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The molars or molar teeth are large, flat
teeth A tooth ( : 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 and omnivores, also use teeth to help with capturing or wounding prey, tea ...
at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during
chewing Chewing or mastication is the process by which food is crushed and ground by teeth. It is the first step of digestion, and it increases the surface area of foods to allow a more efficient break down by enzymes. During the mastication process, th ...
. 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. The third molar of humans is sometimes vestigial.


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 A third molar, commonly called wisdom tooth, 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 this generally occurs betw ...
. 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. Race can also affect the age at which this occurs, with statistical variations between groups. 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 The mandibular first molar or six-year molar is the tooth located distally (away from the midline of the face) from both the mandibular second premolars of the mouth but mesial (toward the midline of the face) from both mandibular second molars. It ...
, mandibular second molar, and
mandibular third molar A third molar, commonly called wisdom tooth, 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 this generally occurs betw ...
.


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 crenulations, are the result of different enamel thickness. Cusps are occasionally joined to form ridges and expanded to form crests.
Cingula ''Cingula'' is a genus of minute sea snails, marine gastropod mollusks or micromollusks in the family Rissoidae Rissoidae is a large family of very small and minute sea snails with an operculum, marine gastropod mollusks in the superfamily ...
are often incomplete ridges that pass around the base of the crown. Mammalian, multicusped
cheek teeth Cheek teeth or post-canines comprise the molar and premolar teeth in mammals. Cheek teeth are multicuspidate (having many folds or tubercles). Mammals have multicuspidate molars (three in placentals, four in marsupials, in each jaw quadrant) and p ...
probably evolved from single-cusped teeth in synapsids, 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.


Morphology

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.


Tribosphenic

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. 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, but, on the other hand, the status of neither of these two groups is well-established. Some Jurassic mammals, such as '' Shuotherium'' and ''
Pseudotribos ''Pseudotribos'' ("false chewing") is an extinct genus of mammal that lived in Northern China during the Middle Jurassic some , possibly more closely related to monotremes than to theria (placental and marsupial Marsupials are any members of ...
'', have "reversed tribosphenic" molars, in which the talonid is towards the front. This variant is regarded as an example of convergent evolution. From the primitive tribosphenic tooth, molars have diversified into several unique morphologies. In many groups, a fourth cusp, the
hypocone A cusp is a pointed, projecting, or elevated feature. In animals, it is usually used to refer to raised points on the crowns of teeth. The concept is also used with regard to the leaflets of the four heart valves. The mitral valve, which has two ...
(hypoconid), subsequently evolved (see below).


Quadrate

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
hedgehog A hedgehog is a spiny mammal of the subfamily Erinaceinae, in the eulipotyphlan family Erinaceidae. There are seventeen species of hedgehog in five genera found throughout parts of Europe, Asia, and Africa, and in New Zealand by introduction ...
s,
raccoon The raccoon ( or , ''Procyon lotor''), sometimes called the common raccoon to distinguish it from other species, is a mammal native to North America. It is the largest of the procyonid family, having a body length of , and a body weight of ...
s, and many primates, including humans. 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.


Bunodont

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. Bunodont molars are effective crushing devices and often basically quadrate in shape.


Hypsodont

Hypsodont Hypsodont is a pattern of dentition with high-crowned teeth and enamel extending past the gum line, providing extra material for wear and tear. Some examples of animals with hypsodont dentition are cows and horses; all animals that feed on gritty ...
dentition is characterized by high-crowned teeth and enamel that extends far past the gum line, which provides extra material for wear and tear. Some examples of animals with hypsodont dentition are
cattle Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus ''Bos''. Adult females are referred to as cows and adult ma ...
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 The Arvicolinae are a subfamily of rodents that includes the voles, lemmings, and muskrats. They are most closely related to the other subfamilies in the Cricetidae (comprising the hamsters and New World rats and mice). Some authorities place ...
(herbivorous rodents). 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 Invagination is the process of a surface folding in on itself to form a cavity, pouch or tube. In developmental biology, invagination is a mechanism that takes place during gastrulation. This mechanism or cell movement happens mostly in the vegetal ...
into the thick layer of dentin.


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. 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.


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 mole Golden moles are small insectivorous burrowing mammals endemic to Sub-Saharan Africa. They comprise the family Chrysochloridae and as such they are taxonomically distinct from the true moles, family Talpidae, and other mole-like families, all ...
s and
solenodon Solenodons (from el, τέλειος , 'channel' or 'pipe' and el, ὀδούς , 'tooth') are venomous, nocturnal, burrowing, insectivorous mammals belonging to the family Solenodontidae . The two living solenodon species are the Cuban solen ...
s.


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
shrew Shrews (family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to differen ...
s,
mole Mole (or Molé) may refer to: Animals * Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America * Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...
s, and some insectivorous
bat Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...
s.


Lophodont

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 Mole-rat or mole rat can refer to several groups of burrowing Old World rodents: * Bathyergidae, a family of about 20 hystricognath species in six genera from Africa also called blesmols. *''Heterocephalus glaber'', the naked mole-rat. * Spalacida ...
, or a complex arrangement of series of ridges and cross-ridges, as those in
odd-toed ungulates Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla (, ), are animals—ungulates—who have reduced the weight-bearing toes to three (rhinoceroses and tapirs, with tapirs still using four toes on the front legs) o ...
, such as equids. 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,
manatee Manatees (family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living species ...
s, and many rodents. 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 The lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families: the Leporidae (hares and rabbits) and the Ochotonidae ( pikas). The name of the order is derived from the Ancient Greek ''lagos'' (λα� ...
(hares, rabbits, and pikas) and some rodents. Extreme forms of lophodonty in
elephant Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae a ...
s and some rodents (such as ''
Otomys African vlei rats (''Otomys''), also known as groove-toothed rats, live in many areas of sub-Saharan Africa. Most species live in marshlands, grasslands, and similar habitats and feed on the vegetation of such areas, occasionally supplementin ...
'') is known as loxodonty. 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.


Secodont

Many carnivorous mammals have enlarged and blade-like teeth especially adapted for slicing and chopping called
carnassial Carnassials are paired upper and lower teeth modified in such a way as to allow enlarged and often self-sharpening edges to pass by each other in a shearing manner. This adaptation is found in carnivorans, where the carnassials are the modified ...
s. A general term for such blade-like teeth is secodont or plagiaulacoid.


See also

* Dental formula *
Polyphyodont A polyphyodont is any animal whose teeth are continually replaced. In contrast, diphyodonts are characterized by having only two successive sets of teeth. Polyphyodonts include most toothed fishes, many reptiles such as crocodiles and geckos, an ...


Notes


References

* * * * * * * *


External links


Overview of molar morphology and terminology
Paleos.com {{Authority control Types of teeth be:Маларэ eo:Vangodento#Molaro