The structures within the molars receive different names according to their position and morphology. This nomenclature was developed by Henry Fairfield Osborn in 1907 and is, although with many variations, the one that continues today.[1][2][3]
Suffixes
Tooth structures bear suffixes in order to note the type of structure they are and whether they are present in the upper or lower molars.
The suffix "-cones /-conids" (upper molar/lower molar) is added to the main cusps: Paracone, Metacone, Protocone and Hypocone on the upper molar, and Paraconid, Metaconid, Protoconid, Hypoconid and Entoconid on the lower molar. This name is used for both bunodont and selenodont molars, that is, as many for "buno" pillar-like cusps as for "selenes" crescent-like cusps.
The suffix "-conule /-conulid" (upper molar/lower molar) is added to the secondary cusps. For example, Metaconule, Hypoconulid.
The suffix "-style/-stylid" (upper molar/lower molar) is added to the peripheral cusps that are found in the cornices or cingulus of the tooth. These cusps are traditionally named according to their proximity to the main cusps, although some anatomists prefer to name them according to their position on the tooth.
The suffix "-loph/-lophid" (upper molar/lower molar) is added to the crests that join cusps together. They include in the name one the cusps involved. For example, the hypolophid is the ridge that unites the hypoconid with the entoconid. These ridges often have secondary ridges: the secondary crest of the ectoloph is called crista; antecrochet to that of the protoloph, and crochet to that of the metaloph.
The suffix "-crista / -cristid" (upper molar / lower molar) is used for the ridges that come out of the cusps but do not connect them with other cusps. It is also used to name the edges of selenes.
To the structures in the lingual part of the molar, the prefix "ento-" ("internal") is often added, while those of the lingual part are added the prefix "ecto-" ("external"). the mesial part of the molar is often added the prefix "pre-" (from "previous") while those of the distal part are added the prefix "post-" ("posterior"). The mesial part is that which is towards the incisors.
The suffix "-flexus / -flexid" (upper molar / lower molar) is used for the open valleys in the occlusal surfaces of the hypsodont teeth. When this valleys are enclosed, they are called fossetes/fossetids (upper molar / lower molar). Sometimes they are used also for the folds of the teeth, although the proper name for the folds is sulcus (pl. sulci).
The cusp prefixes "para-", "meta-", "proto-", "hypo-", etc., are related to the succession and position of the cusps according to the ancient tritubercular theory of the evolution of molars from Cope and Osborn. Although this theory has lost its validity, they continue to use the names for the description of the molars. The prefix "proto" referred to the original cusp which would be homologous to a single cusp tooth according to Osborn, and would be the first cusp to appear not only in evolution but in development.[1] This was criticized early by studies of embryology where it was shown that the first cusp in the embryonic development of the upper molars was the paracone. Later this was shown to be variable and the first cusp to appear in ontogeny would not be related to the evolution of the tooth.[2]
Major features
A diagram of generalized tribosphenic molars with notable features labelled. Upper left molar in pink, lower left molar in blue. Mesial (forwards) is to the left. Teeth are depicted in left lateral view (left side and center of the image) or occlusal view (right side of the image).
The positions of tooth features are described along four directions: mesial (forwards, towards the chin), distal (backwards, towards the jaw joint), lingual (inwards, towards the tongue), and buccal or labial (outwards, towards the cheek).
Upper teeth
Therians (marsupials and placentals) ancestrally have roughly triangular upper molars, with the apex pointing lingually (inwards) and the flat edge positioned labially (outwards). This fundamental three-pointed structure is sometimes called the trigon. Three major cusps are almost always present:
Protocone: A major cusp at the lingual apex of the upper molar. In mammals with tribosphenic teeth, the protocone slides into the talonid basin on the lower teeth in order to grind and crush food. Often the largest cusp in the upper molar, though some insectivorous mammals reduce it substantially.
Paracone: A major cusp outwards from the protocone and in front of the metacone.
Metacone: A major cusp outwards from the protocone and behind the paracone.
Other common features include:
Stylar shelf: A shelf or offset surface along the outer edge of the tooth. Often bears minor cusps (stylar cusps or conules).
Parastyle: A forward extension of the stylar shelf which juts out at the mesiolabial (front outer) corner of the tooth, in front of and outwards from the paracone. May bear one or more minor cusps (including the stylocone, which is observed in some extinct mammals).
Metastyle: A rearward extension of the stylar shelf which juts out at the distolabial (rear outer) corner of the tooth, behind and outwards from the metacone. May bear one or more minor cusps.
Mesostyle: A cusp at the midpoint of the stylar shelf.
Ectoflexus: A notch at the midpoint of the stylar shelf.
Paraconule / Protoconule: A minor cusp on the front edge of the tooth, between the protocone and paracone.
Metaconule: A minor cusp on the rear edge of the tooth, between the protocone and metacone.
Hypocone: A fourth major cusp which only becomes widespread in placentals. It develops at the distolingual (rear inner) corner of the tooth, behind the protocone and inwards from the metacone. Mammals with a large hypocone (such as humans) have upper molars with four cusps set in a quadrate (square-shaped) arrangement rather than a triangular form. Many marsupials and artiodactyls have an enlarged metaconule which serves the same role as the hypocone and is occasionally labelled as a hypocone, despite being derived from a different part of the tooth.[4]
Lower teeth
Therians ancestrally have lower molars which are longer from front-to-back than from side-to-side. Five to six cusps are most common. The trigonid region at the front part of the molar is triangular, with three large cusps:
Protoconid: A major cusp at the labial apex of the trigonid region. Often the largest cusp in the lower molar.
Paraconid: A major cusp on the inner edge of the trigonid region, inwards from the protoconid and in front of the metaconid. Humans and most other primates no longer possess a paraconid in their lower molars, leaving only five cusps.
Metaconid: A major cusp on the inner edge of the trigonid region, inwards from the protoconid and behind the paraconid.
The talonid region at the rear part of the molar has two to three relatively small cusps which define the rear rim of a low basin:
Hypoconid: A major cusp at the outer rear edge of the talonid region. Typically the largest and most consistently present cusp in the talonid, though usually smaller than the trigonid cusps. The hypoconid occludes with the space at the center of the upper molar (between the protocone, paracone, and metacone).
Entoconid: A major cusp which is sometimes present at the inner rear edge of the talonid region.
Hypoconulid: A minor cusp which is sometimes present at the rear edge of the talonid region, between the hypoconid and entoconid.
Other common features include:
Cristid obliqua: A diagonal crest which runs from the hypoconid forwards to the midpoint between the protoconid and metaconid.
Image gallery
Lophodont teeth with ectoloph, protoloph and metaloph, several small cristae, and an entoflexus and postflexus.
The bunolophoselenodont dentition of a horse.
Cricetidae molar teeth nomenclature after Reig (1977)
These are a list of tooth features identified in the rodent family Cricetidae (hamsters, voles, New World mice and rats, etc.). Source:[5]
Cricetidae molar teeth nomenclature after Reig, 1977.
1 2 Osborn, Henry Fairfield, 1857–1935. (1907). Evolution of mammalian molar teeth to and from the triangular type: including collected and revised researches on trituberculy, and new sections on the forms and homologies of the molar teeth in the different orders of mammals. Kessinger Publishing. ISBN978-1-4368-4090-3. OCLC810943856.{{cite book}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
1 2 MacCord, Katherine (2017). Development, evolution, and teeth: how we came to explain the morphological evolution of the mammalian dentition. Arizona State University.
↑ Szalay, Frederick S. (1969). "Mixodectidae, Microsyopidae, and the insectivore-primate transition". Bulletin of the American Museum of Natural History. 140 4: 193–330. hdl:2246/1130.
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