15. The Dentition

15.2. Organization of the Teeth

Homogeneity

Animals whose teeth are all of the same type, such as most non-mammalian vertebrates, have homodont dentition, whereas those whose teeth differ morphologically have heterodont dentition.

Vertebrate teeth are believed to derive from dermal denticles that were present in arthrodiran placoderm fishes in the early Devonian period. They later evolved into the condition seen in living fishes, amphibians, and reptiles: a long row of undifferentiated teeth (homodont). These teeth are pointed or sharp-sided and can be replaced as needed throughout the life of the animal.

Figure 1. Homodont dentition of an alligator.

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, which means that their chewing surfaces are constructed such that the upper and lower teeth fit precisely together. In addition, different types of teeth specialized in cutting, crushing, grinding or shearing food.

Figure 2. Heterodont dentition of a dog.

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, the canine, the premolar, and the molar. 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 specialized teeth—incisors, canines, premolars, and molars—are found in the same order in every mammal.

The majority of dinosaurs had teeth that were similarly shaped throughout their jaws but varied in size. A few groups, however, were heterodonts with teeth that were cylindrical, peg-like, teardrop-shaped, leaf-like, diamond-shaped and blade-like. An example is the group Heterodontosauridae and the enigmatic early dinosaur, Eoraptor.

While most dinosaurs had a single row of teeth on each side of their jaws, others had dental batteries where teeth in the cheek region were fused together to form compound teeth. Individually these teeth were not suitable for grinding food, but when joined together with other teeth they would form a large surface area for the mechanical digestion of tough plant materials. This type of dental strategy is observed in ornithopod and ceratopsian dinosaurs as well as the duck-billed hadrosaurs, which had more than one hundred teeth in each dental battery.

Figure 3. Medial view of the right side of the mandible of the hadrosaur Edmontosaurus. This herbivore dealt with tooth wear by having dozens of teeth aligned next to the other longitudinally, and also had multiple rows of teeth aligned side-by-side from the tongue to the cheek. Teeth erupted earlier, were higher and were worn out earlier near the cheek than near the tongue.

The teeth of carnivorous dinosaurs, called ziphodont, were typically blade-like or cone-shaped, curved, with serrated edges. This dentition was adapted for grasping and cutting through flesh. In some cases, as observed in the railroad-spike sized teeth of Tyrannosaurus rex, the teeth were designed to puncture and crush bone.

Birds have evolved keratinous beaks which replaced the teeth and most of the jaw bone volume. This change was most likely promoted by selection of reduced body weight for improved flight performance. While beaks do not support grinding as well as molar teeth do, many birds feed on hard food items such as seeds. Most birds ingest their food items whole and and conduct the mechanical processing at the stomach. They have a gizzard, a highly muscular portion of the stomach, which can mechanically break down hard food items, sometimes aided by grit or stones (gastroliths) ingested by the bird.

Dental formula

Mammal tooth types are specialized for different functions. Mammal groups diverge in numbers of each type of tooth, according to the selective pressures in their feeding ecology. Tooth form has also undergone evolutionary modification as a result of natural selection for specialized feeding. Mammals have evolved distinct dental shapes and sizes of the chewing surface.

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), resulting in the format I:C:P:M. 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: for example: di:dc:dp.

An animal's dentition for either deciduous or permanent teeth can thus be described in a dental formula, written in the form of a fraction, such as I.C.P.M / I.C.P.M. For example, the following formulae show the deciduous and usual permanent dentition of all catarrhine primates, including humans:

Deciduous: di2.dc1.dm2/di2.dc1.dm2 or 2.1.0.2/2.1.0.2, if the text makes it clear that the formula refers to deciduous teeth.
Permanent: 2.1.2.3/2.1.2.3.

The greatest number of teeth in any known placental land mammal was 48, with a formula of 3.1.5.3/3.1.5.3. Among living placental mammals the maximum dental formula is 3.1.4.3/3.1.4.3. Mammalian teeth usually have the same arrangement in the upper and lower jaws, but not always. Sheep, for example, lack the upper incisives and canines. Bilateral symmetry is a general rule, with the exception of the bat Myonycteris brachycephala, which lacks the left or right lower internal incisor. Another interesting asymmetry is found in the narwhal (Monodon monoceros), a toothed whale in which the left upper canine only forms a single tusk that can be 3 m long. The occurrence of two tusks (bilateral symmetry) is rare, although some cases have been documented.

Figure 4. Skull of a narwhal (Monodon monoceros) showing the single tusk formed by the upper left canine.

Table 1. Examples of dental formulas from several mammalian groups.

Figure credits

Figure 1 by Everglades NPS from Homestead, Florida, United States - Alligator TEETH, NPSPhoto, R. Cammauf, Public Domain, https://commons.wikimedia.org/w/index.php?curid=44378671

Figure 2 by Ivob - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4675607

Figure 3 by Tim Evanson from Washington, D.C., United States of America - Edmontosaurus jaw from above showing rows of inner teeth - Museum of the Rockies - 2013-07-08Uploaded by FunkMonk, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=27363945

Figure 4 by Marco Vinci - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=37203370

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