Tooth

A tooth (plural teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are covered by gums. Teeth are not made of bone, but rather of multiple tissues of varying density and hardness. The cellular tissues that ultimately become teeth originate from the embryonic germ layer, the ectoderm.

The general structure of teeth is similar across the vertebrates, although there is considerable variation in their form and position. The teeth of mammals have deep roots, and this pattern is also found in some fish, and in crocodilians. In most teleost fish, however, the teeth are attached to the outer surface of the bone, while in lizards they are attached to the inner surface of the jaw by one side. In cartilaginous fish, such as sharks, the teeth are attached by tough ligaments to the hoops of cartilage that form the jaw.[1]

Some animals develop only one set of teeth (monophyodont) while others develop many sets (polyphyodont). Sharks, for example, grow a new set of teeth every two weeks to replace worn teeth. Rodent incisors grow and wear away continually through gnawing, which helps maintain relatively constant length. The industry of the beaver is due in part to this qualification. Many rodents such as voles and guinea pigs, but not mice, as well as leporidae like rabbits, have continuously growing molars in addition to incisors.[2][3]

Teeth are not always attached to the jaw, as they are in mammals. In many reptiles and fish, teeth are attached to the palate or to the floor of the mouth, forming additional rows inside those on the jaws proper. Some teleosts even have teeth in the pharynx. While not true teeth in the usual sense, the dermal denticles of sharks are almost identical in structure and are likely to have the same evolutionary origin. Indeed, teeth appear to have first evolved in sharks, and are not found in the more primitive jawless fish – while lampreys do have tooth-like structures on the tongue, these are in fact, composed of keratin, not of dentine or enamel, and bear no relationship to true teeth.[1] Though "modern" teeth-like structures with dentine and enamel have been found in late conodonts, they are now supposed to have evolved independently of later vertebrates' teeth.[4][5]

Living amphibians typically have small teeth, or none at all, since they commonly feed only on soft foods. In reptiles, teeth are generally simple and conical in shape, although there is some variation between species, most notably the venom-injecting fangs of snakes. The pattern of incisors, canines, premolars and molars is found only in mammals, and to varying extents, in their evolutionary ancestors. The numbers of these types of teeth vary greatly between species; zoologists use a standardised dental formula to describe the precise pattern in any given group.[1]

Close up - chimpanzee teeth
A chimpanzee displaying its teeth

Origin

The genes governing tooth development in mammals are homologous to those involved in the development of fish scales.[6] Study of a tooth plate of a fossil of the extinct fish Romundina stellina showed that the teeth and scales were made of the same tissues, also found in mammal teeth, lending support to the theory that teeth evolved as a modification of scales.[7]

Mammals

Teeth are among the most distinctive (and long-lasting) features of mammal species. Paleontologists use teeth to identify fossil species and determine their relationships. The shape of the animal's teeth are related to its diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing and grinding. Carnivores, on the other hand, have canine teeth to kill prey and to tear meat.

Mammals, in general, are diphyodont, meaning that they develop two sets of teeth. In humans, the first set (the "baby," "milk," "primary" or "deciduous" set) normally starts to appear at about six months of age, although some babies are born with one or more visible teeth, known as neonatal teeth. Normal tooth eruption at about six months is known as teething and can be painful. Kangaroos, elephants, and manatees are unusual among mammals because they are polyphyodonts.

Aardvark

In Aardvarks, teeth lack enamel and have many pulp tubules, hence the name of the order Tubulidentata.

Canines

In dogs, the teeth are less likely than humans to form dental cavities because of the very high pH of dog saliva, which prevents enamel from demineralizing.[8] Sometimes called cuspids, these teeth are shaped like points (cusps) and are used for tearing and grasping food[9]

Cetaceans

Like human teeth, whale teeth have polyp-like protrusions located on the root surface of the tooth. These polyps are made of cementum in both species, but in human teeth, the protrusions are located on the outside of the root, while in whales the nodule is located on the inside of the pulp chamber. While the roots of human teeth are made of cementum on the outer surface, whales have cementum on the entire surface of the tooth with a very small layer of enamel at the tip. This small enamel layer is only seen in older whales where the cementum has been worn away to show the underlying enamel.[10]

The toothed whale is a suborder of the cetaceans characterized by having teeth. The teeth differ considerably among the species. They may be numerous, with some dolphins bearing over 100 teeth in their jaws. On the other hand, the narwhals have a giant unicorn-like tusk, which is a tooth containing millions of sensory pathways and used for sensing during feeding, navigation, and mating. It is the most neurologically complex tooth known. Beaked whales are almost toothless, with only bizarre teeth found in males. These teeth may be used for feeding but also for demonstrating aggression and showmanship.

Primates

In humans (and most other primates) there are usually 20 primary ("baby") teeth, 28 to 32 of what's known as permanent teeth, in addition to other four being third molars or wisdom teeth, each of which may or may not grow in.

Among primary teeth, 10 of them are usually found in the maxilla (i.e. upper jaw) and the other 10 in the mandible (i.e. lower jaw). Among permanent teeth, 16 are found in the maxilla and the other 16 in the mandible. Most of the teeth have uniquely distinguishing features.

Horse

An adult horse has between 36 and 44 teeth. The enamel and dentin layers of horse teeth are intertwined.[11] All horses have 12 premolars, 12 molars, and 12 incisors.[12] Generally, all male equines also have four canine teeth (called tushes) between the molars and incisors. However, few female horses (less than 28%) have canines, and those that do usually have only one or two, which many times are only partially erupted.[13] A few horses have one to four wolf teeth, which are vestigial premolars, with most of those having only one or two. They are equally common in male and female horses and much more likely to be on the upper jaw. If present these can cause problems as they can interfere with the horse's bit contact. Therefore, wolf teeth are commonly removed.[12]

Horse teeth can be used to estimate the animal's age. Between birth and five years, age can be closely estimated by observing the eruption pattern on milk teeth and then permanent teeth. By age five, all permanent teeth have usually erupted. The horse is then said to have a "full" mouth. After the age of five, age can only be conjectured by studying the wear patterns on the incisors, shape, the angle at which the incisors meet, and other factors. The wear of teeth may also be affected by diet, natural abnormalities, and cribbing. Two horses of the same age may have different wear patterns.

A horse's incisors, premolars, and molars, once fully developed, continue to erupt as the grinding surface is worn down through chewing. A young adult horse will have teeth which are 4.5-5 inches long, with the majority of the crown remaining below the gumline in the dental socket. The rest of the tooth will slowly emerge from the jaw, erupting about 1/8" each year, as the horse ages. When the animal reaches old age, the crowns of the teeth are very short and the teeth are often lost altogether. Very old horses, if lacking molars, may need to have their fodder ground up and soaked in water to create a soft mush for them to eat in order to obtain adequate nutrition.

Proboscideans

Mammoth ivory hg
Section through the ivory tusk of a mammoth

Elephants' tusks are specialized incisors for digging food up and fighting. Some elephant teeth are similar to those in manatees, and it is notable that elephants are believed to have undergone an aquatic phase in their evolution.

At birth, elephants have a total of 28 molar plate-like grinding teeth not including the tusks. These are organized into four sets of seven successively larger teeth which the elephant will slowly wear through during its lifetime of chewing rough plant material. Only four teeth are used for chewing at a given time, and as each tooth wears out, another tooth moves forward to take its place in a process similar to a conveyor belt. The last and largest of these teeth usually becomes exposed when the animal is around 40 years of age, and will often last for an additional 20 years. When the last of these teeth has fallen out, regardless of the elephant's age, the animal will no longer be able to chew food and will die of starvation.[14][15]

Rabbit

Rabbits and other lagomorphs usually shed their deciduous teeth before (or very shortly after) their birth, and are usually born with their permanent teeth.[16] The teeth of rabbits complement their diet, which consists of a wide range of vegetation. Since many of the foods are abrasive enough to cause attrition, rabbit teeth grow continuously throughout life.[17] Rabbits have a total of 6 incisors, three upper premolars, three upper molars, two lower premolars, and two lower molars on each side. There are no canines. Three to four millimeters of the tooth is worn away by incisors every week, whereas the posterior teeth require a month to wear away the same amount.[18]

The incisors and cheek teeth of rabbits are called aradicular hypsodont teeth. This is sometimes referred to as an elodent dentition. These teeth grow or erupt continuously. The growth or eruption is held in balance by dental abrasion from chewing a diet high in fiber.

Top Rodent Incisor
Buccal view of top incisor from Rattus rattus. Top incisor outlined in yellow. Molars circled in blue.
Lower Incisor of Rattus rattus, Buccal View
Buccal view of the lower incisor from the right dentary of a Rattus rattus.
Lower Incisor of Rattus rattus, Lingual View
Lingual view of the lower incisor from the right dentary of a Rattus rattus.
Top Rodent Incisor, Midsagittal View
Midsagittal view of top incisor from Rattus rattus. Top incisor outlined in yellow. Molars circled in blue.

Rodents

Rodents have upper and lower hypselodont incisors that can continuously grow enamel throughout its life without having properly formed roots.[19] These teeth are also known as aradicular teeth, and unlike humans whose ameloblasts die after tooth development, rodents continually produce enamel, they must wear down their teeth by gnawing on various materials.[20] Enamel and dentin are produced by the enamel organ, and growth is dependent on the presence of stem cells, cellular amplification, and cellular maturation structures in the odontogenic region.[21] Rodent incisors are used for cutting wood, biting through the skin of fruit, or for defense. This allows for the rate of wear and tooth growth to be at equilibrium.[19] The microstructure of rodent incisor enamel has shown to be useful in studying the phylogeny and systematics of rodents because of its independent evolution from the other dental traits. The enamel on rodent incisors are composed of two layers: the inner portio interna (PI) with Hunter-Schreger bands (HSB) and an outer portio externa (PE) with radial enamel (RE).[22] It usually involves the differential regulation of the epithelial stem cell niche in the tooth of two rodent species, such as guinea pigs.[23][24]

Top Rodent Incisor, Lingual View
Lingual view of top incisor from Rattus rattus. Top incisor outlined in yellow. Molars circled in blue.

The teeth have enamel on the outside and exposed dentin on the inside, so they self-sharpen during gnawing. On the other hand, continually growing molars are found in some rodent species, such as the sibling vole and the guinea pig.[23][24][23][24] There is variation in the dentition of the rodents, but generally, rodents lack canines and premolars, and have a space between their incisors and molars, called the diastema region.

Manatee

Manatees are polyphyodont with mandibular molars developing separately from the jaw and are encased in a bony shell separated by soft tissue.

Walrus

Walrus tusks are canine teeth that grow continuously throughout life.[25]

Fish

Fish, such as sharks, may go through many teeth in their lifetime. The replacement of multiple teeth is known as polyphyodontia.

A class of prehistoric shark are called cladodonts for their strange forked teeth.

Amphibians

All amphibians have pedicellate teeth which are modified to be flexible due to connective tissue and uncalcified dentine that separates the crown from the base of the tooth.[26]

Most amphibians exhibit teeth that have a slight attachment to the jaw or acrodont teeth. Acrodont teeth exhibit limited connection to the dentary and have little enervation.[27] This is ideal for organisms who mostly use their teeth for grasping, but not for crushing and allows for rapid regeneration of teeth at a low energy cost. Teeth are usually lost in the course of feeding if the prey is struggling. Additionally, amphibians that undergo a metamorphosis develop bicuspid shaped teeth.[28]

Reptiles

The teeth of reptiles are replaced constantly during their life. Juvenile crocodilians replace teeth with larger ones at a rate as high as one new tooth per socket every month. Once adult, tooth replacement rates can slow to two years and even longer. Overall, crocodilians may use 3,000 teeth from birth to death. New teeth are created within old teeth.

Birds

A skull of Ichthyornis discovered in 2014 suggests that the beak of birds may have evolved from teeth to allow chicks to escape their shells earlier, and thus avoid predators and also to penetrate protective covers such as hard earth to access underlying food.[29][30]

Invertebrates

Svømmende blodigle
The European medicinal leech has three jaws with numerous sharp teeth which function like little saws for incising a host.

True teeth are unique to vertebrates,[31] although many invertebrates have analogous structures often referred to as teeth. The organisms with the simplest genome bearing such tooth-like structures are perhaps the parasitic worms of the family Ancylostomatidae.[32] For example, the hookworm Necator americanus has two dorsal and two ventral cutting plates or teeth around the anterior margin of the buccal capsule. It also has a pair of subdorsal and a pair of subventral teeth located close to the rear.[33]

Historically the European medicinal leech, another invertebrate parasite, has been used in medicine to remove blood from patients.[34] They have three jaws (tripartite) that look like little saws, and on them are about 100 sharp teeth used to incise the host. The incision leaves a mark that is an inverted Y inside of a circle. After piercing the skin and injecting anticoagulants (hirudin) and anaesthetics, they suck out blood, consuming up to ten times their body weight in a single meal.[35]

In some species of Bryozoa, the first part of the stomach forms a muscular gizzard lined with chitinous teeth that crush armoured prey such as diatoms. Wave-like peristaltic contractions then move the food through the stomach for digestion.[36]

Radula docoglosse
The limpet rasps algae from rocks using teeth with the strongest known tensile strength of any biological material

Molluscs have a structure called a radula which bears a ribbon of chitinous teeth. However, these teeth are histologically and developmentally different from vertebrate teeth and are unlikely to be homologous. For example, vertebrate teeth develop from a neural crest mesenchyme-derived dental papilla, and the neural crest is specific to vertebrates, as are tissues such as enamel.[31]

The radula is used by molluscs for feeding and is sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, typically used for scraping or cutting food before the food enters the oesophagus. The radula is unique to molluscs, and is found in every class of mollusc apart from bivalves.

Within the gastropods, the radula is used in feeding by both herbivorous and carnivorous snails and slugs. The arrangement of teeth (also known as denticles) on the radula ribbon varies considerably from one group to another as shown in the diagram on the left.

Predatory marine snails such as the Naticidae use the radula plus an acidic secretion to bore through the shell of other molluscs. Other predatory marine snails, such as the Conidae, use a specialized radula tooth as a poisoned harpoon. Predatory pulmonate land slugs, such as the ghost slug, use elongated razor-sharp teeth on the radula to seize and devour earthworms. Predatory cephalopods, such as squid, use the radula for cutting prey.

In most of the more ancient lineages of gastropods, the radula is used to graze by scraping diatoms and other microscopic algae off rock surfaces and other substrates. Limpets scrape algae from rocks using radula equipped with exceptionally hard rasping teeth.[37] These teeth have the strongest known tensile strength of any biological material, outperforming spider silk.[37] The mineral protein of the limpet teeth can withstand a tensile stress of 4.9 GPa, compared to 4 GPa of spider silk and 0.5 GPa of human teeth.[38]

Fossilization and taphonomy

Because teeth are very resistant, often preserved when bones are not,[39] and reflect the diet of the host organism, they are very valuable to archaeologists and palaeontologists.[40] Early fish such as the thelodonts had scales composed of dentine and an enamel-like compound, suggesting that the origin of teeth was from scales which were retained in the mouth. Fish as early as the late Cambrian had dentine in their exoskeleton, which may have functioned in defense or for sensing their environment.[41] Dentine can be as hard as the rest of teeth and is composed of collagen fibres, reinforced with hydroxyapatite.[41]

Though teeth are very resistant, they also can be brittle and highly susceptible to cracking.[42] However, cracking of the tooth can be used as a diagnostic tool for predicting bite force. Additionally, enamel fractures can also give valuable insight into the diet and behaviour of archaeological and fossil samples.

Decalcification removes the enamel from teeth and leaves only the organic interior intact, which comprises dentine and cementine.[43] Enamel is quickly decalcified in acids,[44] perhaps by dissolution by plant acids or via diagenetic solutions, or in the stomachs of vertebrate predators.[43] Enamel can be lost by abrasion or spalling,[43] and is lost before dentine or bone are destroyed by the fossilisation process.[44] In such a case, the 'skeleton' of the teeth would consist of the dentine, with a hollow pulp cavity.[43] The organic part of dentine, conversely, is destroyed by alkalis.[44]

See also

References

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  2. ^ Tummers M, Thesleff I (March 2003). "Root or crown: a developmental choice orchestrated by the differential regulation of the epithelial stem cell niche in the tooth of two rodent species" (PDF). Development. 130 (6): 1049–57. doi:10.1242/dev.00332. PMID 12571097. Retrieved 28 February 2017.
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  11. ^ "Gummed Out: Young Horses Lose Many Teeth, Vet Says". Archived from the original on 8 July 2014. Retrieved 6 July 2014.
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  18. ^ Ryšavy, Robin. The Missouri House Rabbit Society, hosted by the Kansas City Missouri House Rabbit Society. Page accessed April 9, 2007.
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  36. ^ Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). "Lophoporata". Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 829–845. ISBN 978-0-03-025982-1.CS1 maint: Multiple names: authors list (link)
  37. ^ a b Barber, Lu, Pugno (18 February 2015), "Extreme strength observed in limpet teeth", Interface, Vol. 12 (105): 20141326, doi:10.1098/rsif.2014.1326, PMC 4387522, PMID 25694539CS1 maint: Multiple names: authors list (link)
  38. ^ Zachary Davies Boren (18 February 2015). "The strongest materials in the world: Limpet teeth beats record resistance of spider silk". The Independent. Retrieved 20 February 2015.
  39. ^ Taphonomy: A Process Approach. Ronald E. Martin. Illustrated edition. Cambridge University Press, 1999. ISBN 978-0-521-59833-0
  40. ^ "(PDF) Behavioral inferences from the high levels of dental chipping in Homo naledi". ResearchGate. Retrieved 2019-01-09.
  41. ^ a b Teaford, Mark F and Smith, Moya Meredith, 2007. Development, Function and Evolution of Teeth, Cambridge University Press. ISBN 978-0-521-03372-5, Chapter 5.
  42. ^ Lee, James J.‐W.; Constantino, Paul J.; Lucas, Peter W.; Lawn, Brian R. (2011-11-01). "Fracture in teeth—a diagnostic for inferring bite force and tooth function". Biological Reviews. 86 (4): 959–974. doi:10.1111/j.1469-185x.2011.00181.x. ISSN 1469-185X. PMID 21507194.
  43. ^ a b c d Fisher, Daniel C (1981). "Taphonomic Interpretation of Enamel-Less Teeth in the Shotgun Local Fauna (Paleocene, Wyoming)". Museum of Paleontology Contributions, the University of Michigan. 25 (13): 259–275. hdl:2027.42/48503.
  44. ^ a b c Fernandez-Jalvo, Y.; Sanchez-Chillon, B.; Andrews, P.; Fernandez-Lopez, S.; Alcala Martinez, L. (2002). "Morphological taphonomic transformations of fossil bones in continental environments, and repercussions on their chemical composition". Archaeometry. 44 (3): 353–361. doi:10.1111/1475-4754.t01-1-00068.

External links

Canine tooth

In mammalian oral anatomy, the canine teeth, also called cuspids, dog teeth, fangs, or (in the case of those of the upper jaw) eye teeth, are relatively long, pointed teeth. However, they can appear more flattened, causing them to resemble incisors and leading them to be called incisiform. They developed and are used primarily for firmly holding food in order to tear it apart, and occasionally as weapons. They are often the largest teeth in a mammal's mouth. Individuals of most species that develop them normally have four, two in the upper jaw and two in the lower, separated within each jaw by incisors; humans and dogs are examples. In most species, canines are the anterior-most teeth in the maxillary bone.

The four canines in humans are the two maxillary canines and the two mandibular canines.

Charcot–Marie–Tooth disease

Charcot–Marie–Tooth disease (CMT) is one of the hereditary motor and sensory neuropathies, a group of varied inherited disorders of the peripheral nervous system characterized by progressive loss of muscle tissue and touch sensation across various parts of the body. Currently incurable, this disease is the most commonly inherited neurological disorder, and affects about one in 2,500 people. CMT was previously classified as a subtype of muscular dystrophy.

Eye for an eye

"An eye for an eye", "a tooth for a tooth" or the law of retaliation, is the principle that a person who has injured another person is to be penalized to a similar degree, and the person inflicting such punishment should be the injured party. In softer interpretations, it means the victim receives the [estimated] value of the injury in compensation. The intent behind the principle was to restrict compensation to the value of the loss.The principle is sometimes referred using the Latin term lex talionis or the law of talion. The English word talion (from the Latin talio) means a retaliation authorized by law, in which the punishment corresponds in kind and degree to the injury.

Gear

A gear or cogwheel is a rotating machine part having cut teeth, or in the case of a cogwheel, inserted teeth (called cogs), which mesh with another toothed part to transmit torque. Geared devices can change the speed, torque, and direction of a power source. Gears almost always produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine. The teeth on the two meshing gears all have the same shape. Two or more meshing gears, working in a sequence, are called a gear train or a transmission. A gear can mesh with a linear toothed part, called a rack, producing translation instead of rotation.

The gears in a transmission are analogous to the wheels in a crossed, belt pulley system. An advantage of gears is that the teeth of a gear prevent slippage.

When two gears mesh, if one gear is bigger than the other, a mechanical advantage is produced, with the rotational speeds, and the torques, of the two gears differing in proportion to their diameters.

In transmissions with multiple gear ratios—such as bicycles, motorcycles, and cars—the term "gear" as in "first gear" refers to a gear ratio rather than an actual physical gear. The term describes similar devices, even when the gear ratio is continuous rather than discrete, or when the device does not actually contain gears, as in a continuously variable transmission.

Incisor

Incisors (from Latin incidere, "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, whereas armadillos have none.

Molar (tooth)

The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name molar derives from Latin, molaris dens, meaning "millstone tooth", from mola, millstone and dens, tooth. Molars show a great deal of diversity in size and shape across mammal groups.

Saber-toothed cat

A saber-toothed cat (alternatively spelled sabre-toothed cat) is any member of various extinct groups of predatory mammals that were characterized by long, curved saber-shaped canine teeth. The large maxillary canine teeth extended from the mouth even when it was closed. The saber-toothed cats were found worldwide from the Eocene epoch to the end of the Pleistocene epoch (42 million years ago (mya) – 11,000 years ago), existing for about 42 million years.One of the best-known genera is Smilodon, species of which, especially S. fatalis, are popularly, but incorrectly referred as a "saber-toothed tiger," a genus within the subfamily Machairodontinae of the carnivoran family Felidae. Extant members of Felidae include cats of the subfamilies Felinae and Pantherinae.

However, usage of the word cat is in some cases a misnomer, as many species referred to as saber-toothed "cats" are not closely related to modern cats of Felidae: instead, many are members of other feliform carnivoran families, such as Barbourofelidae and Nimravidae; the oxyaenid "creodont" genera Machaeroides and Apataelurus; and two lineages of metatherian mammals, the thylacosmilids of Sparassodonta, and deltatheroideans, which are more closely related to marsupials than to the placental mammals of the other orders mentioned. In this regard, saber-toothed cats can be viewed as examples of convergent evolution. This convergence is remarkable due not only to the development of elongated canines, but also a suite of other characteristics, such as a wide gape and bulky forelimbs, that is so consistent that it has been termed the "saber-tooth suite."Of the feliform lineages, the family Nimravidae is the oldest, entering the landscape around 42 mya and becoming extinct by 7.2 mya. Barbourofelidae entered around 16.9 mya and were extinct by 9 mya. These two would have shared some habitats.

Smilodon

Smilodon is a genus of the extinct machairodont subfamily of the felids. It is one of the most famous prehistoric mammals, and the best known saber-toothed cat. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats. Smilodon lived in the Americas during the Pleistocene epoch (2.5 mya–10,000 years ago). The genus was named in 1842, based on fossils from Brazil. Three species are recognized today: S. gracilis, S. fatalis, and S. populator. The two latter species were probably descended from S. gracilis, which itself probably evolved from Megantereon. The hundreds of individuals obtained from the La Brea Tar Pits in Los Angeles constitute the largest collection of Smilodon fossils.

Overall, Smilodon was more robustly built than any extant cat, with particularly well-developed forelimbs and exceptionally long upper canine teeth. Its jaw had a bigger gape than that of modern cats, and its upper canines were slender and fragile, being adapted for precision killing. S. gracilis was the smallest species at 55 to 100 kg (120 to 220 lb) in weight. S. fatalis had a weight of 160 to 280 kg (350 to 620 lb) and height of 100 cm (39 in). Both of these species are mainly known from North America, but remains from South America have also been attributed to them. S. populator from South America was the largest species, at 220 to 400 kg (490 to 880 lb) in weight and 120 cm (47 in) in height, and was among the largest known felids. The coat pattern of Smilodon is unknown, but it has been artistically restored with plain or spotted patterns.

In North America, Smilodon hunted large herbivores such as bison and camels, and it remained successful even when encountering new prey species in South America. Smilodon is thought to have killed its prey by holding it still with its forelimbs and biting it, but it is unclear in what manner the bite itself was delivered. Scientists debate whether Smilodon had a social or a solitary lifestyle; analysis of modern predator behavior as well as of Smilodon's fossil remains could be construed to lend support to either view. Smilodon probably lived in closed habitats such as forests and bush, which would have provided cover for ambushing prey. Smilodon died out at the same time that most North and South American megafauna disappeared, about 10,000 years ago. Its reliance on large animals has been proposed as the cause of its extinction, along with climate change and competition with other species, but the exact cause is unknown.

Temple of the Tooth

Sri Dalada Maligawa or the Temple of the Sacred Tooth Relic is a Buddhist temple in the city of Kandy, Sri Lanka. It is located in the royal palace complex of the former Kingdom of Kandy, which houses the relic of the tooth of the Buddha. Since ancient times, the relic has played an important role in local politics because it is believed that whoever holds the relic holds the governance of the country. Kandy was the last capital of the Sri Lankan kings and is a World Heritage Site mainly due to the temple.

Bhikkhus of the two chapters of Malwatte and Asgiriya conduct daily worship in the inner chamber of the temple. Rituals are performed three times daily: at dawn, at noon and in the evenings. On Wednesdays, there is a symbolic bathing of the relic with an herbal preparation made from scented water and fragrant flowers called Nanumura Mangallaya. This holy water is believed to contain healing powers and is distributed among those present.

The temple sustained damage from bombings by Janatha Vimukthi Peramuna in 1989 and by Liberation Tigers of Tamil Eelam in 1998 but was fully restored each time.

Tooth

A tooth (plural teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are covered by gums. Teeth are not made of bone, but rather of multiple tissues of varying density and hardness. The cellular tissues that ultimately become teeth originate from the embryonic germ layer, the ectoderm.

The general structure of teeth is similar across the vertebrates, although there is considerable variation in their form and position. The teeth of mammals have deep roots, and this pattern is also found in some fish, and in crocodilians. In most teleost fish, however, the teeth are attached to the outer surface of the bone, while in lizards they are attached to the inner surface of the jaw by one side. In cartilaginous fish, such as sharks, the teeth are attached by tough ligaments to the hoops of cartilage that form the jaw.Some animals develop only one set of teeth (monophyodont) while others develop many sets (polyphyodont). Sharks, for example, grow a new set of teeth every two weeks to replace worn teeth. Rodent incisors grow and wear away continually through gnawing, which helps maintain relatively constant length. The industry of the beaver is due in part to this qualification. Many rodents such as voles and guinea pigs, but not mice, as well as leporidae like rabbits, have continuously growing molars in addition to incisors.Teeth are not always attached to the jaw, as they are in mammals. In many reptiles and fish, teeth are attached to the palate or to the floor of the mouth, forming additional rows inside those on the jaws proper. Some teleosts even have teeth in the pharynx. While not true teeth in the usual sense, the dermal denticles of sharks are almost identical in structure and are likely to have the same evolutionary origin. Indeed, teeth appear to have first evolved in sharks, and are not found in the more primitive jawless fish – while lampreys do have tooth-like structures on the tongue, these are in fact, composed of keratin, not of dentine or enamel, and bear no relationship to true teeth. Though "modern" teeth-like structures with dentine and enamel have been found in late conodonts, they are now supposed to have evolved independently of later vertebrates' teeth.Living amphibians typically have small teeth, or none at all, since they commonly feed only on soft foods. In reptiles, teeth are generally simple and conical in shape, although there is some variation between species, most notably the venom-injecting fangs of snakes. The pattern of incisors, canines, premolars and molars is found only in mammals, and to varying extents, in their evolutionary ancestors. The numbers of these types of teeth vary greatly between species; zoologists use a standardised dental formula to describe the precise pattern in any given group.

Tooth decay

Tooth decay, also known as dental caries or cavities, is a breakdown of teeth due to acids made by bacteria. The cavities may be a number of different colors from yellow to black. Symptoms may include pain and difficulty with eating. Complications may include inflammation of the tissue around the tooth, tooth loss, and infection or abscess formation.The cause of cavities is acid from bacteria dissolving the hard tissues of the teeth (enamel, dentin and cementum). The acid is produced by the bacteria when they break down food debris or sugar on the tooth surface. Simple sugars in food are these bacteria's primary energy source and thus a diet high in simple sugar is a risk factor. If mineral breakdown is greater than build up from sources such as saliva, caries results. Risk factors include conditions that result in less saliva such as: diabetes mellitus, Sjogren's syndrome and some medications. Medications that decrease saliva production include antihistamines and antidepressants. Caries is also associated with poverty, poor cleaning of the mouth, and receding gums resulting in exposure of the roots of the teeth.Prevention of dental caries includes regular cleaning of the teeth, a diet low in sugar, and small amounts of fluoride. Brushing the teeth twice per day and flossing between the teeth once a day is recommended by many. Fluoride may be from water, salt or toothpaste among other sources. Treating a mother's dental caries may decrease the risk in her children by decreasing the numbers of certain bacteria she may spread to them. Screening can result in earlier detection. Depending on the extent of destruction, various treatments can be used to restore the tooth to proper function or the tooth may be removed. There is no known method to grow back large amounts of tooth. The availability of treatment is often poor in the developing world. Paracetamol (acetaminophen) or ibuprofen may be taken for pain.Worldwide, approximately 2.3 billion people (32% of the population) have dental caries in their permanent teeth. The World Health Organization estimates that nearly all adults have dental caries at some point in time. In baby teeth it affects about 620 million people or 9% of the population. They have become more common in both children and adults in recent years. The disease is most common in the developed world due to greater simple sugar consumption and less common in the developing world. Caries is Latin for "rottenness".

Tooth enamel

Tooth enamel is one of the four major tissues that make up the tooth in humans and many other animals, including some species of fish. It makes up the normally visible part of the tooth, covering the crown. The other major tissues are dentin, cementum, and dental pulp. It is a very hard, white to off-white, highly mineralised substance that acts as a barrier to protect the tooth but can become susceptible to degradation, especially by acids from food and drink. In rare circumstances enamel fails to form, leaving the underlying dentine exposed on the surface.

Tooth fairy

The Tooth Fairy is a fantasy figure of early childhood in Western and Western-influenced cultures. The folklore states that when children lose one of their baby teeth, they should place it underneath their pillow or on their bedside table and the Tooth Fairy will visit while they sleep, replacing the lost tooth with a small payment.The tradition of leaving a tooth under a pillow for the Tooth Fairy to collect is practiced in various countries.

Wisdom tooth

A wisdom tooth or third molar is one of the three molars per quadrant of the human dentition. It is the most posterior of the three. The age at which wisdom teeth come through (erupt) is variable, but generally occurs between late teens and early twenties. Most adults have four wisdom teeth, one in each of the four quadrants, but it is possible to have none, fewer, or more, in which case the extras are called supernumerary teeth. Wisdom teeth may get stuck (impacted) against other teeth if there is not enough space for them to come through normally. While this does not cause movement of other teeth, it can cause tooth decay if the impaction makes oral hygiene difficult. Wisdom teeth which are partially erupted through the gum may also cause inflammation and infection in the surrounding gum tissues, termed pericoronitis. Wisdom teeth are often extracted when or even before these problems occur. However some recommend against the prophylactic extraction of disease-free impacted wisdom teeth.

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