Which teeth are the third molars, and are always the last to0 erupt?
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Tooth Molar (tooth)
A tooth (plural teeth) is a small, calcified, whitish 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 varies greatly between species; zoologists use a standardised dental formula to describe the precise pattern in any given group.

Origin 

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. 

Mammals 

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.

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 cavities because of the very high pH of dog saliva, which prevents enamel from demineralizing. 

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, there are usually 20 primary ("baby") teeth and 28 to 32 permanent teeth, the last four being third molars or "wisdom teeth", each of which may or may not grow in. Among primary teeth, 10 usually are found in the maxilla (upper jaw) and the other 10 in the mandible (lower jaw). Among permanent teeth, 16 are found in the maxilla and the other 16 in the mandible. Most of the teeth have 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 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 study of 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 adequate nutrition.

Proboscideans 

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. 

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.[http://www.peteducation.com/article.cfm?cls18&cat
1798&articleid=3078 Dental Anatomy & Care for Rabbits and Rodents]  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 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 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 

Rodents' incisors grow continuously throughout their lives or are aradicular.  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.

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 multiple replacement of teeth is known as polyphyodontia.

A class of prehistoric shark are called cladodonts for their strange forked teeth.

Reptiles 

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 teeth.

Invertebrates

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 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 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 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 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.Teaford, Mark F and Smith, Moya Meredith, 2007. Development, Function and Evolution of Teeth, Cambridge University Press. ISBN 0-521-03372-1, ISBN 978-0-521-03372-5, Chapter 5.  Dentine can be as hard as the rest of teeth, and is composed of collagen fibres, reinforced with hydroxyapatite.

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. The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name molar derives from Latin, molaris dens, meaning "millstone tooth", from "mola", millstone and dens, tooth. Molars show a great deal of diversity in size and shape across mammal groups.

Human anatomy

In humans, the molar teeth have either four or five cusps. Adult humans have twelve molars, in four groups of three at the back of the mouth. The third, rearmost molar in each group is called a wisdom tooth. It is the last tooth to appear, breaking through the front of the gum at about the age of twenty, although this varies from individual to individual. Ethnicity can also affect the age at which this occurs, with statistical variations between groups.  In some cases, it may not even erupt at all.

The human mouth contains upper (maxillary) and lower (mandibular) molars. They are: maxillary first molar, maxillary second molar, maxillary third molar, mandibular first molar, mandibular second molar, and mandibular third molar.

Mammal evolution

In mammals, the crown of the molars and premolars are folded into a wide range of complex shapes. The basic elements of the crown are the more or less conical projections called cusps and the valleys that separate them. The cusps contain both dentine and enamel, whereas minor projections on the crown, called crenullations, are the result of different enamel thickness. Cusps are occasionally joined to form ridges and expanded to form crests. Cingula are often incomplete ridges that pass around the base of the crown. 

These mammalian multi-cusped cheek teeth probably evolved from single-cusped teeth in reptilians, although the diversity of therapsid molar patterns and the complexity in the molars of the earliest mammals makes it impossible to determine how this happened. According to the widely accepted "differentiation theory", additional cusps have arisen by budding or outgrowth from the crown, while the rivalling "concrescence theory" instead proposes that complex teeth evolved by the clustering of originally separate conical teeth. It is generally agreed that therian mammals (placentals and marsupials) evolved from an ancestor with tribosphenic cheek teeth, with three main cusps arranged in a triangle.

Morphology

Each major cusp on an upper molar is called a cone and is identified by a prefix dependent on its relative location on the tooth: proto-, para-, meta-, hypo-, and ento-. Suffixes are added to these names: -id is added to cusps on a lower molar (e.g., protoconid); -ule to a minor cusp (e.g., protoconulid). A shelf-like ridge on the lower part of the crown (on an upper molar) is called a cingulum; the same feature on the lower molar a cingulid, and a minor cusp on these, for example, a cingular cuspule or conulid. 

Tribosphenic

The design that is considered one of the most important characteristics of mammals is a three-cusped shape called a tribosphenic molar. This molar design has two important features: the trigonid, or shearing end, and the talonid, or crushing heel. In modern tribosphenic molars, the trigonid is towards the front of the jaw and the talonid towards the rear.

The tribosphenic tooth is found in insectivores and young platypuses (adults have no teeth). Upper molars look like three-pointed mountain ranges; lowers look like two peaks and a third off to the side.

The tribosphenic design appears primitively in all groups of mammals. Some paleontologists believe that it developed independently in monotremes (or australosphenidans), rather than being inherited from an ancestor that they share with marsupials and placentals (or boreosphenidans); but this idea has critics and the debate is still going on.  For example, the dentition of the Early Cretaceous monotreme Steropodon is similar to those of Peramus and dryolestoids, which suggests that monotremes are related to some pre-tribosphenic therian mammals,  but, on the other hand, the status of neither of these two groups is well-established.

Some Jurassic mammals, such as Shuotherium and Pseudotribos, have "reversed tribosphenic" molars, in which the talonid is towards the front. This variant is regarded as an example of convergent evolution. 

From the primitive tribosphenic tooth, molars have diversified into several unique morphologies. In many groups, a fourth cusp, the hypocone (hyponconid), subsequently evolved (see below).

Quadrate

Quadrate (also called quadritubercular or euthemorphic) molars have an additional fourth cusp on the lingual (tongue) side called the hypocone, located posterior to the protocone. Quadrate molars appeared early in mammal evolution and are present in many species, including hedgehogs, raccoons, and many primates, including humans.
There may be a fifth cusp.

In many mammals, additional smaller cusps called conules appear between the larger cusps. They are named after their locations, e.g. a paraconule is located between a paracone and a metacone, a hypoconulid is located between a hypoconid and an entoconid.

Bunodont

In bunodont molars, the cusps are low and rounded hills rather than sharp peaks. They are most common among omnivores such as pigs, bears and humans.  Bunodont molars are effective crushing devices and often basically quadrate in shape. 

Hypsodont

Hypsodont dentition is characterized by high-crowned teeth and enamel that extends far past the gum line, which provides extra material for wear and tear.  Some examples of animals with hypsodont dentition are cows and horses; all animals that feed on gritty, fibrous material. Hypsodont molars can continue to grow throughout life, for example in some species of Arvicolinae (herbivorous rodents).

Hypsodont molars lack both crown and neck. The occlusal surface is rough and mostly flat, adapted for crushing and grinding plant material. The body is covered with cementum both above and below the gingival line, below which is a layer of enamel covering the entire length of the body. The cementum and the enamel invaginate into the thick layer of dentin. 

Brachydont

The opposite condition to hypsodont is called brachydont or brachyodont (from brachys, "short"). It is a type of dentition characterized by low-crowned teeth. Human teeth are brachydont.

A brachydont tooth has a crown above the gingival line and a neck just below it, and at least one root. A cap of enamel covers the crown and extends down to the neck. Cementum is only found below the gingival line. The occlusal surfaces tend to be pointed, well-suited for holding prey and tearing and shredding.

Zalambdodont

Zalambdodont molars have three cusps, one larger on the lingual side and two smaller on the labial side, joined by two crests that forms a V- or lambda-shape. The larger inner cusp might be homologous with the paracone in a tribosphenic molar, but can also be fused with the metacone. The protocone is typically missing. The two smaller labial cusps are located on an expanded shelf called the stylar shelf. Zalambdodont molars are found in for example golden moles and solenodons.

Dilambdodont

Like zalambdodont molars, dilambdodont molars have a distinct ectoloph but shaped like two lambdas or a W. On the lingual side, at the bottom of the W, are the metacone and paracone, and the stylar shelf is on the labial side. A protocone is present lingual to the ectoloph. Dilambdodont molars are present in shrews, moles, and some insectivorous bats.

Lophodont

Lophodont teeth are easily identified by the patterns of ridges or lophs of enamel interconnecting the cusps on the crowns. Present in most herbivores, these patterns of lophs can be a simple ring-like edge, like in mole rats, or a complex arrangement of series of ridges and cross-ridges, as those in odd-toed ungulates, such as horses.

Lophodont molars have hard and elongated enamel ridges called lophs oriented either along or perpendicular to the dental row. Lophodont molars are common in herbivores that grind their food thoroughly. Examples include tapirs, manatees, and many rodents.

When two lophs form transverse, often ring-shaped, ridges on a tooth, the arrangement is called bilophodont. This pattern is common in primates, but can also be found in lagomorphs (hare, rabbits, and pikas) and some rodents.

Extreme forms of lophodonty in elephants and some rodents (such as Otomys) is known as loxodonty. The African elephant belongs to a genus called Loxodonta because of this feature.

Selenodont

In selenodont molars (so-named after moon goddess Selene), the major cusp is elongated into crescent-shape ridge. Examples include most even-toed ungulates, such as cows and deer.

Secodont

Many carnivorous mammals have enlarged and blade-like teeth especially adapted for slicing and chopping called carnassials. A general term for such blade-like teeth is secodont or plagiaulacoid.
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Wisdom teeth
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