SNAKES
SCIENTIFIC CLAASSIFICATION
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Superfamily: Varanoidea
(unranked): Pythonomorpha
Suborder: Serpentes
Linnaeus, 1758
Subgroups
Alethinophidia – Nopcsa, 1923
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)neg-o- "to crawl", "to creep", which also gave sneak as well as Sanskrit nagá "snake".[6] The word ousted adder, as adder went on to narrow in meaning, though in Old English næddre was the general word for snake.[7] The other term, serpent, is from French, ultimately from Indo-European *serp- (to creep),[8] which also gave Greek érpo (e?p?) "I crawl".
EvolutionA phylogenetic overview of the extant groups
Modern snakes Scolecophidia
Leptotyphlopidae
Anomalepididae
Typhlopidae
Alethinophidia
Anilius
Core Alethinophidia Uropeltidae
Cylindrophis
Anomochilus
Uropeltinae
Macrostomata Pythonidae
Pythoninae
Xenopeltis
Loxocemus
Caenophidia
Colubridae
Acrochordidae
Atractaspididae
Elapidae
Hydrophiidae
Viperidae
Boidae
Erycinae
Boinae
Calabaria
Ungaliophiinae
Tropidophiinae
Note: the tree only indicates relationships,
not evolutionary branching times.[9]
The fossil record of snakes 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.[10] The earliest known snake fossils come from sites in Utah and Algeria, represented by the genera Coniophis and Lapparentophis, respectively. These fossil sites have been tentatively dated to the Albian or Cenomanian age of the late Cretaceous, between 112 and 94 Ma ago. However, an even greater age has been suggested for one of the Algerian sites, which may be as old as the Aptian, 125 to 112 Ma ago.[11]
Based on comparative anatomy, there is consensus
that snakes descended from lizards.[5]:11[12] 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.[5]:11[13] The Leptotyphlopidae and Typhlopidae groups also possess remnants of the pelvic girdle, sometimes appearing as horny projections when visible.
Front limbs are nonexistent in all known snakes.
This is caused by the evolution of 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, 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 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.
Modern snakes greatly diversified during the Paleocene.
This occurred alongside the adaptive radiation of mammals, following the extinction of (non-avian) dinosaurs. The colubrids, one of the more common snake groups, became particularly diverse due to preying on rodents, an especially successful mammal group.
Origins
The origin of snakes remains an unresolved issue.
There are two main hypotheses competing for acceptance.
Burrowing Lizard Hypothesis
There is fossil evidence to suggest that snakes may have evolved from burrowing lizards,
such as the varanids (or a similar group) during the Cretaceous Period.[14] An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, and was fully terrestrial.[15] One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo (though it also is semiaquatic).[16] Subterranean species evolved bodies streamlined for burrowing, and eventually lost their limbs.[16] 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 the ears.[14][16] 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.[13]
FamiliesInfraorder Alethinophidia 15 families
Family[2] Taxon author[2] Genera[2] Species[2] Common name Geographic range[20]
Acrochordidae Bonaparte, 1831 1 3 Wart snakes Western India and Sri Lanka through tropical Southeast Asia to the Philippines, south through the Indonesian/Malaysian island group to Timor, east through New Guinea to the northern coast of Australia to Mussau Island, the Bismark Archipelago and Guadalcanal Island in the Solomon Islands.
Aniliidae Stejneger, 1907 1 1 False coral snake Tropical South America.
Anomochilidae Cundall, Wallach, 1993 1 2 Dwarf pipe snakes West Malaysia and on the Indonesian island of Sumatra.
Atractaspididae Günther, 1858 12 64 Burrowing asps Africa and the Middle East.[5][21][22]
Boidae Gray, 1825 8 43 Boas Northern, Central and South America, the Caribbean, southeastern Europe and Asia Minor, Northern, Central and East Africa, Madagascar and Reunion Island, the Arabian Peninsula, Central and southwestern Asia, India and Sri Lanka, the Moluccas and New Guinea through to Melanesia and Samoa.
Bolyeriidae Hoffstetter, 1946 2 2 Splitjaw snakes Mauritius.
Colubridae Oppel, 1811 304[3] 1938[3] Typical snakes Widespread on all continents, except Antarctica.[23]
Cylindrophiidae Fitzinger, 1843 1 8 Asian pipe snakes Sri Lanka east through Myanmar, Thailand, Cambodia, Vietnam and the Malay Archipelago to as far east as Aru Islands off the southwestern coast of New Guinea. Also found in southern China (Fujian, Hong Kong and on Hainan Island) and in Laos.
Elapidae Boie, 1827 61 235 Elapids On land, worldwide in tropical and subtropical regions, except in Europe. Sea snakes occur in the Indian Ocean and the Pacific.[24]
Loxocemidae Cope, 1861 1 1 Mexican burrowing snake Along the Pacific versant from Mexico south to Costa Rica.
Pythonidae Fitzinger, 1826 8 26 Pythons Subsaharan Africa, India, Myanmar, southern China, Southeast Asia and from the Philippines southeast through Indonesia to New Guinea and Australia.
Tropidophiidae Brongersma, 1951 4 22 Dwarf boas From southern Mexico and Central America, south to northwestern South America in Colombia, (Amazonian) Ecuador and Peru, as well as in northwestern and southeastern Brazil. Also found in the West Indies.
Uropeltidae Müller, 1832 8 47 Shield-tailed snakes Southern India and Sri Lanka.
Viperidae Oppel, 1811 32 224 Vipers The Americas, Africa and Eurasia.
Xenopeltidae Bonaparte, 1845 1 2 Sunbeam snakes Southeast Asia from the Andaman and Nicobar Islands, east through Myanmar to southern China, Thailand, Laos, Cambodia, Vietnam, the Malay Peninsula and the East Indies to Sulawesi, as well as the Philippines.
Infraorder Scolecophidia 3 families
Family[2] Taxon author[2] Genera[2] Species[2] Common name Geographic range[20]
Anomalepidae Taylor, 1939 4 15 Primitive blind snakes From southern Central America to northwestern South America. Disjunct populations in northeastern and southeastern South America.
Leptotyphlopidae Stejneger, 1892 2 87 Slender blind snakes Africa, western Asia from Turkey to northwestern India, on Socotra Island, from the southwestern United States south through Mexico and Central to South America, though not in the high Andes. In Pacific South America they occur as far south as southern coastal Peru, and on the Atlantic side as far as Uruguay and Argentina. In the Caribbean they are found on the Bahamas, Hispaniola and the Lesser Antilles.
Typhlopidae Merrem, 1820 6 203 Typical blind snakes Most tropical and many subtropical regions around the world, particularly in Africa, Madagascar, Asia, islands in the Pacific, tropical America and in southeastern Europe.