Squamata

Squamates
Temporal range: Early Jurassic–Present
Scientific classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Lepidosauria
Order: Squamata
Oppel, 1811
Subgroups

Squamata (/skwæˈmeɪtə/, Latin squamatus, 'scaly, having scales') is the largest order of reptiles, comprising lizards and snakes. With over 11,500 species, 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, and must periodically engage in molting. They also possess movable quadrate bones, making possible movement of 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. Squamates are the most variably sized living reptiles, ranging from the 16 mm (0.63 in) dwarf gecko (Sphaerodactylus ariasae) to the 6.5 m (21 ft) reticulated python (Malayopython reticulatus). The now-extinct mosasaurs reached lengths over 14 m (46 ft).

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

The holotype of Slavoia darevskii, a fossil squamate

Squamates are a monophyletic sister group to the rhynchocephalians, members of the order Rhynchocephalia. The only surviving member of the Rhynchocephalia is the tuatara. Squamata and Rhynchocephalia form the subclass Lepidosauria, which is the sister group to the 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.

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.

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. However, a 2023 study found that Cryptovaranoides most likely represents an archosauromorph with no apparent squamate affinities. The oldest unambigious fossils of Squamata date to the Middle Jurassic of the Northern Hemisphere, with the first appearance of many modern groups, including snakes, during this period.

Scientists believe crown group squamates probably originated in the Early Jurassic based on the fossil record, with the oldest unambigious fossils of squamates dating to the Middle Jurassic. Squamate morphological and ecological diversity substantially increased over the course of the Cretaceous, including the appeance of groups like iguanians and varanoids, and true snakes. Polyglyphanodontia, an extinct clade of lizards, and mosasaurs, a group of predatory marine lizards that grew to enormous sizes, also appeared in the Cretaceous. Squamates suffered a mass extinction at the Cretaceous–Paleogene (K–Pg) boundary, which wiped out polyglyphanodontians, mosasaurs, and many other distinct lineages.

The relationships of squamates is debatable. Although many of the groups originally recognized on the basis of morphology are still accepted, 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, but genetic data suggest that geckos are the earliest crown group squamates. Iguanians are now united with snakes and anguimorphs in a clade called Toxicofera. Genetic data also suggest that the various limbless groups – snakes, amphisbaenians, and dibamids – are unrelated, and instead arose independently from lizards.

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 mammalian penis. 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. The eggs in oviparous species have a parchment-like shell. The only exception is found in blind lizards and three families of geckos (Gekkonidae, Phyllodactylidae and Sphaerodactylidae), where many lay rigid and calcified eggs. Some species, such as the Komodo dragon, can reproduce asexually through parthenogenesis.

The Japanese striped snake has been studied in sexual selection.

Studies have been conducted on how sexual selection manifests itself in snakes and lizards. Snakes use a variety of tactics in acquiring mates.[dubious ] 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.

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 (cottonmouth snake) can reproduce by facultative parthenogenesis; they are capable of switching from a sexual mode of reproduction to an asexual mode. 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.

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. 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. 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. 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. The fossil record shows the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 million years ago (Mya) to the Late Triassic/Early Jurassic, but the only good fossil evidence is from the Middle Jurassic.

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

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, which leads to a diversification of toxins that allows the ambush predators the ability to attack a wide range of prey. 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.

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. 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.

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. 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. Breeding programs and wildlife parks, though, 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:

Of these, the lizards form a paraphyletic group, since the "lizards" are found in several distinct lineages, with snakes and amphisbaenians recovered as monophyletic groups nested within. Although studies of squamate relationships using molecular biology have found different relationships between some squamata lineagaes, all recent molecular studies suggest that the venomous groups are united in a venom clade. Named Toxicofera, it encompasses a majority (nearly 60%) of squamate species and includes Serpentes (snakes), Iguania (agamids, chameleons, iguanids, etc.), and Anguimorpha (monitor lizards, Gila monster, glass lizards, etc.).

One example of a modern classification of the squamates is shown below.

Squamata
Dibamia

Dibamidae

Bifurcata
Gekkota
Unidentata
Scinciformata
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
Serpentes

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
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
File snakes Marine file snake (Acrochordus granulatus)
Aniliidae
Stejneger, 1907
Coral pipe snakes Burrowing false coral (Anilius scytale)
Anomochilidae
Cundall, Wallach and Rossman, 1993.
Dwarf pipe snakes Leonard's pipe snake, (Anomochilus leonardi)
Boidae
Gray, 1825 (incl. Calabariidae)
Boas Amazon tree boa (Corallus hortulanus)
Bolyeriidae
Hoffstetter, 1946
Round Island boas Round Island burrowing boa (Bolyeria multocarinata)
Colubridae
Oppel, 1811 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
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
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)
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
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
Dawn blind snakes Dawn blind snake (Liotyphlops beui)
Gerrhopilidae
Vidal et al., 2010
Indo-Malayan blindsnakes Andaman worm snake (Gerrhopilus andamanensis)
Leptotyphlopidae
Stejneger, 1892
Slender blind snakes Texas blind snake (Leptotyphlops dulcis)
Typhlopidae
Merrem, 1820
Blind snakes European blind snake (Typhlops vermicularis)
Xenotyphlopidae
Vidal et al., 2010
Malagasy blind snakes Xenotyphlops grandidieri

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