this: Snake (disambiguation) 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. Like lizards, from which they evolved, they have loosely articulated skulls, and most can dislocate their lower jaw in order to swallow prey much larger than their own head. In order to accommodate their narrow bodies, snakes' paired organs (such as kidneys) appear one in front of the other instead of side by side, and they 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. Fifteen families are currently recognized comprising 456 genera and over 2,900 species. Colubridae species list at the TIGR Reptile Database. Accessed 4 December 2008. They range in size from the tiny, 10 cm long thread snake to pythons and anacondas of up to 7.6 m (25 ft) in length. The recently discovered fossil Titanoboa was 13 m or 43 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 non-venomous 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.

Etymology

The word snake comes to English from the Proto-Germanic: *snēk-a- m., originating in the Proto-Indo-European *(s)nēg-o- and has a cognate in the Sanskrit nāgá- m. 'snake' and the English verb sneak. The word serpent comes from Old French, and ultimately from *serp-, "to creep" , also ερπω in Greek.

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.Mehrtens JM. 1987. Living Snakes of the World in Color. New York: Sterling Publishers. 480 pp. ISBN 0-8069-6460-X.rp: 11 There is consensus, on the basis of comparative anatomy, that snakes descended from lizards. rp: 11 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 semi-aquatic. 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 lack a direct connection to the vertebrae. These include fossil species like Haasiophis, Pachyrhachis and Eupodophis, which are slightly older than Najash.