Brief communication Possible third molar impactions in the hominid fossil record.код для вставкиСкачать
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 91~517-521(1993) Brief Communication: Possible Third Molar Impactions in the Horninid Fossil Record KATHLEEN R. GIBSON AND JAMES M. CALCAGNO Department of Anatomical Sciences, University of Texas Dental Branch, Houston, Texas, 77225 (K.R.G.);Department of Sociology and Anthropology, Loyola Uniuersity of Chicago, Chicago, Illinois 60626 (J.M.C.) KEY WORDS Third molar impactions, Australopithecus, Dental evolution Dental crowding, ABSTRACT Impacted third molars affect 15%-20%of modern Americans and Western Europeans. In contrast, third molar impactions have not been reported in the early hominid fossil record. It is uncertain whether the lack of reports reflects an absence of impactions or a failure to recognize them. This communication is intended to raise awareness of the possibility of impactions by describing the appearance of impacted teeth and by noting two possible instances of impaction in early hominids. Specifically, the mandibular third molars of the Sterkfontein specimen, STS52b (Australopithecus africanus), and the left maxillary third molar of the Lake Turkana specimen, KNM-WT 17400 (Australopithecus boisei), are positioned in a manner which suggests that they would not have erupted normally. Both specimens also exhibit strong crowding of the anterior dentition, providing further support for the view that these individuals lacked sufficient space for normal eruption of the third molars. Other published reports of dental crowding in the hominid fossil record are noted, and it is suggested that more attention be paid to dental impaction and dental crowding in hominid evolution. o 1993 Wiley-Liss, Inc. Approximately 15-20% of the modern population of Western Europe and the United States experiences impacted “wisdom t e e t h (Kramer and Williams, 1970). In many other individuals, the third molars erupt only partially or in a manner which causes them to abut against the second molars. Little data exist, however, pertaining to the antiquity of hominid third molar impactions except for a few reports of impactions among the Krapina Neandertals and European Upper Paleolithic peoples (Wolpoff, 1979; Skinner and Sperber, 1982; Frayer and Russell, 1987). These sparse reports of third molar impaction may reflect an extreme rarity of impaction and crowding in ancient hominids. Alternately, perhaps, the incidence of third molar impaction has been underreported in the literature, because it is difficult to recognize impactions 0 1993 WILEY-LISS, INC. in the skeletal record except in cases of extreme displacement. The purpose of this brief communication is to raise the level of awareness, not to provide the final word, of possible third molar impactions in the fossil record. Hopefully, this will result in increased interest in the phenomenon and in better methods of recognition. MANIFESTATIONS OF THIRD MOLAR IMPACTIONS Impacted or partially erupted third molars may assume one of several positions: mesioangular, horizontal, vertical, inverted, or distoangular (Archer, 1966; Kruger, Received March 15,1990; accepted January 31,1993. K.R. GIBSON AND J.M. CALCAGNO 518 r\ , 'L Classification of mandibular impacted teeth. A, Mesioangular; B, horizontal; C, vertical; D, distoangular; E, buccoversion; F, linguoversion; G, low-level; H, high-level. Fig. 1. Types of third molar impactions. (Reprinted from Kruger, 1984, with permission from the publisher.) 1984; Fig. 1).In a mesioangular impaction, the long axis of the tooth inclines in a mesial direction; in a distoangular impaction, the long axis of the tooth inclines distally, and so on. Impacted teeth may also be displaced to the buccal (buccoversion) or lingual (linguoversion) sides or rotated in position, and they may assume several vertical levels. Thus, the highest portion of an impacted third molar may be on a level above the occlusal line of the second molar, below the occlusal line but above the cervical line, or below the cervical line of the second molar. As a result of the many positions an impacted third molar might assume, a variety of complications arise when attempting to assess third molar impaction in a fossil specimen. For instance, most fossil hominids died young. Many australopithecines, in particular, died during late adolescence (Mann, 1975). Thus, age, rather than pathology, might account for the lack of eruption of some third molars. The examples given below are instances in which we believe that the molar is seriously malpositioned and would not have erupted normally. POSSIBLE,UNRECOGNIZED IMPACTIONS IN HOMlNlD FOSSILS An examination of fossil casts distributed among anthropology and anatomy departments throughout the world indicates that the third molars of one well-known fossil is positioned in a manner typical of third molar impaction. Yet, to our knowledge no one has suggested the possibility of impaction in this specimen. Specifically, the Sterkfontein specimen STS-52b (Australopithecus africanus) shows signs of possible impaction of THIRD MOLAR IMPACTIONS IN HOMINID FOSSIL RECORD Fig. 2. Cast of STS-52b. Notice anterior crowding and the displacement of the right M3 which appears to be in buccoversion. both mandibular third molars. In addition, observations of the original Lake Turkana KNM-WT 17400 specimen dating from approximately 2,500,000 years ago and classified as Australopithecus boisei by Leakey and Walker (1988) suggest the presence of an impacted left maxillary third molar. The right M3 of the Sterkfontein 52b mandible is buccally displaced and identical in appearance to third molars impacted in buccoversion (Fig. 2). It is important to note that even in young individuals whose molars have not yet erupted, the molars do not occupy a position of buccoversion. On the left side, the third mandibular molar is erupting in an angulated fashion and the tip of its crown nearly touches the cementoenamel junction of the second molar (Fig. 3). In position, the tooth matches descriptions of mesioangulated midlevel impactions (Archer, 1966). We have shown this tooth to a number of orthodontists and oral surgeons, who, based on their clinical experience, concluded that the tooth would not have erupted normally. It is true that this mandible, like much of the Sterkfontein material, is badly fractured, but it is unlikely that the fracture has distorted the relationship between the second and third molars because the fracture runs anterior to the second molar on both sides. In addition, this fracture is typical of some which commonly occur in modern humans as a result of trauma. In the clinical 519 Fig. 3. Cast of STS-52b left mandibular second and third molars. The left third molar exhibits a mesioangular orientation and its tip nearly touches the second molar. experience of oral surgeons who commonly treat these problems, a fracture of this nature would not have altered the relationships between the second and third molars (McFarland, personal communication). Nor, given its position, is this fracture likely to have altered the third molar position even if the fracture occurred post-mortem. Further indications that the mandibular third molars of STS-52b may have lacked sufficient room for eruption come from the crowded condition of the anterior teeth (Robinson, 1956; Oppenheimer, 1964; Wallace, 1972). Thus, in STS-52b, “insufficient space is available for the (right) canine to fit properly into the normal arch. This has resulted in a buccalward protrusion of the canine” (Robinson, 1956, p. 16). Judging from the condition of the anterior alveoli, Robinson concludes that the left canine, although missing, was also likely to have been crowded and displaced during life. Although crowded anterior teeth may occur in the absence of third molar impactions, the two often coexist, as both conditions may indicate overall dental crowding and insufficient arch space. In fact, crowding of the anterior dentition sometimes manifests itself at the time of eruption of the third molars (Richardson, 1989). The WT-17400 maxilla from Lake Turkana also exhibits signs of possible impaction. In this specimen, the mesiafiuccal corner of the left third molar is in contact with 520 K.R. GIBSON AND J.M. CALCAGNO Fig. 4. Cast of the WT-17400 maxilla from Lake Turkana. Note the contact between the left third molar and the second molar. Also note the crowding of the anterior dentition. the buccalldistal root of the second molar (Fig. 4). Erosion around the roots of the second molar make this clear. Insufficient space exists for the third molar to erupt into the normal position. Indeed the occlusal surface of the tooth exhibits some distal tilting. This specimen also exhibits extreme crowding of the anterior dentition (Leakey and Walker, 1988). The lateral incisors have actually been displaced posteriorly behind the incisal canine tooth row. In addition, the left P4 has been displaced lineally. Despite the paucity of reports of third molar impactions in the fossil record, it is unlikely that STS-52b and WT-17400 were the only members of their populations to suffer from this phenomenon. As stated previously, third molar impactions often coexist with crowding in the remainder of the dentition. A number of authors report dental crowding in Plio-Pleistocene hominids. Additional specimens exhibiting such crowding include “Lucy”(Johanson et al., 1982), “Zinjanthropus” (Tobias, 1967), Swartkrans specimens SK-23, SK-48, SK 838, and SK1590b, Makapansgat specimen MLD-18 (Oppenheimer, 1964; Robinson, 1956; Wallace, 19721, and the Galusi hominid (Protsch, 1981). Similarly, dental crowding and impaction have been reported in the Krapina Neandertals (Wolpoff, 1979; Skinner and Sperber, 1982; Frayer and Russell, 1987). A number of the australopithecine specimens also evidence heavy wear on the first and second molars, but unerupted third molars. While this may reflect a lengthy period of time between the eruption of the second and third molars or heavy masticatory stress on the anterior molars, it could also be a sign of impacted teeth, a possibility which appears to have escaped the attention of previous investigators. Finally, the dental arch length of the australopithecines was relatively long in comparison to the length of the cranial base and overall size of the skull (Rak, 1983; Tobias, 19671, suggesting that among the robust australopithecines tooth size may have been approaching the maximum compatible with the size of the skull. Given that dental crowding and impaction can exert a major impact on health (Calcagno and Gibson, 1988, 1991), we believe that it is time for a more serious examination of the incidence of dental crowding in the fossil record. ACKNOWLEDGMENTS We thank Paul McFarland, former Associate Dean for Research and Professor of Oral Surgery, University of Texas Dental Branch, and Dan West, Chairman of Orthodontics, University of Texas Dental Branch, for expert consultation during the preparation of this paper. We also thank Meave Leakey for granting permission for one of us (Calcagno)to examine the original WT17400 cranium, and to Alice Maunda of the National Museums of Kenya for her onsite assistance. LITERATURE CITED Archer WH (1966) Oral Surgery: A Step-By-StepAtlas of Operative Techniques, 4th edition. Philadelphia: W.B.Saunders Co. Calcagno JM,and Gibson KR (1988) Human dental reduction: Natural selection or the probable mutation effect. Am. J. Phys. Anthropol. 77:505-517. Calcagno JM,and Gibson KFt (1991) Selective compromise: Evolutionary trends and mechanisms in hominid tooth size. In MA Kelley and CS Larsen (eds.): Advances in Dental Anthropology. New York Alan R. Liss, pp. 59-76. 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