Academic literature on the topic 'Adult mandibular first molars'

Now you have an understanding of the anatomy of the maxilla and mandible, the TMJs, and jaw musculature, we can examine how these structures work together to produce the complex actions involved in the biting and chewing of food. Technically, incision is biting a piece from a larger chunk of food and mastication is the grinding down of that piece into smaller components and mixing them with saliva. Mastication is often used to cover both actions. Box 26.1 briefly compares the anatomy of the human dentition to that of other mammals. As well as knowledge of the TMJ, muscles of mastication, and other muscles used in jaw movements, it is necessary to appreciate some aspects of the static and dynamic relationships of the teeth to understand chewing movements. The first thing to notice is the bigger width of the upper dental arch compared to the lower arch, a condition known as anisognathy. In Figure 26.1A , you can see that the maxillary molars overhang the mandibular teeth by half a cusp width so the buccal cusps of the lower molars and premolars occlude between the buccal and palatal cusps of the maxillary teeth. Observe also that the long axis of the maxillary molars and premolars incline buccally while the corresponding axis of the mandibular teeth incline lingually; the occlusal plane of the posterior teeth is thus curved transversely as illustrated in Figure 26.1A . It would be possible to chew food simply by moving the teeth up and down without any side-to-side movement, but this would be inefficient and not make full use of the cusps on the occlusal surfaces of posterior teeth. However, we can only chew on one side at a time because of the anisognathy of the upper and lower teeth. Due to anisognathic jaw positions, the maxillary anterior teeth are also going to protrude in front of the mandibular anterior teeth. Figure 26.1B illustrates the normal relationships of the anterior teeth. The maxillary incisors overhang the mandibular incisors by about 2–3 mm in the horizontal plane; this is called the overjet. The upper incisors usually have a vertical overhang, the overbite, of about the same amount. As mentioned in Chapter 24 , the mouth at rest is closed by tonic contraction of the muscles of mastication and facial expression.

Sunghir 1, 2, and 3 retain most of their maxillary and mandibular teeth, although those of Sunghir 1 are heavily worn and those of Sunghir 2 and especially Sunghir 3 were developing at the times of their deaths. As a result, the two immature individuals provide extensive data on their dental crown discrete morphology and crown metrics, but there are limited data on the third molars of Sunghir 2 and on the premolars and second molars of Sunghir 3 (and none on her third molars beyond their calcification stage; see chapter 6). In addition, although they retain none of their teeth, Sunghir 5 and especially 6 preserve alveolar bone, and they thereby provide limited dentoalveolar data. The Sunghir dentitions and alveoli thus have the potential to provide paleobiological data on their crown configurations, crown dimensions, some root lengths and configurations, in addition to wear patterns. The condition and salient aspects of each are provided first, followed by comparisons of their dimensions and shapes in a Late Pleistocene context. As noted in chapter 4, Sunghir 1 retains 31 of his original 32 teeth, and the one missing tooth, the left I2, was probably lost shortly before death. All of the teeth are heavily worn, thereby limiting morphological and morphometric observations principally to the M3s. But the other teeth provide considerable information regarding their wear patterns. The right I1 consists of worn dentin with a partial thin enamel ring around the labial margin of the crown. The dentin is occlusally flat to convex, the convexity produced mostly by a rounding of the lingual edge of the crown. There is a small area of secondary dentin exposed in the middle of the occlusal dentin. Note that the protruding nature of the tooth is a postmortem artifact, and it probably was originally at the same level as the left I1. There is no unusual wear in the mandibular incisors to match its procumbent state. The left I1 has similar wear, except that it retains more of the thin enamel ring around the lingual side and hence lacks the lingual rounding evident on the right one.

A.L. 444-2 is the first specimen to preserve the cranium and mandible of a single adult individual of A. afarensis. Pairing this specimen with A.L. 417-1, which includes a mandible and maxilla, enables us to compare comprehensively the craniofacial morphology of male and female individuals of the species for the first time. The occluded mandibles and maxillae of A.L. 444-2 and A.L. 417-1 reveal a distinctive hominoid snout contour, combining a strongly inclined, convexly sloping nasoalveolar clivus with a relatively upright mandibular symphysis, a straight to slightly rounded anterior symphyseal outline, and an anteriorly placed gnathion. Both A. afarensis specimens feature a very deep mandibular corpus, whose height occupies close to 70% of the orbitoalveolar height of the face. In the African great apes, this value ranges from 36% to 54%, and in modern humans, it is 66%. The high value in humans is due to a short orbitoalveolar region rather than to a deep mandible. A. afarensis appears to share a relatively deep corpus with A. robustus (the only robust species in which the feature can be determined for a single individual) but not with A. africanus. Relative to the calvarial length, the A.L. 444-2 braincase height is apelike, falling between the tall modern human braincase and the low braincase of A. boisei and A. aethiopicus. In A. africanus (Sts. 5) and H. habilis (KNMER 1813) the relative braincase height is like that of A.L. 444-2 and the great apes. According to Le Gros Clark’s (1950) index expressing the height of the calvaria above the roof of the orbit as a percentage of total calvarial height, Sts. 5 and KNM-ER 1813 have tall, “humanlike” braincases, whereas A.L. 444-2, A. boisei, A. aethiopicus, and the African great apes group together with low braincases. In contrast to the rounded, nearly circular midsagittal outline of the chimpanzee calvaria, the posterior parietal/ occipital arc in A.L. 444-2 is steep and deviates anteriorly from the circle. This is also true of the A. boisei calvaria. As expected from the calvarial height comparison, the slope of the A.L. 444-2 frontal squama is smaller than that of A. africanus and H. habilis.