Tooth?qsrc=3044

Teeth (singular tooth) are small, calcified, whitish structures found in the jaws (or mouths) of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are covered by gums. Teeth are not made of bone, but rather of multiple tissues of varying density and hardness.

Teeth are among the most distinctive (and long-lasting) features of mammal species. Paleontologists use teeth to identify fossil species and determine their relationships. The shape of the animal's teeth are related to its diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing and grinding. Carnivores, on the other hand, need canines to kill prey and to tear meat.

Mammals are diphyodont, meaning that they develop two sets of teeth. In humans, the first set (the "baby," "milk," "primary" or "deciduous" set) normally starts to appear at about six months of age, although some babies are born with one or more visible teeth, known as neonatal teeth. Normal tooth eruption at about six months is known as teething and can be painful.

Some animals develop only one set of teeth (monophyodont) while others develop many sets (polyphyodont). Sharks, for example, grow a new set of teeth every two weeks to replace worn teeth. Rodent incisors grow and wear away continually through gnawing, which helps maintain relatively constant length. The industry of the beaver is due in part to this qualification. Many rodents such as voles (but not mice) and guinea pigs, as well as rabbits, have continuously growing molars in addition to incisors.^[1][2]

Mammals

Aardvark

In Aardvarks, teeth lack enamel and have many pulp tubules, hence the name of the order Tubulidentata.

Cetaceans

Toothed whales is a suborder of the cetaceans characterized by having teeth. The teeth differ considerably between the species. They may be numerous, with some dolphins bearing over 100 teeth in their jaws. On the other hand, the narwhals have a giant unicorn-like tusk, which is a tooth containing millions of sensory pathways and used for sensing during feeding, navigation and mating. It is the most neurologically complex tooth known. Beaked whales are almost toothless, with only bizarre teeth found in males. These teeth may be used for feeding but also for demonstrating aggression and showmanship.

Primates

Proboscideans

Section through the ivory tusk of a mammoth

Elephants' tusks are specialized incisors for digging food up and fighting. Some of elephant teeth are similar to those in 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.^[3]

Rodent

Rodents' incisors grow continuously throughout their lives, a process known as aradicular. Unlike humans whose ameloblasts die after tooth development, rodents continually produce enamel and must wear down their teeth by gnawing on various materials.^[4] 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.^[5][6] 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.

Fish

Fish, such as sharks, may go through many teeth in their lifetime. The multiple replacement of teeth is known as polyphydontia.

Reptiles

The teeth of reptiles are replaced constantly during their life. Juvenile crocodilians replace teeth with larger ones at a rate as high as 1 new tooth per socket every month. Once adult, tooth replacement rates can slow to two years and even longer. Over all, crocodilians may use 3,000 teeth from birth to death. New teeth are created within old teeth.

Fossilization and taphonomy

Because teeth are very resistant, often preserved when bones are not,^[7] and reflect the diet of the host organism, they are very valuable to archaeologists and palaeontologists. Early fish such as the thelodonts had teeth for scales, suggesting that the origin of teeth was scales which were retained in the mouth. Fish as early as the late Cambrian had dentine in their exoskeleton, which may have functioned in defense or for sensing their environment.^[8] Dentine can be as hard as the rest of teeth, and is composed of collagen fibres, reinforced with hydroxyapatite.^[8]

Decalcification removes the enamel from teeth and leaves only the organic interior intact, which comprises dentine and cementine.^[9] Enamel is quickly decalcified in acids,^[10] perhaps by dissolution by plant acids or via diagenetic solutions, or in the stomachs of vertebrate predators.^[9] Enamel can be lost by abrasion or spalling,^[9] and is lost before dentine or bone are destroyed by the fossilisation process.^[10] In such a case, the 'skeleton' of the teeth would consist of the dentine, with a hollow pulp cavity.^[9] The organic part of dentine, conversely, is destroyed by alkalis.^[10]

References

External links

•  "Teeth". The New Student's Reference Work. Chicago: F. E. Compton and Co.. 1914.