Incisor teeth are positioned anterior to the canines and in the upper jaw are further defined as developing within the frontonasal process that becomes the premaxilla. 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. In all Hominids there are eight incisors, two in each quadrant of the mouth. Incisor teeth are single rooted. Maxillary incisor roots are conical and tapering whereas mandibular incisor roots are more ‘blade-like’ being flattened mesodistally. The cervical margin between the crown and the root is more sinuous than in other tooth types rising higher towards the occlusal edge mesially and distally. Incisor crowns are spatulate and generally have a rounded distal incisal edge but a sharper more angular mesial incisal edge. The mesial and distal margins of incisor crowns (marginal ridges) may be weakly or strongly developed, extreme development giving rise to a ‘shovel-shaped’ morphology on their lingual aspect. Upper central incisors are wider mesiodistally at their incisal edge than upper lateral incisors whereas lower central incisors are narrower than lower lateral incisors. Incisors of great apes are larger than those of modern humans relative to post-canine tooth size. This is especially so in Pan where post-canine teeth are smaller compared to Gorilla. Compared with modern humans, the distal incisal margin of great ape lower lateral incisors may slope more strongly downwards and the incisal edge of upper lateral incisors may come to a pointed rather than be straight-edged. In great apes a central cingulum, or tubercle, lingually may extend as a central ridge as far as the incisal edge.
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