Snakes

 Snakes 

Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes. Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads. To accommodate their narrow bodies, snakes' paired organs 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. Lizards have evolved elongate bodies without limbs or with greatly reduced limbs about twenty-five times independently via convergent evolution, leading to many lineages of legless lizards. These resemble snakes, but several common groups of legless lizards have eyelids and external ears, which snakes lack, although this rule is not universal.

Living snakes are found on every continent except Antarctica, and on most smaller land masses; exceptions include some large islands, such as Ireland, Iceland, Greenland, the Hawaiian archipelago, and the islands of New Zealand, as well as many small islands of the Atlantic and central Pacific oceans. Additionally, sea snakes are widespread throughout the Indian and Pacific oceans. More than twenty families are currently recognized, comprising about 520 genera and about 3,900 species. They range in size from the tiny, Barbados threadsnake The fossil species Titanoboa cerrejonensis was long. The diversity of modern snakes appeared during the Paleocene epoch. The oldest preserved descriptions of snakes can be found in the Brooklyn Papyrus.

Most species of snake are nonvenomous and those that have venom use it primarily to kill and subdue prey rather than for self-defense. Some possess venom that is 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, from Proto-Indo-European root 'to crawl to creep', which also gave sneak as well as Sanskrit 'snake'. The other term, serpent'', is from French, ultimately from Indo-European 'to creep', which also gave Ancient Greek ἕρπω 'I crawl'.

Evolution 

The fossil record of snakes is relatively poor because snake skeletons are typically small and fragile making fossilization uncommon. Fossils readily identifiable as snakes first appear in the fossil record during the Cretaceous period. The earliest known true snake fossils come from the marine simoliophiids, the oldest of which is the Late Cretaceous Haasiophis terrasanctus, dated to between 112 and 94 million years old.

Based on comparative anatomy, there is consensus that snakes descended from lizards. Pythons and boas—primitive groups among modern snakes—have vestigial hind limbs: tiny, clawed digits known as anal spurs, which are used to grasp during mating. Previously, snakes were a minor component of the North American fauna, but during the Miocene, the number of species and their prevalence increased dramatically with the first appearances of vipers and elapids in North America and the significant diversification of Colubridae. during the Cretaceous Period. An early fossil snake relative, 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.