101-409: 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 the mammal groups. The third molar of humans
202-447: A cingulum ; the same feature on the lower molar a cingulid, and a minor cusp on these, for example, a cingular cuspule or conulid. The design that is considered one of the most important characteristics of therian mammals is called a tribosphenic molar. Among living mammals, the tribosphenic tooth is found in most insectivorous mammals as well as young platypuses , even though adults platypuses are toothless. In tribosphenic teeth,
303-483: A clade termed Australosphenida , a group of mammals from the Jurassic and Cretaceous of Madagascar, South America and Australia, that share tribosphenic molars . However, in a 2022 review of monotreme evolution, it was noted that Teinolophos , the oldest ( Barremian ~ 125 million years ago) and the most primitive monotreme differed substantially from non-monotreme australosphenidans in having five molars as opposed to
404-423: A separate genital tract , whereas most placental mammalian females have separate openings for reproduction (the vagina ), urination (the urethra ), and defecation (the anus ). In monotremes, only semen passes through the penis while urine is excreted through the male's cloaca. The monotreme penis is similar to that of turtles and is covered by a preputial sac. Monotreme eggs are retained for some time within
505-518: A constant body temperature in a variety of circumstances, such as the platypus in icy mountain streams. Early researchers were misled by two factors: firstly, monotremes maintain a lower average temperature than most mammals; secondly, the short-beaked echidna , much easier to study than the reclusive platypus, maintains normal temperature only when active; during cold weather, it conserves energy by "switching off" its temperature regulation. Understanding of this mechanism came when reduced thermal regulation
606-399: 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 upper molars have at least three main cusps, one larger on the lingual side and two smaller on
707-715: 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 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
808-681: 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 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. Some rodents, such as voles and guinea pigs (but not mice ), as well as lagomorpha ( rabbits , hares and pikas ), have continuously growing molars in addition to incisors. Also, tusks (in tusked mammals) grow almost throughout life. Teeth are not always attached to
909-456: A paraconule is located between a paracone and a metacone, a hypoconulid is located between a hypoconid and an entoconid. 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 dentition is characterized by high-crowned teeth and enamel that extends far past
1010-473: A reptile-like gait, with legs on the sides of, rather than underneath, their bodies. The monotreme leg bears a spur in the ankle region; the spur is not functional in echidnas, but contains a powerful venom in the male platypus. This venom is derived from β-defensins , proteins that are present in mammals that create holes in viral and bacterial pathogens. Some reptile venom is also composed of different types of β-defensins, another trait shared with reptiles. It
1111-483: A simple, ring-like edge, as in mole rats , or a complex arrangement of series of ridges and cross-ridges, as those in odd-toed ungulates , such as equids . 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
SECTION 10
#17328687705711212-441: A single bone in their lower jaw; and have three middle-ear bones. In common with reptiles and marsupials , monotremes lack the connective structure ( corpus callosum ) which in placental mammals is the primary communication route between the right and left brain hemispheres. The anterior commissure does provide an alternate communication route between the two hemispheres, though, and in monotremes and marsupials it carries all
1313-463: A soft mush for them to eat in order to obtain adequate nutrition. Elephants ' tusks are specialized incisors for digging food up and fighting. Some elephant teeth are similar to those in manatees , and 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
1414-475: A substantial reduction at the elevated temperature of 28 °C (82 °F). Monotreme milk contains a highly expressed antibacterial protein not found in other mammals, perhaps to compensate for the more septic manner of milk intake associated with the absence of teats. During the course of evolution the monotremes have lost the gastric glands normally found in mammalian stomachs as an adaptation to their diet. Monotremes synthesize L- ascorbic acid only in
1515-426: A tooth, the arrangement is called bilophodont . This pattern is common in primates, but can also be found in lagomorphs (hares, 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. In selenodont molars (so-named after moon goddess Selene ),
1616-399: A triangle. 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 left lower part of the crown (on an upper molar) is called
1717-475: Is a similar pattern of tooth replacement seen in monotremes and marsupials, which originally provided the basis for the competing " Marsupionta " hypothesis in which the divergence between monotremes and marsupials happened later than the divergence between these lineages and the placental mammals. Van Rheede (2005) concluded that the genetic evidence favors the Theria hypothesis, and this hypothesis continues to be
1818-427: 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. 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
1919-508: Is disputed. Nonetheless, findings on the extinct species Teinolophos confirm that suspended ear bones evolved independently among monotremes and therians. The external opening of the ear still lies at the base of the jaw. The sequencing of the platypus genome has also provided insight into the evolution of a number of monotreme traits, such as venom and electroreception , as well as showing some new unique features, such as monotremes possessing 5 pairs of sex chromosomes and that one of
2020-607: 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 . 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
2121-503: 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
SECTION 20
#17328687705712222-474: 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. The word tooth comes from Proto-Germanic * tanþs , derived from
2323-403: Is sometimes vestigial . In humans, the molar teeth have either four or five cusps . Adult humans have 12 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 20, although this varies among individuals and populations, and in many cases
2424-558: 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. 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
2525-539: Is the oldest known platypus-like fossil. The durophagous Kollikodon , the pseudotribosphenic Steropodon , and Stirtodon , Dharragarra , Opalios , and Parvopalus occur in the same Cenomanian deposits. Oligo-Miocene fossils of the toothed platypus Obdurodon have also been recovered from Australia, and fossils of a 63 million-year old platypus relative occur in southern Argentina ( Monotrematum ), see fossil monotremes below. The extant platypus genus Ornithorhynchus in also known from Pliocene deposits, and
2626-586: Is the opposite of that hypothesized for Australia's other dominant mammal group, the marsupials , which likely migrated across Antarctica to Australia from South America. In 2024, a prominent assemblage of early monotremes was described from the Cenomanian deposits (100–96.6 Ma) of the Griman Creek Formation in Lightning Ridge, New South Wales. One of these, the fossil jaw fragment of Dharragarra ,
2727-479: Is thought to be an ancient mammalian characteristic, as many non-monotreme archaic mammal groups also possess venomous spurs . The key anatomical difference between monotremes and other mammals gives them their name; monotreme means "single opening" in Greek, referring to the single duct (the cloaca ) for their urinary, defecatory, and reproductive systems. Like reptiles, monotremes have a single cloaca. Marsupials have
2828-492: Is thought to provide some insight into the most recent common ancestor of the synapsid lineage leading to mammals and the sauropsid lineage leading to birds and modern reptiles, which are believed to have split about 315 million years ago during the Carboniferous . The presence of vitellogenin genes (a protein necessary for egg yolk formation) is shared with birds; the presence of this symplesiomorphy suggests that
2929-490: 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
3030-602: The Cretaceous , indicating that monotremes were diversifiying by the early Late Cretaceous. Monotremes have been found in the latest Cretaceous and Paleocene of southern South America, so one hypothesis is that monotremes arose in Australia in the Late Jurassic or Early Cretaceous , and that some migrated across Antarctica to South America , both of which were still united with Australia at that time. This direction of migration
3131-488: The Early Cretaceous monotreme Steropodon is similar to those of Peramus and dryolestoids , which suggests that monotremes are related to some pre-tribosphenic mammals, but, on the other hand, the status of neither of these two groups is well-established. Some Jurassic mammalia forms , such as docodonts and shuotheriids , have "reversed tribosphenic" molars, in which a talonid-like structure develops towards
Molar (tooth) - Misplaced Pages Continue
3232-470: The Proto-Indo-European * h₁dent- , which was composed of the root * h₁ed- ' to eat ' plus the active participle suffix * -nt , therefore literally meaning ' that which eats ' . The irregular plural form teeth is the result of Germanic umlaut whereby vowels immediately preceding a high vocalic in the following syllable were raised. As the nominative plural ending of
3333-591: 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, have canine teeth 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
3434-736: The commissural fibers arising from the neocortex , whereas in placental mammals the anterior commissure carries only some of these fibers. Extant monotremes lack teeth as adults. Fossil forms and modern platypus young have a "tribosphenic" form of molars (with the occlusal surface formed by three cusps arranged in a triangle), which is one of the hallmarks of extant mammals. Some recent work suggests that monotremes acquired this form of molar independently of placental mammals and marsupials, although this hypothesis remains disputed. Tooth loss in modern monotremes might be related to their development of electrolocation . Monotreme jaws are constructed somewhat differently from those of other mammals, and
3535-414: 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
3636-471: The jaws (or mouths ) of many vertebrates and used to break down food . Some animals, particularly carnivores and omnivores , 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 . Teeth are not made of bone, but rather of multiple tissues of varying density and hardness that originate from
3737-477: 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
3838-540: 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. 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
3939-641: The Late Cretaceous and Paleocene epochs in southern South America, implying that they were also present in Antarctica, though remains have not yet been found there. The name monotreme derives from the Greek words μονός ( monós 'single') and τρῆμα ( trêma 'hole'), referring to the cloaca . Like other mammals, monotremes are endothermic with a high metabolic rate (though not as high as other mammals; see below); have hair on their bodies; produce milk through mammary glands to feed their young; have
4040-767: The Proto-Germanic consonant stems (to which * tanþs belonged) was * -iz , the root vowel in the plural form * tanþiz (changed by this point to * tą̄þi via unrelated phonological processes) was raised to /œː/, and later unrounded to /eː/, resulting in the tōþ/tēþ alternation attested from Old English . Cf. also Old English bōc/bēċ ' book/books ' and ' mūs/mȳs ' ' mouse/mice ' , from Proto-Germanic * bōks/bōkiz and * mūs/mūsiz respectively. Cognate with Latin dēns , Greek ὀδούς ( odous ), and Sanskrit dát . Teeth are assumed to have evolved either from ectoderm denticles (scales, much like those on
4141-415: 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 teeth are easily identified by the differentiating patterns of ridges or lophs of enamel interconnecting the cusps on the crowns. Present in most herbivores, these patterns of lophs can be
Molar (tooth) - Misplaced Pages Continue
4242-523: The X ;chromosomes resembles the Z ;chromosome of birds, suggesting that the two sex chromosomes of marsupial and placental mammals evolved after the split from the monotreme lineage. Additional reconstruction through shared genes in sex chromosomes supports this hypothesis of independent evolution. This feature, along with some other genetic similarities with birds, such as shared genes related to egg-laying,
4343-515: The already eutherian Juramaia is dated to 161–160 million years ago). Teinolophos like modern monotremes displays adaptations to elongation and increased sensory perception in the jaws, related to mechanoreception or electroreception . Molecular clock and fossil dating give a wide range of dates for the split between echidnas and platypuses, with one survey putting the split at 19–48 million years ago, but another putting it at 17–89 million years ago. It has been suggested that both
4444-465: The averages of 35 °C (95 °F) for marsupials and 37 °C (99 °F) for placental mammals . Research suggests this has been a gradual adaptation to the harsh, marginal environmental niches in which the few extant monotreme species have managed to survive, rather than a general characteristic of extinct monotremes. Monotremes may have less developed thermoregulation than other mammals, but recent research shows that they easily maintain
4545-421: 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. 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
4646-520: The common ancestor of monotremes, marsupials, and placental mammals was oviparous , and that this trait was retained in monotremes but lost in all other extant mammal groups. DNA analyses suggest that although this trait is shared and is synapomorphic with birds, platypuses are still mammals and that the common ancestor of extant mammals lactated. The monotremes also have extra bones in the shoulder girdle , including an interclavicle and coracoid , which are not found in other mammals. Monotremes retain
4747-568: The course of feeding if the prey is struggling. Additionally, amphibians that undergo a metamorphosis develop bicuspid shaped teeth. 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. A skull of Ichthyornis discovered in 2014 suggests that
4848-403: 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
4949-474: 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
5050-411: The direct ancestors of all three living mammal groups, but it was most likely not ancestral to mammals as a whole. Many paleontologists argue that it developed independently in monotremes (from australosphenidans ), rather than being inherited from a common ancestor that they share with marsupials and placentals (from boreosphenidans ); this idea still has some critics. For example, the dentition of
5151-497: The divergence of the monotreme lineage from the Metatheria ( marsupial ) and Eutheria ( placental ) lineages happened prior to the divergence between marsupials and placental mammals, and this explains why monotremes retain a number of primitive traits presumed to have been present in the synapsid ancestors of later mammals, such as egg-laying. Most morphological evidence supports the Theria hypothesis, but one possible exception
SECTION 50
#17328687705715252-582: The diversity of therapsid molar patterns and the complexity in the molars of the earliest mammals make determining how this happened impossible. 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. Therian mammals (placentals and marsupials) are generally agreed to have evolved from an ancestor with tribosphenic cheek teeth, with three main cusps arranged in
5353-418: 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
5454-507: The female monotremes nurse their young with milk . Monotremes have been considered by some authors to be members of Australosphenida , a clade that contains extinct mammals from the Jurassic and Cretaceous of Madagascar, South America, and Australia, but this categorization is disputed and their taxonomy is under debate. All extant species of monotremes are indigenous to Australia and New Guinea , although they were also present during
5555-460: 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
5656-412: The foods are abrasive enough to cause attrition, rabbit teeth grow continuously throughout life. Rabbits have a total of six incisors, three upper premolars, three upper molars, two lower premolars, and two lower molars on each side. There are no canines. Dental formula is 2.0.3.3 1.0.2.3 = 28. Three to four millimeters of the tooth is worn away by incisors every week, whereas the cheek teeth require
5757-426: The front of the lower molar, rather than towards the rear. 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 (hypoconid), subsequently evolved (see below). Quadrate (also called quadritubercular or euthemorphic) molars have a hypocone, an additional fourth cusp on
5858-466: The gum line, which provides extra material for wear and tear. Some examples of animals with hypsodont dentition are cattle 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 a crown and a neck. The occlusal surface is rough and mostly flat, adapted for crushing and grinding plant material. The body
5959-430: 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 studying
6060-460: 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
6161-417: 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. Monophyodonts are animals that develop only one set of teeth, while diphyodonts grow an early set of deciduous teeth and a later set of permanent or "adult" teeth . Polyphyodonts grow many sets of teeth. For example, sharks , grow
SECTION 60
#17328687705716262-442: The jaw and are encased in a bony shell separated by soft tissue. Walrus tusks are canine teeth that grow continuously throughout life. 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 forked teeth. Unlike the continuous shedding of functional teeth seen in modern sharks,
6363-424: The jaw opening muscle is different. As in all true mammals, the tiny bones that conduct sound to the inner ear are fully incorporated into the skull, rather than lying in the jaw as in non-mammal cynodonts and other premammalian synapsids ; this feature, too, is now claimed to have evolved independently in monotremes and therians , although, as with the analogous evolution of the tribosphenic molar, this hypothesis
6464-469: 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
6565-461: The kidneys. Both the platypus and echidna species have spurs on their hind limbs. The echidna spurs are vestigial and have no known function, while the platypus spurs contain venom. Molecular data show that the main component of platypus venom emerged before the divergence of platypus and echidnas, suggesting that the most recent common ancestor of these taxa was also possibly a venomous monotreme. The traditional " Theria hypothesis" states that
6666-422: The labial side. The large cusp is joined to the other two by crests, forming a narrow V- or λ (lambda)-shape. The term "zalambdodont" roughly translates to "very lambda-toothed". Zalambdodont molars are found in tenrecs , golden moles , solenodons , and marsupial moles among living mammals. In zalambdodont placentals, the larger inner cusp is homologous with the paracone in a tribosphenic upper molar, while
6767-419: 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
6868-401: The lingual (tongue) side of the upper molar, 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.
6969-441: The lingual (tongue) side. Upper molars look like three-pointed mountain ranges, with their features mirrored from the lower molars. The protocone cusp is on the lingual side of the tooth, while the anterior paracone and posterior metacone are on the buccal side. The protocone of the upper molar and talonid basin of the lower molar mesh together as a crushing system similar to a mortar and pestle . Tribosphenic molars were present in
7070-412: The lower molar is divided into two regions: the three-cusped 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 is towards the rear. The trigonid is defined by three large cusps: the protoconid is on the buccal/labial (cheek) side of the tooth, while the anterior paraconid and posterior metaconid are on
7171-412: The lower molars, the talonid region is reduced or absent, having lost its role as a crushing basin against the protocone. Zalambdodonty reduces tooth contact to a few simple shearing surfaces, though the evolutionary advantage of this tooth type is unclear. Like zalambdodont molars, dilambdodont molars have a distinct ectoloph, but are shaped like two lambdas or a W. On the lingual side, at the bottom of
7272-413: The major cusp is elongated into crescent-shaped ridge. Examples include most even-toed ungulates, such as cattle and deer . 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. Tooth A tooth ( pl. : teeth ) is a hard, calcified structure found in
7373-443: The majority of stem chondrichthyan lineages retained all tooth generations developed throughout the life of the animal. This replacement mechanism is exemplified by the tooth whorl-based dentitions of acanthodians , which include the oldest known toothed vertebrate, Qianodus duplicis . All amphibians have pedicellate teeth , which are modified to be flexible due to connective tissue and uncalcified dentine that separates
7474-434: 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 3 mm ( 1 ⁄ 8 in) 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
7575-411: The metacone is absent, reduced or fused. Marsupial moles show the opposite condition, with the large cusp equivalent to the metacone, and the paracone absent instead. The protocone is either absent (as in some golden moles and tenrecs) or reduced to a small fourth cusp, positioned lingual to the large cusp at the tip of the V. The two labial cusps are located on an expanded shelf called the stylar shelf. In
7676-456: 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
7777-452: The monotremes diverged from the mammalian lineage before the marsupials and placental mammals arose. The only Mesozoic monotremes are Teinolophos (Barremian, 126 Ma), Sundrius and Kryoryctes (Albian, 113–108 Ma), and Dharragarra , Kollikodon , Opalios , Parvopalus , Steropodon , and Stirtodon (all Cenomanian, 100.2–96.6 Ma) from Australian deposits, and Patagorhynchus (Maastrichtian) from Patagonian deposits in
7878-491: 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 crenulations, 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. Mammalian, multicusped cheek teeth probably evolved from single-cusped teeth in synapsids, although
7979-580: 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
8080-433: The more widely accepted one. Monotremes are conventionally treated as comprising a single order Monotremata. The entire grouping is also traditionally placed into a subclass Prototheria , which was extended to include several fossil orders, but these are no longer seen as constituting a group allied to monotreme ancestry. A controversial hypothesis now relates the monotremes to a different assemblage of fossil mammals in
8181-533: The mother and receive nutrients directly from her, generally hatching within 10 days after being laid – much shorter than the incubation period of sauropsid eggs. Much like newborn marsupials (and perhaps all non-placental mammals ), newborn monotremes, called "puggles", are larval- and fetus-like and have relatively well-developed forelimbs that enable them to crawl around. Monotremes lack nipples , so puggles crawl about more frequently than marsupial joeys in search of milk. This difference raises questions about
8282-469: The order Monotremata . They are the only group of living mammals that lay eggs , rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas . Monotremes are typified by structural differences in their brains, jaws, digestive tract, reproductive tract, and other body parts, compared to the more common mammalian types. Although they are different from almost all mammals in that they lay eggs, like all mammals,
8383-428: The outermost 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
8484-488: 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
8585-514: The ovum splits into multiple, divisible daughter cells. In contrast, monotreme zygotes, like those of birds and reptiles, undergo meroblastic (partial) division. This means that the cells at the yolk's edge have cytoplasm continuous with that of the egg, allowing the yolk and embryo to exchange waste and nutrients with the surrounding cytoplasm. Monotremes' metabolic rate is remarkably low by mammalian standards. The platypus has an average body temperature of about 31 °C (88 °F) rather than
8686-633: 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 resemble saws in both appearance and function, and on them are about 100 sharp teeth used to incise
8787-463: 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 . The teeth have enamel on
8888-542: 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
8989-404: 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
9090-430: 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
9191-475: The short-beaked and long-beaked echidna species are derived from a platypus-like ancestor. The precise relationships among extinct groups of mammals and modern groups such as monotremes are uncertain, but cladistic analyses usually put the last common ancestor (LCA) of placentals and monotremes close to the LCA of placentals and multituberculates , whereas some suggest that the LCA of placentals and multituberculates
9292-531: The skin of sharks ) that folded and integrated into the mouth (called the "outside–in" theory), or from endoderm pharyngeal teeth (primarily formed in the 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 to those involved in
9393-426: 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. Monotreme Monotremes ( / ˈ m ɒ n ə t r iː m z / ) are mammals of
9494-461: 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 . 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
9595-432: The supposed developmental restrictions on marsupial forelimbs. Rather than through nipples, monotremes lactate from their mammary glands via openings in their skin. All five extant species show prolonged parental care of their young, with low rates of reproduction and relatively long life-spans. Monotremes are also noteworthy in their zygotic development: Most mammalian zygotes go through holoblastic cleavage , where
9696-511: The three present in non-monotreme australosphenidians. Aptian and Cenomanian monotremes of the family Kollikodontidae (113–96.6 ma) have four molars. This suggests that the monotremes are likely to be unrelated to the australosphenidan tribosphenids. The time when the monotreme line diverged from other mammalian lines is uncertain, but one survey of genetic studies gives an estimate of about 220 million years ago, while others have posited younger estimates of 163 to 186 million years ago (though
9797-448: The tooth can be used as a diagnostic tool for predicting bite force. 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 cementine . Enamel is quickly decalcified in acids, perhaps by dissolution by plant acids or via diagenetic solutions, or in
9898-401: The tooth is missing. 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 . In mammals, the crown of the molars and premolars is folded into a wide range of complex shapes. The basic elements of the crown are
9999-475: 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 110–130 mm (4.5–5 inches) long, with
10100-511: Was more recent than the LCA of placentals and monotremes. Ornithorhynchus anatinus Tachyglossus aculeatus Z. bartoni Z. attenboroughi Z. bruijnii Ornithorhynchus anatinus Tachyglossus aculeatus Zaglossus bruijnii The first Mesozoic monotreme to be discovered was the Cenomanian (100–96.6 Ma) Steropodon galmani from Lightning Ridge, New South Wales. Biochemical and anatomical evidence suggests that
10201-487: Was observed in the hyraxes , which are placental mammals . The echidna was originally thought to experience no rapid eye movement sleep . However, a more recent study showed that REM sleep accounted for about 15% of sleep time observed on subjects at an environmental temperature of 25 °C (77 °F). Surveying a range of environmental temperatures, the study observed very little REM at reduced temperatures of 15 °C (59 °F) and 20 °C (68 °F), and also
#570429