Tooth
Tooth | |
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Details | |
Identifiers | |
Latin | dens |
MeSH | D014070 |
FMA | 12516 |
Anatomical terminology |
A tooth (pl.: teeth) is a hard,
The general structure of teeth is similar across the vertebrates, although there is considerable variation in their form and position. The
Monophyodonts are animals that develop only one set of teeth, while diphyodonts grow an early set of deciduous teeth and a later set of permanent or "adult" teeth. Polyphyodonts grow many sets of teeth. For example, sharks, grow a new set of teeth every two weeks to replace worn teeth. Most extant mammals including humans are diphyodonts, but there are exceptions including elephants, kangaroos, and manatees, all of which are polyphyodonts.
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. Some rodents, such as voles and guinea pigs (but not mice), as well as lagomorpha (rabbits, hares and pikas), have continuously growing molars in addition to incisors.[2][3] Also, tusks (in tusked mammals) grow almost throughout life.[4]
Teeth are not always attached to the jaw, as they are in mammals. In many
Living
Etymology
The word tooth comes from
The irregular plural form teeth is the result of Germanic umlaut whereby vowels immediately preceding a high vocalic in the following syllable were raised. As the nominative plural ending of the Proto-Germanic consonant stems (to which *tanþs belonged) was *-iz, the root vowel in the plural form *tanþiz (changed by this point to *tą̄þi via unrelated phonological processes) was raised to /œː/, and later unrounded to /eː/, resulting in the tōþ/tēþ alternation attested from Old English. Cf. also Old English bōc/bēċ 'book/books' and 'mūs/mȳs' 'mouse/mice', from Proto-Germanic *bōks/bōkiz and *mūs/mūsiz respectively.
Cognate with Latin dēns, Greek ὀδούς (odous), and Sanskrit dát.
Origin
Teeth are assumed to have evolved either from
The genes governing tooth development in mammals are
Mammals
Teeth are among the most distinctive (and long-lasting) features of
Mammals, in general, are
Aardvark
In
Canines
In
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.[14]
The toothed whale is a
Primates
In
Among primary teeth, 10 of them are usually found in the
Horse
An adult
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 110–130 mm (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 3 mm (1⁄8 in) 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
Elephants' tusks are specialized incisors for digging food up and fighting. Some elephant teeth are similar to those in manatees, and 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.[19][20]
Rabbit
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.
Rodents
The teeth have enamel on the outside and exposed dentin on the inside, so they
Manatee
Walrus
Walrus tusks are canine teeth that grow continuously throughout life.[32]
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.
Unlike the continuous shedding of functional teeth seen in modern sharks,
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.[38]
Most amphibians exhibit teeth that have a slight attachment to the jaw or
Reptiles
The teeth of
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.[42][43]
Invertebrates
True teeth are unique to vertebrates,
Historically, the
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.[49]
The radula is used by molluscs for feeding and is sometimes compared rather inaccurately to a
Within the
. 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
In most of the more ancient lineages of gastropods, the radula is used to graze by scraping
Fossilization and taphonomy
Because teeth are very resistant, often preserved when bones are not,
Though teeth are very resistant, they also can be brittle and highly susceptible to cracking.[55] 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.[56] Enamel is quickly decalcified in acids,[57] perhaps by dissolution by plant acids or via diagenetic solutions, or in the stomachs of vertebrate predators.[56] Enamel can be lost by abrasion or spalling,[56] and is lost before dentine or bone are destroyed by the fossilisation process.[57] In such a case, the 'skeleton' of the teeth would consist of the dentine, with a hollow pulp cavity.[56] The organic part of dentine, conversely, is destroyed by alkalis.[57]
See also
References
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- ^ "Ancylostoma duodenale". Nematode.net Genome Sequencing Center. Archived from the original on 2008-05-16. Retrieved 2009-10-27.
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Sources
- Shoshani, Jeheskel (2002). "Tubulidentata". In Robertson, Sarah (ed.). Encyclopedia of Life Sciences. Vol. 18: Svedberg, Theodor to Two-hybrid and Related Systems. London, UK: Nature Publishing Group. ISBN 978-1-56159-274-6.
External links
- Beach, Chandler B., ed. (1914). . . Chicago: F. E. Compton and Co.