Molare: sights, history and trails

The tribosphenic molar is considered one of the most important characteristics of the teri mammals. Among the living mammals, this type of tooth is present in most insectivorous mammals and in young ornitorins, although adult individuals of this species are devoid of teeth.

In tribosphenic teeth, the lower molar is divided into two regions: trigonide, equipped with three cusps and specialized in cutting, and the Talonides, the rear portion used for crushing. In modern tribosphenic molars, the trigonid faces the anterior part of the mandible, while the talonid is located posteriorly. The trigonid is delimited by three large cusps: the protoconid, located on the buccal or labial side of the tooth, facing the cheek, and the anterior paraconid and the posterior metaconid, both located on the lingual side, facing the tongue.

The upper molars can be compared to mountain chains with three peaks, with a specular arrangement than the lower molars. The protocono is located on the tongue side of the tooth, while the anterior paracono and the back methane are located on the buccal side. The upper molar protocono is embedded in the lower molar thaladonide depression, forming a crushing system comparable to a mortar with pestle.

The tribosphenic molars were present in the direct ancestors of all three major groups of living mammals, but probably did not constitute the ancestral condition of the entire class. According to many paleontologists, in monotremists this structure would have developed independently from the australosfenids, instead of being inherited by a common ancestor shared with marsupial and placentati, descended from the boreosphenids. However, this interpretation retains some opponents.^[4] The detachment of the monotreme of the lower Cretaceous St., for example, is similar to that of Peramus and driolestoids, suggesting that monotremes can be related to some pretribosphenic mammals.^[5] The systematic position of these groups, however, is not yet definitively established.

Some juror mammalianforms, such as the docodonti and shuotheriids, possessed molars defined tribosphenic inversi, in which a structure similar to thaladonide developed at the front of the lower molar instead of in the back one. This configuration is considered an example of convergent evolution.^[6]

Starting from the primitive tribosfenic tooth, molars have diversified taking on numerous special morphology. In many groups a fourth cusp was developed, called hypocono in the upper and hypoconid molars in the lower ones. The square molars, also called quadritubercolati or heutemorphic, have a hypocono, that is an additional fourth cuspide located on the tongue side of the upper molar, posteriorly to the protocono. The square molars appeared early in the evolution of mammals and are present in numerous species, including curls, raccoons and many primates, including the human being.^[7] In some cases there may also be a fifth cusp.

In many mammals, the smaller headsets also appear smaller cusp called conuli, named according to their position. The paraconul, for example, is located between the paracono and the methane, while the hypoconulide is located between the hypoconid and the entoconide.^[7]

In the dark molars, the cuspids are low and rounded, similar to small reliefs, rather than high and pointed. This morphology is particularly common in omnivores, such as pigs, bears and human beings.^[7] The bunodontic molars are effective crushing tools and often have a quadrangular overall shape.^[8]

The hypsodont teeth are characterized by high crowns and a layer of enamel that extends considerably below the gum, providing a greater amount of material intended to compensate for wear.^[9] Examples of animals with ipsodont toothing are cattle and horses, which feed on fibrous vegetable material and often contaminated by abrasive particles. In some species of Arvicolinae, a subfamily of herbivorous rodents, molars can continue to grow throughout their lives.^[7]

In ipsodont molars it is not always possible to distinguish clearly a crown and a collar. The occlusal surface is rough and mainly flat, adapted to crush and grind the plant material. The body of the tooth is covered with cement both above and below the gum margin. Below the concrete is a layer of enamel that extends along the tooth body. Cement and enamel form folds within the thick layer of dentine.^[10]

The condition opposite to hypsodontia is called brachidontia or brachiodontia, from Greek ♪, «short». It is a type of dentition characterized by teeth with low crowns. Human teeth are brachydont.^[7]

A brachydon tooth possesses a crown located above the gum, a collar immediately below it and at least one root. A layer of enamel covers the crown and extends to the collar, while cement is present only below the gum margin. Occlusal surfaces tend to be detected or pointed and, in carnivores, they are particularly suitable for holding prey and tearing or deminuting tissues.^[10]

The upper molars zalambdonti have at least three main cuspids: one larger on the tongue side and two smaller on the labiale side. The larger cusp is connected to the other two by means of crests forming a narrow V or a figure similar to the Greek letter λ, lambda. The term "zalambdonte" means approximately "strongly provided with lambda-shaped teeth". Among the living mammals, zalambdonti molars are present in tenrec, golden moles, sunnodon and marsupial moles.^[3][11]

In zalambdonti placentates, the large internal cusp is homologated to the tribosfenic upper molar paracono, while the methane is absent, reduced or merged with it. Marsupial moles show the opposite condition: the larger cusp corresponds to the methane, while it is the paracono to be absent. The protocono may be missing altogether, as in some golden moles and in some tenrec, or to be reduced to a small fourth cusp located tonguely compared to the major cusp, at the peak of V. The two labyrinth cusps are placed on a wide platform called stylus shelf. In the lower molars, thalaide is reduced or absent, since it has lost its function as a crushing basin opposed to the protocono.^[3][11] The zalambdodontia reduces contact between teeth to a few simple sharp surfaces, although the evolutionary advantage of this morphology is still not entirely clear.^[11]

Like zalambdonti molars, even dilambdonte molars have a well developed ectolofo, but its shape resembles two lambda letters flanked or a W. On the tongue side, at the lower part of the W, you will find methane and paracono, while the stylus shelf is located on the labiale side. The protocono is present on the tongue side with respect to the eclope. The dilambdonti molars are present in the toporagni, in the moles and in some insectivorous bats.^[7]

The lophodond teeth are easily recognizable for the characteristic systems of enamel ridges, called lophies, which connect among them the crown cusp. Present in most herbivores, these systems can consist of a simple annular margin, such as rats talpa, or in a complex succession of longitudinal and transversal ridges, as in the perissodactyl ungulates, including equides.^[8]

The lophurous molars have hard and elongated enamel ridges, oriented parallelly or perpendicularly to the dentary row. They are common in herbivores that shred the food thoroughly. Examples of mammals with molars are taphire, lamantine and numerous rodents.^[7]

When two lofies form transverse crests, often arcuate or almost anular, the disposition is defined bilophone. This configuration is common in primates, but also appears in lagomorphs, including hares, rabbits and pica, and in some rodents.^[7][8]

The extreme forms of lophodontia present in elephants and in some rodents, such as those of the genus Otomys, are called loxodontia.^[7] The name of the genus comes from this characteristic. Loxodontic, to which African elephants belong.

In the selenodont molars, whose name comes from Selene, Greek goddess of the Moon, the main cusps are elongated in crescent-shaped crests. This morphology is present in most ungulated artiodactyls, such as cattle and cervids.^[7][8]

Many carnivorous mammals have enlarged teeth and similar to blades, specialized in cutting and handing meat, called carnassial teeth. The general term used to indicate teeth with this laminar morphology is secodonte or plagiaulacoide.^[7]