Snake

Snakes are elongated, . Like any other squamates, snakes are ectothermic, amniote vertebrates spoke in overlapping scales. Many generation of snakes hold skulls with several more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads cranial kinesis. To accommodate their narrow bodies, snakes' paired organs such(a) as kidneysone in front of a other instead of side by side, as living as most work only one functional lung. Some types retain the 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 approximately twenty-five times independently via convergent evolution, main to numerous lineages of legless lizards. These resemble snakes, but several common groups of legless lizards have eyelids together with external ears, which snakes lack, although this advice is non universal see Amphisbaenia, Dibamidae, in addition to Pygopodidae.

Living snakes are found on every continent except Antarctica, and on most smaller land masses; exceptions add some large islands, such(a) as Ireland, Iceland, Greenland, the Barbados threadsnake to the Titanoboa cerrejonensis was 12.8 meters 42 ft long. Snakes are thought to have evolved from either burrowing or aquatic lizards, perhaps during the Ma ago. The diversity of sophisticated snakes appeared during the Cretaceous–Paleogene extinction event. 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.

Evolution

The fossil record of snakes is relatively poor because snake skeletons are typically small and fragile devloping fossilization uncommon. Fossils readily identifiable as snakes though often retaining hind limbs first appear in the fossil record during the Cretaceous period. The earliest so-called 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, dated to between 112 and 94 million years old.

Based on Pythons and Leptotyphlopidae and Typhlopidae also possess remnants of the pelvic girdle, appearing as horny projections when visible.

Front limbs are nonexistent in all call snakes. This is caused by the evolution of their Hox genes, controlling limb morphogenesis. The axial skeleton of the snakes' common ancestor, like almost other tetrapods, had regional specializations consisting of cervical neck, thoracic chest, lumbar lower back, sacral pelvic, and caudal tail vertebrae. Early in snake evolution, the Hox gene expression in the axial skeleton responsible for the developing of the thorax became dominant. As a result, the vertebrae anterior to the hindlimb buds when presented all have the same thoracic-like identity except from the atlas, axis, and 1–3 neck vertebrae. In other words, most of a snake's skeleton is an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. Neck, lumbar and pelvic vertebrae are very reduced in number only 2–10 lumbar and pelvic vertebrae are present, while only a short tail continues of the caudal vertebrae. However, the tail is still long enough to be of important usage in many species, and is modified in some aquatic and tree-dwelling species.

Many modern snake groups originated during the Paleocene, alongside the adaptive radiation of mammals coming after or as a written of. the extinction of non-avian dinosaurs. The expansion of grasslands in North America also led to an explosive radiation among snakes. Previously, snakes were a minor element 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 including the origin of many modern genera such as Nerodia, Lampropeltis, Pituophis, and Pantherophis.

There is fossil evidence tothat snakes may have evolved from burrowing lizards, during the Subterranean species evolved bodies streamlined for burrowing, and eventually 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 difficulties, such(a) as scratched corneas and dirt in the ears. Some primitive snakes are known to have possessed hindlimbs, but their pelvic bones lacked a direct link to the vertebrae. These include fossil species like Haasiophis, Pachyrhachis and Eupodophis, which are slightly older than Najash.

This hypothesis was strengthened in 2015 by the discovery of a 113-million-year-old fossil of a four-legged snake in Brazil that has been named Tetrapodophis amplectus. It has many snake-like features, is adapted for burrowing and its stomach indicates that it was preying on other animals. this is the currently uncertain if Tetrapodophis is a snake or another species, in the squamate order, as a snake-like body has independently evolved at least 26 times. Tetrapodophis does not have distinctive snake qualifications in its spine and skull. A examine in 2021 places the animal in a multinational of extinct marine lizards from the Cretaceous period known as dolichosaurs and not directly related to snakes.

An selection hypothesis, based on ]

Genetic studies in recent years have referenced snakes are not as closely related to monitor lizards as was once believed—and therefore not to mosasaurs, the presented ancestor in the aquatic scenario of their evolution. However, more evidence links mosasaurs to snakes than to varanids. Fragmented maintains found from the Jurassic and Early Cretaceous indicate deeper fossil records for these groups, which may potentially refute either hypothesis.

Tetrapodophis

Eupodophis descouensi

Eupodophis descouensi hind leg

Both fossils and phylogenetic studiesthat snakes evolved from lizards, hence the question became which genetic revise led to limb destruction in the snake ancestor. Limb loss is actually very common in extant reptiles and has happened dozens of times within skinks, anguids, and other lizards.

In 2016, two studies reported that limb loss in snakes is associated with DNA mutations in the Zone of Polarizing Activity Regulatory Sequence ZRS, a regulatory region of the sonic hedgehog gene which is critically required for limb development. More advanced snakes have no remnants of limbs, but basal snakes such as pythons and boas do have traces of highly reduced, vestigial hind limbs. Python embryos even have fully developed hind limb buds, but their later development is stopped by the DNA mutations in the ZRS.