Teeth are common to most vertebrates, but mammalian teeth are distinctive in having a variety of shapes and functions. This feature first arose among early therapsids during the Permian, and has continued to the present day. All therapsid groups with the exception of the mammals are now extinct, but each of these groups possessed different tooth patterns, which aids with the classification of fossils.
Most extant mammals including humans are diphyodonts, i.e. they have an early set of deciduous teeth and a later set of permanent or "adult" teeth. Notable exceptions are elephants, kangaroos, and manatees, all of which are polyphyodonts, i.e. having teeth that are continuously being replaced.
Mammal teeth include incisors, canines, premolars, and molars, not all of which are present in all mammals. Various evolutionary modifications have occurred, such as the lack of canines in Glires, the development of tusks from either incisors (elephants) or canines (pigs and walruses), the adaptation of molars into flesh-shearing carnassials in Carnivora, and others.
Diversity
The extant mammalian infraclasses each have a set dental formula; the Eutheria (placental mammals) commonly have three pairs of molars and four premolars per jaw, whereas the Metatheria (marsupials) generally have four pairs of molars and between three or two premolars. For example, the tiger quoll (Dasyurus maculatus) is a dasyurid marsupial native to Australia. The quoll possesses four upper incisors and three lower incisors per left and right-hand side [I = 14]; two upper premolars [PM] and two lower premolars per side [PM = 8]; and four upper and four lower molars per side [M = 16], giving the animal a complement of thirty-eight teeth. The tiger quoll's dental formula is therefore as follows: .
Rabbits
Rabbits and other lagomorphs usually shed their deciduous teeth before (or very shortly after) their birth, and are usually born with their permanent teeth. The teeth of rabbits complement their diet, which consist of a wide range of vegetation. Since many of the foods are abrasive enough to cause attrition, rabbit teeth grow continuously throughout life. Rabbits have a dental formula of . There are no canines. Three to four millimeters of tooth is worn away by incisors every week, whereas the posterior teeth require a month to wear away the same amount.
Anatomy of rabbit teeth
The incisors and cheek teeth of rabbits are called aradicular hypsodont teeth. Aradicular teeth never form a true root with an apex, and hypsodont teeth have a high crown to root ratio (providing more room for wear and tear). This is sometimes referred to as an elodont dentition, meaning ever-growing. These teeth grow or erupt continuously. The growth or eruption is held in balance by dental abrasion from chewing a diet high in fiber.
Rodents
Like those of rabbits, rodents' incisors grow continuously throughout their lives, and are both aradicular and hypsodont. Unlike humans whose ameloblasts die after tooth development, rodents continually produce enamel and must wear down their teeth by gnawing on various materials. These teeth are used for cutting wood, biting through the skin of fruit, or for defense. The teeth have enamel on the outside and exposed dentin on the inside, so they self-sharpen during gnawing. On the other hand, continually growing molars are found in some rodent species, such as the sibling vole and the guinea pig. There is variation in the dentition of the rodents, but generally, rodents lack canines and premolars, and have a space between their incisors and molars, called the diastema region.
Humans
Afrotherians
Elephants
Elephants' tusks are specialized incisors for digging food up and fighting. Elephants are polyphyodonts whose teeth are similar to those of manatees, and it is notable that elephants are believed to have undergone an aquatic phase in their evolution.
Elephants have four molars, one on each side of the upper and lower jaw. Until age 40, these are replaced by larger molars. The new molars shift forward from the back of the jaw as the old wear down. The final set of molars last for about twenty years.
Manatees
Manatees are polyphyodonts with mandibular molars developing separately from the jaw and are encased in a bony shell separated by soft tissue.
Aardvarks
In Aardvarks, teeth lack enamel and have many pulp tubules, hence the name of the order Tubulidentata.
