Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes. Like all completely different squamates, snakes are ectothermic, amniote vertebrates lined in overlapping scales. Many species of snakes dangle skulls with various further joints than their lizard ancestors, enabling them to swallow prey good elevated than their heads (cranial kinesis). To accommodate their slim our our bodies, snakes’ paired organs (equal to kidneys) appear one in entrance of the opposite in state of affairs of facet by facet, and most dangle totally one purposeful lung. Some species withhold a pelvic girdle with a pair of vestigial claws on every facet of the sentina. Lizards dangle developed elongate our our bodies with out limbs or with vastly diminished limbs about twenty-5 situations independently by the use of convergent evolution, leading to many lineages of legless lizards. These resemble snakes, nonetheless various now not new teams of legless lizards dangle eyelids and exterior ears, which snakes lack, though this rule is now now not widespread (look Amphisbaenia, Dibamidae, and Pygopodidae).
Residing snakes are discovered on every continent excluding Antarctica, and on most smaller land plenty; exceptions embody some expansive islands, equal to Eire, Iceland, Greenland, the Hawaiian archipelago, and the islands of New Zealand, along with many small islands of the Atlantic and medial Pacific oceans. Moreover, sea snakes are in fashion at some stage all through the Indian and Pacific oceans. Round thirty households are in the mean time recognized, comprising about 520 genera and about 3,900 species. They fluctuate in measurement from the slight, 10.4 cm-long (4.1 in) Barbados threadsnake to the reticulated python of 6.95 meters (22.8 toes) in measurement. The fossil species Titanoboa cerrejonensis was as soon as 12.8 meters (42 toes) prolonged. Snakes are idea to carry developed from every burrowing or aquatic lizards, per likelihood at some stage all through the Jurassic interval, with the earliest acknowledged fossils courting to between 143 and 167 Ma up to now. The variety of well-liked snakes regarded at some stage all through the Paleocene epoch (c. 66 to 56 Ma up to now, after the Cretaceous–Paleogene extinction occasion). The oldest preserved descriptions of snakes might per likelihood perhaps successfully be discovered all through the Brooklyn Papyrus.
Most species of snake are nonvenomous and folks which dangle venom use it primarily to assassinate and subdue prey fairly then for self-defense. Some private venom that’s potent adequate to set off painful ache or demise to folks. Nonvenomous snakes every swallow prey alive or assassinate by constriction.
The English phrase snake comes from Earlier English snaca, itself from Proto-Germanic *snak-an- (cf. Germanic Schnake ‘ring snake’, Swedish snok ‘grass snake’), from Proto-Indo-European root *(s)nēg-o- ‘to maneuver to stride’, which moreover gave sneak along with Sanskrit nāgá ‘snake’. The phrase ousted adder, as adder went on to slim whereby technique, though in Earlier English næddre was as soon as the totalidade phrase for snake. The different time frame, serpent, is from French, in a roundabout plot from Indo-European *serp- ‘to stride’, which moreover gave Inclined Greek ἕρπω (hérpō) ‘I recede’.
A phylogenetic overview of well-liked snakes.
Cloak: the tree totally signifies relationships, now now not evolutionary branching situations.
The fossil chronicle of snakes is relatively miserable attributable to snake skeletons are typically small and fragile making fossilization uncommon. Fossils readily identifiable as snakes (though normally holding hind limbs) first appear all through the fossil chronicle at some stage all through the Cretaceous interval. The earliest acknowledged factual snake fossils (members of the crown neighborhood Serpentes) advance from the marine simoliophiids, the oldest of which is the Slack Cretaceous (Cenomanian age) Haasiophis terrasanctus, dated to between 112 and 94 million years primitive.
In line with comparative anatomy, there’s consensus that snakes descended from lizards. Pythons and boas—historic teams amongst well-liked snakes—dangle vestigial hind limbs: slight, clawed digits acknowledged as anal spurs, which might per likelihood perhaps even be veteran to grab all by means of mating. The households Leptotyphlopidae and Typhlopidae moreover private remnants of the pelvic girdle, displaying as horny projections when seen.
Entrance limbs are nonexistent in all acknowledged snakes. This is introduced on by the evolution of their Hox genes, controlling limb morphogenesis. The axial skeleton of the snakes’ now not new ancestor, love most completely different tetrapods, had regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower assist), sacral (pelvic), and caudal (tail) vertebrae. Early in snake evolution, the Hox gene expression all through the axial skeleton answerable for the yell of the thorax grew to change into dominant. Which performance that, the vertebrae anterior to the hindlimb buds (when present) all dangle the equal thoracic-love identification (excluding from the atlas, axis, and 1–3 neck vertebrae). In completely different phrases, most of a snake’s skeleton is a very extended thorax. Ribs are discovered solely on the thoracic vertebrae. Neck, lumbar and pelvic vertebrae are very diminished in quantity (totally 2–10 lumbar and pelvic vertebrae are present), whereas totally a short tail stays of the caudal vertebrae. Nonetheless, the tail is unexcited prolonged adequate to be of significant use in a lot of species, and is modified in some aquatic and tree-characteristic species.
Many well-liked snake teams originated at some stage all through the Paleocene, alongside the adaptive radiation of mammals following the extinction of (non-avian) dinosaurs. The enlargement of grasslands in North America moreover resulted in an explosive radiation amongst snakes. Beforehand, snakes had been a minor ingredient of the North American fauna, nonetheless at some stage all through the Miocene, the sequence of species and their prevalence elevated dramatically with the foremost appearances of vipers and elapids in North America and the very important diversification of Colubridae (together with the beginning of many well-liked genera equal to Nerodia, Lampropeltis, Pituophis, and Pantherophis).
There could also be fossil proof to counsel that snakes can also dangle developed from burrowing lizards, at some stage all through the Cretaceous Size. An early fossil snake relative, Najash rionegrina, was as soon as a two-legged burrowing bicho with a sacrum, and was as soon as completely terrestrial. One extant analog of these putative ancestors is the earless video present Lanthanotus of Borneo (though it moreover is semiaquatic). Subterranean species developed our our bodies streamlined for burrowing, and at remaining misplaced their limbs. In line with this hypothesis, factors equal to the clear, fused eyelids (brille) and lack of exterior ears developed to deal with fossorial difficulties, equal to scratched corneas and mud all through the ears. Some historic snakes are acknowledged to carry possessed hindlimbs, nonetheless their pelvic bones lacked a yell connection to the vertebrae. These embody fossil species love Haasiophis, Pachyrhachis and Eupodophis, which might per likelihood perhaps even be a runt older than Najash.
This hypothesis was as soon as bolstered in 2015 by the invention of a 113-million-three hundred and sixty 5 days-primitive fossil of a four-legged snake in Brazil that has been named Tetrapodophis amplectus. It has many snake-love factors, is tailored for burrowing and its abdomen signifies that it was as soon as preying on completely different animals. It’s miles in the mean time unsure if Tetrapodophis is a snake or however every completely different species, all through the squamate converse, as a snake-love physique has independently developed now now not now now not as fairly a bit as 26 situations. Tetrapodophis does now now not dangle distinctive snake factors in its backbone and cranium. A seek for in 2021 areas the bicho in a neighborhood of extinct marine lizards from the Cretaceous interval acknowledged as dolichosaurs and can be found what might per likelihood related to snakes.
An fully completely different hypothesis, in accordance with morphology, suggests the ancestors of snakes had been related to mosasaurs—extinct aquatic reptiles from the Cretaceous—forming the clade Pythonomorpha. In line with this hypothesis, the fused, clear eyelids of snakes are idea to carry developed to battle marine stipulations (corneal water loss by the use of osmosis), and the surface ears had been misplaced by the use of disuse in an aquatic ambiance. This in a roundabout plot resulted in an bicho the an identical to nowadays’s sea snakes. Within the Slack Cretaceous, snakes recolonized land, and persevered to diversify into nowadays’s snakes. Fossilized snake stays are acknowledged from early Slack Cretaceous marine sediments, which is in step with this hypothesis; notably so, as they’re older than the terrestrial Najash rionegrina. An equal cranium improvement, diminished or absent limbs, and completely different anatomical factors present in every mosasaurs and snakes lead to a particular cladistical correlation, though these originate of factors are shared with varanids.
Genetic experiences as of late dangle indicated snakes are literally now not as rigorously related to video present lizards as was as soon as as quickly as believed—and attributable to this actuality now to now not mosasaurs, the proposed ancestor all through the aquatic state of affairs of their evolution. Nonetheless, further proof hyperlinks mosasaurs to snakes than to varanids. Fragmented stays discovered from the Jurassic and Early Cretaceous present deeper fossil info for these teams, which might per likelihood perhaps moreover seemingly refute every hypothesis.
Genetic foundation of snake evolution
Each fossils and phylogenetic experiences present that snakes developed from lizards, subsequently the query grew to change into which genetic modifications resulted in limb loss all through the snake ancestor. Limb loss is normally very now not new in extant reptiles and has befell dozens of situations inside skinks, anguids, and completely different lizards.
In 2016, two experiences reported that limb loss in snakes is expounded with DNA mutations all through the Zone of Polarizing Train Regulatory Sequence (ZRS), a regulatory space of the sonic hedgehog gene which is severely required for limb pattern. Extra vital snakes invent now now not dangle any remnants of limbs, nonetheless basal snakes equal to pythons and boas attain dangle traces of extraordinarily diminished, vestigial hind limbs. Python embryos even dangle completely developed hind limb buds, nonetheless their later pattern is stopped by the DNA mutations all through the ZRS.
Approximate world distribution of snakes
There are about 3,900 species of snakes, ranging as a good way northward as a result of the Arctic Circle in Scandinavia and southward by the use of Australia. Snakes might per likelihood perhaps successfully be discovered on every continent excluding Antarctica, along with all through the ocean, and as extreme as 16,000 toes (4,900 m) all through the Himalayan Mountains of Asia.: 143 There are substitute islands from which snakes are absent, equal to Eire, Iceland, and New Zealand (though New Zealand’s waters are often visited by the yellow-bellied sea snake and the banded sea krait).
All well-liked snakes are grouped at some stage all through the suborder Serpentes in Linnean taxonomy, fragment of the converse Squamata, though their right placement inside squamates stays controversial.
The 2 infraorders of Serpentes are: Alethinophidia and Scolecophidia. This separation is in accordance with morphological traits and mitochondrial DNA sequence similarity. Alethinophidia is normally smash up into Henophidia and Caenophidia, with the latter consisting of “colubroid” snakes (colubrids, vipers, elapids, hydrophiids, and atractaspids) and acrochordids, whereas the opposite alethinophidian households comprise Henophidia. Whereas now now not extant nowadays, the Madtsoiidae, a family of broad, historic, python-love snakes, was as soon as spherical till 50,000 years up to now in Australia, represented by genera equal to Wonambi.