View source for Molar (tooth)

{{Short description|Large tooth at the back of the mouth}}
{{Redirect|Molars|the unit of molar concentration|Molar (unit)|other uses|Molar (disambiguation)}}
{{Use dmy dates|date=July 2022}}
{{Infobox anatomy
| Name        =  Molar
| Latin       = dentes molares

| Image       = Lower wisdom tooth.jpg
| Caption     = A lower [[wisdom tooth]] after extraction.
| Width       = 
| Image2      = Gray997.png
| Caption2    = Permanent teeth of right half of lower dental arch, seen from above: In this diagram, a healthy [[wisdom tooth]] (third, rearmost molar) is included

| Precursor   = 
| System      = 
| Artery      = [[Posterior superior alveolar artery]]
| Vein        = 
| Nerve       = 
| Lymph       = 
}}The '''molars''' or '''molar teeth''' are large, flat [[tooth|teeth]] at the back of the [[mouth]]. They are more developed in [[mammal|mammals]]. They are used primarily to [[comminution|grind food]] during [[mastication|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 the mammal groups. The third molar of humans is sometimes [[vestigial]].

==Human anatomy==
{{Further|Dental anatomy|Human tooth}}
In humans, the molar teeth have either four or five [[cusp (dentistry)|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 [[Third molar|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 among individuals and populations, and in many cases the tooth is missing.<ref>{{cite journal|last=Rozkovcová|first=E|author2=Marková, M |author3=Dolejsí, J |title=Studies on agenesis of third molars amongst populations of different origin.|journal=Sbornik Lekarsky|year=1999|volume=100|issue=2|pages=71–84|pmid=11220165}}</ref>

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==
{{See also|Mammal tooth}}

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 crenulations, are the result of different enamel thickness. Cusps are occasionally joined to form ridges and expanded to form crests. [[Cingulum (tooth)|Cingula]] are often incomplete ridges that pass around the base of the crown.<ref name="Zhao-etal-2000-154" />

<onlyinclude>Mammalian, multicusped [[cheek teeth]] probably evolved from single-cusped teeth in synapsids, although the diversity of [[Therapsida|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. [[Theria]]n 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.<ref name="Zhao-etal-2000-154">{{Harvnb|Zhao|Weiss|Stock|2000|loc= Acquisition of multi-cusped cheek teeth in mammals, p. 154}}</ref>
[[File:Cambridge Natural History Mammalia Fig 039.png|thumb|center|650px|Comparison of cheek teeth in various taxa: '''1''', a single-cusped [[pelycosaur]]; 2, ''[[Dromatherium]]'' (a Triassic [[cynodont]]); 3, ''[[Microconodon]]'' (a Triassic [[eucynodont]]); 4, ''[[Spalacotherium]]'' (a Cretaceous "[[symmetrodont]]"); 5, ''[[Amphitherium]]'' (a Jurassic [[Prototribosphenida|prototribosphenid]] mammal)]]</onlyinclude>

==Morphology==
{{See also|Glossary of mammalian dental topography}}[[File:3D Medical Animation Still Showing Types of Teeth.jpg|thumb|365x365px|Image showing molar teeth and their arrangement in the mouth of an adult human]]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 (tooth)|cingulum]]; the same feature on the lower molar a cingulid, and a minor cusp on these, for example, a cingular cuspule or conulid.<ref name=":0">{{Harvnb|Myers|Espinosa|Parr|Jones|2013b}}</ref>

===Tribosphenic===
[[File:Tribosphenic molar diagram.png|left|thumb|297x297px|A diagram of generalized tribosphenic molars with notable features labelled. Upper left molar in pink, lower left molar in blue.]]
The design that is considered one of the most important characteristics of [[Theria|therian]] mammals is called a tribosphenic molar. Among living mammals, the tribosphenic tooth is found in most [[insectivore|insectivorou]]s mammals as well as young [[platypus]]es, even though adult platypuses are toothless.

In tribosphenic teeth, the lower molar is divided into two regions: the three-cusped '''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 trigonid is defined by three large cusps: the protoconid is on the buccal/labial (cheek) side of the tooth, while the anterior paraconid and posterior metaconid are on the lingual (tongue) side.[[File:Tribosphenic molar.svg|thumb|Generalized tribosphenic left upper molar, showing the protocone, paracone, and metacone.]]

Upper molars look like three-pointed mountain ranges, with their features mirrored from the lower molars. The protocone cusp is on the lingual side of the tooth, while the anterior paracone and posterior metacone are on the buccal side. The protocone of the upper molar and talonid basin of the lower molar mesh together as a crushing system similar to a [[mortar and pestle]].

Tribosphenic molars were present in the direct ancestors of all three living mammal groups, but it was most likely not ancestral to mammals as a whole. Many paleontologists argue that it developed independently in [[monotreme]]s (from [[Australosphenida|australosphenidans]]), rather than being inherited from a common ancestor that they share with [[marsupial]]s and [[placental]]s (from [[Boreosphenida|boreosphenidans]]); this idea still has some critics.<ref>{{Harvnb|Stokstad|2001}}</ref> For example, the dentition of the [[Early Cretaceous]] monotreme ''[[Steropodon]]'' is similar to those of ''[[Peramuridae|Peramus]]'' and [[Dryolestoidea|dryolestoids]], which suggests that monotremes are related to some pre-tribosphenic mammals,<ref>{{Harvnb|Luo|Cifelli|Kielan-Jaworowska|2001}}</ref> but, on the other hand, the status of neither of these two groups is well-established.

Some [[Jurassic]] [[Mammaliaformes|mammalia forms]], such as [[Docodonta|docodonts]] and [[Shuotheriidae|shuotheriids]], have "reversed tribosphenic" molars, in which a talonid-like structure develops towards the front of the lower molar, rather than towards the rear. This variant is regarded as an example of [[convergent evolution]].<ref>{{Harvnb|Luo|Ji|Yuan|2007}}</ref>

===Quadrate===
[[File:Pig tooth.jpg|thumb|Pig tooth]]
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 (also called quadritubercular or euthemorphic) molars have a hypocone, an additional fourth cusp on the lingual (tongue) side of the upper molar, located posterior to the protocone. Quadrate molars appeared early in mammal evolution and are present in many species, including [[hedgehog]]s, [[raccoon]]s, and many [[primate]]s, including humans.<ref name="ADW-div" /> 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.<ref name="ADW-div" />

===Bunodont===
<!-- linked from redirect [[bunodont]] -->
[[File:Chimpjaw.png|thumb|Upper and lower dentition of a [[Common chimpanzee|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.<ref name="ADW-div">{{Harvnb|Myers|Espinosa|Parr|Jones|2013a}}</ref> Bunodont molars are effective crushing devices and often basically quadrate in shape.<ref name="Lawlor-13">{{Harvnb|Lawlor|1979|pp=13–4}}</ref>

===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.<ref>{{Harvnb|Flynn|Wyss|Charrier|2007}}</ref> 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).<ref name="ADW-div" />

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 [[gingiva]]l line, below which is a layer of enamel covering the entire length of the body. The cementum and the enamel [[Invagination|invaginate]] into the thick layer of dentin.<ref name="Kwan-Tufts">{{Cite web
 | last = Kwan | first = Paul W.L.
 | title = Digestive system I
 | publisher = Tufts University | year = 2007
 | url = http://ocw.tufts.edu/data/4/531949.pdf | access-date = 2013-05-18
 | archive-url = https://web.archive.org/web/20120913162229/http://ocw.tufts.edu/data/4/531949.pdf | archive-date = 2012-09-13
}}</ref>

===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.<ref name="ADW-div" />

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.<ref name="Kwan-Tufts" />

===Zalambdodont===
Zalambdodont upper molars have at least three main cusps, one larger on the lingual side and two smaller on the labial side. The large cusp is joined to the other two by crests, forming a narrow V- or [[lambda|λ]] (lambda)-shape. The term "zalambdodont" roughly translates to "very lambda-toothed". Zalambdodont molars are found in [[Tenrec|tenrecs]], [[golden mole]]s, [[solenodon]]s, and [[Marsupial mole|marsupial moles]] among living mammals.<ref name=":0" /><ref name=":1">{{Cite journal |last1=Asher |first1=Robert J. |last2=Sánchez-Villagra |first2=Marcelo R. |date=2005 |title=Locking Yourself Out: Diversity Among Dentally Zalambdodont Therian Mammals |url=http://link.springer.com/10.1007/s10914-005-5725-3 |journal=Journal of Mammalian Evolution |language=en |volume=12 |issue=1–2 |pages=265–282 |doi=10.1007/s10914-005-5725-3 |issn=1064-7554|url-access=subscription }}</ref>

In zalambdodont placentals, the larger inner cusp is [[Homology (biology)#Anatomical homology|homologous]] with the paracone in a tribosphenic upper molar, while the metacone is absent, reduced or fused. Marsupial moles show the opposite condition, with the large cusp equivalent to the metacone, and the paracone absent instead. The protocone is either absent (as in some golden moles and tenrecs) or reduced to a small fourth cusp, positioned lingual to the large cusp at the tip of the V. The two labial cusps are located on an expanded shelf called the stylar shelf. In the lower molars, the talonid region is reduced or absent, having lost its role as a crushing basin against the protocone.<ref name=":0" /><ref name=":1" /> Zalambdodonty reduces tooth contact to a few simple shearing surfaces, though the evolutionary advantage of this tooth type is unclear.<ref name=":1" />

===Dilambdodont===
Like zalambdodont molars, dilambdodont molars have a distinct ectoloph, but are shaped like two [[lambda]]s 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 [[shrew]]s, [[Mole (animal)|mole]]s, and some insectivorous [[bat]]s.<ref name="ADW-div" />

===Lophodont===
[[File:Elefantenzaehne-drawing.jpg|thumb|left|Lophodont molars of ''[[Elephas]]'' (left) and ''[[Loxodonta]]'' (center), compared to the nonlophodont [[mastodon]] (right)]]
[[File:Elephant molars compared to rodent molars (48058012058).jpg|thumb|left|Rodent molars (left) compared to an elephant molar, 2019]]
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 [[Blesmol|mole rats]], or a complex arrangement of series of ridges and cross-ridges, as those in [[Perissodactyla|odd-toed ungulates]], such as [[Equidae|equids]].<ref name="Lawlor-13" />

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 [[tapir]]s, [[manatee]]s, and many rodents.<ref name="ADW-div" />

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 [[Lagomorpha|lagomorphs]] (hares, rabbits, and pikas) and some rodents.<ref name="ADW-div" /><ref name="Lawlor-13" />

Extreme forms of lophodonty in [[elephant]]s and some rodents (such as ''[[Otomys]]'') is known as loxodonty.<ref name="ADW-div" /> 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]].<ref name="ADW-div" /><ref name="Lawlor-13" />

===Secodont===
[[File:Carnassials.jpg|thumb|Carnassials of a [[Eurasian wolf]]]]
Many carnivorous mammals have enlarged and blade-like teeth especially adapted for slicing and chopping called [[carnassial]]s. A general term for such blade-like teeth is secodont or plagiaulacoid.<ref name="ADW-div" />

==See also==
{{Portal|Medicine}}
*[[Dental formula]]
*[[Polyphyodont]]

==Notes==
{{Reflist}}

==References==
{{Refbegin|30em}}
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 | title = The Diversity of Cheek Teeth | publisher = Animal Diversity Web, University of Michigan
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{{Refend}}

==External links==
* [http://palaeos.com/vertebrates/bones/teeth/molars.html Overview of molar morphology and terminology]- Paleos.com
{{Tooth anatomy}}
{{Authority control}}

[[Category:Types of teeth]]

[[be:Маларэ]]
[[eo:Vangodento#Molaro]]