Giraffe

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Giraffe[1]
Conservation status
Least Concern (IUCN 3.1)[2]
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Artiodactyla Family: Giraffidae Genus: Giraffa Species: G. camelopardalis
Binomial name
Giraffa camelopardalis Linnaeus, 1758
Range map

The giraffe (Giraffa camelopardalis) is an African even-toed ungulate mammal, the tallest of all land-living animal species, and the largest ruminant. It is covered in large, irregular patches of yellow to black fur separated by white, off-white, or dark yellowish brown background. The average mass for an adult male giraffe is 1,191 kilograms (2,630 lb) while the average mass for an adult female is 828 kilograms (1,830 lb).^[3][4] It is approximately 4.3 metres (14 ft) to 5.2 metres (17 ft) tall, although the tallest male recorded stood almost 6 metres (20 ft).^[3][4]

The giraffe is related to deer and cattle, but is placed in a separate family, the Giraffidae, consisting of only the giraffe and its closest relative, the okapi. Its range extends from Chad in Central Africa to South Africa.

Giraffes usually inhabit savannas, grasslands, or open woodlands. However, when food is scarce they will venture into areas with denser vegetation. They prefer areas with plenty of acacia growth. They will drink large quantities of water when available, which enables them to live for extended periods in dry, arid areas.

Contents [hide] 1 Etymology 2 Taxonomy and evolution 2.1 Subspecies 3 Anatomy and morphology 3.1 Horns 3.2 Neck 3.3 Legs and pacing 3.4 Circulatory system 4 Behaviour 4.1 Social structure and breeding habits 4.2 Reproduction 4.3 Necking 4.4 Diet 4.5 Sleeping 4.6 Communication 4.7 Stereotypic behavior 5 Human interactions 5.1 Conservation 5.2 Scientific inspiration 5.3 In art and culture 6 References 7 External links

Etymology

The species name camelopardalis (camelopard) is derived from its early Roman name, where it was described as having characteristics of both a camel and a leopard.^[5] The English word camelopard first appeared in the 14th century and survived in common usage well into the 19th century. The Afrikaans language retained it. The Arabic word الزرافة ziraafa or zurapha, meaning "assemblage" (of animals), or just "tall", was used in English from the sixteenth century on, often in the Italianate form giraffa.

Taxonomy and evolution

The giraffe is one of only two living species of the family Giraffidae, along with the okapi. The family was once much more extensive, with numerous other species. The giraffids evolved from a 3 metres (9.8 ft) tall antelope-like mammal that roamed Europe and Asia some 30–50 million years ago.^[6]

The earliest known giraffid was Climacoceras, which still resembled deer, having large antler-like ossicones. It first appeared in the early Miocene epoch. Later examples include the genera Palaeotragus and Samotherium, which appeared in the early to mid-Miocene. They were both tall at the shoulder, and had developed the simple, unbranched ossicones of modern giraffids, but still had relatively short necks.^[7]

Comparison of the African Miocene giraffids: Palaeotragus (two top) and Climacoceras (two bottom)

From the late Pliocene onwards, the variety of giraffids drastically declined, until only the two surviving species remained. The modern genus Giraffa evolved during the Pliocene epoch, and included a number of other long-necked species, such as Giraffa jumae, that do not survive today.^[7] Alan Turner proposes, in the 2004 book Evolving Eden, that giraffe ancestors initially had a dark coat with pale spots, and that the spots gradually became star-shaped, before eventually forming the reticulated pattern found today.^[8] The modern species, Giraffa camelopardalis, appeared during the Pleistocene 1 million years ago.^{[citation needed]}

The evolution of the long necks of giraffes has been the subject of much debate. The standard story is that they were evolved to allow the giraffes to browse vegetation that was out of the reach of other herbivores in the vicinity, giving them a competitive advantage.^[7] However, an alternative theory proposes that the long necks evolved as a secondary sexual characteristic, giving males an advantage in "necking" contests (see below) to establish dominance and obtain access to sexually receptive females. This theory notes that giraffes frequently feed from relatively low-lying shrubs, and that the necks of males are significantly longer than those of females.^[9] However, this theory is not universally accepted, and some of the data supporting it has recently been challenged, lending support to the original proposal that neck length is related to browsing habits.[10

From Wikipedia, the free encyclopedia

Jump to: navigation, search

For other uses, see Giraffe (disambiguation).

Giraffe[1]
Conservation status
Least Concern (IUCN 3.1)[2]
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Artiodactyla Family: Giraffidae Genus: Giraffa Species: G. camelopardalis
Binomial name
Giraffa camelopardalis Linnaeus, 1758
Range map

The giraffe (Giraffa camelopardalis) is an African even-toed ungulate mammal, the tallest of all land-living animal species, and the largest ruminant. It is covered in large, irregular patches of yellow to black fur separated by white, off-white, or dark yellowish brown background. The average mass for an adult male giraffe is 1,191 kilograms (2,630 lb) while the average mass for an adult female is 828 kilograms (1,830 lb).^[3][4] It is approximately 4.3 metres (14 ft) to 5.2 metres (17 ft) tall, although the tallest male recorded stood almost 6 metres (20 ft).^[3][4]

The giraffe is related to deer and cattle, but is placed in a separate family, the Giraffidae, consisting of only the giraffe and its closest relative, the okapi. Its range extends from Chad in Central Africa to South Africa.

Giraffes usually inhabit savannas, grasslands, or open woodlands. However, when food is scarce they will venture into areas with denser vegetation. They prefer areas with plenty of acacia growth. They will drink large quantities of water when available, which enables them to live for extended periods in dry, arid areas.

Contents [hide] 1 Etymology 2 Taxonomy and evolution 2.1 Subspecies 3 Anatomy and morphology 3.1 Horns 3.2 Neck 3.3 Legs and pacing 3.4 Circulatory system 4 Behaviour 4.1 Social structure and breeding habits 4.2 Reproduction 4.3 Necking 4.4 Diet 4.5 Sleeping 4.6 Communication 4.7 Stereotypic behavior 5 Human interactions 5.1 Conservation 5.2 Scientific inspiration 5.3 In art and culture 6 References 7 External links

Etymology

The species name camelopardalis (camelopard) is derived from its early Roman name, where it was described as having characteristics of both a camel and a leopard.^[5] The English word camelopard first appeared in the 14th century and survived in common usage well into the 19th century. The Afrikaans language retained it. The Arabic word الزرافة ziraafa or zurapha, meaning "assemblage" (of animals), or just "tall", was used in English from the sixteenth century on, often in the Italianate form giraffa.

Taxonomy and evolution

The giraffe is one of only two living species of the family Giraffidae, along with the okapi. The family was once much more extensive, with numerous other species. The giraffids evolved from a 3 metres (9.8 ft) tall antelope-like mammal that roamed Europe and Asia some 30–50 million years ago.^[6]

The earliest known giraffid was Climacoceras, which still resembled deer, having large antler-like ossicones. It first appeared in the early Miocene epoch. Later examples include the genera Palaeotragus and Samotherium, which appeared in the early to mid-Miocene. They were both tall at the shoulder, and had developed the simple, unbranched ossicones of modern giraffids, but still had relatively short necks.^[7]

Comparison of the African Miocene giraffids: Palaeotragus (two top) and Climacoceras (two bottom)

From the late Pliocene onwards, the variety of giraffids drastically declined, until only the two surviving species remained. The modern genus Giraffa evolved during the Pliocene epoch, and included a number of other long-necked species, such as Giraffa jumae, that do not survive today.^[7] Alan Turner proposes, in the 2004 book Evolving Eden, that giraffe ancestors initially had a dark coat with pale spots, and that the spots gradually became star-shaped, before eventually forming the reticulated pattern found today.^[8] The modern species, Giraffa camelopardalis, appeared during the Pleistocene 1 million years ago.^{[citation needed]}

The evolution of the long necks of giraffes has been the subject of much debate. The standard story is that they were evolved to allow the giraffes to browse vegetation that was out of the reach of other herbivores in the vicinity, giving them a competitive advantage.^[7] However, an alternative theory proposes that the long necks evolved as a secondary sexual characteristic, giving males an advantage in "necking" contests (see below) to establish dominance and obtain access to sexually receptive females. This theory notes that giraffes frequently feed from relatively low-lying shrubs, and that the necks of males are significantly longer than those of females.^[9] However, this theory is not universally accepted, and some of the data supporting it has recently been challenged, lending support to the original proposal that neck length is related to browsing habits.[10

Koala

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This article is about the Australian marsupial. For other uses, see Koala (disambiguation).

Koala[1]
female male
Conservation status
Least Concern (IUCN 3.1)[2]
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Diprotodontia Family: Phascolarctidae Genus: Phascolarctos Species: P. cinereus
Binomial name
Phascolarctos cinereus (Goldfuss, 1817)

The Koala (Phascolarctos cinereus) is a thickset arboreal marsupial herbivore native to Australia, and the only extant representative of the family Phascolarctidae.

The koala is found in coastal regions of eastern and southern Australia, from near Adelaide to the southern part of Cape York Peninsula. Populations also extend for considerable distances inland in regions with enough moisture to support suitable woodlands. The koalas of South Australia were largely exterminated during the early part of the 20th century, but the state has since been repopulated with Victorian stock. The koala is not found in Tasmania or Western Australia.

Contents [hide] 1 Names 2 Variation 3 Physical description 4 Life cycle 5 Diet and behaviour 6 Conservation status 7 See also 8 References 9 External links

Names

The word koala comes from the Dharuk gula. Although the vowel /u/ was originally written in the Latin alphabet as "oo" (in spellings such as coola or koolah), it was changed to "oa" possibly due to an error.^[3] The word is erroneously said to mean "doesn't drink".^[3]

The scientific name of the koala's genus, Phascolarctos, is derived from Greek phaskolos "pouch" and arktos "bear". Its species name, cinereus, is Latin and means "ash-coloured".^[4]

Although the koala is not a bear, when first adopted by English speakers, the name koala bear became popular, as this roughly evoked the species' similarity in appearance to the Teddy bear, to people unfamiliar with it. Although taxonomically incorrect, the name koala bear is still in use today outside Australia^[5] — its use is discouraged because of the inaccuracy in the name.^{[6][7][8][9][10]} Other descriptive English names based on "bear" have included monkey bear, native bear, and tree-bear.^[3]

Variation

A Southern koala on Kangaroo Island, not native to the island

Although three subspecies have been described, these are arbitrary selections from a cline and are not generally accepted as valid. Following Bergmann's Rule, southern individuals from the cooler climates are larger. A typical Victorian koala (formerly P. cinereus victor) has longer, thicker fur, is a darker, softer grey, often with chocolate-brown highlights on the back and forearms, and has a more prominently light-coloured ventral side and fluffy white ear tufts. Typical and New South Wales koala weights are 12 kg (26 lb) for males and 8.5 kg (19 lb) for females. In tropical and sub-tropical Queensland, however, the koala is smaller (at around 6.5 kg (14 lb) for an average male and just over 5 kg (11 lb) for an average female), a lighter often rather scruffy grey in colour, and has shorter, thinner fur. In Queensland, the koala was previously classified as the subspecies P. cinereus adustus, and the intermediate forms in New South Wales as P. cinereus cinereus. A fourth variation, though not technically a subspecies, is Phascolarctos cinereus aurum, or in English "golden koala," which has a slight golden tinge to the fur as a result of an absence of the melanin pigment that produces albinism in most other mammalian species. The variation from one form to another is continuous and there are substantial differences between individual koalas in any given region such as hair colour. The origins of the koala are unclear, although almost certainly they descended from terrestrial wombat-like animals. Koala fossils are quite rare, but some have been found in northern Australia dating to 20 million years ago. During this time, the northern half of Australia was rainforest. The koala did not specialise in a diet of eucalyptus until the climate cooled and eucalypt forests grew in the place of rainforests. The fossil record indicates that before 50,000 years ago, giant koalas inhabited the southern regions of Australia. The koala fills the same ecological role as the sloth of South America.

Physical description

Koalas have a slow metabolism and sleep for most of the day

The koala is broadly similar in appearance to the wombat (its closest living relative),^[1] but has a thicker coat, much larger ears, and longer limbs. The koala has large, sharp claws to assist with climbing tree trunks. Weight varies from about 14 kg (31 lb) for a large southern male, to about 5 kg (11 lb) for a small northern female. The koala's five fingers are arranged with opposable thumbs, providing better gripping ability. The first two fingers are positioned in apposition on the front paws, and the first three fingers for the hind paws.^[11] The koala is one of the few mammals (other than primates) that has fingerprints. Koala fingerprints are similar to human fingerprints; even with an electron microscope, it can be quite difficult to distinguish between the two.^[12]

The teeth of the koala are adapted to their herbivorous diet, and are similar to those of other diprotodont marsupials, such as kangaroos and wombats. They have sharp incisors to clip leaves at the front of the mouth, separated from the grinding cheek teeth by a wide diastema. The dental formula for koalas is:

Dentition
3.1.1.4
1.0.1.4

The male koala, like many marsupials, has a bifurcated penis. The female has two lateral vaginas and two separate uteri, which is common to all marsupials.^[13]

Koala walking along a branch. Koalas also walk on all four legs when walking on the ground

The brain in the ancestors of the modern koala once filled the whole cranial cavity, but has become drastically reduced in the present species, a degeneration scientists suspect is an adaptation to a diet low in energy.^[14] One of the smallest in marsupials with no more than 0.2% of its body weight,^[15] about 40% of the cranial cavity is filled with cerebrospinal fluid, while the brain's two cerebral hemispheres are like "a pair of shrivelled walnut halves on top of the brain stem, in contact neither with each other nor the bones of the skull. It is the only animal on Earth with such a strangely reduced brain."^[16]

It is generally a silent animal, but males have a very loud advertising call that can be heard from almost a kilometre away during the breeding season. When under stress, koalas may issue a loud cry, which has been reported as similar to that of a human baby.^[17] There is little reliable information about the lifespan of the koala, but in captivity they have been observed to reach the age of 18 years.^[11]

Porpoise

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For other uses, see Porpoise (disambiguation).

Porpoises Fossil range: 15.970–0 Ma PreЄ Є O S D C P T J K Pg N Miocene to Recent
Phocoena phocoena, Harbour Porpoise at the Fjord & Bæltcentret in Denmark
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Cetacea Suborder: Odontoceti Superfamily: Delphinoidea Family: Phocoenidae Gray, 1825
Genera
See text

Porpoises are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are distinct from dolphins, although the word "porpoise" (pronounced /ˈpɔrpəs/) has been used to refer to any small dolphin, especially by sailors and fishermen. The most obvious visible difference between the two groups is that porpoises have flattened, spade-shaped teeth distinct from the conical teeth of dolphins, and shorter beaks.

The name derives from French pourpois, originally from Medieval Latin porcopiscus (porcus pig + piscus fish).

Porpoises, divided into six species, live in all oceans, mostly near the shore. Freshwater populations of the Finless Porpoise also exist. Probably the best known species is the Harbour Porpoise, which can be found across the Northern Hemisphere. Like all toothed whales, porpoises are predators, using sounds to locate prey and to coordinate with others. They hunt fish, squid, and crustaceans.

Porpoises apparently diverged from dolphins about 15 million years ago in the northern Pacific, then spread across the globe much later.

Contents [hide] 1 Taxonomy and evolution 2 Physical characteristics 3 Life history 4 Behaviour 5 Human impact 6 See also 7 References 8 External links

[edit] Taxonomy and evolution

See also: Evolution of cetaceans

Porpoises, along with whales and dolphins, are descendants of land-living ungulates (hoofed animals) that first entered the oceans around 50 million years ago. During the Miocene (23 to 5 MYA), mammals were fairly modern. The cetaceans diversified, and fossil evidence suggests that porpoises diverged from dolphins and other cetaceans around 15 MYA. The oldest fossils are known from the shallow seas around the north Pacific, with animals spreading to the European coasts and southern hemisphere only much later, during the Pliocene.^[1]

Suborder Odontoceti Toothed whales

• Infraorder Delphinida
  • Superfamily Delphinoidea
    • Family Phocoenidae: Porpoises
      • Genus †Haborophocoena^[2]
        • Haborophocoena toyoshimai
      • Genus Neophocaena
        • Finless Porpoise, Neophocaena phocaeniodes
      • Genus †Numataphocoena^[3]
        • Numataphocoena yamashitai
      • Genus Phocoena
        • Harbour Porpoise, Phocoena phocoena
        • Vaquita, Phocoena sinus
        • Spectacled Porpoise, Phocoena dioptrica
        • Burmeister's Porpoise, Phocoena spinipinnis
      • Genus Phocoenoides
        • Dall's Porpoise, Phocoenoides dalli
      • Genus †Septemriocetus^[4]
        • Septemriocetus bosselaersi

Recently-discovered hybrids between male Harbour porpoises and female Dall's Porpoises indicate that the two species may actually be members of the same genus.^[5]

[edit] Physical characteristics

A Harbour Porpoise at an aquarium. In the wild, porpoises rarely jump out of the water.

Porpoises tend to be smaller but stouter than dolphins. They have small, rounded heads and blunt jaws instead of beaks. While dolphins have a round, bulbous "melon", porpoises do not. Their teeth are spade-shaped, whereas dolphins have conical teeth. In addition, a porpoise's dorsal fin is generally triangular, rather than curved like that of many dolphins and large whales. Some species have small bumps, known as tubercles, on the leading edge of the dorsal fin. The function of these bumps is unknown. ^[5]

These animals are the smallest cetaceans, reaching body lengths up to 2.5 metres (8 ft); the smallest species is the Vaquita, reaching up to 1.5 m (5 ft). In terms of weight the lightest is the Finless Porpoise at 30-45 kilograms (65-100 lb) and the heaviest is Dall's Porpoise at 130-200 kg (280-440 lb). Because of their small size, porpoises lose body heat to the water more rapidly than other cetaceans. Their stout shape, which minimizes surface area, may be an adaptation to reduce heat loss. Thick blubber also insulates them from the cold. The small size of porpoises requires them to eat frequently, rather than depending on fat reserves.^[5]

Snake

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Snakes Fossil range: 145–0 Ma PreЄ Є O S D C P T J K Pg N Cretaceous – Recent
Common garter snake, Thamnophis sirtalis
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Reptilia Order: Squamata Suborder: Serpentes Linnaeus, 1758
Infraorders
Alethinophidia – Nopcsa, 1923 Scolecophidia – Cope, 1864
World range of snakes (rough range of sea snakes in blue)

This article is about the animal. For other uses, see Snake (disambiguation).

Snakes are elongate legless carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with many more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads with their highly mobile jaws. In order 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. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca.

Living snakes are found on every continent except Antarctica and most islands. Fifteen families are currently recognized comprising 456 genera and over 2,900 species.^[1][2] They range in size from the tiny, 10 cm long thread snake to pythons and anacondas of up to 7.6 metres (25 ft) in length. The recently discovered fossil Titanoboa was 13 metres (43 ft) long. Snakes are thought to have evolved from either burrowing or aquatic lizards during the Cretaceous period (c 150 Ma). The diversity of modern snakes appeared during the Paleocene period (c 66 to 56 Ma).

Most species are non-venomous and those that have venom use it primarily to kill and subdue prey rather than self-defense. Some possess venom potent enough to cause painful injury or death to humans. Those which are non-venomous either swallow prey alive or kill it via constriction.

Contents [hide] 1 Etymology 2 Evolution 3 Taxonomy 4 Families 5 Skeleton 6 Size comparison 7 Skin 7.1 Moulting 8 Perception 8.1 Eyesight 8.2 Smell 8.3 Vibration sensitivity 8.4 Infrared sensitivity 9 Feeding and diet 10 Internal organs 11 Locomotion 11.1 Lateral undulation 11.1.1 Terrestrial lateral undulation 11.1.2 Aquatic 11.2 Sidewinding 11.3 Concertina locomotion 11.4 Rectilinear locomotion 11.5 Other 12 Reproduction 13 Venom 14 Interactions with humans 14.1 Snake bite 14.2 Snake charmers 14.3 Snake trapping 14.4 Human consumption 14.5 As pets 14.6 Symbolism 14.7 In religion 15 See also 16 References 17 Further reading 18 External links

Etymology

The English word snake comes from Old English snaca, itself from Proto-Germanic *snak-an- (cf. German Schnake "ring snake", Swedish snok "grass snake"), from Proto-Indo-European root *(s)nēg-o- "to crawl, creep", which also gave sneak as well as Sanskrit nāgá "snake".^[3] The word ousted adder as it went on to narrow in meaning, though in Old English næddre was the general word for snake.^[4] The other term, serpent, is a french word, ultimately from Indo-European *serp- "to creep"^[5] which also gave Greek érpein (ερπω) "to crawl".

Evolution

Modern snakes Scolecophidia Leptotyphlopidae Anomalepididae Typhlopidae Alethinophidia Anilius Core Alethinophidia Uropeltidae Cylindrophis Anomochilus Uropeltinae Macrostomata Pythonidae Pythoninae Xenopeltis Loxocemus Caenophidia Colubroidea Acrochordidae Boidae Erycinae Boinae Calabaria Ungaliophiinae Tropidophiinae Note that the tree only indicates relationships not evolutionary branching times.[6]

The fossil record of snakes is relatively poor because snake skeletons are typically small and fragile, making fossilization uncommon. However 150 million-year-old specimens, readily identifiable as snakes, yet with lizard-like skeletal structures, have been uncovered in South America and Africa.^[7]:11 There is consensus, on the basis of comparative anatomy, that snakes descended from lizards.^[7]:11[8] Fossil evidence suggests that snakes may have evolved from burrowing lizards, such as the varanids or a similar group during the Cretaceous Period.^[9] An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, and was fully terrestrial.^[10] One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo, although it also is semi-aquatic.^[11] Subterranean forms evolved bodies that were streamlined for burrowing and lost their limbs.^[11] According to this hypothesis, features such as the transparent, fused eyelids (brille) and loss of external ears evolved to cope with fossorial difficulies such as scratched corneas and dirt in the ears.^[9][11] Some primitive snakes are known to have possessed hindlimbs, but their pelvic bones lack a direct connection to the vertebrae. These include fossil species like Haasiophis, Pachyrhachis and Eupodophis, which are slightly older than Najash.^[12]

Fossil of Archaeophis proavus

Primitive groups among the modern snakes, pythons and boas, have vestigial hind limbs; tiny, clawed digits known as anal spurs which are used to grasp during mating.^[7]:11[12] Leptotyphlopidae and Typhlopidae are other groups where remnants of the pelvic girdle are present, sometimes appearing as horny projections when visible. The frontal limbs are non-existent in all snakes and this loss is associated with the evolution of the 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. The Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant early in snake evolution and 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 one to three neck vertebrae), making most of the snake's skeleton being composed of an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. The neck, lumbar and pelvic vertebrae are very reduced in number (only two to ten lumbar and pelvic vertebrae are still present), while only a short tail remains of the caudal vertebrae, although the tail is still long enough to be of good use in many species, and is modified in some aquatic and tree dwelling species. An alternative hypothesis, based on morphology, suggests that the ancestors of snakes were related to mosasaurs — extinct aquatic reptiles from the Cretaceous — which in turn are thought to have derived from varanid lizards.^[8] Under this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water-loss through osmosis), while the external ears were lost through disuse in an aquatic environment, ultimately leading to an animal similar in appearance to sea snakes of today. In the Late Cretaceous, snakes re-colonized land to appear as they are today. Fossil snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis, particularly as they are older than the terrestrial Najash rionegrina. Similar skull structure; reduced/absent limbs; and other anatomical features found in both mosasaurs and snakes lead to a positive cladistical correlation, although some of these features are shared with varanids. In recent years, genetic studies have indicated that snakes are not as closely related to monitor lizards as it was once believed, and therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, there is more evidence linking mosasaurs to snakes than to varanids. Fragmentary remains that have been found from the Jurassic and Early Cretaceous indicate deeper fossil records for these groups, which may eventually refute either hypothesis.

The great diversity of modern snakes appeared in the Paleocene, correlating with the adaptive radiation of mammals following the extinction of the non-avian dinosaurs. One of the more common groups today, the colubrids, became particularly diverse due to their preying on rodents, a mammal group that has been particularly successful. There are over 2,900 species of snakes ranging as far northward as the Arctic Circle in Scandinavia and southward through Australia and Tasmania.^[8] Snakes can be found on every continent (with the exception of Antarctica), dwelling in the sea, and as high as 16,000 feet (4,900 m)in the Himalayan Mountains of Asia.^[8][13]:143 There are numerous islands from which snakes are conspicuously absent such as Ireland, Iceland, and New Zealand.