are the rearmost and most complicated kind of tooth
in most mammals
. In many mammals they grind food; hence the Latin name mola
Adult humans have twelve molars, in four groups of three at the back of the mouth. The third (rearmost) molar in each group is called a wisdom tooth
. It is the last tooth to appear, breaking through the surface of the gum at about the age of twenty, although this varies from individual to individual. Ethnicity can also have an impact on the age at which this occurs, with statistical variations between groups.
The types of molars in the human mouth are:
Molars among species
Molars differ considerably from one species to another, so there are many terms describing them:
- Tribosphenic: This kind is found in insectivores and young platypuses (adults have no teeth). Upper molars look like three-pointed mountain ranges; lowers look like two peaks and a third off to the side.
- Quadrate: This kind is found in humans and various other species. Four cusps are arranged in a rectangle; there may be a fifth.
- Bunodont: The cusps, instead of being sharp peaks, are rounded hills. The entire tooth is covered in enamel, and is most common among omnivores such as the pig, the bear and humans.
- Hypsodont: There is a lot of enamel and dentine above the gumline and the top of the pulp. This kind of molar is found in mammals that wear their teeth a lot, such as the horse.
- Zalambdodont: The tooth has two ridges that meet at an angle, forming the letter lambda.
- Dilambdodont: Like zalambdodont, but there are two lambdas on one tooth.
- Lophodont: The tooth has a few ridges perpendicular to the jaw.
- Selenodont: The tooth has a crescent-shaped ridge or ridges.
- Loxodont: The tooth has several parallel oblique ridges on its surface. The elephant Loxodonta is named for this feature.
design that is considered one of the most important characteristics of mammals is a three-cusped shape called a tribosphenic molar
. This design of molar has two important features: the trigonid
, or shearing end, and the talonid
, or crushing heel.
In modern mammals that have tribosphenic molars the trigonid is towards the front and the talonid towards the rear.
The tribosphenic design appears in all groups of mammals. Some paleontologists believe that it developed independently in monotremes, rather than being inherited from an ancestor that they share with marsupials and placentals; but this idea has critics and the debate is still going on.
Also some Jurassic mammals, such as Shuotherium and Pseudotribos, have "reversed tribosphenic" molars in which the talonid is towards the front. This variant is regarded as an example of convergent evolution
For pictures of various molars see The Diversity of Cheek Teeth