Snakes are limbless reptiles of the squamate suborder Serpentes. Closely related to lizards and the tuatara, snakes are ectothermic, amniote, air-breathing vertebrates with an elongated rope-Iike body covered by overlapping scales, and they move around using undulatory, rectilinear, concertina, sidewinding or slide-pushing locomotions, with some arboreal species even capable of short-ranged gliding flight by flaring out the ribs to flatten and widen the body for aerodynamic lift. Snakes have skulls with significantly fewer sutures and more hypermobile articulations (i.e. cranial kinesis) than their lizard relatives, enabling them to greatly widen the mouth and swallow prey much larger than their heads. To accommodate their narrow bodies, snakes' paired organs (such as kidneys) appear one in front of the other instead of side by side, and most only have one functional lung. Despite being tetrapods from a cladistic prospective, snakes lack limbs completely, although pythons and boids retain a vestigial pelvic girdle. Some lizards have also independently evolved a snake-like elongated body without limbs or with greatly reduced limbs at least twenty-five times via convergent evolution, leading to many lineages of legless lizards, who have eyelids, external ears and a fleshy non-forked tongue that snakes lack, although this rule is not universal. Snakes are mostly terrestrial with a cosmopolitan distribution, and extant species are found on every continent except Antarctica, and on most islands — exceptions include Ireland, Iceland, Greenland and the islands of New Zealand, as well as many small islands of the Atlantic and Central Pacific oceans. Additionally, secondarily aquatic groups such as the sea snakes are widespread throughout the Indo-Pacific regions, while water snakes are found worldwide. Around thirty families are currently recognized, comprising about 520 genera and about more than 4,170 species. They range in size from the tiny, 10.4 cm-long (4.1 in) Barbados threadsnake to the reticulated python up to 6.95 m (22.8 ft) in length. The fossil species Titanoboa cerrejonensis was 12.8 meters (42 ft) long. Snakes are thought to have evolved from either burrowing or aquatic lizards, perhaps during the Jurassic period, with the earliest known fossils dating to between 143 and 167 Ma ago. The diversity of modern snakes appeared during the Paleocene epoch (c. 66 to 56 Ma ago, after the Cretaceous–Paleogene extinction event). The oldest preserved descriptions of snakes can be found in the Brooklyn Medical Papyrus. Most species of snake are non-venomous, and venomous snake species use their venoms primarily to paralyze and kill prey rather than for self-defense. Snake venoms are delivered during biting via hollow/grooved fangs and contain a wide variety of toxins including neurotoxins, hemotoxins, cardiotoxins and myotoxins, and some are potent enough to cause severe injury or death to humans with just one snakebite. Non-venomous snakes typically subdue prey by constriction before either swallow the prey alive or kill it by strangulation before swallowing whole. In captivity, snakes are typically fed dead prey because live foods such as rodents may fight and injure snakes during feeding.
Etymology The English word snake comes from Old English snaca, itself from Proto-Germanic *snak-an- (cf. Swedish snok 'grass snake'), from Proto-Indo-European root *(s)nēg-o- 'to crawl to creep', which also gave sneak as well as Sanskrit nāgá 'snake'. The word ousted adder, as adder went on to narrow in meaning, though in Old English næddre was the general word for snake. The other term, serpent, is from French, ultimately from Indo-European *serp- 'to creep', which also gave Ancient Greek ἕρπω (hérpō) 'I crawl' and Sanskrit sarpá 'snake'.
Taxonomy
All modern snakes are grouped within the suborder Serpentes in Linnean taxonomy, part of the order Squamata, though their precise placement within squamates remains controversial. The two infraorders of Serpentes are Alethinophidia and Scolecophidia. This separation is based on morphological characteristics and mitochondrial DNA sequence similarity. Alethinophidia is sometimes split into Henophidia and Caenophidia, with the latter consisting of "colubroid" snakes (colubrids, vipers, elapids, hydrophiids, and atractaspids) and acrochordids, while the other alethinophidian families comprise Henophidia. While not extant today, the Madtsoiidae, a family of giant, primitive, python-like snakes, lived until 50,000 years ago in Australia, represented by genera such as Wonambi. Recent molecular studies support the monophyly of the clades of modern snakes, scolecophidians, typhlopids + anomalepidids, alethinophidians, core alethinophidians, uropeltids (Cylindrophis, Anomochilus, uropeltines), macrostomatans, booids, boids, pythonids and caenophidians.
Families
Legless lizards
While snakes are limbless reptiles, evolved from (and grouped with) lizards, there are many other species of lizards that have lost their limbs independently but which superficially look similar to snakes. These include glass snakes (including the slowworm), and amphisbaenians.
Evolution
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