Mammal tooth

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An adult cheetah showing its long, sharp canine teeth. South Africa - Cheetah Experience (48974093898) (cropped).jpg
An adult cheetah showing its long, sharp canine teeth.

Teeth are common to most vertebrates, but mammalian teeth are distinctive in having a variety of shapes and functions. This feature first arose among early therapsids during the Permian, and has continued to the present day. All therapsid groups with the exception of the mammals are now extinct, but each of these groups possessed different tooth patterns, which aids with the classification of fossils.

Contents

Most extant mammals including humans are diphyodonts, i.e. they have an early set of deciduous teeth and a later set of permanent or "adult" teeth. Notable exceptions are elephants, kangaroos, and manatees, all of which are polyphyodonts, i.e. having teeth that are continuously being replaced.

Mammal teeth include incisors, canines, premolars, and molars, not all of which are present in all mammals. Various evolutionary modifications have occurred, such as the lack of canines in Glires, the development of tusks from either incisors (elephants) or canines (pigs and walruses), the adaptation of molars into flesh-shearing carnassials in Carnivora, and others.

Diversity

The extant mammalian infraclasses each have a set dental formula; the Eutheria (placentals) commonly have three pairs of molars and four premolars per jaw, whereas the Metatheria (marsupials) generally have four pairs of molars and between three or two premolars. For example, the tiger quoll (Dasyurus maculatus) is a dasyurid marsupial native to Australia. The quoll possesses four upper incisors and three lower incisors per left and right-hand side [I = 14]; two upper premolars [PM] and two lower premolars per side [PM = 8]; and four upper and four lower molars per side [M = 16], giving the animal a complement of thirty-eight teeth. The tiger quoll's dental formula is therefore as follows: 4.1.2-3.43.1.2-3.4.

Rabbits

Rabbits and other lagomorphs usually shed their deciduous teeth before (or very shortly after) their birth, and are usually born with their permanent teeth. [1] 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. [2] Rabbits have a dental formula of 2.0.3.31.0.2.2. 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. [3]

Anatomy of rabbit teeth

The incisors and cheek teeth of rabbits are called aradicular hypsodont teeth. Aradicular teeth never form a true root with an apex, and hypsodont teeth have a high crown to root ratio (providing more room for wear and tear). [4] This is sometimes referred to as an elodont dentition, meaning ever-growing. 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

Like those of rabbits, rodents' incisors grow continuously throughout their lives, and are both aradicular and hypsodont. Unlike humans whose ameloblasts die after tooth development, rodents continually produce enamel and must wear down their teeth by gnawing on various materials. [5] These teeth are used for cutting wood, biting through the skin of fruit, or for defense. The teeth have enamel on the outside, which is often orange-yellow due to the incorporation of iron-containing pigments, [6] 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. [7] [8] 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.

Humans

Afrotherians

Elephants

Section through the ivory tusk of a mammoth Mammoth ivory hg.jpg
Section through the ivory tusk of a mammoth

The tusks of an elephant are specialized incisors for digging food up and fighting. Elephants are polyphyodonts whose teeth are similar to those of 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. [9]

Manatees

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

Aardvarks

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

Ungulates

Horses

An adult horse has between 36 and 44 teeth. All horses have twelve premolars, twelve molars, and twelve incisors. [10] 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. [11] 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. [10]

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.

Pigs

Tusks, which grow larger in males and in wild boars, are derived from the canine teeth, a trait shared with warthogs.

Cetaceans

The toothed whales, comprising the Odontoceti parvorder of the cetaceans, are differentiated from the baleen whales by the presence of their teeth. The number of teeth and their function can vary widely between species, with some dolphins having over a hundred teeth in their jaws, while the narwhal has two functional teeth in its upper jaw which grow into long tusks in males. The tusk is used in feeding, navigation and mating and contains millions of sensory pathways, making it the most neurologically complex tooth known. In contrast, the sperm whale has up to forty teeth in its bottom jaw and none functional in its upper.

Carnivorans

In the order Carnivora, many different diets are not always meat. Carnivores possess diverse carnassial teeth. The carnassials are specialized teeth for different diets of different animals. [12] These teeth are used to cut through flesh. [12] Either the molars or both the premolars and molars in combination may be adapted into shearing carnassials. Tooth roots can be great indicators of diets. [13] [14]   In a study done on the bear tooth root area, [14] it was found that the amount of space a tooth root took up was an indicator of the bite force required from the species. For example, Panda teeth had the deepest root systems because of their bamboo diet. [14] Ursidae  (bears) is not the only carnivorous family whose root area has been studied. It has been found that in general, carnivore tooth roots are highly dependent on their diets. [12]

Walrus

Walrus tusks are canine teeth that grow continuously throughout life. [15]

Dogs

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. [16]

Related Research Articles

<span class="mw-page-title-main">Horse teeth</span> Aspect of equine anatomy

Horse teeth refers to the dentition of equine species, including horses and donkeys. Equines are both heterodontous and diphyodontous, which means that they have teeth in more than one shape, and have two successive sets of teeth, the deciduous and permanent sets.

<span class="mw-page-title-main">Cattle age determination</span>

The age of cattle is determined chiefly by examination of the teeth, and less perfectly by the horn rings or the length of the tail brush; due to bang-tailing, which is the act of cutting the long hairs at the tip of the tail short to identify the animal after management practices, the last method is the least reliable.

<span class="mw-page-title-main">Human tooth</span> Calcified whitish structure in humans mouths used to break down food

Human teeth function to mechanically break down items of food by cutting and crushing them in preparation for swallowing and digesting. As such, they are considered part of the human digestive system. Humans have four types of teeth: incisors, canines, premolars, and molars, which each have a specific function. The incisors cut the food, the canines tear the food and the molars and premolars crush the food. The roots of teeth are embedded in the maxilla or the mandible and are covered by gums. Teeth are made of multiple tissues of varying density and hardness.

<span class="mw-page-title-main">Dentition</span> Development and arrangement of teeth

Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiology of the teeth of an animal.

<span class="mw-page-title-main">Molar (tooth)</span> Large tooth at the back of the mouth

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 is sometimes vestigial.

<span class="mw-page-title-main">Canine tooth</span> Long pointed tooth in mammals

In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, eye teeth, vampire teeth, or fangs, are the relatively long, pointed teeth. In the context of the upper jaw, they are also known as fangs. They can appear more flattened, however, causing them to resemble incisors and leading them to be called incisiform. They developed and are used primarily for firmly holding food in order to tear it apart, and occasionally as weapons. They are often the largest teeth in a mammal's mouth. Individuals of most species that develop them normally have four, two in the upper jaw and two in the lower, separated within each jaw by incisors; humans and dogs are examples. In most species, canines are the anterior-most teeth in the maxillary bone. The four canines in humans are the two upper maxillary canines and the two lower mandibular canines. They are specially prominent in dogs (Canidae), hence the name.

<span class="mw-page-title-main">Incisor</span> Front teeth of most mammals

Incisors are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight. Opossums have 18, whereas armadillos have none.

Hypsodont is a pattern of dentition characterized by teeth with high crowns, providing 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. The opposite condition is called brachydont.

<span class="mw-page-title-main">Deciduous teeth</span> First set of teeth in diphyodonts

Deciduous teeth or primary teeth, also informally known as baby teeth, milk teeth, or temporary teeth, are the first set of teeth in the growth and development of humans and other diphyodonts, which include most mammals but not elephants, kangaroos, or manatees, which are polyphyodonts. Deciduous teeth develop during the embryonic stage of development and erupt during infancy. They are usually lost and replaced by permanent teeth, but in the absence of their permanent replacements, they can remain functional for many years into adulthood.

<span class="mw-page-title-main">Human tooth development</span> Process by which teeth form

Tooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. For human teeth to have a healthy oral environment, all parts of the tooth must develop during appropriate stages of fetal development. Primary (baby) teeth start to form between the sixth and eighth week of prenatal development, and permanent teeth begin to form in the twentieth week. If teeth do not start to develop at or near these times, they will not develop at all, resulting in hypodontia or anodontia.

<span class="mw-page-title-main">Carnassial</span> Mammal tooth type

Carnassials are paired upper and lower teeth modified in such a way as to allow enlarged and often self-sharpening edges to pass by each other in a shearing manner. This adaptation is found in carnivorans, where the carnassials are the modified fourth upper premolar and the first lower molar. These teeth are also referred to as sectorial teeth.

<span class="mw-page-title-main">Permanent teeth</span> Second set of teeth in diphyodont mammals

Permanent teeth or adult teeth are the second set of teeth formed in diphyodont mammals. In humans and old world simians, there are thirty-two permanent teeth, consisting of six maxillary and six mandibular molars, four maxillary and four mandibular premolars, two maxillary and two mandibular canines, four maxillary and four mandibular incisors.

<span class="mw-page-title-main">Polyphyodont</span> Animal whose teeth are continually replaced

A polyphyodont is any animal whose teeth are continually replaced. In contrast, diphyodonts are characterized by having only two successive sets of teeth.

<span class="mw-page-title-main">Animal tooth development</span> Process by which teeth form and grow into the mouth

Tooth development or odontogenesis is the process in which teeth develop and grow into the mouth. Tooth development varies among species.

<span class="mw-page-title-main">Tooth eruption</span> Process in tooth development

Tooth eruption is a process in tooth development in which the teeth enter the mouth and become visible. It is currently believed that the periodontal ligament plays an important role in tooth eruption. The first human teeth to appear, the deciduous (primary) teeth, erupt into the mouth from around 6 months until 2 years of age, in a process known as "teething". These teeth are the only ones in the mouth until a person is about 6 years old creating the primary dentition stage. At that time, the first permanent tooth erupts and begins a time in which there is a combination of primary and permanent teeth, known as the mixed dentition stage, which lasts until the last primary tooth is lost. Then, the remaining permanent teeth erupt into the mouth during the permanent dentition stage.

<span class="mw-page-title-main">Dental anatomy</span> Field of anatomy dedicated to human teeth

Dental anatomy is a field of anatomy dedicated to the study of human tooth structures. The development, appearance, and classification of teeth fall within its purview. Tooth formation begins before birth, and the teeth's eventual morphology is dictated during this time. Dental anatomy is also a taxonomical science: it is concerned with the naming of teeth and the structures of which they are made, this information serving a practical purpose in dental treatment.

<span class="mw-page-title-main">Durophagy</span> Eating of hard-shelled or exoskeleton bearing organisms, such as corals, shelled mollusks, or crabs

Durophagy is the eating behavior of animals that consume hard-shelled or exoskeleton-bearing organisms, such as corals, shelled mollusks, or crabs. It is mostly used to describe fish, but is also used when describing reptiles, including fossil turtles, placodonts and invertebrates, as well as "bone-crushing" mammalian carnivores such as hyenas. Durophagy requires special adaptions, such as blunt, strong teeth and a heavy jaw. Bite force is necessary to overcome the physical constraints of consuming more durable prey and gain a competitive advantage over other organisms by gaining access to more diverse or exclusive food resources earlier in life. Those with greater bite forces require less time to consume certain prey items as a greater bite force can increase the net rate of energy intake when foraging and enhance fitness in durophagous species.

<span class="mw-page-title-main">Tooth</span> Hard structure of the mouth

A tooth is a hard, calcified structure found in the jaws 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 the outermost embryonic germ layer, the ectoderm.

Changes to the dental morphology and jaw are major elements of hominid evolution. These changes were driven by the types and processing of food eaten. The evolution of the jaw is thought to have facilitated encephalization, speech, and the formation of the uniquely human chin.

Archaeopithecus is an extinct genus of notoungulate, belonging to the suborder Typotheria. It lived during the Middle Eocene, in what is today Argentina.

References

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