Cast of a human upper jaw showing incisors, canines, premolars, and 2 of the 3 sets of molars.

Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age.^[1] That is, the number, type, and morpho-physiology of the teeth of an animal.^[2]

Animals whose teeth are all of the same type, such as most non-mammalian vertebrates, are said to have homodont dentition, whereas those whose teeth differ morphologically are said to have heterodont dentition. The dentition of animals with two successions of teeth (deciduous, permanent) is referred to as diphyodont, while the dentition of animals with only one set of teeth throughout life is monophyodont. The dentition of animals in which the teeth are continuously discarded and replaced throughout life is termed polyphyodont.^[2]

Overview[link]

Vertebrate dentition originated from a folding in of the placoderm's armour, evolving into the familiar condition of living reptiles, amphibians, and fish: a long row of pointed or sharp-sided, undifferentiated teeth (homodont) that are completely replaceable. The mammalian pattern is significantly different. The teeth in the upper and lower jaws in mammals have evolved a close-fitting relationship such that they operate together as a unit. "They occlude, that is, the chewing surfaces of the teeth are so constructed that the upper and lower teeth are able to fit precisely together, cutting, crushing, grinding or shearing the food caught between".^[3]

All mammals except the monotremes, the xenarthrans, the pangolins, and the cetaceans^{[citation needed]} have up to four distinct types of teeth, with a maximum number for each. These are the incisor (cutting), the canine, the premolar, and the molar (grinding). The incisors occupy the front of the tooth row in both upper and lower jaws. They are normally flat, chisel-shaped teeth that meet in an edge-to-edge bite. Their function is cutting, slicing, or gnawing food into manageable pieces that fit into the mouth for further chewing. The canines are immediately behind the incisors. In many mammals, the canines are pointed, tusk-shaped teeth, projecting beyond the level of the other teeth. In carnivores, they are primarily offensive weapons for bringing down prey. In other mammals such as some primates, they are used to split open hard surfaced food. The premolars and molars are at the back of the mouth. Depending on the particular mammal and its diet, these two kinds of teeth prepare pieces of food to be swallowed by grinding, shearing, or crushing. The specialised teeth - incisors, canines, premolars, and molars - are found in the same order in every mammal.^[4] In many mammals the infants have a set of teeth that fall out and are replaced by adult teeth. These are called deciduous teeth, primary teeth, baby teeth or milk teeth.^[5][6] Animals that have two sets of teeth, one followed by the other, are said to be diphyodont. Normally the dental formula for milk teeth is the same as for adult teeth except that the molars are missing.

Dental formula[link]

Because mammalian teeth are specialised for different functions, many mammal groups have lost teeth not needed in their adaptation. Tooth form has also undergone evolutionary modification as a result of natural selection for specialised feeding or other adaptations. Over time, different mammal groups have evolved distinct dental features, both in the number and type of teeth, and in the shape and size of the chewing surface.^[7]

The number of teeth of each type is written as a dental formula for one side of the mouth, or quadrant, with the upper and lower teeth shown on separate rows. The number of teeth in a mouth is twice that listed as there are two sides. In each set, incisors (I) are indicated first, canines (C) second, premolars (P) third, and finally molars (M), giving I:C:P:M.^[7][8] So for example, the formula 2.1.2.3 for upper teeth indicates 2 incisors, 1 canine, 2 premolars, and 3 molars on one side of the upper mouth.

The deciduous dental formula is notated in lowercase lettering preceded by the letter d: e.g. di:dc:dp.^[8] An animal's dentition for either deciduous or permanent teeth can thus be expressed as a dental formula, written in the form of a fraction, which can be written as Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ I.C.P.M}{ I.C.P.M} , or I.C.P.M / I.C.P.M.^[8][9] For example, the following formulae show the deciduous and permanent dentition of all catarrhine primates, including humans:

1. Deciduous: Failed to parse (Missing texvc executable; please see math/README to configure.): (di^2-dc^1-dp^2) / (di_2-dc_1-dp_2) x2 =20.

^[5] This can also be written as Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ di2.dc1.dp2}{ di2.dc1.dp2} . (Superscript and subscript denote upper and lower jaw, i.e. do not indicate mathematical operations; the numbers are the count of the teeth of each type. The '-' in the formula are likewise not mathematical operators, but spacers. 'd' denotes deciduous, i.e. milk or baby teeth, and lower case also indicates temporary teeth. This is a redundancy.) Another annotation is Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ 2.1.2}{ 2.1.2} , if the fact that it pertains to deciduous teeth is clearly stated, per examples found in some texts such as The Cambridge Dictionary of Human Biology and Evolution^[8]

1. Permanent: Failed to parse (Missing texvc executable; please see math/README to configure.): (I^2-C^1-P^2-M^3) / (I_2-C_1-P_2-M_3) x2 =32.

^[5] This can also be written as Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ 2.1.2.3}{ 2.1.2.3} . When the upper and lower dental formulae are the same, some texts write the formula without a fraction (in this case, 2.1.2.3), on the implicit assumption that the reader will realise it must apply to both upper and lower quadrants. This is seen for example throughout The Cambridge Dictionary of Human Biology and Evolution.

The greatest number of teeth in any known placental mammal was 48, with a formula of Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ 3.1.5.3}{ 3.1.5.3} .^[7] However, no existing (or extant) placental mammal has this number. In extant placental mammals, the maximum dental formula is: Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ 3.1.4.3}{ 3.1.4.3}

Mammal teeth are usually symmetrical, but not always. For example, the aye-aye has a formula of Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ 1.0.1.3}{ 1.0.0.3} 

, demonstrating the need for both upper and lower quadrant counts.^[8]

Tooth naming discrepancies[link]

Teeth are numbered starting at 1 in each group, except the premolars which end at 4. Thus the human teeth are I1, I2, C1, P3, P4, M1, M2, and M3.^[10] In humans, the third molar pertains to the wisdom tooth, whether or not it has erupted.^[11]

Regarding premolars, there is disagreement regarding whether the third type of deciduous tooth is a premolar (the general consensus among mammalogists) or a molar (commonly held among human anatomists).^[6] There is thus some discrepancy between nomenclature in zoology and in dentistry. This is because the terms of human dentistry, which have generally prevailed over time, have not included mammalian dental evolutionary theory. There were originally four premolars in each quadrant of early mammalian jaws. However, all living primates have lost at least the first premolar. "Hence most of the prosimians and platyrrhines have three premolars. Some genera have also lost more than one. A second premolar has been lost in all catarrhines. The remaining permanent premolars are then properly identified as P2, P3 and P4 or P3 and P4; however, traditional dentistry refers to them as P1 and P2".^[5]

Dental eruption sequence[link]

The order in which teeth emerge through the gums is known as the dental eruption sequence. Rapidly developing anthropoid primates such as macaques, chimpanzees, and australopithecines have an eruption sequence of M1 I1 I2 M2 P3 P4 C M3, whereas anatomically modern humans have the sequence M1 I1 I2 C P3 P4 M2 M3. The later that tooth emergence begins, the earlier the anterior teeth (I1-P4) appear in the sequence.^[10]

Dental formulae examples[link]

or Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ 2.1.2.3}{ 1.1.2.3} 

or Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ 2.1.3.2}{ 2.1.3.2} 

or Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ 2.1.2.3}{ 1.1.2.3} 

Dentition use in archaeology[link]

Dentition, or the study of teeth, is an important area of study for archaeologists, especially those specializing in the study of older remains.^[36][37] Dentition affords many advantages over studying the rest of the skeleton itself (osteometry). The structure and arrangement of teeth is constant and, although it is inherited, does not undergo extensive change during environmental change, dietary specializations, or alterations in use patterns. The rest of the skeleton is much more likely to exhibit change because of adaptation. Teeth also preserve better than bone, and so the sample of teeth available to archaeologists is much more extensive and therefore more representative.

Dentition is particularly useful in tracking ancient populations' movements, because, although all humans have the same basic 32 teeth, there are subtle differences in the shapes of incisors, the number of grooves on molars, and extra cusps on particular teeth. These differences can not only be associated with different populations across space, but also change over time so that the study of the characteristics of teeth could say which population one is dealing with, and at what point in that population's history they are.

See also[link]

• Odontometrics
• Phalangeal formula

Dentition discussions in other articles[link]

Some articles have helpful discussions on dentition, which will be listed as identified.

• Lemur

Further reading[link]

• Daris R. Swindler (2002), "Chapter 1: Introduction (pp.1-11) and Chapter 2: Dental anatomy (pp.12-20).", Primate Dentition: An Introduction to the Teeth of Non-human Primates, Cambridge: Cambridge University Press, ISBN 0-521-65289-8, http://catdir.loc.gov/catdir/samples/cam031/2001037356.pdf  See also preview pages in Google books

• Feldhamer, George A., Lee C. Drickhamer, Stephen H. Vessey, Joseph F. Merritt, Carey Krajewski (2007), "4: Evolution and Dental Characteristics", Mammalogy: Adaptation, Diversity, Ecology, Baltimore, Maryland: Johns Hopkins University Press, pp. 48–67, ISBN 978-0-8018-8695-9, http://books.google.com.au/books?id=udCnKce9hfoC&printsec=frontcover&dq=Mammalogy#v=onepage&q&f=false, retrieved 7 June 2010  (link provided to title page to give reader choice of scrolling straight to relevant chapter or perusing other material).

Notes[link]

References[link]

Adovasio, J. M. and David Pedler. "The Peopling of North America." North American Archaeology. Blackwell Publishing, 2005. p. 35–36.

External links[link]

• Colorado State's Dental Anatomy Page
• For image of skulls and more information on dental formula of mammals.