Molar Morphology

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Permanent molar teeth (M1, M2, M3) are post-canine teeth that lack deciduous precursors. Tooth surfaces that face the anterior midline of the jaws are termed mesial and those that face away from it distal. Surfaces that face the tongue are lingual and surfaces on the outer aspect of the tooth arch are buccal. The occlusal surface articulates with one or more teeth in the opposing jaw. Upper human molar teeth are usually three-rooted with two positioned buccally and one lingually. Molar roots in M1 are more splayed but tend to be fused together and shorter in M3. Molar roots tend to be longer in great ape teeth than in modern human teeth. Upper molars in all Hominids usually have 4 cusps. The mesially positioned paracone (buccal) and protocone (lingual) are larger than the distally positioned metacone (buccal) and hypocone (lingual). Upper molars are characterized by an oblique ridge, which connects the protocone and metacone. In both apes and humans there is progressive reduction in hypocone size from M1 to M3. Among great apes this reduction is greatest in Pan, intermediate in Pongo and least in Gorilla. Lower molar teeth in all Hominids usually have five cusps and two blade-like roots, positioned mesially and distally. Human M2s, however, often only have 4 cusps and accessory cusps are common on all molars. The protoconid (buccal) and the metaconid (lingual) are positioned mesially with the hypoconid (buccal) and the entoconid (lingual) distal to these. The fifth most distal cusp (hypoconulid) of lower molars is also the smallest. M1 crowns are uniformly the smallest of the molar series because they are constrained by the size of the neonatal jaws within which they develop. Among humans and great apes, the M3 crown size is most variable and usually, but not always, smaller than M2.


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