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A tooth (plural teeth) is a hard,
calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. D. Cardiovascular calcification: Orbicular origi ...
structure found in the
jaw The jaw is any opposable articulated structure at the entrance of the mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an ...

jaw
s (or
mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is ...

mouth
s) of many
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
s and used to break down
food Food is any substance consumed to provide nutritional Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organism In biology, an organism (from Anci ...

food
. Some animals, particularly
carnivore A carnivore , meaning "meat eater" (Latin, ''caro'', genitive ''carnis'', meaning "meat" or "flesh" and ''vorare'' meaning "to devour"), is an organism, animal whose food and energy requirements derive solely from animal Tissue (biology), tissue ...
s and
omnivore An omnivore () is an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular ...
s, also use teeth to help with capturing or wounding prey, tearing food, for defensive purposes, to intimidate other animals often including their own, or to carry prey or their young. The roots of teeth are covered by
gums The gums or gingiva (plural: ''gingivae'') consist of the mucosal tissue that lies over the mandible and maxilla inside the mouth. Gum health and disease can have an effect on general health. Structure The gums are part of the soft tissue lining ...

gums
. Teeth are not made of bone, but rather of multiple tissues of varying density and hardness that originate from the embryonic
germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce tw ...
, the
ectoderm The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrate Vertebrates () comprise all species of animal Animals (also ...

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
crocodilian Crocodilia (or Crocodylia, both ) is an Order (biology), order of mostly large, predatory, List of semiaquatic tetrapods, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous Period (g ...
s. In most
teleost Teleostei ( Greek: ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts , is by far the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish ...
fish, however, the teeth are attached to the outer surface of the bone, while in
lizard Lizards (suborder Lacertilia) are a widespread group of Squamata, squamate reptiles, with over 6,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic as it excludes the s ...

lizard
s they are attached to the inner surface of the jaw by one side. In
cartilaginous fish Chondrichthyes (; ) is a class (biology), class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed ...
, such as sharks, the teeth are attached by tough
ligament A ligament is the Connective tissue#Types, fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body incl ...

ligament
s to the hoops of
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic fibers (or yellow fibers) are an essential component of the extracellular matrix In biology Biology is the natural science that studies life and living org ...

cartilage
that form the jaw. Some animals develop only one set of teeth (monophyodonts) while others are
diphyodontA diphyodont is any animal with two successive sets of teeth A tooth (plural teeth) is a hard, calcification, calcified structure found in the jaws (or mouths) of many vertebrates and used to Mastication, break down food. Some animals, particular ...
s, i.e. they have an early set of
deciduous teeth Deciduous teeth or primary teeth, also informally known as baby teeth, milk teeth, or temporary teeth,Illustrated Dental Embryology, Histology, and Anatomy, Bath-Balogh and Fehrenbach, Elsevier, 2011, page 255 are the first set of in the grow ...
and a later set of permanent or "adult" teeth. Still others develop many sets (
polyphyodont A polyphyodont is any animal whose teeth A tooth (plural teeth) is a hard, calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, but calcium can be deposi ...
s).
Shark Sharks are a group of elasmobranch Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including shark Sharks are a group of elasmobranch fish characterized by a Chondrichthyes#Skeleton, cartilaginous skeleton, ...

Shark
s, for example, grow a new set of teeth every two weeks to replace worn teeth. Most extant mammals including humans are diphyodonts, but there are exceptions including elephants, kangaroos, and manatees, all of which are polyphyodonts.
Rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents ...

Rodent
incisors grow and wear away continually through gnawing, which helps maintain relatively constant length. The industry of the
beaver Beavers are large, semiaquatic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interaction ...

beaver
is due in part to this qualification. Many rodents such as
vole Voles are small rodents that are relatives of lemmings and hamsters, but with a stouter body; a shorter, hairy tail; a slightly rounder head; smaller ears and eyes; and differently formed molar (tooth), molars (high-crowned with angular cusps i ...
s and
guinea pig The guinea pig or domestic guinea pig (''Cavia porcellus''), also known as the cavy or domestic cavy (), is a species of rodent Rodents (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic bra ...

guinea pig
s, but not
mice A mouse, plural mice, is a small mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which i ...

mice
, as well as
leporidae Leporidae is the family (biology), family of rabbits and hares, containing over 60 species of extant mammals in all. The Latin word ''Leporidae'' means "those that resemble ''lepus''" (hare). Together with the pikas, the Leporidae constitute the ...

leporidae
like
rabbit Rabbits, also known as bunnies or bunny rabbits, are small s in the (along with the ) of the (along with the ). ''Oryctolagus cuniculus'' includes the species and its descendants, the world's of . ''Sylvilagus'' includes 13 wild rabbit ...

rabbit
s, have continuously growing molars in addition to incisors. Also,
tusks Tusks are elongated, continuously growing front teeth that protrude well beyond the mouth of certain mammal species. They are most commonly canine tooth, canine teeth, as with pigs, hippos, and walruses, or, in the case of elephants, elongated inc ...

tusks
(in tusked mammals) grow almost throughout life. Teeth are not always attached to the jaw, as they are in mammals. In many
reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). Living reptiles comprise turtl ...

reptile
s and fish, teeth are attached to the
palate The palate () is the roof of the mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ...

palate
or to the floor of the mouth, forming additional rows inside those on the jaws proper. Some
teleost Teleostei ( Greek: ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts , is by far the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish ...
s even have teeth in the
pharynx The pharynx (plural: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea – the tubes going down to the stomach and the lungs. It is found in vertebrates and invertebrates, thou ...

pharynx
. While not true teeth in the usual sense, the
dermal denticle A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most fishes is covered with these protective scale (zoology), scales, which can also provide effective Underwater camouflage, camouflage through the use of ani ...
s 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 Agnatha (, Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycen ...
– while
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of Agnatha, jawless fish of the order (biology), order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by ...

lamprey
s do have tooth-like structures on the tongue, these are in fact, composed of
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up Scale (anatomy), scales, hair, Nail ...

keratin
, not of dentine or enamel, and bear no relationship to true teeth. Though "modern" teeth-like structures with
dentine Dentin () (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American English ...
and enamel have been found in late
conodont Conodonts (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as ...
s, they are now supposed to have evolved independently of later vertebrates' teeth. Living
amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial animal, ter ...
s 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
fang The four canines, or fangs, of a domestic cat. (The largest two teeth of the top and bottom rows of teeth.) A fang is a long, pointed tooth A tooth (plural teeth) is a hard, calcified structure found in the jaws (or mouths) of many vert ...

fang
s of
snake Snakes are elongated, , s of the Serpentes . Like all other , snakes are , s covered in overlapping . Many species of snakes have s with several more joints than their ancestors, enabling them to swallow prey much larger than their heads w ...

snake
s. 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 This is a list of authors of names published under the International Code of Zoological Nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Convention (norm), convention in zoology that rules the formal sci ...
use a standardised
dental formula Dentition pertains to the development of teeth and their arrangement in the mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biolo ...
to describe the precise pattern in any given group.


Etymology

The word ''tooth'' comes from
Proto-Germanic Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the reconstructed Reconstruction may refer to: Politics, history, and sociology *Reconstruction (law), the transfer of a company's (or several companies') business to a new ...
*''tanthu''- which in turn comes from
Proto-Indo-European Proto-Indo-European (PIE) is the theorized common ancestor of the Indo-European language family The Indo-European languages are a language family A language is a structured system of communication used by humans, including speech ( ...
*''h₁dent''-, which was composed of the root *''h₁ed''- ("to eat") plus the active participle suffix -''nt'', therefore it literally meant "that which eats". Cognate with
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became the dominant la ...

Latin
''dēns'',
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
ὀδούς ''odous'', and
Sanskrit Sanskrit (; attributively , ; , , ) is a of that belongs to the branch of the . It arose in South Asia after its predecessor languages had there from the northwest in the late . Sanskrit is the of , the language of classical , and of h ...

Sanskrit
''dát''.


Origin

Teeth are assumed to have evolved either from
ectoderm The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrate Vertebrates () comprise all species of animal Animals (also ...

ectoderm
denticles (scales, much like those on the skin of sharks) that folded and integrated into the mouth (called the "outside–in" theory), or from endoderm Pharynx, pharyngeal teeth (primarily formed in the
pharynx The pharynx (plural: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea – the tubes going down to the stomach and the lungs. It is found in vertebrates and invertebrates, thou ...

pharynx
of jawless vertebrates) (the "inside–out" theory). In addition, there is another theory stating that neural crest gene regulatory network, and neural crest-derived ectomesenchyme are the key to generate teeth (with any epithelium, either ectoderm or endoderm). The genes governing tooth development in mammals are homologous gene, homologous to those 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.


Mammals

Teeth are among the most distinctive (and long-lasting) features of mammal species. Paleontology, 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 Molar (tooth), molars for mastication, chewing and grinding. Carnivores, on the other hand, have canine (tooth), canine teeth to kill prey and to tear meat. Mammals, in general, are
diphyodontA diphyodont is any animal with two successive sets of teeth A tooth (plural teeth) is a hard, calcification, calcified structure found in the jaws (or mouths) of many vertebrates and used to Mastication, break down food. Some animals, particular ...
, meaning that they develop two sets of teeth. In humans, the first set (the "baby," "milk," "primary" or "deciduous teeth, 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
polyphyodont A polyphyodont is any animal whose teeth A tooth (plural teeth) is a hard, calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, but calcium can be deposi ...
s.


Aardvark

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


Canines

In dogs, the teeth are less likely than humans to form dental caries, 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.


Cetaceans

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 showmanship.


Primates

In humans (and most other primates) there are usually 20 primary (also "baby" or "milk") teeth, and later up to 32 permanent teeth. Four of these 32 may be third molars or wisdom teeth, although these are not present in all adults, and may be removed surgically later in life. Among primary teeth, 10 of them are usually found in the maxilla (i.e. upper jaw) and the other 10 in the human mandible, 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.


Horse

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 (horse), bit contact. Therefore, wolf teeth are commonly removed. 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 (horse), 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 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 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 adequate nutrition.


Proboscideans

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 animal, 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.


Rabbit

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.


Rodents

Rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents ...

Rodent
s have upper and lower hypselodont incisors that can continuously grow Tooth enamel, 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 Tooth enamel, 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 region. 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 pigs.Tummers M and Thesleff I. Root or crown: a developmental choice orchestrated by the differential regulation of the epithelial stem cell niche in the tooth of two rodent species. Development (2003). 130(6):1049-57.AM Hunt. A description of the molar teeth and investing tissues of normal guinea pigs. J Dent Res. (1959) 38(2):216-31. The teeth have Tooth enamel, 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 The guinea pig or domestic guinea pig (''Cavia porcellus''), also known as the cavy or domestic cavy (), is a species of rodent Rodents (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic bra ...

guinea pig
. There is variation in the dentition of the rodents, but generally, rodents lack canine tooth, canines and premolars, and have a space between their incisors and molars, called the Diastema (dentistry), diastema region.


Manatee

Manatees are polyphyodont with mandibular molars developing separately from the jaw and are encased in a bony shell separated by soft tissue.


Walrus

Walrus tusks are canine teeth that grow continuously throughout life.


Fish

Fish, such as sharks, may go through many teeth in their lifetime. The replacement of multiple teeth is known as
polyphyodont A polyphyodont is any animal whose teeth A tooth (plural teeth) is a hard, calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, but calcium can be deposi ...
ia. A class of prehistoric shark are called cladodonts for their strange forked teeth.


Amphibians

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 Mandible, dentary and have little Enervate, 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 shaped teeth.


Reptiles

The teeth of reptiles are replaced constantly throughout their lives. Crocodilian juveniles replace teeth with larger ones at a rate as high as one new tooth per socket every month. Once mature, 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 teeth.


Birds

A skull of Ichthyornis discovered in 2014 suggests that the beak of birds may have evolved from teeth to allow chicks to escape their shells earlier, and thus avoid predators and also to penetrate protective covers such as hard earth to access underlying food.


Invertebrates

True teeth are unique to vertebrates, although many invertebrates have Analogy (biology), 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 Anatomical terms of location, dorsal and two Anatomical terms of location, ventral cutting plates or teeth around the anterior margin of the Cheek, 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 single meal. 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 peristalsis, peristaltic contractions then move the food through the stomach for digestion. 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 Homology (biology), 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 Tooth enamel, 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 Bivalvia, bivalves. Within the gastropods, the radula is used in feeding by both herbivore, herbivorous and carnivore, 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 Cone snail#Harpoon and venoms, 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 stress (mechanics), 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 Cambrian had dentine in their exoskeletons, which may have functioned in defense or for sensing their environments.Teaford, Mark F and Smith, Moya Meredith, 2007. ''Development, Function and Evolution of Teeth'', Cambridge University Press. , Chapter 5. Dentine can be as hard as the rest of teeth and is composed of collagen fibres, reinforced with hydroxyapatite. Though teeth are very resistant, they also can be brittle and highly susceptible to cracking. However, cracking of the tooth can be used as a diagnostic tool for predicting bite force. Enamel fracture, Additionally, enamel fractures can also give valuable insight into the diet and behaviour of archaeological and fossil samples. Decalcification removes the enamel from teeth and leaves only the organic interior intact, which comprises dentine and Cementite, 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.


See also

* Animal tooth development * Dragon's teeth (mythology)


References


External links

* {{Authority control Animal anatomy Teeth, Speech organs