Mole and Muskrat skulls

Teeth are structures found in fishes, some amphibians, reptiles, and mammals that range in form from single crowned scale-like or horny plates in fishes to more complex multiple crowned ones in mammals. Teeth are lacking in birds but modifications such as hooks, notches, and serrations of their beaks serve to function somewhat as a substitute.

In mammals, teeth vary in number from 0 in the anteaters, to 50 in the opossum and 100 small ones in the giant armadillo. Each mammal species has a specific total number (or formula) of permanent teeth which is distinctive as well in certain features and numbers of certain kinds of teeth-incisors, canines, premolars, and molars. The combination of dental formula and tooth forms is often required to distinguish correctly certain very similar species of rodents, shrews and bats from each other.

Different groups of mammals tend to have different dental characteristics. Carnivores (meat eaters), for example, have all their teeth with very narrow points or edges and no grinding surfaces; omnivores, those feeding equally on plants and animals, have the front teeth (incisors) wedge shaped and the rear teeth (molars) with broad grooves and crowned surfaces. Ungulates (hoofed animals) have wide molars with multiridged patterns, and rodents have continuously growing incisors and highly variable molar patterns. Greatly modified teeth are conspicuous in the tusks of walruses, elephants, and narwhal.

Shown here are the skulls of a Common Mole (Scalopus aquaticus), an insectivore, and a Muskrat (Ondatra zibethicus), a rodent. Note especially the very complex design of closed loops and triangles on the enamel surfaces of the muskrat molars. These are particularly unique in number to each species of microtine rodent known collectively as voles, which includes the muskrat.

For the mole, the dental formula is:

The muskrat dental formula is:

An unusual condition exists in elephants whereby as the front molar wears down, it is replaced by another tooth from the rear in a process suggestive of a conveyor belt!

For more information about teeth, refer to the Encyclopedia of the Biological Sciences, edited by Peter Gray (Reinhold Publishing Corporation, New York, 1961).