this: Snake (disambiguation)

A snake is an elongate reptile of the suborder Serpentes. Like all reptiles, snakes are covered in scales. All snakes are carnivorous and can be distinguished from legless lizards by their lack of eyelids, hind limbs, external ears, and the presence of only vestigal forelimbs. The 2,700+ species of snakes spread across every continent except Antarctica ranging in size from the tiny, 10 cm long thread snake to pythons and anacondas at 9 m (30 ft) long. In order to accommodate snakes' narrow bodies, paired organs (such as kidneys) appear one in front of the other instead of side by side.

While venomous snakes comprise a minority of the species, some possess potent venom capable of causing painful injury or death to humans. However, venom in snakes is primarily for killing and subduing prey rather than for self-defense. Snakes may have evolved from a lizard which adapted to burrowing during the Cretaceous period (c 150 Ma), though some scientists have postulated an aquatic origin. The diversity of modern snakes appeared during the Paleocene period (c 66 to 56 Ma).

A literary word for snake is serpent (a Middle English word which comes from Old French, and ultimately from *serp-, "to creep" , also ερπω in Greek). The serpent is also a symbol of the healing arts.

Evolution

Phylogeny of snakes is poorly known because snake skeletons are typically small and fragile, making fossilization uncommon. However 150 million-year-old specimens readily definable as snakes with lizardlike skeletal structures have been uncovered in South America and Africa.Mehrtens (1987) p. 11 It has been agreed, on the basis of morphology, that snakes descended from lizards. Fossil evidence suggests that snakes may have evolved from burrowing lizards, such as 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. As these ancestors became more subterranean, they lost their limbs and their bodies became more streamlined for burrowing. According to this hypothesis, features such as the transparent, fused eyelids (brille) and loss of external ears evolved to combat subterranean conditions such as scratched corneas and dirt in the ears with snakes re-emerged onto the surface of the earth 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.