The extant mammalian infraclasses each have a set
dental formula; the
Eutheria (placentals) 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, which is often orange-yellow due to the incorporation of iron-containing pigments, 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 The
tusks of an
elephant 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.
Ungulates Horses An adult
horse has between 36 and 44 teeth. All horses have twelve premolars, twelve molars, and twelve incisors. Generally, all male equines also have four
canine teeth (called tushes) between the molars and incisors. However, few female horses (less than 28%) have canines, and those that do usually have only one or two, which many times are only partially erupted. A few horses have one to four
wolf teeth, which are
vestigial premolars, with most of those having only one or two. They are equally common in male and female horses and much more likely to be on the upper jaw. If present these can cause problems as they can interfere with the horse's
bit contact. Therefore, wolf teeth are commonly removed. These teeth are used to cut through flesh. In a study done on the bear tooth root area,
Dogs In dogs, the teeth are less likely than humans to form dental cavities because of the very high pH of dog saliva, which prevents enamel from demineralizing. ==References==