Snake

Snakes (from Old English snaca, and ultimately from the Proto-Indo-European base snag- or sneg-, "to crawl"), also known as ophidians, are cold blooded legless reptiles closely related to lizards, which share the order Squamata. more...

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There are also several species of legless lizard which superficially resemble snakes, but are not otherwise related to them. A love of snakes is called ophiophilia, a fear of snakes is called ophidiophobia (or snakephobia). A specialist in snakes is an ophiologist.

An old synonym for snake is serpent (which comes from Old French, and ultimately from PIE *serp-, "to creep"); in modern usage this usually refers to a mythic or symbolic snake, and information about such creatures will be found under serpent (symbolism). This article deals with the biology of snakes.

Evolution

The phylogeny of snakes is poorly known due to the fact that snake skeletons are typically small and fragile, making fossilization unlikely. It has however been generally agreed, on the basis of morphology, that snakes descended from lizard-like ancestors. Recent research based on genetics and biochemistry confirms this; snakes form a venom clade with several extant lizard families.

Recent fossil evidence suggests that snakes directly evolved from burrowing lizards, either varanids or some other group. An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, fully terrestrial. One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo, although it also is semi-aquatic. As these ancestors became more subterranean, they lost their limbs and became more streamlined for burrowing. Features such as the transparent, fused eyelids and loss of external ears, according to this hypothesis, evolved to combat subterranean conditions (scratched corneas, dirt in the ears). According to this hypothesis, snakes re-emerged onto the surface of the land much as they are today. Other primitive snakes are known to have possessed hindlimbs but lacked a direct connection of the pelvic bones to the vertebrae, including Haasiophis, Pachyrhachis and Eupodophis) which are slightly older than Najash.

Modern boas do have vestigal hind limbs, tiny, clawed digits known as anal spurs and used to grasp during mating.

The alternative hypothesis, based on morphology, suggests that ancestors were related to mosasaurs — extinct aquatic reptiles from the Cretaceous — which in turn are thought to have derived from varanid lizards. Under this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), while the external ears were lost through disuse in an aquatic environment, ultimately leading to an animal similar in appearance to sea snakes of today. In the Late Cretaceous, snakes re-colonized the land much like they are today. Fossil snake remains are known from early Late Cretateous marine sediments, which is consistent with this hypothesis, particularly as they are older than the terrestrial Najash rionegrina. Similar skull structure; reduced/absent limbs; and other anatomical features found in both mosasaurs and snakes lead to a positive cladistical correlation, though some features are also shared with varanids. Supposedly similar locomotion for both groups is also used as support for this hypothesis. Genetic studies have indicated that snakes are not especially related to monitor lizards, and (it has been claimed) therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, there is more evidence linking mosasaurs to snakes than to varanids. Fragmentary remains that have been found from the Jurassic and Early Cretaceous indicate deeper fossil records for these groups, which may eventually refute either hypothesis.

Read more at Wikipedia.org