Tree modified from Estes et al. (1988)
Arnold, E. N. 1984. Variation in the cloacal and hemipenial muscles of lizards and its bearing on their relationships. Pages 47-85 in M. W. J. Ferguson, ed., The structure, development and evolution of reptiles, Academic Press, London.
Camp, C. L. 1923. Classification of the lizards. Bull. Am. Mus. Nat. Hist. 48:289-481.
Estes, R. 1983. Sauria terrestria, Amphisbaenia Vol. 10A. Handbuch der Paleoherpetologie (Encyclopedia of Paleoherpetology). Gustav Fisher Verlag, Stuttgart.
Estes, R., K. de Queiroz, and J. Gauthier. 1988. Phylogenetic relationships within Squamata. IN: Phylogenetic Relationships of the Lizard Families: Essays commemorating Charles L. Camp. (R. Estes and G. Pregill, eds.). Stanford University Press: Stanford, CA.
Etheridge, R. 1967. Lizard caudal vertebrae. Copeia 1967:699-721.
Jollie, M. T. 1960. The head skeleton of the lizard. Acta Zool. 41:1-64.
Julien, R., and S. Renous-Lecuru. 1972. Variations du trajet du nerf ulnaire (ulnaris) et de l'innervation des muscles dorsaux de la jambe chez les lacertiliens (reptiles, squamates): Valeur systematique et application phylogenetique. Bull. Mus. Natl. Hist. Nat. 23:207-245.
Kluge, A. G. 1989. Progress in squamate classification "Review of Phylogenetic relationships of the lizard families: Essays commemorating Charles L. Camp". Herpetologica 45(3):368-379.
Lecuru, S. 1968. Myologie et innervation du membre anterieur des lacertiliens. Mem. Mus. Nat. d'Hist. Nat. Zool. 48:127-215.
Lecuru, S. 1968. Remarques sur le scapulo-coraco de des lacertiliens. Ann. Sci. Nat. Zool. 10:475-510.
Lecuru, S. 1968. Etude des variations morphologiques du sternum, des clavicules et de l'interclavicule des lacertiliens. Ann. Sci. Nat. Zool. 10:511-544.
Malan, M. E. 1946. Contributions to the comparative anatomy of the nasal capsule and the organ of Jacobson of the Lacertilia. Ann. Univ. Stellenbosch 24:69-137.
McDowell, S. B., Jr., and C. M. Bogert. 1954. The systematic position of Lanthanotus and the affinities of the anguinomorphan lizards. Bull. Am. Mus. Nat. Hist. 105:1-142.
Northcutt, R. G. 1978. Forebrain and midbrain organization in lizards and its phylogenetic significance. Pages 11-64 in N. Greenberg and P. D. MacLean, eds., Behavior and neurology of lizards, U.S. Department of Health, Education, and Welfare, Rockville, Maryland.
Rage, J.-C. 1982. La phylogenie des lepidosauriens (Reptilia): Une approche cladistique. C. R. Acad. Sc. Paris 294:399-402.
Renous-Lecuru, S. 1973. Morphologie comparee du carpe chez lepidosauriens actuels (rhynchocephales, lacertiliens, amphisbeniens). Gegenbaurs morph. Jahrb., Leipzig 119:727-766.
Renous, S. 1980. Developpement de l'aspect historique de la biogeographie par la superposition de deux theses: Proposition d'une hypothese phylogenetique batie selon les principes Hennigiens et theorie de la derive des continents. C. R. Soc. Biogeogr. 57:81-102.
Rieppel, O. 1988. The classification of the Squamata. Pages 261-293 in M. J. Benton, ed., The phylogeny and classification of tetrapods, Volume 1: Amphibians, reptiles, birds, Clarendon Press, Oxford.
Russell, A. P. 1988. Limb muscles in relation to lizard systematics: A reappraisal. Pages 493-568 in R. Estes and G. Pregill, eds., Phylogenetic relationships of the lizard families. Essays commemorating Charles L. Camp Stanford University Press, Stanford, California.
Schwenk, K. 1988. Comparative morphology of the lepidosaur tongue and its relevance to squamate phylogeny. Pages 569-598 in R. Estes and G. Pregill, eds., Phylogenetic relationships of the lizard families. Essays commemorating Charles L. Camp. Stanford University Press, Stanford, California.
Sukhanov, S. B. 1976. Some problems of the phylogeny and systematics of Lacertilia. Smithson. Herpetol. Inf. Serv. (38):1-15.
Townsend, T. M., A. Larson, E. Louis, and J. R. Macey. 2004. Molecular phylogenetic of Squamata: The Position of Snakes, amphisbaenians, and dibamids, and the root of the squamate tree. Systematic Biology 53(3):735-757.
Underwood, G. L. 1971. A modern appreciation of Camp's "Classification of the lizards". Pages iii-xvii in Camp's classification of the lizards, Society for the Study of Amphibians and Reptiles.
Information on the Internet
- Assembling the Tree of Life: Deep Scaly. A large-scale, collaborative effort by eight investigators at seven institutions (one Australian and six U.S. institutions) to determine the evolutionary relationships among the major lineages of squamate reptiles.
About This Page
Kevin de Queiroz
Smithsonian National Museum of Natural History, Washington, D. C., USA
Emilia P. Martins
Indiana University, Bloomington, Indiana, USA
Correspondence regarding this page should be directed to Emilia P. Martins at
Page copyright © 1996 Kevin de Queiroz and Emilia P. Martins
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This article is about the Squamata order of reptiles. For the Roman scale armour, see Lorica squamata.
The Squamata, or the scaled reptiles, are the largest recent order of reptiles, comprising all lizards and snakes. With over 10,000 species, it is also the second-largest order of extant vertebrates, after the perciform fish, and roughly equal in number to the Saurischia (one of the two major groups of dinosaurs). 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. They are 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 14 m (46 ft).
Among the other reptiles, squamates are most closely related to the tuatara, which strongly resembles lizards.
Squamates are a monophyleticsister group to the tuatara. The squamates and tuatara together are a sister group to crocodiles and birds, the extant archosaurs. Fossils of the squamate sister group, the Rhynchocephalia, appear in the Early Triassic, meaning that the lineage leading to squamates must have existed as well. Modern squamates probably originated in the mid Jurassic, when fossil relatives of geckos and skinks and snakes appear; other groups, including iguanians and varanoids, first appear in the Cretaceous period. Also appearing in the Cretaceous are the polyglyphanodonts, a lizard group of uncertain affinities, and the mosasaurs, a group of predatory, marine lizards that grew to enormous sizes. At the end of the Cretaceous, squamates suffered a major extinction at the K-T boundary, which wiped out polyglyphanodonts, mosasaurs, and a number of other groups.
The relationships of squamates have been debated. 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 DNA. From morphological data, the iguanians were long thought to be the most ancient branch of the tree; however, studies of the DNA suggest that the geckos represent the most ancient branch. Iguanians are now united with snakes and anguimorphs in a group called Toxicofera. DNA also suggests that the various limbless groups—snakes, amphisbaenians, and dibamids—are unrelated, and instead arose independently from lizards.
See also: Sexual selection in scaled reptiles
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. 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 actually reproduce asexually through parthenogenesis.
There have been studies on how sexual selection manifests itself in snakes and lizards. Snakes use a variety of tactics in acquiring mates.[dubious– discuss] 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.
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. 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.
When female sand lizards mate with two or more males, sperm competition within the females 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
Main article: Evolution of snake venom
See also: 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 Mya to the Late Triassic/Early Jurassic. But the only good fossil evidence is from the 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
See also: Snakebite
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.
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. The Gila monster and beaded lizards of North and Central America are venomous, but not deadly to humans.
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
Historically, the order Squamata has been divided into three suborders:
Of these, the lizards form a paraphyletic group, 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