall or true lizards	Ocellated lizard (Lacerta lepida)
Scincidae Oppel, 1811	Skinks	Western blue-tongued skink (Tiliqua occipitalis)
Teiidae	Tegus or whiptails	Gold tegu (Tupinambis teguixin)
Xantusiidae	Night lizards	Granite night lizard (Xantusia henshawi)
Alethinophidia
Family	Common Names	Example Species	Example Photo
Acrochordidae Bonaparte, 1831[17]	File snakes	Marine file snake (Acrochordus granulatus)
Aniliidae Stejneger, 1907[18]	Coral pipe snakes	Burrowing false coral (Anilius scytale)
Anomochilidae Cundall, Wallach and Rossman, 1993.[19]	Dwarf pipe snakes	Leonard's pipe snake, (Anomochilus leonardi)
Atractaspididae Günther, 1858[20]	Mole vipers	Bibron's burrowing asp (Atractaspis bibroni)
Boidae Gray, 1825[17]	Boas	Amazon tree boa (Corallus hortulanus)
Bolyeriidae Hoffstetter, 1946	Round Island boas	Round Island burrowing boa (Bolyeria multocarinata)
Colubridae Oppel, 1811[17]	Colubrids	Grass snake (Natrix natrix)
Cylindrophiidae Fitzinger, 1843	Asian pipe snakes	Red-tailed pipe snake (Cylindrophis ruffus)	100px
Elapidae Boie, 1827[17]	Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids	King cobra (Ophiophagus hannah)
Loxocemidae Cope, 1861	Mexican burrowing snakes	Mexican burrowing snake (Loxocemus bicolor)
Pythonidae Fitzinger, 1826	Pythons	Ball python (Python regius)
Tropidophiidae 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[17]	Vipers, pitvipers, rattlesnakes	European asp (Vipera aspis)
Xenopeltidae Bonaparte, 1845	Sunbeam snakes	Sunbeam snake (Xenopeltis unicolor)
Scolecophidia
Family	Common Names	Example Species	Example Photo
Anomalepidae Taylor, 1939[17]	Dawn blind snakes	Dawn blind snake (Liotyphlops beui)
Leptotyphlopidae Stejneger, 1892[17]	Slender blind snakes	Texas blind snake (Leptotyphlops dulcis)
Typhlopidae Merrem, 1820[21]	Blind snakes	European blind snake (Typhlops vermicularis)

Notes[link]

Wikimedia Commons has media related to: Squamata

Wikispecies has information related to: Squamata

References[link]

External links[link]

• Squamata
• Squamata at National Center for Biotechnology Information (NCBI)
• "Squamata". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=173861. 
• Squamata at the Encyclopedia of Life