Temporal range: Late Triassic-recent
Eastern blue-tongued lizard Scientific classification Kingdom: Animalia Phylum: Chordata Class: Reptilia Order: Squamata
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) Jaragua Sphaero (Sphaerodactylus ariasae) to the 8-metre (26 ft) Green Anaconda (Eunectes murinus) and the now-extinct mosasaurs, which reached lengths of 14 metres (46 ft).
Squamates are a monophyletic group that is a 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 late Triassic, but a mitochondrial phylogeny suggests that 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. Because snakes have a faster molecular clock than other squamates, and there are few early snake and snake ancestor fossils, it is difficult to resolve the relationship between snakes and other squamate groups.
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. 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 the hemipenis 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, like the Komodo dragon, can actually reproduce asexually and undergo parthenogenesis.
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 families Caenophidia, Anguimorpha, and Iguania and has been shown that it evolved a single time along these lineages before the three families diverged because all lineages share nine common toxins. The fossil record does show that the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 MYA to the Late Triassic/Early Jurassic.
It has been shown that snake venom 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 venom were modifications of salivary or pancreatic proteins, but it has been discovered that different toxins have been recruited from numerous different protein bodies and are diverse as the functions themselves.
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 sit-and-wait predators the ability to attack a wide range of prey. 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.
Humans and squamates
Bites and fatalities
It is estimated that 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. In addition, large pet constrictors, like boas and pythons, have been known to kill humans through constriction on rare occasions.
Lizard bites, unlike venomous snake bites, are not fatal. The Komodo dragon has been known to kill people due to its size. The two known venomous species of lizard, the Gila monster and Mexican beaded lizard have never caused a human death by envenomation.
Even though they survived the worst changes in Earth's history, today many squamate species are endangered due to habitat loss, hunting and poaching, the pet trade, alien species being introduced to their habitat (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 & breeders educate people about the importance of snakes and lizards.
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 sub-clade 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
The relationships between these suborders is not yet certain, though recent research suggests that 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:
- 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
Amphisbaenia Family Common Names Example Species Example Photo Amphisbaenidae
Tropical worm lizards Darwin's worm lizard (Amphisbaena darwinii) - Bipedidae
Bipes worm lizards Mexican mole lizard (Bipes biporus) Rhineuridae
North American worm lizards North American worm lizard (Rhineura floridana) Trogonophidae
Palearctic worm lizards Checkerboard worm lizard (Trogonophis wiegmanni) - Anguidea or Diploglossa Family Common Names Example Species Example Photo Anguidae
Glass lizards, alligator lizards & slow worms Slow worm (Anguis fragilis) Anniellidae
American legless lizards California legless lizard (Anniella pulchra) Xenosauridae
Knob-scaled lizards Chinese crocodile lizard (Shinisaurus crocodilurus) Gekkota Family Common Names Example Species Example Photo Dibamidae
Blind lizards Dibamus nicobaricum - Gekkonidae
Geckos Thick-tailed gecko (Underwoodisaurus milii) Pygopodidae
Legless lizards Burton's snake lizard (Lialis burtonis) Iguania Family Common Names Example Species Example Photo Agamidae
Agamas Eastern bearded dragon (Pogona barbata) Chamaeleonidae
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
Wall or true lizards Ocellated lizard (Lacerta lepida) Scincidae
Skinks Western blue-tongued skink (Tiliqua occipitalis) Teiidae Tegus or whiptails Blue tegu (Tupinambis teguixin) Xantusiidae Night lizards Granite night lizard (Xantusia henshawi) Alethinophidia Family Common Names Example Species Example Photo Acrochordidae
File snakes Marine file snake (Acrochordus granulatus) Aniliidae
Coral pipe snakes Burrowing false coral (Anilius scytale) Anomochilidae
Cundall, Wallach and Rossman, 1993.
Dwarf pipe snakes Leonard's pipe snake, (Anomochilus leonardi) Atractaspididae
Mole vipers Bibron's burrowing asp (Atractaspis bibroni) Boidae
Boas Amazon tree boa (Corallus hortulanus) Bolyeriidae
Round Island boas Round Island burrowing boa (Bolyeria multocarinata) Colubridae
Colubrids Grass snake (Natrix natrix) Cylindrophiidae
Asian pipe snakes Red-tailed pipe snake (Cylindrophis ruffus) Elapidae
Cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids King cobra (Ophiophagus hannah) Loxocemidae
Mexican burrowing snakes Mexican burrowing snake (Loxocemus bicolor) Pythonidae
Pythons Ball python (Python regius) Tropidophiidae
Dwarf boas Northern eyelash boa (Trachyboa boulengeri) Uropeltidae
Shield-tailed snakes, short-tailed snakes Cuvier's shieldtail (Uropeltis ceylanica) Viperidae
Vipers, pitvipers, rattlesnakes European asp (Vipera aspis) Xenopeltidae
Sunbeam snakes Sunbeam snake (Xenopeltis unicolor) Scolecophidia Family Common Names Example Species Example Photo Anomalepidae
Dawn blind snakes Dawn blind snake (Liotyphlops beui) Leptotyphlopidae
Slender blind snakes Texas blind snake (Leptotyphlops dulcis) Typhlopidae
Blind snakes European blind snake (Typhlops vermicularis)
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Extant reptile orders by subclassKingdom: Animalia · Phylum: Chordata · Subphylum: Vertebrata Anapsida DiapsidaSphenodontia · Squamata
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