Dentition

Cast of a human upper jaw showing incisors, canines, premolars, and 2 out of 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

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

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:dm.^[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 $Upper: I.C.P.M, lower: I.C.P.M$, 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: (di² − dc¹ − dp²) / (di₂ − dc₁ − dp₂)x2 = 20.^[5] This can also be written as $Upper: di2.dc1.dp2, lower: di2.dc1.dp2$. (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 redunancy.) Another anotation is $Upper: 2.1.2, lower: 2.1.2$, 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]
2. Permanent: (I² − C¹ − P² − M³) / (I₂ − C₁ − P₂ − M₃)x2 = 32.^[5] This can also be written as $Upper: 2.1.2.3, lower: 2.1.2.3$. 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 $Upper: 3.1.5.3, lower: 3.1.5.3$.^[7] However, no existing (or extant) placental mammal has this number. In extant placental mammals, the maximum dental formula is: $Upper: 3.1.4.3, lower: 3.1.4.3$ Mammal teeth are usually symmetrical, but not always. For example, the aye-aye has a formula of $Upper: 1.0.1.3, lower: 1.0.0.3$, demonstrating the need for both upper and lower quadrant counts.^[8]

Tooth naming discrepancies

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

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

Some examples of dental formulae^[a]

MAMMAL	DENTAL FORMULA	COMMENT
Non placental	.	Non-placental mammals such as marsupials can have more teeth than placentals. For example, the opossum (below)
Kangaroo	$Upper: 3.1.2.4, lower: 1.0.2.4$[12]
Musky Rat kangaroo	$Upper: 3.1.1.4, lower: 2.0.1.4$ [13]
All of Potoroidae except Musky Rat-kangaroo	$Upper: 3.1.1.4, lower: 1.0.1.4$ [13]	The marsupial family Potoroidae includes the bettongs, potoroos, and two of the rat-kangaroos. All are rabbit-sized, brown, jumping marsupials and resemble a large rodent or a very small wallaby.
Opossum	$Upper: 5.1.3.4, lower: 4.1.3.4$
Placental	.	Some examples of dental formulae for placental mammals
Armadillo	$Upper: 0.0.7.1, lower: 0.0.7.1$[14]
Aye-aye	$Upper: 1.0.1.3, lower: 1.0.0.3$[15]	A Prosimian. The Aye-aye's deciduous dental formula (dI:dC:dM)is $Upper: 2.1.2, lower: 2.1.2$[8]
Badger	$Upper: 3.1.3.1, lower: 3.1.3.2$[16]
Big brown bat	$Upper: 2.1.1.3, lower: 3.1.2.3$[17]
Red Bat, Hoary Bat, Seminole Bat, Mexican Freetail Bat	$Upper: 1.1.2.3, lower: 3.1.2.3$[17]
Cat (deciduous)	$Upper: di3.dc1.dp3.dm0, lower: di3.dc1.dp2.dm0$[18]
Cat	$Upper: 3.1.3.1, lower: 3.1.2.1$[9]	The last upper premolar and first lower molar of the cat, since it is a carnivore, are called carnassials and are used to slice meat and skin. This means that the carnassials are always the fourth upper premolar and the first lower molar
Cow	$Upper: 3.1.3.3, lower: 3.1.3.3$[16]
Dog (deciduous)	$Upper: di3.dc1.dp3.dm0, lower: di3.dc1.dp3.dm0$[18]
Dog (permanent)	$Upper: 3.1.4.2, lower: 3.1.4.3$[16]
Eulemur	$Upper: 3.1.3.3, lower: 3.1.3.3$	Prosimian genus to which the large Malagasy or 'true' lemurs belong.[19] Ruffed lemurs (genus Varecia),[20] Dwarf lemurs (genus Mirza),[21] and Mouse lemurs (genus Microcebus) also have this dental formula, but the mouse lemurs have a dental comb[22]
Euoticus	$Upper: 2.1.3.3, lower: 2.1.3.3$	Prosimian genus to which the needle-clawed bushbabies (or galagos) belong. Specialised morphology for gummivory includes procumbent dental comb and caniniform upper anterior premolars.[19]
Fox (red)	$Upper: 3.1.4.2, lower: 3.1.4.3$[16]
Guinea pig	$Upper: 1.0.1.3, lower: 1.0.1.3$[23]
Hedgehog	$Upper: 3.1.3.3, lower: 2.1.2.3$[16]
Horse (deciduous)	$Upper: 3.0.3.0, lower: 3.0.3.0$[24][25]
Horse (permanent)	$Upper: 3.0-1.3-4.3, lower: 3.0-1.3.3$	Permanent dentition varies from 36-42, depending on the presence or absence of canines and the number of premolars.[26] The first premolar (wolf tooth) may be absent or rudimentary,[24][25] and is mostly present only in the upper (maxillary) jaw.[25] The canines are small and spade-shaped, and usually present only in males.[26] Canines appear in 20-25 % of females and are usually smaller than in males.[25][b]
Human (deciduous teeth)	$Upper: 2.1.2, lower: 2.1.2$
Human (permanent teeth)	$Upper: 2.1.2.3, lower: 2.1.2.3$[9]	This pattern is shared with Apes and Old World monkeys (excluding Prosimii), and is sometimes known as the cercopithecoid dental formula.[11]
Indri	See comment	A prosimian. Dental formula disputed. Either $Upper: 2.1.2.3, lower: 2.0.2.3$ or $Upper: 2.1.2.3, lower: 1.1.2.3$. Proponents of both formulae agree there are 30 teeth and that there are only four teeth in the dental comb.[27]
Lepilemur	$Upper: 0.1.3.3, lower: 2.1.3.3$	A prosimian. The upper incisors are lost in the adult, but are present in the deciduous dentition.[28]
Lion	$Upper: 3.1.3.1, lower: 3.1.2.1$ [29]
Mole	$Upper: 3.1.4.3, lower: 3.1.4.3$[16]
Mouse (House)	$Upper: 1.0.0.3, lower: 1.0.0.3$[16]	Plains pocket mouse(Perognathus flavescens) have dental formula of $Upper: 1.0.1.3, lower: 1.0.1.3$[30]
New World anthropoids	See comment	All New World anthropoids have a dentition formula of $Upper: 2.1.3.3, lower: 2.1.3.3$ or $Upper: 2.1.3.2, lower: 2.1.3.2$[11]
Opossum	$Upper: 5.1.3.4, lower: 4.1.3.4$[17]
Pig (deciduous)	$Upper: di3.dc1.dp4.dm0, lower: di3.dc1.dp4.dm0$[18]
Pig	$Upper: 3.1.4.3, lower: 3.1.4.3$[16]
Rabbit	$Upper: 2.0.3.3, lower: 1.0.2.3$ [9]
Raccoon	$Upper: 3.1.4.2, lower: 3.1.4.2$
Rat	$Upper: 1.0.0.3, lower: 1.0.0.3$[16]
Sheep (deciduous)	$Upper: di0.dc0.dp3.dm0, lower: di4.dc0.dp3.dm0$[18]
Sheep (permanent)	$Upper: 3.1.3.3, lower: 1.1.1.3$[17]
Shrew	$Upper: 3.1.3.3, lower: 3.1.3.3$[16]
Sifakas	See comment	Prosimians. Dental formula disputed. Either $Upper: 2.1.2.3, lower: 2.0.2.3$ or $Upper: 2.1.2.3, lower: 1.1.2.3$. Possess dental comb comprising fourt teeth.[31]
Slender loris Slow loris	$Upper: 2.1.3.3, lower: 2.1.3.3$	Prosimians. Lower incisors and canines form a dental comb; upper anterior dentition is peg-like and short.[32][33]
Squirrel	$Upper: 1.0.2.3, lower: 1.0.1.3$[16]
Tarsiers	$Upper: 2.1.3.3, lower: 1.1.3.3$	Prosimians.[34]
Vole (field)	$Upper: 1.0.0.3, lower: 1.0.0.3$[16]
Weasel	$Upper: 3.1.3.1, lower: 3.1.3.2$[16]

Dentition use in archaeology

Dentition, or the study of teeth, is an important area of study for archaeologists, especially those specializing in the study of older remains.^[35][36] 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

• Odontometrics
• Phalangeal formula

Dentition discussions in other articles

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

• Lemur

Further reading

• 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

a. ^{^} Unless otherwise stated, the formulae can be assumed to be for adult, or permanent dentition.

b. ^{^} Regarding horse dentition, Pence gives erroneous upper and lower figures of 40 to 44 for the dental range.^[24] It is not possible to arrive at this range from the figures she provides. The figures from Cirelli^[25] and Ultimate Ungulates^[26] are more reliable, although there is a self-evident error for Cirelli's calculation of the upper female range of 40, which is not possible from the figures he provides. One can only arrive at an upper figure of 38 without canines, which for females Cirelli shows as 0/0. It appears canines do sometimes appear in females, hence the sentence in Ultimate Ungulates that canines are "usually present only in males",^[26] However, Pence's and Cirelli's references are clearly otherwise useful, hence the inclusion, but with the caveat of this footnote.

References

1. ^ Angus Stevenson, ed. (2007), "Dentition definition", Shorter Oxford English Dictionary, 1: A-M (6th ed.), Oxford: Oxford University Press, p. 646, ISBN 978-0-19-920687-2 
2. ^ ^{a b} E.A. Martin (1983), Macmillan Dictionary of Life Sciences (2nd revised ed.), London: Macmillan Press, p. 103, ISBN 0-333-38647-2 
3. ^ Weiss, M.L., & Mann, A.E (1985), 'Human Biology and Behaviour: An anthropological perspective (4th ed.), Boston: Little Brown, pp. 130–131, ISBN 0-673-39013-6 
4. ^ Weiss, M.L., & Mann, A.E (1985), p.132-135.
5. ^ ^{a b c d} 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, p. 11, ISBN 0-521-65289-8, http://catdir.loc.gov/catdir/samples/cam031/2001037356.pdf 
6. ^ ^{a b} Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), The Cambridge Dictionary of Human Biology and Evolution, Cambridge & New York: Cambridge University Press, p. 135, ISBN 978-0-521-66486-8 
7. ^ ^{a b c} Weiss, M.L., & Mann, A.E (1985), p.134.
8. ^ ^{a b c d e f} Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.139
9. ^ ^{a b c d} E.A. Martin (1983), p.102
10. ^ ^{a b} Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.139. See section on dental eruption sequence, where numbering used is per this text.
11. ^ ^{a b c} Marvin Harris (1988), Culture, People, Nature: An Introduction to General Anthropology (5th ed.), New York: Harper & Row, ISBN 06-042697-7 
12. ^ http://www.1902encyclopedia.com/K/KAN/kangaroo.html
13. ^ ^{a b} Bettongs, potoroos and the musky rat-kangaroo. Csiro Publishing. 2007. pp. 182 pages. ISBN 0643093419, 9780643093416. 
14. ^ Freeman, Patricia W., & Genoways, Hugh H (December 1998), "Recent Northern Records of the Nine-banded Armadillo (Dasypodidae) in Nebraska", The Southwestern Naturalist 43 (4): 491–504, http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1012&context=museummammalogy, retrieved 7 June 2010 
15. ^ Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), pp.134,139.
16. ^ ^{a b c d e f g h i j k l m} "The Skulls". Chunnie's British Mammal Skulls. http://www.geocities.ws/chunniemonster/adultsandbabies.html. Retrieved 15 October 2011. 
17. ^ ^{a b c d} http://www.wildwoodtracking.com/skulls/dentalformulae.html
18. ^ ^{a b c d} http://www.provet.co.uk/health/diagnostics/dentalformulae.htm
19. ^ ^{a b} Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.177
20. ^ Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.550
21. ^ Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.340
22. ^ Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.335
23. ^ http://www.adelaide.edu.au/ANZCCART/publications/Guinea%20Pig_17.pdf
24. ^ ^{a b c} Patricia Pence (2002), Equine Dentistry: A Practical Guide, Baltimore: Lippincott Williams & Wilkins, p. 7, ISBN 0-683-30403-8, http://books.google.com.au/books?id=o4KVGceXBNIC&printsec=frontcover&dq=%22Equine+dentistry%22&cd=1#v=onepage&q&f=false 
25. ^ ^{a b c d e} Al Cirelli, Equine Dentition, Nevada: University of Nevada, http://www.unce.unr.edu/publications/files/ag/2000/sp0008.pdf, retrieved 7 June 2010 
26. ^ ^{a b c d} Family Equidae: Horses, asses, and zebras, Ulitimate Unqulate.com, http://www.ultimateungulate.com/Perissodactyla/Equidae.html, retrieved 7 June 2010 
27. ^ Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.267
28. ^ Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.300
29. ^ http://www.geocities.ws/chunniemonster/dental_formula.html
30. ^ http://www.nsrl.ttu.edu/tmot1/perflave.htm
31. ^ Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.438
32. ^ Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.309
33. ^ Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.371
34. ^ Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005), p.520
35. ^ Weiss, M.L., & Mann, A.E (1985), p.130-135.
36. ^ Larry L Mai; Marcus Young Owl; M Patricia Kersting (2005). The utility of dental formulae in species identification is indicated throughout this dictionary. Dental formulae are noted for many species, both extant and extinct, and where unknown (in some extinct species) this is noted.

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

External links

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