Squamata

Scaled reptiles Fossil range: Jurassic-recent
Blotched Blue-Tongued Skink
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Sauropsida Order: Squamata Oppel, 1811
black: range of Squamata
Suborders
see text

Squamata (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 widely to accommodate comparatively large prey. They are the most variably-sized order of reptiles, ranging from the 16 mm (0.63 in) Jaragua Sphaero to the 8 m (26 ft) Green Anaconda.

Classification

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. 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
  • Infraorder Gekkota (the geckos)
  • Infraorder Anguimorpha (the monitors, Gila monster, alligator lizards, galliwasps, slow-worms and others)
  • Infraorder Scincomorpha (skinks, whiptail lizards and common European lizards)
• Suborder Serpentes (the snakes)
  • Infraorder Alethinophidia (vipers, boas, cobras, etc.)
  • Infraorder Scolecophidia (blind snakes)
• Suborder Amphisbaenia

The relationships between these suborders is not yet certain, though recent research^[1] 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^[1]:

• 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 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)
Rhineuridae Vanzolini, 1951	North American worm lizards	North American worm lizard (Rhineura floridana)
Trogonophidae Gray, 1865	Palearctic worm lizards	Checkerboard Worm Lizard (Trogonophis wiegmanni)
Diploglossa
Family	Common Names	Example Species	Example Photo
Anguidae Oppel, 1811	Glass lizards	Slow Worm (Anguis fragilis)
Anniellidae Gray, 1852	American legless lizards	California Legless Lizard (Anniella pulchra)
Xenosauridae Cope, 1866	Knob-scaled lizards	Chinese Crocodile Lizard (Shinisaurus crocodilurus)
Gekkota
Family	Common Names	Example Species	Example Photo
Dibamidae Boulenger, 1884	Blind lizards	Dibamus nicobaricum	-
Gekkonidae Gray, 1825	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	Club-tail Iguana (Hoplocercus spinosus)	-
Iguanidae	Iguanas	Marine Iguana (Amblyrhynchus cristatus)
Leiosauridae Frost et al., 2001	-	Darwin's Iguana (Diplolaemus darwinii)	-
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	Caronlina Anole (Anolis carolinensis)
Tropiduridae Frost & Etheridge, 1989	Neotropical ground lizards	(Microlophus peruvianus)
Platynota
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	-	-
Lacertidae Oppel, 1811	Wall or true lizards	Eyed Lizard (Lacerta lepida)
Scincidae Oppel, 1811	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 Bonaparte, 1831[2]	file snakes	Marine File Snake (Acrochordus granulatus)
Aniliidae Stejneger, 1907[3]	coral pipe snakes	Burrowing False Coral (Anilius scytale)
Anomochilidae Cundall, Wallach and Rossman, 1993.[4]	dwarf pipe snakes	Leonard's Pipe Snake, (Anomochilus leonardi)
Atractaspididae Günther, 1858[5]	mole vipers	Bibron's burrowing asp (Atractaspis bibroni)
Boidae Gray, 1825[2]	boas	Amazon tree boa (Corallus hortulanus)
Bolyeriidae Hoffstetter, 1946	Round Island boas	Round Island Burrowing Boa (Bolyeria multocarinata)
Colubridae Oppel, 1811[2]	colubrids	Grass Snake (Natrix natrix)
Cylindrophiidae Fitzinger, 1843	Asian pipe snakes	Red-tailed Pipe Snake (Cylindrophis ruffus)
Elapidae Boie, 1827[2]	cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids	King Cobra (Ophiophagus hannah)
Loxocemidae Cope, 1861	Mexican burrowing snakes	Mexican burrowing snake (Loxocemus bicolor)
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[2]	vipers, pitvipers, rattlesnakes	European asp (Vipera aspis)
Xenopeltidae Bonaparte, 1845	sunbeam snakes	Sunbeam snake (Xenopeltis unicolor)
Scolecophidia
Family	Common Names	Example Species	Example Photo
Anomalepidae Taylor, 1939[2]	dawn blind snakes	Dawn Blind Snake (Liotyphlops beui)
Leptotyphlopidae Stejneger, 1892[2]	slender blind snakes	Texas Blind Snake (Leptotyphlops dulcis)
Typhlopidae Merrem, 1820[6]	blind snakes	Black Blind Snake (Typhlops reticulatus)

Evolution

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 extanct archosaurs. Squamate fossils first appear in the early Jurassic, 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^[7]. Because snakes have a faster molecular clock than other squamates,^[7] and there are few early snake and snake ancestor fossils,^[8] it is difficult to resolve the relationship between snakes and other squamate groups.

Reproduction

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.^[9] 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, in order 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. ^[10]

Venom

Venom is modified saliva, delivered through fangs.^[11] The fangs of 'advanced' venomous snakes like viperids and elapids are hollow in order to inject venom more effectively, while the fangs of rear-fanged snakes such as the Boomslang merely have a groove on the posterior edge to channel venom into the wound. Snake venoms are often prey specific, its role in self-defense is secondary.^[11] Venom, like all salivary secretions, is a pre-digestant which initiates the breakdown of food into soluble compounds allowing for proper digestion and even "non-venomous" snake bites (like any animal bite) will cause tissue damage.^[12]

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 Helodermatidae, Elapidae, Viperidae, and some members of the Colubridae. However, all snakes, some agamid lizards and most monitor lizards are now believed by some to have proteins very closely related to venom.^[13][14]

Humans and Squamates

Bites and fatalities

See also: Snakebite

Map showing global distribution of snakebite morbidity.

In the US alone, more than 8,000 venomous snake bites are reported each year.^[15] It is estimated that 125,000 people a year die from venomous snake bites.^[16] In addition, large pet constrictors, like boas and pythons, have been known to kill their owners through strangulation on rare occasions. ^[17]

Lizard bites, unlike venomous snake bites, are not fatal. The Komodo dragon has been known to kill people due to its size. ^[18] The two known venomous species of lizard, the Gila monster and Mexican beaded lizard have never caused a human death by envenomation.

Conservation

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 unfair competition & 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.

References

• Bebler, John L.; King, F. Wayne (1979). The Audubon Society Field Guide to Reptiles and Amphibians of North America. Alfred A. Knopf. pp. 581. ISBN 0394508246. 
• Capula, Massimo; Behler (1989). Simon & Schuster's Guide to Reptiles and Amphibians of the World. New York: Simon & Schuster. ISBN 0671690981. 
• Cogger, Harold; Zweifel, Richard (1992). Reptiles & Amphibians. Sydney, Australia: Weldon Owen. ISBN 0831727861. 
• Conant, Roger; Collins, Joseph (1991). A Field Guide to Reptiles and Amphibians Eastern/Central North America. Boston, Massachusetts: Houghton Mifflin Company. ISBN 0395583896. 
• Ditmars, Raymond L (1933). Reptiles of the World: The Crocodilians, Lizards, Snakes, Turtles and Tortoises of the Eastern and Western Hemispheres. New York: Macmillian. pp. 321. 
• Freiberg, Dr. Marcos; Walls, Jerry (1984). The World of Venomous Animals. New Jersey: TFH Publications. ISBN 0876665679. 
• Gibbons, J. Whitfield; Gibbons, Whit (1983). Their Blood Runs Cold: Adventures With Reptiles and Amphibians. Alabama: University of Alabama Press. pp. 164. ISBN 978-0817301354. 
• McDiarmid, RW; Campbell, JA; Touré, T (1999). Snake Species of the World: A Taxonomic and Geographic Reference. 1. Herpetologists' League. pp. 511. ISBN 1893777006. 
• Mehrtens, John (1987). Living Snakes of the World in Color. New York: Sterling. ISBN 0806964618. 
• Rosenfeld, Arthur (1989). Exotic Pets. New York: Simon & Schuster. pp. 293. ISBN 067147654. 

Cited references

External links

• Squamata