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47-497: Snakes are elongated, limbless reptiles of the suborder Serpentes ( / s ɜːr ˈ p ɛ n t iː z / ). 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 ( cranial kinesis ). To accommodate their narrow bodies, snakes' paired organs (such as kidneys) appear one in front of

94-461: A cohors (plural cohortes ). Some of the plant families still retain the names of Linnaean "natural orders" or even the names of pre-Linnaean natural groups recognized by Linnaeus as orders in his natural classification (e.g. Palmae or Labiatae ). Such names are known as descriptive family names. In the field of zoology , the Linnaean orders were used more consistently. That is,

141-570: A distinct rank of biological classification having its own distinctive name (and not just called a higher genus ( genus summum )) was first introduced by the German botanist Augustus Quirinus Rivinus in his classification of plants that appeared in a series of treatises in the 1690s. Carl Linnaeus was the first to apply it consistently to the division of all three kingdoms of nature (then minerals , plants , and animals ) in his Systema Naturae (1735, 1st. Ed.). For plants, Linnaeus' orders in

188-712: A family of giant, primitive, python-like snakes, was around until 50,000 years ago in Australia, represented by genera such as Wonambi . Recent molecular studies support the monophyly of the clades of modern snakes, scolecophidians, typhlopids + anomalepidids, alethinophidians, core alethinophidians, uropeltids ( Cylindrophis , Anomochilus , uropeltines), macrostomatans, booids, boids, pythonids and caenophidians. While snakes are limbless reptiles, evolved from (and grouped with) lizards, there are many other species of lizards that have lost their limbs independently but which superficially look similar to snakes. These include

235-470: A group of related families. What does and does not belong to each order is determined by a taxonomist , as is whether a particular order should be recognized at all. Often there is no exact agreement, with different taxonomists each taking a different position. There are no hard rules that a taxonomist needs to follow in describing or recognizing an order. Some taxa are accepted almost universally, while others are recognized only rarely. The name of an order

282-659: 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 (including the origin of many modern genera such as Nerodia , Lampropeltis , Pituophis , and Pantherophis ). There is fossil evidence to suggest that snakes may have evolved from burrowing lizards, during

329-413: A positive cladistical correlation, although some of these features are shared with varanids. Genetic studies in recent years have indicated snakes are not as closely related to monitor lizards as was once believed—and therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, more evidence links mosasaurs to snakes than to varanids. Fragmented remains found from

376-518: A short tail remains of the caudal vertebrae. However, the tail is still long enough to be of important use 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 following the extinction of (non-avian) dinosaurs . The expansion of grasslands in North America also led to an explosive radiation among snakes. Previously, snakes were

423-420: Is a finer one, barely visible; the cavities are connected internally, separated only by a membrane with nerves that are extraordinarily attuned to detecting temperature changes between. As in the overlapping vision fields of human eyes, the forward-facing pit on either side of the face combined produces a field of vision: a pit viper can distinguish between objects and their environments, as well as accurately judge

470-412: Is adapted for burrowing and its stomach indicates that it was preying on other animals. It is 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 features in its spine and skull. A study in 2021 places the animal in a group of extinct marine lizards from

517-476: 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

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564-460: Is based on morphological characteristics and mitochondrial DNA sequence similarity. Alethinophidia is sometimes split into Henophidia and Caenophidia , with the latter consisting of "colubroid" snakes ( colubrids , vipers , elapids , hydrophiids , and atractaspids ) and acrochordids, while the other alethinophidian families comprise Henophidia. While not extant today, the Madtsoiidae ,

611-458: Is not universal (see Amphisbaenia , Dibamidae , and Pygopodidae ). 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, 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

658-486: Is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy . It is classified between family and class . In biological classification , the order is a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes . An immediately higher rank, superorder , is sometimes added directly above order, with suborder directly beneath order. An order can also be defined as

705-623: Is potent enough to cause painful injury or death to humans. Nonvenomous snakes either swallow prey alive or kill by constriction . The English word snake comes from Old 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 crawl to creep', which also gave sneak as well as Sanskrit nāgá 'snake'. The word ousted adder , as adder went on to narrow in meaning, though in Old English næddre

752-584: Is relatively poor because snake skeletons are typically small and fragile making fossilization uncommon. Fossils readily identifiable as snakes (though often retaining hind limbs) first appear in the fossil record during the Cretaceous period. 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

799-805: Is stopped by the DNA mutations in the ZRS. There are about 3,900 species of snakes, ranging as far northward as the Arctic Circle in Scandinavia and southward through Australia. Snakes can be found on every continent except Antarctica, as well as in the sea, and as high as 16,000 feet (4,900 m) in the Himalayan Mountains of Asia. There are numerous islands from which snakes are absent, such as Ireland , Iceland , and New Zealand (although New Zealand's northern waters are infrequently visited by

846-533: Is usually written with a capital letter. For some groups of organisms, their orders may follow consistent naming schemes . Orders of plants , fungi , and algae use the suffix -ales (e.g. Dictyotales ). Orders of birds and fishes use the Latin suffix -iformes meaning 'having the form of' (e.g. Passeriformes ), but orders of mammals and invertebrates are not so consistent (e.g. Artiodactyla , Actiniaria , Primates ). For some clades covered by

893-477: The Cretaceous Period . An early fossil snake relative, Najash rionegrina , was a two-legged burrowing animal with a sacrum , and was fully terrestrial . Najash , which lived 95 million years ago, also had a skull with several features typical for lizards, but had evolved some of the mobile skull joints that define the flexible skull in most modern snakes. The species did not show any resemblances to

940-528: The International Code of Zoological Nomenclature , several additional classifications are sometimes used, although not all of these are officially recognized. In their 1997 classification of mammals , McKenna and Bell used two extra levels between superorder and order: grandorder and mirorder . Michael Novacek (1986) inserted them at the same position. Michael Benton (2005) inserted them between superorder and magnorder instead. This position

987-555: The Jurassic and Early Cretaceous indicate deeper fossil records for these groups, which may potentially refute either hypothesis. Both fossils and phylogenetic studies demonstrate that snakes evolved from lizards , hence the question became which genetic changes led to limb loss 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

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1034-630: The Jurassic period, with the earliest known fossils dating to between 143 and 167  Ma ago. The diversity of modern snakes appeared during the Paleocene epoch ( c.  66 to 56 Ma ago, after 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

1081-815: The Systema Naturae and the Species Plantarum were strictly artificial, introduced to subdivide the artificial classes into more comprehensible smaller groups. When the word ordo was first consistently used for natural units of plants, in 19th-century works such as the Prodromus Systematis Naturalis Regni Vegetabilis of Augustin Pyramus de Candolle and the Genera Plantarum of Bentham & Hooker, it indicated taxa that are now given

1128-524: The West Bank , dated to between 112 and 94 million years old. Based on genomic analysis it is certain that snakes descend from lizards . This conclusion is also supported by comparative anatomy , and the fossil record. 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. The families Leptotyphlopidae and Typhlopidae also possess remnants of

1175-478: The slowworm , glass snake , and amphisbaenians . Leptotyphlopidae Gerrhopilidae Typhlopidae Xenophidiidae Anomalepididae Aniliidae Tropidophiidae Xenopeltidae Loxocemidae Pythonidae Boidae Bolyeridae Xenophidiidae Uropeltidae Anomochilidae Cylindrophiidae Acrochordidae Xenodermidae Pareidae Viperidae Homalopsidae Colubridae Lamprophiidae Elapidae The fossil record of snakes

1222-430: The yellow-bellied sea snake and the banded sea krait ). The now extinct Titanoboa cerrejonensis was 12.8 m (42 ft) in length. By comparison, the largest extant snakes are the reticulated python , measuring about 6.95 m (22.8 ft) long, and the green anaconda , which measures about 5.21 m (17.1 ft) long and is considered the heaviest snake on Earth at 97.5 kg (215 lb). At

1269-506: The Cretaceous period known as dolichosaurs and not directly related to snakes. An alternative hypothesis, based on morphology , suggests the ancestors of snakes were related to mosasaurs —extinct aquatic reptiles from the Cretaceous —forming the clade Pythonomorpha . According to this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), and

1316-584: The Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant. As a result, the vertebrae anterior to the hindlimb buds (when present) 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

1363-523: The Indian and Pacific oceans. Around thirty families are currently recognized, comprising about 520 genera and about 3,900 species . 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. The fossil species Titanoboa cerrejonensis was 12.8 meters (42 ft) long. Snakes are thought to have evolved from either burrowing or aquatic lizards, perhaps during

1410-452: The distance between objects and itself. The heat sensing ability of a pit viper is so great that it can react to a difference as small as one third of a degree Fahrenheit. Other infrared-sensitive snakes have multiple, smaller labial pits lining the upper lip, just below the nostrils. Limbless vertebrate The jawless fish ( hagfish and lamprey ) do not have appendage-like fins. They may not have lost them, but rather, simply retained

1457-528: The ears. Some primitive snakes are known to have possessed hindlimbs, but their pelvic bones lacked a direct connection 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,

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1504-621: The external ears were lost through disuse in an aquatic environment. This ultimately led to an animal similar to today's sea snakes . In the Late Cretaceous , snakes recolonized land, and continued to diversify into today's snakes. Fossilized snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis; particularly so, as they are older than the terrestrial Najash rionegrina . Similar skull structure, reduced or absent limbs, and other anatomical features found in both mosasaurs and snakes lead to

1551-484: The extinct moa and elephant birds . The moa in particular are notable for having completely lost their wings, without even vestigial wings remaining outside their bodies. Despite its name, the finless porpoise has two fins. Legless forms of reptiles and amphibians probably evolved so as to be able to move underground or in water more easily. Some analyses suggest that elongation and undulatory locomotion (slithering) evolved first, before limb loss. The debate about

1598-1023: The families Pygopodidae and Dibamidae and species of Anguis , Isopachys , and Ophisaurus ) . Several species of legless lizards have tiny useless legs, such as pygopodids which retain rudimentary flaps. Contrarily, the worm lizard Bipes as its scientific name suggests has two stubby forelimbs which actually assist in digging similar to a mole. All other amphisbaenians have reduced or absent forelimb girdles. Larval amphibians, tadpoles, are often limbless. Adult amphibians have also evolved limblessness multiple times – caecilians , Sirenidae (a clade of salamanders that are limbless except for atrophied front limbs), Amphiumidae (a clade of salamander with extremely atrophied limbs that appear non-functional) and at least three extinct groups ( Aïstopoda , Lysorophia , and Adelospondyli ). There are no known limbless species of mammal or bird, although partial limb-loss and reduction has occurred in several groups, including whales and dolphins , sirenians , kiwis , and

1645-404: The form that vertebrates had before the evolution of limbs. There are also a number of fish with elongated bodies that have no fins or reduced appendage-like fins, for example eels and swamp eels . Reptiles have on a number of occasions evolved into limbless forms – snakes , amphisbaenians , and legless lizards (limb loss in lizards has evolved independently several times, examples include

1692-437: The head, between the nostrils and the eyes. In fact the pit looks like an extra pair of nostrils. All snakes have the ability to sense warmth with touch and heat receptors like other animals ;however, the highly developed pit of the pit vipers is distinctive. Each pit is made of a pit cavity and an inner cavity, the larger one lies just behind and generally below the level of the nostril, and opens forward. Behind this larger cavity

1739-542: The modern burrowing blind snakes, which have often been seen as the most primitive group of extant forms. One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo (though it is also semiaquatic ). 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 as scratched corneas and dirt in

1786-708: The orders in the zoology part of the Systema Naturae refer to natural groups. Some of his ordinal names are still in use, e.g. Lepidoptera (moths and butterflies) and Diptera (flies, mosquitoes, midges, and gnats). In virology , the International Committee on Taxonomy of Viruses 's virus classification includes fifteen taxomomic ranks to be applied for viruses , viroids and satellite nucleic acids : realm , subrealm , kingdom , subkingdom, phylum , subphylum , class, subclass, order, suborder, family, subfamily , genus, subgenus , and species. There are currently fourteen viral orders, each ending in

1833-472: The origin of limblessness led to a temporary hypothesis about a marine origin for snakes, which is no longer favored since the discovery of snake fossils with hindlimbs. In the case of limb loss during evolution, vestigial structures testify to this change (remains of the pelvis, rudimentary femur or spurs in boas , pythons and Typhlops ). The evolutionary process of transforming quadrupedal lizards into legless forms results in three main characteristics:

1880-573: The other end of the scale, the smallest extant snake is Leptotyphlops carlae , with a length of about 10.4 cm (4.1 in). Most snakes are fairly small animals, approximately 1 m (3.3 ft) in length. Some of the most highly developed sensory systems are found in the Crotalidae, or pit vipers—the rattlesnakes and their associates. Pit vipers have all the sense organs of other snakes, as well as additional aids. Pit refers to special infrared-sensitive receptors located on either side of

1927-514: 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 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 . These resemble snakes, but several common groups of legless lizards have eyelids and external ears, which snakes lack, although this rule

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1974-451: The pelvic girdle, appearing as horny projections when visible. Front limbs are nonexistent in all known snakes. This is caused by the evolution of their Hox genes , controlling limb morphogenesis . The axial skeleton of the snakes' common ancestor, like most other tetrapods, had regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic), and caudal (tail) vertebrae. Early in snake evolution,

2021-564: The precursor of the currently used International Code of Nomenclature for algae, fungi, and plants . In the first international Rules of botanical nomenclature from the International Botanical Congress of 1905, the word family ( familia ) was assigned to the rank indicated by the French famille , while order ( ordo ) was reserved for a higher rank, for what in the 19th century had often been named

2068-502: The rank of family (see ordo naturalis , ' natural order '). In French botanical publications, from Michel Adanson 's Familles naturelles des plantes (1763) and until the end of the 19th century, the word famille (plural: familles ) was used as a French equivalent for this Latin ordo . This equivalence was explicitly stated in the Alphonse Pyramus de Candolle 's Lois de la nomenclature botanique (1868),

2115-421: The regression of the limbs is carried out gradually, via the reduction in their size and the reduction in the number of phalanges or fingers; the multiplication of the vertebrae (up to 600 in some snakes) induces a lengthening and a gain in flexibility of the trunk; and the vertebral axis is homogenized from the neck to the cloaca, evoking an interminable ribcage. Infraorder Order ( Latin : ordo )

2162-418: Was adopted by Systema Naturae 2000 and others. In botany , the ranks of subclass and suborder are secondary ranks pre-defined as respectively above and below the rank of order. Any number of further ranks can be used as long as they are clearly defined. The superorder rank is commonly used, with the ending -anae that was initiated by Armen Takhtajan 's publications from 1966 onwards. The order as

2209-570: Was the general word for snake. 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’. All modern snakes are grouped within the suborder Serpentes in Linnean taxonomy , part of the order Squamata , though their precise placement within squamates remains controversial. The two infraorders of Serpentes are Alethinophidia and Scolecophidia . This separation

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