Dentition

Most mammals are diphyodont, meaning they have two sets of teeth during the course of their life—deciduous (baby teeth) and adult teeth. Some animals, called polyphyodonts, have teeth that are continuously replaced in life, including sharks and alligators. There are four types of teeth: incisors, canines, premolars, and molars (Figure 11.16). Animals that have two or more types of teeth are referred to as heterodonts, while those with a single type of tooth, like various reptiles and fish, are called homodonts.

Incisors (I) tend to be small, chisel-shaped teeth with a single cusp and a single root. Artiodactyls, or even-toed ungulates (i.e., cattle, deer), do not have any maxillary incisors (except pig). Other species, particularly rodents, have modified or enlarged incisors. These teeth are often used for nipping, scraping, and tearing.

Canines (C) tend to be conical and single-cusped with a single root. In horses and other ungulates, they have become rudimentary and incisor-like in appearance, while in large carnivores they can become quite large and distinctive. These teeth are often used to puncture, hold, and tear.

Premolars (P) and molars (M) are used for chewing or grinding food. Premolars are single or double rooted, and molars tend to have two or more roots. Molars are at the rear of the mouth and tend to be larger than premolars, but in some species, such as rodents, rabbits, and ungulates, they are very similar in size and appearance.

The numbers of types of teeth present can be useful in genus determination. A dental formula is used to describe how many of each type of tooth is present in one half of the maxilla and mandible. A dental formula is always given in the same order, from front to back I:C:P:M. The number of teeth

Figure 11.16 Tooth types indicated on inferior view of a dog skull.

can vary between the maxilla and mandible, so it is important to report both in the formula. For example, the dental formula for all dogs, coyotes, and wolves is 3/3:1/1:4/4:273, meaning they have three incisors in both the maxilla and mandible (3/3), one canine in each (1/1), four premolars in each (4/4), two molars in the maxilla and three molars in the mandible (2/3) (Figure 11.17). This is very different from a white-tail deer, which has a dental formula of 0/3:0/l:3/3:3/3, indicating they have no incisors or canines in the maxilla. When looking at closely related species, the domestic cat has a dental formula of 3/3:l/l :3/2:1/1, while the bobcat has a dental formula of 3/3:1/1:2/2:1/1 (Figure 11.18).

Occasionally, the dentition found will vary from the expected dental formula, and this is important to document. For example, the humans the dental formula is 2:1:2:3 (we have the same number of teeth in the mandible and maxilla); however, many people do not have third molars, often they are removed through surgery, which is noticeable by the space present or indications on radiographs. However, an increasing number of people never developed third molars, which tends to be evident from the very small retromolar space, and radiographs often confirm their congenital absence. At this time, the dental formula is still 2:1:2:3, and it is important to document the absence of the third molar and explain why. Eventually, the human dental formula may change to 2:1:2:2. Returning to animals, the same is found in some breeds; teeth are being lost due to dental crowding or other genetic modifications. Therefore, checking the literature on the expected dental formula and variations seen is important so one can then determine which teeth are missing and why, if possible (e.g., lost premortem due to trauma or disease, lost postmortem, juvenile so unerupted or not developed, or individual anomaly). For example, a right second maxillary premolar and a right mandibular third molar are absent on the dog dentition example (Figure 11.17); the premolar appears to be due to individual anomaly, it just never developed, while the molar was lost postmortem.

Figure 11.18 View of dentition: Cat skull.