Snake

Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with many more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads with their highly mobile jaws. 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 have only one functional lung. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca.

Living snakes are found on every continent except Antarctica and on most islands. Five main families are currently recognized, comprising 456 genera and over 2,900 species. They range in size from the tiny, 10 cm-long thread snake to pythons and anacondas of up to 7.6 metres (25 ft) in length. The recently discovered fossil Titanoboa was 15 metres (49 ft) long. Snakes are thought to have evolved from either burrowing or aquatic lizards during the Cretaceous period (c 150 Ma). The diversity of modern snakes appeared during the Paleocene period (c 66 to 56 Ma).

Most species are nonvenomous and those that have venom use it primarily to kill and subdue prey rather than for self-defense. Some possess venom potent enough to cause painful injury or death to humans. Nonvenomous snakes either swallow prey alive or kill by constriction.

Etymology
The English word snake comes from Old English snaca, itself from Proto-Germanic *snak-an- (cf. German Schnake "ring snake," Swedish snok "grass snake"), from Proto-Indo-European root *(s)nēg-o- "to crawl, 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 Greek érpo (ερπω) "I crawl."

Evolution
The fossil record of snakes is relatively poor because snake skeletons are typically small and fragile, making fossilization uncommon. However, 150 million-year-old specimens, readily identifiable as snakes, yet with lizard-like skeletal structures, have been uncovered in South America and Africa. There is consensus, on the basis of comparative anatomy, that snakes descended from lizards. Fossil evidence suggests that snakes may have evolved from burrowing lizards, such as the varanids or a similar group during the Cretaceous Period. An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, and was fully terrestrial. One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo, although it also is semiaquatic. Subterranean forms evolved bodies that were streamlined for burrowing and lost their limbs. According to this hypothesis, features such as the transparent, fused eyelids (brille) and loss of external ears evolved to cope with fossorial difficulies, such as scratched corneas and dirt in the ears. Some primitive snakes are known to have possessed hindlimbs, but their pelvic bones lacked a direct connection to the vertebrae. These include fossil species like Haasiophis, Pachyrhachis and Eupodophis, which are slightly older than Najash.

Primitive groups among the modern snakes, pythons and boas, have vestigial hind limbs; tiny, clawed digits known as anal spurs, which they use to grasp during mating. Typhlopidae is another group where remnants of the pelvic girdle are present, sometimes appearing as horny projections when visible. The frontal limbs are nonexistent in all snakes, and this loss is associated with the evolution of the Hox genes controlling limb morphogenesis. The axial skeleton of the snakes' common ancestor, like most other tetrapods, had regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic) and caudal (tail) vertebrae. The Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant early in snake evolution and as a result, the vertebrae anterior to the hindlimb buds (when present) all have the same thoracic-like identity (except from the atlas, axis and one to three neck vertebrae), making most of the snake's skeleton being composed of an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. The neck, lumbar and pelvic vertebrae are very reduced in number (only two to ten lumbar and pelvic vertebrae are still present), while only a short tail remains of the caudal vertebrae, although the tail is still long enough to be of good use in many species, and is modified in some aquatic and tree dwelling species.

An alternative hypothesis, based on morphology, suggests the ancestors of snakes 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 recolonized land to appear as they are today. Fossil snake remains are known from early Late Cretaceous 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, although some of these features are shared with varanids. In recent years, genetic studies have indicated snakes are not as closely related to monitor lizards as it was once believed, and 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.

The great diversity of modern snakes appeared in the Paleocene, correlating with the adaptive radiation of mammals following the extinction of the nonavian dinosaurs. One of the more common groups today, the colubrids, became particularly diverse due to their preying on rodents, a mammal group that has been particularly successful. There are over 2,900 species of snakes ranging as far northward as the Arctic Circle in Scandinavia and southward through Australia and Tasmania. Snakes can be found on every continent (with the exception of Antarctica), dwelling in the sea, and as high as 16,000 feet (4,900 m)in the Himalayan Mountains of Asia. There are numerous islands from which snakes are absent, such as Ireland, Iceland, and New Zealand.