This is caused by the evolution of their Hox genes, controlling limb morphogenesis. : 11 The families Leptotyphlopidae and Typhlopidae also possess remnants of the pelvic girdle, appearing as horny projections when visible.įront limbs are nonexistent in all known snakes. : 11 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. īased on comparative anatomy, there is consensus that snakes descended from lizards. The earliest known true snake fossils (members of the crown group Serpentes) come from the marine simoliophiids, the oldest of which is the Late Cretaceous ( Cenomanian age) Haasiophis terrasanctus from the West Bank, dated to between 112 and 94 million years old. Fossils readily identifiable as snakes (though often retaining hind limbs) first appear in the fossil record during the Cretaceous period. The fossil record of snakes is relatively poor because snake skeletons are typically small and fragile making fossilization uncommon. Note: the tree only indicates relationships, not evolutionary branching times. Evolution A phylogenetic overview of modern snakes. The other term, serpent, is from French, ultimately from Indo-European * serp- 'to creep', which also gave Ancient Greek ἕρπω ( hérpō) 'I crawl' and Sanskrit sarpá ‘snake’. The word ousted adder, as adder went on to narrow in meaning, though in Old English næddre was the general word for snake. Germanic Schnake 'ring snake', Swedish snok 'grass snake'), from Proto-Indo-European root * (s)nēg-o- 'to crawl to creep', which also gave sneak as well as Sanskrit nāgá 'snake'. The English word snake comes from Old English snaca, itself from Proto-Germanic * snak-an- ( cf. Nonvenomous snakes either swallow prey alive or kill by constriction. Some possess venom that is potent enough to cause painful injury or death to humans. Most species of snake are nonvenomous and those that have venom use it primarily to kill and subdue prey rather than for self-defense. The oldest preserved descriptions of snakes can be found in the Brooklyn Papyrus. The diversity of modern snakes appeared during the Paleocene epoch ( c. 66 to 56 Ma ago, after the Cretaceous–Paleogene extinction event). Snakes are thought to have evolved from either burrowing or aquatic lizards, perhaps during the Jurassic period, with the earliest known fossils dating to between 143 and 167 Ma ago. The fossil species Titanoboa cerrejonensis was 12.8 meters (42 ft) long. They range in size from the tiny, 10.4 cm-long (4.1 in) Barbados threadsnake to the reticulated python of 6.95 meters (22.8 ft) in length. Around thirty families are currently recognized, comprising about 520 genera and about 3,900 species. Additionally, sea snakes are widespread throughout the Indian and Pacific oceans. 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. These resemble snakes, but several common groups of legless lizards have eyelids and external ears, which snakes lack, although this rule is not universal (see Amphisbaenia, Dibamidae, and Pygopodidae). Lizards have independently evolved elongate bodies without limbs or with greatly reduced limbs at least twenty-five times via convergent evolution, leading to many lineages of legless lizards. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca. 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. Many species of snakes have skulls with several more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads ( cranial kinesis). Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes ( / s ɜːr ˈ p ɛ n t iː z/). Approximate world distribution of snakes, all species
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