lanthanotus borneensis

Squamata

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:

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:

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 (agamas, chameleons, iguanas, 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
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)
Atractaspididae
Günther, 1858

mole vipers Bibron's burrowing asp (Atractaspis bibroni)
Boidae
Gray, 1825
boas Amazon tree boa (Corallus hortulanus)
Bolyeriidae
Hoffstetter, 1946
Round Island boas Round Island Burrowing Boa (Bolyeria multocarinata)
Colubridae
Oppel, 1811
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)
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
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
dawn blind snakes Dawn Blind Snake (Liotyphlops beui)
Leptotyphlopidae
Stejneger, 1892
slender blind snakes Texas Blind Snake (Leptotyphlops dulcis)
Typhlopidae
Merrem, 1820
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. 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.

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. 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.

Venom

Venom is modified saliva, delivered through fangs. 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. 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. 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.

Humans and Squamates

Bites and fatalities

In the US alone, more than 8,000 venomous snake bites are reported each year. It is estimated that 125,000 people a year die from venomous snake bites. However, large pet constrictors, like boas and pythons, have been known to kill their owners through strangulation 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.

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

Cited references

External links

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