A tooth (plural teeth) is a hard, calcified structure found in the
jaws (or mouths) of many vertebrates and 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. The cellular tissues that ultimately become teeth
originate from the embryonic germ layer, the ectoderm.
The general structure of teeth is similar across the vertebrates,
although there is considerable variation in their form and position.
The teeth of mammals have deep roots, and this pattern is also found
in some fish, and in crocodilians. In most teleost fish, however, the
teeth are attached to the outer surface of the bone, while in lizards
they are attached to the inner surface of the jaw by one side. In
cartilaginous fish, such as sharks, the teeth are attached by tough
ligaments to the hoops of cartilage that form the jaw.
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 and guinea pigs, but
not mice, as well as leporidae like rabbits, have continuously growing
molars in addition to incisors.
Teeth are not always attached to the jaw, as they are in mammals. In
many reptiles and fish, teeth are attached to the palate or to the
floor of the mouth, forming additional rows inside those on the jaws
proper. Some teleosts even have teeth in the pharynx. While not true
teeth in the usual sense, the dermal denticles of sharks are almost
identical in structure and are likely to have the same evolutionary
origin. Indeed, teeth appear to have first evolved in sharks, and are
not found in the more primitive jawless fish – while lampreys do
have tooth-like structures on the tongue, these are in fact, composed
of keratin, not of dentine or enamel, and bear no relationship to true
teeth. Though "modern" teeth-like structures with dentine and
enamel have been found in late conodonts, they are now supposed to
have evolved independently of later vertebrates' teeth.
Living amphibians typically have small teeth, or none at all, since
they commonly feed only on soft foods. In reptiles, teeth are
generally simple and conical in shape, although there is some
variation between species, most notably the venom-injecting fangs of
snakes. The pattern of incisors, canines, premolars and molars is
found only in mammals, and to varying extents, in their evolutionary
ancestors. The numbers of these types of teeth vary greatly between
species; zoologists use a standardised dental formula to describe the
precise pattern in any given group.
7 Fossilization and taphonomy
8 See also
10 External links
The genes governing tooth development in mammals are homologous to
these involved in the development of fish scales. Study of a tooth
plate of a fossil of the extinct fish
Romundina stellina showed that
the teeth and scales were made of the same tissues, also found in
mammal teeth, lending support to the theory that teeth evolved as a
modification of scales.
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, in general, 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. Kangaroos, elephants, and
manatees are unusual among mammals because they are polyphyodonts.
In Aardvarks, teeth lack enamel and have many pulp tubules, hence the
name of the order Tubulidentata.
Main article: Canine tooth
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. Sometimes called cuspids,these teeth are shaped
like points(cusps) and are used for tearing and grasping food
Like human teeth, whale teeth have polyp-like protrusions located on
the root surface of the tooth. These polyps are made of cementum in
both species, but in human teeth, the protrusions are located on the
outside of the root, while in whales the nodule is located on the
inside of the pulp chamber. While the roots of human teeth are made of
cementum on the outer surface, whales have cementum on the entire
surface of the tooth with a very small layer of enamel at the tip.
This small enamel layer is only seen in older whales where the
cementum has been worn away to show the underlying enamel.
The toothed whale is a suborder of the cetaceans characterized by
having teeth. The teeth differ considerably among 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
Human tooth and Dental anatomy
In humans (and most other primates) there are usually 20 primary
("baby") teeth, 28 to 32 of what's known as permanent teeth, in
addition to other four being third molars or wisdom teeth, each of
which may or may not grow in.
Among primary teeth, 10 of them are usually found in the maxilla (i.e.
upper jaw) and the other 10 in the mandible (i.e. lower jaw). Among
permanent teeth, 16 are found in the maxilla and the other 16 in the
mandible. Most of the teeth have uniquely distinguishing features.
An adult horse has between 36 and 44 teeth. The enamel and dentin
layers of horse teeth are intertwined. All horses have 12
premolars, 12 molars, and 12 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
Horse teeth can be used to estimate the animal's age. Between birth
and five years, age can be closely estimated by observing the eruption
pattern on milk teeth and then permanent teeth. By age five, all
permanent teeth have usually erupted. The horse is then said to have a
"full" mouth. After the age of five, age can only be conjectured by
studying the wear patterns on the incisors, shape, the angle at which
the incisors meet, and other factors. The wear of teeth may also be
affected by diet, natural abnormalities, and cribbing. Two horses of
the same age may have different wear patterns.
A horse's incisors, premolars, and molars, once fully developed,
continue to erupt as the grinding surface is worn down through
chewing. A young adult horse will have teeth which are
4.5-5 inches long, with the majority of the crown remaining below
the gumline in the dental socket. The rest of the tooth will slowly
emerge from the jaw, erupting about 1/8" each year, as the horse ages.
When the animal reaches old age, the crowns of the teeth are very
short and the teeth are often lost altogether. Very old horses, if
lacking molars, may need to have their fodder ground up and soaked in
water to create a soft mush for them to eat in order to obtain
Section through the ivory tusk of a mammoth
Elephants' tusks are specialized incisors for digging food up and
fighting. Some 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.
At birth, elephants have a total of 28 molar plate-like grinding teeth
not including the tusks. These are organized into four sets of seven
successively larger teeth which the elephant will slowly wear through
during its lifetime of chewing rough plant material. Only four teeth
are used for chewing at a given time, and as each tooth wears out,
another tooth moves forward to take its place in a process similar to
a conveyor belt. The last and largest of these teeth usually becomes
exposed when the animal is around 40 years of age, and will often last
for an additional 20 years. When the last of these teeth has fallen
out, regardless of the elephant's age, the animal will no longer be
able to chew food and will die of starvation.
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
consists 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 total of 6 incisors, three upper
premolars, three upper molars, two lower premolars, and two lower
molars on each side. There are no canines. Three to four millimeters
of the tooth is worn away by incisors every week, whereas the
posterior teeth require a month to wear away the same amount.
The incisors and cheek teeth of rabbits are called aradicular
hypsodont teeth. This is sometimes referred to as an elodent
dentition. These teeth grow or erupt continuously. The growth or
eruption is held in balance by dental abrasion from chewing a diet
high in fiber.
Buccal view of top incisor from Rattus rattus. Top incisor outlined in
Molars circled in blue.
Buccal view of the lower incisor from the right dentary of a Rattus
Lingual view of the lower incisor from the right dentary of a Rattus
Midsagittal view of top incisor from Rattus rattus. Top incisor
outlined in yellow.
Molars circled in blue.
Rodents have upper and lower hypselodont incisors that can
continuously grow enamel throughout its life without having properly
formed roots. These teeth are also known as aradicular teeth, and
unlike humans whose ameloblasts die after tooth development, rodents
continually produce enamel, they must wear down their teeth by gnawing
on various materials. Enamel and dentin are produced by the enamel
organ, and growth is dependent on the presence of stem cells, cellular
amplification, and cellular maturation structures in the odontogenic
Rodent incisors are used for cutting wood, biting through
the skin of fruit, or for defense. This allows for the rate of wear
and tooth growth to be at equilibrium. The microstructure of
rodent incisor enamel has shown to be useful in studying the phylogeny
and systematics of rodents because of its independent evolution from
the other dental traits. The enamel on rodent incisors are composed of
two layers: the inner portio interna (PI) with Hunter-Schreger bands
(HSB) and an outer portio externa (PE) with radial enamel (RE). It
usually involves the differential regulation of the epithelial stem
cell niche in the tooth of two rodent species, such as guinea
Lingual view of top incisor from Rattus rattus. Top incisor outlined
Molars circled in blue.
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.
Manatees are polyphyodont with mandibular molars developing separately
from the jaw and are encased in a bony shell separated by soft tissue.
Walrus tusks are canine teeth that grow continuously throughout
Teeth of Great White Shark
Pharyngeal teeth and
Fish, such as sharks, may go through many teeth in their lifetime. The
replacement of multiple teeth is known as polyphyodontia.
A class of prehistoric shark are called cladodonts for their strange
All amphibians have pedicellate teeth which are modified to be
flexible due to connective tissue and uncalcified dentine that
separates the crown from the base of the tooth.
Most amphibians exhibit teeth that have a slight attachment to the jaw
or acrodont teeth.
Acrodont teeth exhibit limited connection to the
dentary and have little enervation. This is ideal for organisms
who mostly use their teeth for grasping, but not for crushing and
allows for rapid regeneration of teeth at a low energy cost. Teeth are
usually lost in the course of feeding if the prey is struggling.
Additionally, amphibians that undergo a metamorphosis develop bicuspid
The teeth of reptiles are replaced constantly during their life.
Juvenile crocodilians replace teeth with larger ones at a rate as high
as one new tooth per socket every month. Once adult, tooth replacement
rates can slow to two years and even longer. Overall, crocodilians may
use 3,000 teeth from birth to death. New teeth are created within old
European medicinal leech
European medicinal leech has three jaws with numerous sharp teeth
which function like little saws for incising a host.
True teeth are unique to vertebrates, although many invertebrates
have analogous structures often referred to as teeth. The organisms
with the simplest genome bearing such tooth-like structures are
perhaps the parasitic worms of the family Ancylostomatidae. For
example, the hookworm
Necator americanus has two dorsal and two
ventral cutting plates or teeth around the anterior margin of the
buccal capsule. It also has a pair of subdorsal and a pair of
subventral teeth located close to the rear.
Historically the European medicinal leech, another invertebrate
parasite, has been used in medicine to remove blood from patients.
They have three jaws (tripartite) that look like little saws, and on
them are about 100 sharp teeth used to incise the host. The incision
leaves a mark that is an inverted Y inside of a circle. After piercing
the skin and injecting anticoagulants (hirudin) and anaesthetics, they
suck out blood, consuming up to ten times their body weight in a
In some species of Bryozoa, the first part of the stomach forms a
muscular gizzard lined with chitinous teeth that crush armoured prey
such as diatoms. Wave-like peristaltic contractions then move the food
through the stomach for digestion.
The limpet rasps algae from rocks using teeth with the strongest known
tensile strength of any biological material
Molluscs have a structure called a radula which bears a ribbon of
chitinous teeth. However, these teeth are histologically and
developmentally different from vertebrate teeth and are unlikely to be
homologous. For example, vertebrate teeth develop from a neural crest
mesenchyme-derived dental papilla, and the neural crest is specific to
vertebrates, as are tissues such as enamel.
The radula is used by molluscs for feeding and is sometimes compared
rather inaccurately to a tongue. It is a minutely toothed, chitinous
ribbon, typically used for scraping or cutting food before the food
enters the oesophagus. The radula is unique to molluscs, and is found
in every class of mollusc apart from bivalves.
Within the gastropods, the radula is used in feeding by both
herbivorous and carnivorous snails and slugs. The arrangement of teeth
(also known as denticles) on the radula ribbon varies considerably
from one group to another as shown in the diagram on the left.
Predatory marine snails such as the
Naticidae use the radula plus an
acidic secretion to bore through the shell of other molluscs. Other
predatory marine snails, such as the Conidae, use a specialized radula
tooth as a poisoned harpoon. Predatory pulmonate land slugs, such as
the ghost slug, use elongated razor-sharp teeth on the radula to seize
and devour earthworms. Predatory cephalopods, such as squid, use the
radula for cutting prey.
In most of the more ancient lineages of gastropods, the radula is used
to graze by scraping diatoms and other microscopic algae off rock
surfaces and other substrates. Limpets scrape algae from rocks using
radula equipped with exceptionally hard rasping teeth. These teeth
have the strongest known tensile strength of any biological material,
outperforming spider silk. The mineral protein of the limpet teeth
can withstand a tensile stress of 4.9 GPa, compared to 4 GPa of spider
silk and 0.5 GPa of human teeth.
Fossilization and taphonomy
Because teeth are very resistant, often preserved when bones are
not, and reflect the diet of the host organism, they are very
valuable to archaeologists and palaeontologists. Early fish such as
the thelodonts had scales composed of dentine and an enamel-like
compound, suggesting that the origin of teeth was from scales which
were retained in the mouth.
Fish as early as the late
dentine in their exoskeleton, which may have functioned in defense or
for sensing their environment.
Dentine can be as hard as the rest
of teeth and is composed of collagen fibres, reinforced with
Though teeth are very resistant, they also can be brittle and highly
susceptible to cracking.It is fact that cracking of the tooth can be
used as a diagnostic tool for predicting bite force.
Decalcification removes the enamel from teeth and leaves only the
organic interior intact, which comprises dentine and cementine.
Enamel is quickly decalcified in acids, perhaps by dissolution by
plant acids or via diagenetic solutions, or in the stomachs of
vertebrate predators. Enamel can be lost by abrasion or
spalling, and is lost before dentine or bone are destroyed by the
fossilisation process. In such a case, the 'skeleton' of the teeth
would consist of the dentine, with a hollow pulp cavity. The
organic part of dentine, conversely, is destroyed by alkalis.
Animal tooth development
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Wikimedia Commons has media related to tooth.
Beach, Chandler B., ed. (1914). "Teeth". The New Student's
Reference Work. Chicago: F. E. Compton and Co.