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Premolar teeth replace deciduous molar (premolar) teeth and lie immediately behind the canines. 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. The hypothetical ancestral eutherian mammalian dentition contained four deciduous molars of which at least the last three were replaced by premolars. Old World monkeys, apes and humans retain only Pm3 and Pm4 of this series. Modern human premolars have one root with the exception of the upper Pm3, which has two, one buccal and one lingual. Lower human premolars are usually single rooted. Premolar teeth are bicuspid but whereas the human upper Pm3 is larger than the upper Pm4 the lower Pm3 is smaller with a greatly reduced lingual cusp. The upper Pm3 is distinguished by a depression mesially, the canine fossa, purportedly created during development as the canine crown lies against it. Great ape upper premolar teeth are three-rooted with two roots placed buccally and one lingually. The crowns are bicuspid. The taller larger cusp (paracone) buccally and the smaller (protocone) lingually are separated by a deep mesiodistal fissure. The lower Pm3 in great apes is usually three-rooted with a strong stout mesial root and two smaller distal roots. The crown is distinct and described as sectorial with an expanded blade-like mesial aspect (a hypertrophied protoconid) that occludes with the distal aspect of the sexually dimorphic upper canine. A small cusp (metaconid) is present on the lingual slope of the protoconid. The two-rooted lower Pm4 is often multicuspid with one or more additional cusps distally (a hypoconid distobuccally and an entoconid distolingually).
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