Squamata

Squamata (/skwæˈmeɪtə/, Latin squamatus (“scaly, having scales”)) is the largest order of reptiles, comprising lizards, snakes and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species,[3] it is also the second-largest order of extant (living) 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. Squamata is 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 over 14 m (46 ft).

Squamata Temporal range: Early Jurassic – Present, 199–0 Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	Lepidosauria
Order:	Squamata Oppel, 1811
Subgroups[2]
Dibamidae Gekkota Scinciformata Laterata Toxicofera Anguimorpha Iguania Pythonomorpha

Among other reptiles, squamates are most closely related to the tuatara, which superficially resembles lizards.

Evolution

Slavoia darevskii, a fossil squamate

Squamates are a monophyletic sister group to the rhynchocephalians, members of the order Rhynchocephalia. The only surviving member of Rhynchocephalia is the tuatara. Squamata and Rhynchocephalia form the subclass Lepidosauria, which is the sister group to Archosauria, the clade that contains crocodiles and birds, and their extinct relatives. Fossils of rhynchocephalians first appear in the Early Triassic, meaning that the lineage leading to squamates must have also existed at the time.[4] Scientists believe crown group squamates probably originated in the Early Jurassic based on the fossil record.[4] The first fossils of geckos, skinks and snakes appear in the Middle Jurassic.[5] Other groups like iguanians and varanoids appeared in the Cretaceous. Polyglyphanodontians, a distinct clade of lizards, and mosasaurs, a group of predatory marine lizards that grew to enormous sizes, also appeared in the Cretaceous.[6] Squamates suffered a mass extinction at the Cretaceous–Paleogene (K–PG) boundary, which wiped out polyglyphanodontians, mosasaurs and many other distinct lineages.[7]

The relationships of squamates is debatable. 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 genomes. Iguanians were long thought to be the earliest crown group squamates based on morphological data,[6] however, genetic data suggests that geckoes are the earliest crown group squamates.[8] Iguanians are now united with snakes and anguimorphs in a clade called Toxicofera. Genetic data also suggests that the various limbless groups; snakes, amphisbaenians and dibamids, are unrelated, and instead arose independently from lizards.

A study in 2018 found that Megachirella, an extinct genus of lepidosaur 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. This data was 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.[9][10]

Reproduction

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.[11] 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 reproduce asexually through parthenogenesis.[12]

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.[13] 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.[14]

Facultative parthenogenesis

The effects of central fusion and terminal fusion on heterozygosity

Parthenogenesis is a natural form of reproduction in which the 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.[15] 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.[15]

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.[16] 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.[17] 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.[17] 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 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.[18] The fossil record shows the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 Mya to the Late Triassic/Early Jurassic.[18] 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.[19] Previous literature hypothesized that venoms were modifications of salivary or pancreatic proteins,[20] but different toxins have been found to have been recruited from numerous different protein bodies and are as diverse as their functions.[21]

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,[22] which leads to a diversification of toxins that allows the ambush predators the ability to attack a wide range of prey.[23] 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.[24]

Humans and squamates

Bites and fatalities

Map showing the global distribution of venomous snakebites

An estimated 125,000 people a year die from venomous snake bites.[25] In the US alone, more than 8,000 venomous snake bites are reported each year, but only 1 in 50 million people (5-6 fatalities per year in the USA) will die from venomous snake bites.[26][27]

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.[28] The Gila monster and beaded lizards of North and Central America are venomous, but not deadly to humans.

Conservation

Though they survived the Cretaceous–Paleogene extinction event, many squamate species are now endangered 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 squamate species have recently become extinct, with Africa having the most extinct species. 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 and phylogeny

Desert iguana from Amboy Crater, Mojave Desert, California

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 paraphyletic group,[29] 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 is[2][30]

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       Xenodermidae   Colubroidea   Pareidae       Viperidae   Proteroglypha   Homalopsidae       Colubridae       Lamprophiidae     Elapidae

All recent molecular studies[18] 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.).[18]

List of extant families

The over 10,900 extant squamates are divided into 58 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	Mediterranean worm lizards	Mediterranean worm lizard (Blanus cinereus)
Cadeidae Vidal & Hedges, 2008[31]	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 (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)
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	Enyalioides binzayedi
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 (+ Dactyloidae)	Anoles	Carolina anole (Anolis carolinensis)
Tropiduridae Frost & Etheridge, 1989	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		Alopoglossus vallensis
Gymnophthalmidae Fitzinger, 1826	Spectacled lizards	Bachia bicolor
Lacertidae Oppel, 1811	Wall or true lizards	Ocellated lizard (Lacerta lepida)
Teiidae	Tegus or whiptails	Gold tegu (Tupinambis teguixin)
Neoanguimorpha
Family	Common names	Example species	Example photo
Anguidae Oppel, 1811	Glass lizards, alligator lizards and slowworms	Slowworm (Anguis fragilis)
Anniellidae Gray, 1852	American legless lizards	California legless lizard (Anniella pulchra)
Helodermatidae	Gila monsters	Gila monster (Heloderma suspectum)
Xenosauridae Cope, 1866	Knob-scaled lizards	Mexican knob-scaled lizard (Xenosaurus grandis)
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)
Varanidae	Monitor lizards	Perentie (Varanus giganteus)
Scincoidea
Family	Common Names	Example Species	Example Photo
Cordylidae	Spinytail lizards	Girdle-tailed lizard (Cordylus warreni)
Gerrhosauridae	Plated lizards	Sudan plated lizard (Gerrhosaurus major)
Scincidae Oppel, 1811	Skinks	Western blue-tongued skink (Tiliqua occipitalis)
Xantusiidae	Night lizards	Granite night lizard (Xantusia henshawi)
Alethinophidia
Family	Common names	Example species	Example photo
Acrochordidae Bonaparte, 1831[32]	File snakes	Marine file snake (Acrochordus granulatus)
Aniliidae Stejneger, 1907[33]	Coral pipe snakes	Burrowing false coral (Anilius scytale)
Anomochilidae Cundall, Wallach and Rossman, 1993.[34]	Dwarf pipe snakes	Leonard's pipe snake, (Anomochilus leonardi)
Boidae Gray, 1825[32] (incl. Calabariidae)	Boas	Amazon tree boa (Corallus hortulanus)
Bolyeriidae Hoffstetter, 1946	Round Island boas	Round Island burrowing boa (Bolyeria multocarinata)
Colubridae Oppel, 1811[32] sensu lato (incl. Dipsadidae, Natricidae, Pseudoxenodontidae)	Colubrids	Grass snake (Natrix natrix)
Cylindrophiidae Fitzinger, 1843	Asian pipe snakes	Red-tailed pipe snake (Cylindrophis ruffus)
Elapidae Boie, 1827[32]	Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids	King cobra (Ophiophagus hannah)
Homalopsidae Bonaparte, 1845			–
Lamprophiidae Fitzinger, 1843[35]		Bibron's burrowing asp (Atractaspis bibroni)
Loxocemidae Cope, 1861	Mexican burrowing snakes	Mexican burrowing snake (Loxocemus bicolor)
Pareatidae Romer, 1956			–
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[32]	Vipers, pitvipers, rattlesnakes	European asp (Vipera aspis)
Xenodermatidae Fitzinger, 1826			–
Xenopeltidae Gray, 1849	Sunbeam snakes	Sunbeam snake (Xenopeltis unicolor)
Scolecophidia (incl. Anomalepidae)
Family	Common names	Example species	Example photo
Anomalepidae Taylor, 1939[32]	Dawn blind snakes	Dawn blind snake (Liotyphlops beui)
Gerrhopilidae Vidal et al., 2010[31]			–
Leptotyphlopidae Stejneger, 1892[32]	Slender blind snakes	Texas blind snake (Leptotyphlops dulcis)
Typhlopidae Merrem, 1820[36]	Blind snakes	European blind snake (Typhlops vermicularis)
Xenotyphlopidae Vidal et al., 2010[31]		Xenotyphlops grandidieri	–

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32. Cogger(1991), p.23
33. "Aniliidae". Integrated Taxonomic Information System. Retrieved 12 December 2007.
34. "Anomochilidae". Integrated Taxonomic Information System. Retrieved 13 December 2007.
35. "Atractaspididae". Integrated Taxonomic Information System. Retrieved 13 December 2007.
36. "Typhlopidae". Integrated Taxonomic Information System. Retrieved 13 December 2007.

Further reading

• Bebler, John L.; King, F. Wayne (1979). The Audubon Society Field Guide to Reptiles and Amphibians of North America. New York: Alfred A. Knopf. pp. 581. ISBN 978-0-394-50824-5.
• Capula, Massimo; Behler (1989). Simon & Schuster's Guide to Reptiles and Amphibians of the World. New York: Simon & Schuster. ISBN 978-0-671-69098-4.
• Cogger, Harold; Zweifel, Richard (1992). Reptiles & Amphibians. Sydney: Weldon Owen. ISBN 978-0-8317-2786-4.
• Conant, Roger; Collins, Joseph (1991). A Field Guide to Reptiles and Amphibians Eastern/Central North America. Boston, Massachusetts: Houghton Mifflin Company. 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)". Biological Reviews, Cambridge. 78 (4): 513–551. doi:10.1017/S1464793103006134. PMID 14700390. S2CID 4845536.
• 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). Ithaca, 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. Lecture Notes in Earth Sciences. 132. pp. 27–44. doi:10.1007/978-3-642-10311-7_2. ISBN 978-3-642-10310-0.
• Freiberg, Dr. Marcos; Walls, Jerry (1984). The World of Venomous Animals. New Jersey: TFH Publications. ISBN 978-0-87666-567-1.
• Gibbons, J. Whitfield; Gibbons, Whit (1983). Their Blood Runs Cold: Adventures With Reptiles and Amphibians. Alabama: University of Alabama Press. pp. 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 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: Simon & Schuster. p. 293. ISBN 978-0-671-47654-0.

External links

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