OVERVIEW OF MAMMAL TEETH

This post is the second one concerning background information about the ancient history of various mammal groups that I shall be focused on in my upcoming series of posts (see my immediately previous posts about land bridges),

Before proceeding into the paleontologic details of various groups of land mammals, I discuss here the various types of mammal teeth. The shapes and arrangements of teeth in mammals are extremely important for the classification and identification of fossil vertebrates (as they are for living ones).

Some vertebrates (fish [including sharks], amphibians, and reptiles) generally have continuous replacement of teeth, so that as one is shed or lost, another erupts to takes its place. Land mammals, however, are fundamentally different is this regard; they have a mostly pre-determined number of teeth and very limited replacement of teeth.

Mammal teeth are differentiated into four types: incisors, canines, premolars, and molars. The premolars and molars are grouped together as cheek teeth. The teeth are, furthermore, very characteristic of the orders of the mammals to which they belong, and the morphology of the teeth reflect the diets of the mammals.

The number of teeth varies greatly in different groups of mammals. Dental formulae (i.e., the “ratios”) designate the number of teeth (upper/lower) on just one half of one side of both jaws. For example, the formula for dogs is 3/3, 1/1, 4/4, 2/3 = 40. This means that there are at least 10 teeth (namely, three incisors, one canine, four premolars, and two molars) on each quadrant (= one quarter) of the skull. But, as there are quadrants, the number of teeth is 10 x four ] = a total of 40 teeth on the entire adult skull.

The above diagram shows the side view of each of the jaws in a skull of a dog and the types of teeth present. The canine teeth are large, whereas the incisors are small. The large and pointy shape of the most forward molar tooth in the lower jaw and the most backward placed premolar in the upper jaw move pass one other, like blades in a pair of scissors, in such a way to allow for a shearing action (allows for eating flesh and bone). These two teeth (present are both sides of the skull) are referred to as carnassial teeth.  

The formula for a typical adult human is 2/2, 1/1, 2/2, 3/3, and it refers to the presence of two incisors, one, canine, two premolars, and 3 molars; for a total of 8 x 4 = 32 teeth. In some individuals there is an additional molar in each quadrant of the skull. This molar is the most posterior one and is referred to as a “wisdom tooth”. If there are four wisdom teeth, then the formula is 2/2, 1/1, 2/2, 4/4 = total of 36 teeth. 

In many mammals, including humans, there are two sets of teeth: the deciduous (“baby” or “milk” teeth) and the adult teeth. In humans, the “baby” teeth (2/2, 1/1, 0/0, 2/2), for a total of 20 teeth, are replaced by the adult teeth, which erupt starting around 6 years old or so. Note: baby humans do not have any premolar teeth.