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.

Molar
Lower wisdom tooth
A lower wisdom tooth after extraction.
Gray997
Permanent teeth of right half of lower dental arch, seen from above: In this diagram, a healthy wisdom tooth (third, rearmost molar) is included.
Details
ArteryPosterior superior alveolar artery
Identifiers
Latindentes molares
MeSHD008963
TAA05.1.03.007
FMA55638
Anatomical terminology
3D Medical Animation Still Showing Types of Teeth
Image showing molar teeth and their arrangement in the mouth of an adult human being

Human anatomy

In humans, the molar teeth have either four or five cusps. Adult humans have 12 molars, in four groups of three at the back of the mouth. The third, rearmost molar in each group is called a wisdom tooth. It is the last tooth to appear, breaking through the front of the gum at about the age of 20, although this varies from individual to individual. Ethnicity can also affect the age at which this occurs, with statistical variations between groups.[1] In some cases, it may not even erupt at all.

The human mouth contains upper (maxillary) and lower (mandibular) molars. They are: maxillary first molar, maxillary second molar, maxillary third molar, mandibular first molar, mandibular second molar, and mandibular third molar.

Mammal evolution

In mammals, the crown of the molars and premolars is folded into a wide range of complex shapes. The basic elements of the crown are the more or less conical projections called cusps and the valleys that separate them. The cusps contain both dentine and enamel, whereas minor projections on the crown, called crenullations, are the result of different enamel thickness. Cusps are occasionally joined to form ridges and expanded to form crests. Cingula are often incomplete ridges that pass around the base of the crown.[2]

These mammalian, multicusped cheek teeth probably evolved from single-cusped teeth in reptilians, although the diversity of therapsid molar patterns and the complexity in the molars of the earliest mammals make determining how this happened impossible. According to the widely accepted "differentiation theory", additional cusps have arisen by budding or outgrowth from the crown, while the rivalling "concrescence theory" instead proposes that complex teeth evolved by the clustering of originally separate conical teeth. Therian mammals (placentals and marsupials) are generally agreed to have evolved from an ancestor with tribosphenic cheek teeth, with three main cusps arranged in a triangle.[2]

Cambridge Natural History Mammalia Fig 039
Comparison of cheek teeth in various taxa: 1, reptile; 2, Dromatherium (a Triassic cynodont); 3, Microconodon (a Triassic eucynodont); 4, Spalacotherium (a Cretaceous symmetrodont); 5, Amphitherium (a Jurassic mammal)

Morphology

Australosphenidan molar labeled
Generalized cusp of a mammalian molar: ant, anterior; pos, posterior; ci, lingual cingulum; pa, paraconid; pr, protoconid; me, metaconid; hy, hypoconid; hl, hypoconulid; ec, entocristid; tb, talonid basin

Each major cusp on an upper molar is called a cone and is identified by a prefix dependent on its relative location on the tooth: proto-, para-, meta-, hypo-, and ento-. Suffixes are added to these names: -id is added to cusps on a lower molar (e.g., protoconid); -ule to a minor cusp (e.g., protoconulid). A shelf-like ridge on the lower part of the crown (on an upper molar) is called a cingulum; the same feature on the lower molar a cingulid, and a minor cusp on these, for example, a cingular cuspule or conulid.[3]

Tribosphenic

Tribosphenic molar
Generalized tribosphenic molar: The protocone is on the lingual (tongue) side, while the anterior 'paracone and posterior metacone are on the buccal (cheek) side of the jaw).

The design that is considered one of the most important characteristics of mammals is a three-cusped shape called a tribosphenic molar. This molar design has two important features: the trigonid, or shearing end, and the talonid, or crushing heel. In modern tribosphenic molars, the trigonid is towards the front of the jaw and the talonid is towards the rear.

The tribosphenic tooth is found in insectivores and young platypuses (adults have no teeth). Upper molars look like three-pointed mountain ranges; lowers look like two peaks and a third off to the side.

The tribosphenic design appears primitively in all groups of mammals. Some paleontologists believe that it developed independently in monotremes (or australosphenidans), rather than being inherited from an ancestor that they share with marsupials and placentals (or boreosphenidans); but this idea has critics and the debate is still going on.[4] For example, the dentition of the Early Cretaceous monotreme Steropodon is similar to those of Peramus and dryolestoids, which suggests that monotremes are related to some pre-tribosphenic therian mammals,[5] but, on the other hand, the status of neither of these two groups is well-established.

Some Jurassic mammals, such as Shuotherium and Pseudotribos, have "reversed tribosphenic" molars, in which the talonid is towards the front. This variant is regarded as an example of convergent evolution.[6]

From the primitive tribosphenic tooth, molars have diversified into several unique morphologies. In many groups, a fourth cusp, the hypocone (hypoconid), subsequently evolved (see below).

Quadrate

Pig tooth
Pig tooth

Quadrate (also called quadritubercular or euthemorphic) molars have an additional fourth cusp on the lingual (tongue) side called the hypocone, located posterior to the protocone. Quadrate molars appeared early in mammal evolution and are present in many species, including hedgehogs, raccoons, and many primates, including humans.[7] There may be a fifth cusp.

In many mammals, additional smaller cusps called conules appear between the larger cusps. They are named after their locations, e.g. a paraconule is located between a paracone and a metacone, a hypoconulid is located between a hypoconid and an entoconid.[7]

Bunodont

Chimpjaw
Upper and lower dentition of a chimpanzee

In bunodont molars, the cusps are low and rounded hills rather than sharp peaks. They are most common among omnivores such as pigs, bears, and humans.[7] Bunodont molars are effective crushing devices and often basically quadrate in shape.[8]

Hypsodont

Hypsodont dentition is characterized by high-crowned teeth and enamel that extends far past the gum line, which provides extra material for wear and tear.[9] Some examples of animals with hypsodont dentition are cattle and horses, all animals that feed on gritty, fibrous material. Hypsodont molars can continue to grow throughout life, for example in some species of Arvicolinae (herbivorous rodents).[7]

Hypsodont molars lack both a crown and a neck. The occlusal surface is rough and mostly flat, adapted for crushing and grinding plant material. The body is covered with cementum both above and below the gingival line, below which is a layer of enamel covering the entire length of the body. The cementum and the enamel invaginate into the thick layer of dentin.[10]

Brachydont

The opposite condition to hypsodont is called brachydont or brachyodont (from brachys, "short"). It is a type of dentition characterized by low-crowned teeth. Human teeth are brachydont.[7]

A brachydont tooth has a crown above the gingival line and a neck just below it, and at least one root. A cap of enamel covers the crown and extends down to the neck. Cementum is only found below the gingival line. The occlusal surfaces tend to be pointed, well-suited for holding prey and tearing and shredding.[10]

Zalambdodont

Zalambdodont molars have three cusps, one larger on the lingual side and two smaller on the labial side, joined by two crests that form a V- or λ-shape. The larger inner cusp might be homologous with the paracone in a tribosphenic molar, but can also be fused with the metacone. The protocone is typically missing. The two smaller labial cusps are located on an expanded shelf called the stylar shelf. Zalambdodont molars are found in, for example, golden moles and solenodons.[7]

Dilambdodont

Like zalambdodont molars, dilambdodont molars have a distinct ectoloph, but are shaped like two lambdas or a W. On the lingual side, at the bottom of the W, are the metacone and paracone, and the stylar shelf is on the labial side. A protocone is present lingual to the ectoloph. Dilambdodont molars are present in shrews, moles, and some insectivorous bats.[7]

Lophodont

Elefantenzaehne-drawing
Lophodont molars of Elephas (left) and Loxodonta (center), compared to the nonlophodont mastodon (right)

Lophodont teeth are easily identified by the differentiating patterns of ridges or lophs of enamel interconnecting the cusps on the crowns. Present in most herbivores, these patterns of lophs can be a simple, ring-like edge, as in mole rats, or a complex arrangement of series of ridges and cross-ridges, as those in odd-toed ungulates, such as equids.[8]

Lophodont molars have hard and elongated enamel ridges called lophs oriented either along or perpendicular to the dental row. Lophodont molars are common in herbivores that grind their food thoroughly. Examples include tapirs, manatees, and many rodents.[7]

When two lophs form transverse, often ring-shaped, ridges on a tooth, the arrangement is called bilophodont. This pattern is common in primates, but can also be found in lagomorphs (hares, rabbits, and pikas) and some rodents.[7][8]

Extreme forms of lophodonty in elephants and some rodents (such as Otomys) is known as loxodonty.[7] The African elephant belongs to a genus called Loxodonta because of this feature.

Selenodont

In selenodont molars (so-named after moon goddess Selene), the major cusp is elongated into crescent-shaped ridge. Examples include most even-toed ungulates, such as cattle and deer.[7][8]

Secodont

Carnassials
Carnassials of a Eurasian wolf

Many carnivorous mammals have enlarged and blade-like teeth especially adapted for slicing and chopping called carnassials. A general term for such blade-like teeth is secodont or plagiaulacoid.[7]

See also

Notes

  1. ^ Rozkovcová, E; Marková, M; Dolejsí, J (1999). "Studies on agenesis of third molars amongst populations of different origin". Sbornik Lekarsky. 100 (2): 71–84. PMID 11220165.
  2. ^ a b Zhao, Weiss & Stock 2000, Acquisition of multi-cusped cheek teeth in mammals, p. 154
  3. ^ Myers et al. 2013b
  4. ^ Stokstad 2001
  5. ^ Luo, Cifelli & Kielan-Jaworowska 2001
  6. ^ Luo, Ji & Yuan 2007
  7. ^ a b c d e f g h i j k l Myers et al. 2013a
  8. ^ a b c d Lawlor 1979, pp. 13–4
  9. ^ Flynn, Wyss & Charrier 2007
  10. ^ a b Kwan, Paul W.L. (2007). "Digestive system I" (PDF). Tufts University. Retrieved May 2013. Check date values in: |accessdate= (help)

References

External links

Anachlysictis

Anachlysictis gracilis is an extinct carnivorous mammal belonging to the group Sparassodonta, which were metatherians (a group including marsupials and their close relatives) that inhabited South America during the Cenozoic. Unlike other remains assigned to the family Thylacosmilidae (a group of metatherian predators equipped with "saber teeth") that had been found previously, Anachlysictis is the first record of such borhyaenoids in northern South America, and also most primitive and ancient in the family (in fact, is the first confirmed record that did not belong to the genus Thylacosmilus, until the official publication of Patagosmilus in 2010). This species was found in the Villavieja Formation in the area of La Venta in Colombia, a famous fossil deposit in the Middle Miocene (Laventan; 13.8-11.8 million years ago), based on fragments that include a front portion of the lower jaw, with an incipient molar tooth and a piece of carnassial from the front of the maxilla.

Cervix (disambiguation)

A cervix or collum is a neck, that is, a narrowed region of an object (such as a body or a body part). In anatomy, various body parts are called necks, with the neck (of the body) and the neck of the uterus (the uterine cervix) being major examples. A list of examples includes:

Neck, the narrowed region of the body between the torso and the head

Uterine cervix, usually just called the cervix when the context is implicit

Cervix vesicae urinariae, the neck of the urinary bladder

Cervix cornus dorsalis medullae spinalis or cervix cornus posterioris medullae spinalis, the neck of the posterior grey column (the posterior horn of the spinal cord)

Cervix dentis, the neck of a tooth (a slightly narrowed area where the crown meets the root, such as on a molar tooth)

Cervix (insect anatomy), a membrane that separates the head from the thorax in insects

Coronoid process of the mandible

The mandible's coronoid process (from Greek korone, "like a crown") is a thin, triangular eminence, which is flattened from side to side and varies in shape and size.

Its anterior border is convex and is continuous below with the anterior border of the ramus.

Its posterior border is concave and forms the anterior boundary of the mandibular notch.

Its lateral surface is smooth, and affords insertion to the Temporalis and Masseter.

Its medial surface gives insertion to the Temporalis, and presents a ridge which begins near the apex of the process and runs downward and forward to the inner side of the last molar tooth.

Between this ridge and the anterior border is a grooved triangular area, the upper part of which gives attachment to the Temporalis, the lower part to some fibers of the Buccinator.

Cutthroat Peak

Cutthroat Peak is a granitic mountain located on the boundary of Chelan County and Skagit County, in Washington. The mountain is part of the Okanagan Range in the Cascade Range. Cutthroat Peak is about two miles west of Washington Pass and one mile east of Rainy Pass. It's a prominent landmark along the North Cascades Highway. There is also a Cutthroat Lake, Cutthroat Creek, and Cutthroat Pass on its north and east aspects. Molar Tooth is a granite pillar half a mile north on the ridge extending to Cutthroat Pass.

European dhole

The European dhole (Cuon alpinus europaeus) was a paleosubspecies of the dhole which ranged throughout much of Western and Central Europe during the Middle and Late Pleistocene. Like the modern Asiatic populations, it was a more progressive form than other prehistoric members of the genus Cuon, having transformed its lower molar tooth into a single cusped slicer. It was virtually indistinguishable from its modern counterpart, save for its greater size, which closely approached that of the gray wolf.It became extinct in much of Europe during the late Würm period, though it may have survived in the Iberian Peninsula up until the early Holocene. One factor contributing to its extinction may have been interspecific competition with grey wolves and other wolf-like canids.

Flat-faced fruit-eating bat

The flat-faced fruit-eating bat (Artibeus planirostris) is a South American species of bat in the family Phyllostomidae. It is sometimes considered a subspecies of the Jamaican fruit bat, but can be distinguished by its larger size, the presence of faint stripes on the face, and of a third molar tooth on each side of the upper jaw. Genetic analysis has also shown that the two species may not be closely related.

Gondtherium

Gondtherium is a genus of extinct mammaliaform from the Kota Formation in India. It was considered a docodontan mammal by those who described it, but it remains unclear if this is the case.

Gondtherium was found in the Kota Formation, which is considered to be between Middle Jurassic and Early Cretaceous in age. Other Mesozoic mammals found there include members of Morganucodonta and Amphilestidae. The authors who described Gondtherium - which is known from only a single worn and fragmentary molar tooth - considered it to be a docodontan mammal based on the tooth cusps. However, this has been disputed by several subsequent researchers, and so the exact identity of Gondtherium remains unresolved.

Guimarotodon

Guimarotodon is an extinct mammal of the Upper Jurassic. It was a relatively early member of the also extinct order of Multituberculata. It made its living nibbling plants as great big, and small, dinosaurs roamed the world. (For the technically minded, suborder Plagiaulacida, family Paulchoffatiidae.)

Guimarotodon ("Guimarota tooth") Hahn G, 1969. "Guimarotodon Hahn 1969 exhibits a more slender Corpus mandibulae than either Paulchoffatia or Meketibolodon. The most conspicuous character of this genus is the morphology of the P3-4 and the M1.". The Corpus mandibulae is the part of the jaw below the tooth row. P3-4 are upper premolars, whilst M1 is an upper molar tooth.

"The incisor is relatively little curved and its root is of similar length as that of Meketibolodon and extends to underneath the posterior premolars." (Both quotations from Hahn & Hahn 2000, p. 105-106).

Inzigkofen

Inzigkofen is a municipality in the district of Sigmaringen in Baden-Württemberg in Germany. Historically, it is part of the Swabian north Alpine foreland basin.It consists of three districts:

Within Engelswies is the now-abandoned Talsberg quarry, known for its fossiliferous layers, and the site of evidence of the oldest Eurasian hominoids; a molar tooth found there in June 1973 was reported in June 2011 to have been "dated with relative precision at 17 to 17.1 Ma" (million years ago).

Kozarnika

Kozarnika or Peshtera Kozarnika (Bulgarian: Пещера Козарника, "The Goat Shed") is a cave in northwestern Bulgaria that was used as a hunters’ shelter as early as the Lower Paleolithic (1.6-1.4 million BP). It marks an older route of early human migration from Africa to Europe via the Balkans, prior to the currently suggested route across Gibraltar. The cave probably keeps the earliest evidence of human symbolic behaviour and the earliest European Gravette flint assemblages came to light here.

Kozarnika cave is located 6 km (4 mi) from the town of Belogradchik in northwestern Bulgaria, on the northern slopes of the Balkan Mountains, close to the Danubian Plain. It is opened to the south, at 85 m (279 ft) above the valley. With its length of 210 m (689 ft), the cave is among the small-sized in the Belogradchick karst region. Studies over the course of two decades uncovered 21 geological layers there, containing (bottom to top) archaeological complexes of Early Lower Paleolithic (layers 13 - 11a), Middle Paleolithic (layers 10b - 9a), Early Upper Paleolithic (layer 6/7), a sequence of an original Paleolithic bladelets industry with backed pieces that scholars called Kozarnikian (layers 5c - 3a), Early Neolithic, Late Copper age, Late Bronze Age, Medieval and Late medieval periods.The Kozarnika cave project started in 1984. Since 1996, it has been headed by Dr. Prof. Nikolay Sirakov (Archaeological Institute and Museum of Bulgarian Academy of Sciences, Sofia, Bulgaria) and Dr. Jean-Luc Guadelli (IPGQ-UMR5199 of French National Center for Scientific Research, Bordeaux-France).

In the ground layers, dated to 1.6–1.4 million BP (using palaeomagnetism, which determines age using past patterns of reversals in the Earth's magnetic field and analyses of both the microfauna and the macrofauna), archaeologists have discovered a human molar tooth (considered to be the earliest human—Homo erectus/Homo ergaster—traces discovered in Europe outside Caucasian region), lower palaeolithic assemblages that belong to a core-and-flake non-Acheulian industry, and incised bones that may be the earliest example of human symbolic behaviour.The findings from Middle Paleolithic layers (East Balkan Levallois cores and side-scrapers as well as East Balkan Levallois and Le Moustier points), rather bifacial points, dating from 300,000–50,000 BP prove presence of hunters’ groups possibly of Homo neanderthalensis. Upper Paleolithic layers consist flint assemblages from the earliest European Gravette complex dating from 43,000 up to 39,000 BP belonging to Homo sapiens.

In fieldwork since 2015, researchers have started to investigate the nature and impact paleo-human presence had on local fauna in order to establish a more accurate chronology of the occupation periods. In this context the research team also attempts to get a better understanding on the relationship between the Campanian Ignimbrite Eruption and its consequences on human occupation in the Kozarnika cave.

Krusatodon

Krusatodon is a genus of extinct docodont mammaliaform from the Middle Jurassic of the United Kingdom. It is known from the Forest Marble Formation, Kirtlington, in England, and also from a single molar tooth in the Kilmaluag Formation on the Isle of Skye, Scotland. Krusatodon is only known from a handful of individual molar teeth, but like all docodontans, these teeth have more complex cusps than other groups of early mammaliaformes. The name Krusatodon honours the palaeontologist Dr. George Krusat, who carried out important research on docodonts.

Molar

Molar may refer to:

Molar (tooth), the fourth kind of tooth in mammals

Molar (grape), another name for the Spanish wine grape Listan Negro

Molar (unit), a unit of concentration equal to 1 mole per litre

Molar mass

Molar volume

El Molar, Tarragona, a village in the comarca (county) of Priorat, province of Tarragona in the autonomous region of Catalonia, Spain

El Molar, Madrid, a town in the north of the Community of Madrid in the road to Burgos, after San Agustín de Guadalix

Molar Massif

Molar Massif (71°38′S 163°45′E) is a large mountain massif immediately east of the Lanterman Range in the Bowers Mountains of Antarctica. It was mapped by the United States Geological Survey from ground surveys and U.S. Navy air photos, 1960–64. The descriptive name was applied by the Advisory Committee on Antarctic Names; when viewed in plan, the outline of the massif resembles a molar tooth.

Molar Tooth (Washington)

Molar Tooth is a 7,547 ft granite summit located on the shared border of Okanogan County and Skagit County, in Washington state, USA. The mountain is part of the Okanagan Range which is a subset of the Cascade Range. Set in the Okanogan-Wenatchee National Forest, Molar Tooth is situated south of Cutthroat Pass, and 0.57 miles (0.92 km) north of Cutthroat Peak, which is also its nearest higher peak. The easiest climbing route is class 4 scrambling, but solid rock provides class 5 technical routes as well. Precipitation runoff from the east side of Molar Tooth drains into tributaries of the Methow River, whereas runoff from the west side drains into tributaries of the Skagit River.

Mount Azov

Azov (Russian: Азов) is a mountain in Central Ural, Russia. It is located 8 km from Polevskoy next to the Zyuzelsky village. It's one of the natural monuments of Russia.According to Aleksandr Matveyev, the configuration of the rocks gives a reason to believe that the name of the mountain derives from the Tatar word azaw teš (азау теш), meaning "molar tooth". In other Turkic languages the word means "fang", "edge", or "sting". The archaeologist E. Bers believed that in the Iron Age it served as the sacrificial place.

According to popular beliefs, the creature from the local folktales called Azovka (lit. "the Azov girl") lives inside Mount Azov.

Nanjing Man

Nanjing Man (Homo erectus nankinensis) is a subspecies of Homo erectus found in China. Large fragments of one male and one female skull and a molar tooth of H. e. nankinensis were discovered in 1993 in the Hulu cave on the Tangshan hills near Nanjing, the former capital city of China. The term Nanjing man is used to describe the subspecies of Homo erectus but is also used when referring to the three fossils. The specimens were found in the Hulu limestone cave at a depth of 60–97 cm by Liu Luhong, a local worker. Dating the fossils yielded an estimated age of 580,000 to 620,000 years old.

Paradental cyst

Paradental cysts constitute a family of inflammatory odontogenic cyst, that typically appear in relation to crown or root of partially erupted molar tooth. When the cyst is developed in the distal region of partially erupted third molar or in other locations in the dentition, it called simply paradental cyst, but the unique cyst that developed in the buccal bifurcation region of the mandibular first molars in the second half of the first decade of life is called buccal bifurcation cyst and has unique clinical features and management considerations in comparison to the other paradental cysts.

Posterior superior alveolar nerve

The posterior superior alveolar branches (posterior superior dental branches) arise from the trunk of the maxillary nerve just before it enters the infraorbital groove; they are generally two in number, but sometimes arise by a single trunk.

They descend on the tuberosity of the maxilla and give off several twigs to the gums and neighboring parts of the mucous membrane of the cheek.

They then enter the alveolar canals on the infratemporal surface of the maxilla, and, passing from behind forward in the substance of the bone, communicate with the middle superior alveolar nerve, and give off branches to the lining membrane of the maxillary sinus and gingival and dental branches to each molar tooth from a superior dental plexus; these branches enter the apical foramina at the roots of the teeth.

The posterior superior alveolar nerve innervates the second and third maxillary molars, and two of the three roots of the maxillary first molar (all but the mesiobuccal root). When giving a posterior superior alveolar nerve block, it will anesthetize the mesialbuccal root of the maxillary first molar approximately 72% of the time.

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 commonly affect other teeth as they develop, becoming impacted. They are often extracted when or even before this occurs.

Nomenclature
Teeth
Maxillary teeth
Mandibular teeth
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