Squamata (/skwæˈmeɪtə/,Latinsquamatus, 'scaly, having scales') is the largestorder ofreptiles, comprisinglizards andsnakes. With over 11,991species,[3] it is also the second-largest order ofextant (living)vertebrates, after theperciform fish.Squamates are distinguished by their skins, which bear hornyscales or shields, and must periodically engage inmolting. They also possess movablequadrate bones, making possible movement of theupper jaw relative to theneurocranium. This is particularly visible in snakes, which are able to open their mouths very widely 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-extinctmosasaurs reached lengths over 14 m (46 ft).
Among other reptiles, squamates are most closely related to thetuatara, the last surviving member of the once diverseRhynchocephalia, with both groups being placed in the cladeLepidosauria.
Squamates are amonophyleticsister group to therhynchocephalians, members of the order Rhynchocephalia. The only surviving member of the Rhynchocephalia is thetuatara. Squamata and Rhynchocephalia form the superorderLepidosauria, which is the sister group to theArchosauria, theclade that contains crocodiles and birds, and their extinct relatives.Fossils of rhynchocephalians first appear in theEarly Triassic, meaning that the lineage leading to squamates must have also existed at the time.[4][5]
A study in 2018 found thatMegachirella, an extinct genus oflepidosaurs that lived about 240 million years ago during theMiddle Triassic, was astem-squamate, making it the oldest known squamate. Thephylogenetic analysis was conducted by performing high-resolutionmicrofocus X-ray computed tomography (micro-CT) scans on the fossil specimen ofMegachirella to gather detailed data about itsanatomy. These data were then compared with a phylogenetic dataset combining the morphological andmolecular data of 129 extant and extinct reptiliantaxa. The comparison revealedMegachirella had certain features that are unique to squamates. The study also found that geckos are the earliest crown group squamates, not iguanians.[6][7] However, a 2021 study found the genus to be a lepidosaur of uncertain position, in apolytomy with Squamata andRhynchocephalia.[8]
In 2022, the extinct genusCryptovaranoides was described from theLate Triassic (Rhaetian age) ofEngland as a highly derived squamate belonging to the groupAnguimorpha, which contains many extant lineages such asmonitor lizards,beaded lizards andanguids. The presence of an essentially moderncrown group squamate so far back in time was unexpected, as their diversification was previously thought to have occurred during the Jurassic and Cretaceous.[9] A 2023 study found thatCryptovaranoides most likely represents anarchosauromorph with no apparent squamate affinities,[10] though the original describers maintained their original conclusion that this taxon represents a squamate.[11] The oldest unambiguous fossils of Squamata date to theBathonian age of the Middle Jurassic of the Northern Hemisphere,[1] with the first appearance of many modern groups, including snakes, during this period.[12]
Scientists believecrown group squamates probably originated in theEarly Jurassic based on the fossil record,[4] with the oldest unambiguous fossils of squamates dating to the Middle Jurassic.[1] Squamate morphological and ecological diversity substantially increased over the course of theCretaceous,[12] including the appeance of groups likeiguanians andvaranoids, and true snakes.Polyglyphanodontia, an extinct clade of lizards, andmosasaurs, a group of predatory marine lizards that grew to enormous sizes, also appeared in the Cretaceous.[13] Squamates suffered a mass extinction at theCretaceous–Paleogene (K–Pg) boundary, which wiped out polyglyphanodontians, mosasaurs, and many other distinct lineages.[14]
The relationships of squamates are 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 theirgenomes. Iguanians were long thought to be the earliest crown group squamates based onmorphological data,[13] butgenetic data suggest that geckos are the earliest crown group squamates.[15] Iguanians are now united with snakes andanguimorphs in a clade calledToxicofera. Genetic data also suggest that the various limbless groups – snakes,amphisbaenians, anddibamids – are unrelated, and instead arose independently from lizards.
The male members of the group Squamata havehemipenes, which are usually held inverted within their bodies, and are everted for reproduction viaerectile tissue like that in the mammalianpenis.[16] Only one is used at a time, and some evidence indicates that males alternate use betweencopulations. The hemipenis has a variety of shapes, depending on the species. Often itbears 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 ofsperm, but rather a seminal groove that seals as the erectile tissue expands. This is also the only reptile group in which bothviviparous andovoviviparous species are found, as well as the usualoviparous reptiles. The eggs in oviparous species have a parchment-like shell. The only exception is found inblind lizards and three families ofgeckos (Gekkonidae, Phyllodactylidae and Sphaerodactylidae), where many lay rigid and calcified eggs.[17][18] Some species, such as theKomodo dragon, can reproduceasexually throughparthenogenesis.[19]
The Japanese striped snake has been studied in sexual selection.
Studies have been conducted on howsexual selection manifests itself in snakes andlizards. Snakes use a variety of tactics in acquiring mates.[20][dubious –discuss] Ritual combat between males for the females with which they want tomate includes topping, a behavior exhibited by mostviperids, 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.[21]
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) andAgkistrodon piscivorus (cottonmouth snake) can reproduce by facultative parthenogenesis; they are capable of switching from a sexual mode of reproduction to an asexual mode.[22] The type of parthenogenesis that likely occurs is automixis with terminal fusion (see figure), a process in which two terminal products from the samemeiosis fuse to form a diploidzygote. This process leads to genome-widehomozygosity, expression of deleterious recessive alleles, and often to developmental abnormalities. Both captive-born and wild-bornA. contortrix andA. piscivorus appear to be capable of this form of parthenogenesis.[22]
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.[23] The WW females are likely produced by terminal automixis.
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.[24] 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.[24] This preference may enhance the fitness of progeny by reducinginbreeding depression.
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 calledToxicofera. Venom has been known in the cladesCaenophidia,Anguimorpha, andIguania, 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.[25] The fossil record shows the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 million years ago (Mya) to theLate Triassic/Early Jurassic,[25] but the only good fossil evidence is from the Middle Jurassic.[26]
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.[27] Previous literature hypothesized that venoms were modifications of salivary or pancreatic proteins,[28] but different toxins have been found to have been recruited from numerous different protein bodies and are as diverse as their functions.[29]
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 venomproteome, they form large, multigene families and evolve via the birth-and-death model of protein evolution,[30] which leads to a diversification of toxins that allows the ambush predators the ability to attack a wide range of prey.[31] The rapid evolution and diversification is thought to be the result of a predator–preyevolutionary arms race, where both are adapting to counter the other.[32]
Map showing the global distribution of venomous snakebites
An estimated 125,000 people a year die from venomous snake bites.[33] 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.[34][35]
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.[36] TheGila monster andbeaded lizards of North and Central America are venomous, but not deadly to humans.
Though they survived theCretaceous–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 becomeextinct, with Africa having themost 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.
Of these, the lizards form aparaphyletic group,[37] 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 lineages, all recent molecular studies[25] 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.), andAnguimorpha (monitor lizards, Gila monster, glass lizards, etc.).[25]
One example of a modern classification of the squamates is shown below.[2][38]
^Simōes, Tiago R.; Caldwell, Michael W.; Talanda, Mateusz; Bernardi, Massimo; Palci, Alessandro; Vernygora, Oksana; Bernardini, Federico; Mancini, Lucia; Nydam, Randall L. (30 May 2018). "The origin of squamates revealed by a Middle Triassic lizard from the Italian Alps".Nature.557 (7707):706–709.Bibcode:2018Natur.557..706S.doi:10.1038/s41586-018-0093-3.PMID29849156.S2CID44108416.
^abGauthier, Jacques; Kearney, Maureen; Maisano, Jessica Anderson; Rieppel, Olivier; Behlke, Adam D. B. (April 2012). "Assembling the squamate tree of life: perspectives from the phenotype and the fossil record".Bulletin of the Peabody Museum of Natural History.53:3–308.doi:10.3374/014.053.0101.S2CID86355757.
^Shine, Richard; Langkilde, Tracy; Mason, Robert T (2004). "Courtship tactics in garter snakes: How do a male's morphology and behaviour influence his mating success?".Animal Behaviour.67 (3):477–83.doi:10.1016/j.anbehav.2003.05.007.S2CID4830666.
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 (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.Bibcode:2010LNES..132...27E.doi:10.1007/978-3-642-10311-7_2.ISBN978-3-642-10310-0.
McDiarmid, RW; Campbell, JA; Touré, T (1999).Snake Species of the World: A Taxonomic and Geographic Reference. Vol. 1. Herpetologists' League. p. 511.ISBN978-1-893777-00-2.
.svg)
.jpg)
.jpg)
.jpg)
.jpg)
_(Lacerta_agilis).jpg)
.jpg)
_(Varanus_exanthematicus).png)
_(Chamaeleo_calyptratus).jpg)
.jpg)
_1977%2c_Cyclura_cornuta.jpg)
_flipped.jpg)
.jpg)
.jpg)
.jpg)
_(8636512143).jpg)
_Nosy_Komba.jpg)
.jpg)
.jpg)
.jpg)
_close-up_(13929278050).jpg)
.jpg)
_(8577519420).jpg)
