Measurements of tooth wear among Australian Aborigines: I. Serial loss of the enamel crown

The dental casts made from Aboriginal children during the course of a longitudinal growth study in Central Australia provided material for analyzing tooth wear under known environmental conditions. The wear facets produced on the occlusal surfaces were clearly preserved on the dental stone casts and recorded the progress of enamel attrition from ages 6 to 18. These casts were photographed and traced by electronic planimetric methods that automatically recorded the location and size of wear facets on the first and second permanent molars. These areas of worn tooth surface were compared to the total tooth surface. The worn surface was regressed on age to calculate wear rates of each tooth. Discriminant analyses were also performed to determine the significance of dental attrition differences between the sexes at each age group. The total wear on each tooth was highly correlated with age as expected but females wore their teeth at a significantly higher rate than males. The mandibular molars wore more rapidly than maxillary teeth in both sexes. The discriminant analysis successfully grouped 91% of the cases according to age and sex. Pattern of wear, the location, and size of wear facets also differed between age groups and sex. The questions of why there is a difference between male and female wear or why there is greater wear on one arch or arch region have no ready answers. The differing rates and pattern of dental wear do suggest that arch shape and growth rates may be the answer though it has yet to be tested. However, the occlusal surface wear is useful for age estimation in a population and provides a record of shifting masticatory forces during growth.     

Development of the helicoidal plane

In this study of thebelicoidal occlusal plane the relationships between tooth wear, the transverse slopes of mandibular molars and dental arch breadths were examined in 74 pre-contemporary Australian Aboriginal skulls. With increasing age and tooth wear the orientation of the mandibular occlusal surfaces increased towards the buccal. The differential occlusal orientation from first to third molars, present at eruption, tended to increase progressively with tooth wear. These functionally induced changes, together with regional differences in relative breadths of the maxillary and mandibular dental arches, are important in the development of abelicoidal occlusal plane.     

Modular Wear Facet Nomenclature for mammalian post-canine dentitions

Abstract

Dental wear facets on the occlusal surface of premolars and molars are traces of their main function, the mastication and therefore reflect masticatory movements and also paramasticatory (i.e. non-dietary use of teeth) behavior. Here we present the Modular Wear Facet Nomenclature applicable to most mammalian dentitions. Topographic positions of wear facets in relation to the major cusps and crests of the teeth are used to designate the areas of the occlusal surface the facets occupy (e.g. their mesial, distal, lingual, or buccal position). Previous published systems for labeling wear facets have been inconsistent with each other. Therefore, we provide a synoptic review of the most widely-used terminologies, and introduce the alternative Modular Wear Facet Nomenclature. This nomenclature aims to overcome the difficulties caused by the existing inconsistent wear facet terminologies. Our new approach is applicable to dentitions where the occlusal morphology does not change significantly for most of the lifetime of the animal. In those dentitions, the primary occlusal surfaces are not significantly modified as wear facets become more extensive with wearing. This appears to be a common pattern in pre-tribosphenic, tribosphenic molars, and the teeth derived from tribosphenic precursors (e.g. bunodont molar morphologies). In teeth where the secondary occlusal surface is functionally intensely modified (i.e. high-crowned and evergrowing teeth with large areas of dentine exposed) any facet labeling system appears to be challenging, since the identification of individual facets is blurred and their spatial position may be indeterminable.     

Tooth morphology and other aspects of the Teso dentition

The teeth of over 5,000 Teso schoolchildren members of a Nilo-Hamitic tribe in East Africa, were examined for morphological traits. There was a significant difference between the sexes in the number of cusps on the lower first and second molars, in the prevalence of the cusp of Carabelli, and in variability and agenesis of the upper lateral incisor. The results showed that females consistently favoured tooth reduction. There was also a tendency among those possessing extra cusps on one molar to have extra cusps or other molars. Records kept of the prevalence of the tribal custom of extracting lower central incisors indicated that this practise is rapidly dying out. On another group of teeth which had been extracted from adults common variations of root morphology were noted, together with the fissure pattern of the lower molars. Measurements were made of those teeth which were unworn and were not broken down by dental decay, and the lower third molar was found to be the largest tooth of the series. Observations on the pattern of molar tooth wear showed that the buccal as well as the occlusal surface was strongly affected.     

Occlusion in an Adult Male Gorilla with a Supernumerary Maxillary Premolar

Supernumerary teeth, or teeth that develop in addition to the normal number of deciduous and permanent dentition, have been widely described in human and nonhuman primates. Most studies have focused on the morphology and on the etiology of supernumerary teeth, and little is known about their occlusal relationships with adjacent and antagonistic teeth, and their effects on individuals’ masticatory efficiency. We analyzed the occlusal wear pattern of an adult male Western lowland gorilla (Gorilla gorilla gorilla) with a fully erupted extra maxillary right premolar. We used a virtual method, occlusal fingerprint analysis, to reconstruct the major mandibular occlusal pathways responsible for the creation of wear facets on the tooth crowns. This approach is based on analysis of facet parameters such as inclination, directions, and areas, all measured using high-resolution 3-D virtual models of dental crowns. The results show unusual wear patterns in the supernumerary premolar and on its antagonist contacts (lower P4 and M1) that cannot be associated with a normal masticatory behavior. Occlusal simulation and kinematic analyses reveal a high level of directional overlapping combined with the absence of common occlusal contacts. This indicates a case of malocclusion that must have caused discomfort in this gorilla when biting or chewing, and may represent the first evidence of bruxism (grinding the teeth and clenching the jaw) in wild great apes.     

Age-related variability in occlusal wear planes

Investigation of two populations of 136 individuals shows several patterns of occlusal wear plane change which are positively correlated with age. For individuals up to the age of 18, there is a characteristic pattern in which the occlusal wear planes of the mandibular teeth are lingually sloped and those of the maxillary dentition buccally sloped, with the exception of the maxillary premolars, which are also lingually sloped. The long axes of the mandibular teeth give them a lingual orientation relative to the maxillary teeth, and the long axes of the maxillary molars, by contrast, are buccally oriented. In the 18-30 age range for all sexes, the mandibular M1 becomes buccally sloped on its occlusal surface while the occlusal wear plane on the maxillary M1 becomes lingually sloped. Later age changes indicate a trend for the mandibular premolars to become buccally sloped, while the wear planes of the maxillary premolars remain lingually sloped. There is a corresponding tendency for the maxillary and mandibular second molars to undergo changes in the initial orientation of the occlusal wear planes.     

Interproximal wear facets and tooth associations in the Paşalar hominoid sample

Interproximal wear facets were examined on hominoid teeth from the middle Miocene site at Pa?alar, Turkey. The aim was to find matches between adjacent premolar and molar teeth from single individuals that were collected in the field as isolated teeth and use them to reconstruct tooth rows. These were then used to investigate: (1) the wear gradient on the molar teeth; (2) the dispersal of teeth from single mandibles and maxillae; (3) the size ratios among the molars; and (4) the number of individuals represented by the hominoid sample. Facets were scored for size and shape and were assessed visually using photographs and superimposed outline drawings on acetate transparencies. Out of a sample of approximately 1,500 teeth collected between 1983 and 1996, 532 molars and 258 premolars produced apparent matches making up 160 tooth rows. These were then examined rigorously for morphological consistency and state of wear, and, employing the criterion that only the most unequivocal associations should be used, the final number was reduced to 48 tooth rows-31 mandibular and 17 maxillary. The tooth associations represent a minimum of 21 individuals and probably as many as 34. Molar wear was rapid, with M1s having almost twice as much wear as M3s, as measured by a wear-gradient index. The M2s are intermediate but generally closer to M1s in degree of wear, as are P4s. This wear pattern suggests either delayed eruption of M3s or extremely abrasive diets causing rapid, heavy wear. There is some indication that the wear patterns in Griphopithecus alpani and Kenyapithecus kizili are different, with the latter perhaps having a lower wear gradient, but the K. kizili sample is very small. In both species, the M2 is the largest molar and the M1 is the smallest. Separation of individual teeth in the 48 tooth associations varied from widely separated-up to 8.5m apart-to within a few centimeters of each other. One tooth row (D922) was found with the teeth in contact but the maxillary bone had dissolved away. Two dispersal mechanisms have been identified from earlier taphonomic work: transport of disarticulated elements to the fossil site and reworking of sediments by spring action.     

MOLAR OCCLUSION IN LATE TRIASSIC MAMMALS

1. A new genus and species of late Triassic mammal, Megazostrodon rudnerae, from Lesotho in southern Africa is described. The molars are similar to those of the British Eozostrodon parvus except that they are slightly larger and the upper molars have a large external cingulum supporting well-developed cusps. 2. Molar occlusion is discussed in two groups of late Triassic mammals: Eoxostrodon and the closely related Megazostrodon on one the hand and the unnamed primitive symmetrodonts on the other. It is shown that in Eoxostrodon the upper and lower molars did not have matching occlusal surfaces upon eruption but that wear produced matching occlusal surfaces. These surfaces are confined to the internal surface of the upper molars and the external surface of the lower molars and form a series of wide-angled triangles. The main cusp of an upper molar occluded between the main and posterior subsidiary cusp of the lower molar and the main cusp of the lower molar occluded between the main and anterior subsidiary cusp of the upper molar, 3. It is shown that the molars of Docodon and HaIdanodon were possibly derived from those of a primitive mammal such as Eozostrodon. The transition involved the development on the upper molars of an internal extension which, as it increased in size, established contact with the dorsal surfaces of two adjacent lower molars. The process involved is fundamentally different from that leading to tribosphenic molars. 4. In Megaxostrodon the main cusp of the upper molars occluded between the posterior and anterior subsidiary cusps of two adjacent lower molars, i.e. more posteriorly than in Eozostrodon. Primitive Rhaetic symmetrodonts were derived from mammals which had this type of occlusion and which were also closely related to Eoxostrodon and Megaxostrodon. The transition involved a rotation of the subsidiary cusps of the upper molars externally and those of the lower molars internally. This rotation increased the shearing surfaces between occluding upper and lower molars. Cusp rotation was carried further in the acute-angled symmetrodonts (Peralestes and Spalacotherium) and pantotheres. It appears that marked cusp rotation was coupled with the acquisition of transverse movements of the lower jaw during mastication. Transverse movement was apparently not possible in cynodonts, in Eoxostrodon (and related forms) and in Docodon. 5. The evolution of therian molars involves cusp rotation as originally proposed by the Cope—Osborn theory. Criticisms of the Cope—Osborn theory are re-evaluated in light of the new late Triassic material. 6. In Rhaetic symmetrodonts, molar wear produces matching occlusal facets, but the amount of attrition necessary to produce these facets was considerably less than in Eoxostrodon. In acute-angled symmetrodonts and in pantotheres, the molars erupt with more precise occlusal surfaces and attrition was not necessary to produce matching surfaces. 7. On the basis of the structure of the molar teeth it was concluded that Eozostrodon, Megazostrodon and Erythrotherium were closely related to the Rhaetic symmetrodonts. Slightly different occlusal relationships between upper and lower molars indicated that in these early mammals constant occlusal relations were being established. 8. Primitive cynodonts, such as Thrinaxodon, are characterized by alternate tooth replacement; there is a total lack of a constant occlusal relationship between upper and lower postcanine teeth. In Thrinaxodon individual postcanines were replaced several times. The crown structures of successive generations of postcanines were different so that a freshly erupted postcanine tooth had a crown structure quite distinct from the tooth which it replaced. It has been shown that the crown structure of one of the generations of postcanine teeth of Thrinaxodon is almost identical to that of Eozostrodon except that Thrinaxodon postcanines have a single root, On the basis of this similarity and the over-all structure of the primitive cynodont skull, it was concluded that Rhaetic mammals (excluding ictidosaurs and haramyids) could be derived from primitive cynodonts. 9. All the orders of Jurassic mammals (with the possible exception of multituber-culates) were probably derived from late Triassic mammals. The apparent close relationship of late Triassic mammals is evidence of a monophyletic origin of this class.     

Cheek tooth erosion in male babirusa (genus Babyrousa)

The wear on the occlusal surfaces of male babirusa cheek teeth was evaluated in 53 skulls of Babyrousa babyrussa from Buru and the Sula Islands and 87 skulls of B. celebensis from Sulawesi, Indonesia. Based on the comparative lengths of their continually growing maxillary canine teeth, the skulls were divided into five ‘age categories’ (A–E). Numerical and symbolic codes representing tooth wear were applied to each pillar (cusp region) of the mandibular and maxillary permanent third and fourth premolar teeth, and the first, second and third permanent molar teeth. There was no significant difference between the tooth wear patters of skulls in groups A and B, or in groups C and D, and so these were amalgamated. There was close correspondence in wear patterns between each side of the mouth in both species and in each age group. The wear patterns of the mandibular and maxillary teeth, although not identical, were very similar, as were the wear patterns of both species. In group A + B for both species tooth wear was relatively slight, with the M1 teeth experiencing most relative wear. There was almost no wear of the M3 teeth. In group C + D substantial wear of upper and lower M1 was evident. In group E more widespread wear of the cheek teeth was seen, with increased severity of M1 tooth wear, yet there was comparatively much less M2 and M3 tooth wear. The pattern of cheek tooth wear of the Babyrousa spp. was different from that shown by Sus scrofa. Differences in diet selection and processing were highlighted as potential contributing factors. The pattern of cheek tooth wear in male babirusa was not adequate for use to monitor their age.     

Enamel thickness and the helicoidal occlusal plane

In the present study 38 unworn maxillary molars (M¹ = 16, M²= 12, M³ = 10) of modern humans from a Slavic necropolis were sectioned through the mesial cusps in a plane perpendicular to the cervical margin of the crown. Five slightly worn M¹s and one slightly worn M³ were also used thus increasing the total sample to 44, but measurements made on the worn areas were coded as missing values. Seven measurements of enamel thickness as well as the heights of the protocone and the paracone dentine horns were recorded in order to analyze whether changes in these dimensions in anteroposterior direction can be related to the helicoidal occlusal plane. Uni- and multivariate analyses revealed that the distribution of enamel thickness within and between maxillary molars corresponds to a helicoidal occlusal wear pattern. Enamel thickness along the occlusal basin increases from anterior to posterior, which may lead to rapid development of a reverse curve of Monson in first molars when compared to posterior teeth. However, although these overall differences together with the serial, especially delayed eruption pattern of human molars, contribute to the marked expression of the helicoidal occlusal plane in Homo, differences in enamel patterning between molars indicate that a helicoidal plane is a structural feature of the orofacial skeleton. In contrast to first upper molars, second and third molars show absolutely and relatively thicker enamel under the Phase I wear facet of the paracone, i. e., the lingual slope of the paracone, than under the Phase II facet of the protocone, i. e., the buccal slope of that cusp. These proportional differences are most pronounced in M³, as evidenced by uni- and multivariate statistics. It thus appears that the pattern of enamel thickness distribution from M¹ to M³ follows a trend towards providing additional tooth material in areas that are under greater functional demands, that is, corresponding to a lingual slope of wear anteriorly and to a flat or even buccal one posteriorly. In addition, the heights of the dentine horns in anteroposterior direction change in a way that lends support to the hypothesis that the axial inclination of teeth could be one of the most important factors for the development of the helicoidal occlusal plane. Finally, the changes in morphology and enamel thickness distribution from first to third upper molars found in this study suggest that molars could be “specialized” in their function, i. e., from performing proportionally more shearing anteriorly to increased crushing and grinding activities posteriorly. © 1994 Wiley-Liss, Inc.     

Tooth eruption in Reeves' muntjac (Muntiacus reevesi) and its use as a method of age estimation (Mammalia: Cervidae)

The sequence of tooth eruption and replacement in Reeves' muntjac was determined from captive animals of known age. Pronounced sexual dimorphism is shown by the permanent upper canine which in the male is large, tusk-like and is used as a weapon. The upper canine was the first deciduous tooth to be replaced in males, at approximately 21 weeks of age, compared with 53–57 weeks in the female. The permanent mandibular teeth erupted in the order: molars, first and second incisors, premolars, third incisor and canine. The maxillary teeth erupted in the order: first molar, canine (in male), second and third molars, canine (in female), premolars. The full complement of 34 functional permanent teeth was attained by 83–92 weeks of age.     

Dental reductions and dental caries

Although first permanent molar hypoconulid absence, third molar agenesis, and small tooth size are all part of the evolutionary trend of dental reduction, each bears a different relationship to dental caries. Caries prevalence in the maxillary and mandibular permanent first molars of the Burlington Research Centre serial experimental group at age 16 years was less in the children whose first molars were missing the hypoconulid. Conversely, caries prevalence in mandibular first molars was greater in the children who had agenesis of third molars. The extraction of first molars due to caries was more frequent in children with agenesis of third molars, less frequent in those with absence of hypoconulids of the first molars and unrelated to tooth size. Caries prevalence was less in small teeth, and occurred least in the small mandibular first molars with four cusps. Whereas this is in harmony with the hypothesis that evolutionary dental reductions resulted from the pressure of caries, the increased prevalence of caries and extractions coinciding with third molar agenesis does not support this view. In addition, agenesis of hypoconulids and agenesis of third molars were related to changes in structures unrelated to caries.     

Occlusal relief changes with molar wear in Pan troglodytes troglodytes and Gorilla gorilla gorilla

Most research on primate tooth form-function relationships has focused on unworn teeth. This study presents a morphological comparison of variably worn lower second molars (M(2)s) of lowland gorillas (Gorilla gorilla gorilla; n=47) and common chimpanzees (Pan troglodytes troglodytes; n=54) using dental topographic analysis. High-resolution replicas of occlusal surfaces were prepared and scanned in 3D by laser scanning. The resulting elevation data were used to create a geographic information system (GIS) for each tooth. Occlusal relief, defined as the ratio of 3D surface area to 2D planometric area of the occlusal table, was calculated and compared between wear stages, taxa, and sexes. The results failed to show a difference in occlusal relief between males and females of a given taxon, but did evince differences between wear stages and between taxa. A lack of significant interaction between wear stage and taxon factors suggests that differences in occlusal relief between chimpanzees and gorillas are maintained throughout the wear sequence. These results add to a growing body of information on how molar teeth change with wear, and how differences between primate species are maintained at comparable points throughout the wear sequence. Such studies provide new insights into form-function relationships, which will allow us to infer certain aspects of diet in fossils with worn teeth.     

The role of effectors of the activin signalling pathway, activin receptors IIA and IIB, and Smad2, in patterning of tooth development.

The gene for activin betaA is expressed in the early odontogenic mesenchyme of all murine teeth but mutant mice show a patterning defect where incisors and mandibular molars fail to develop but maxillary molars develop normally. In order to understand why maxillary molar tooth development can proceed in the absence of activin, we have explored the role of mediators of activin signalling in tooth development. Analysis of tooth development in activin receptor II and Smad2 mutants shows that a similar tooth phenotype to activin betaA mutants can be observed. In addition, we identify a novel downstream target of activin signalling, the Iroquois-related homeobox gene, Irx1, and show that its expression in activin betaA mutant embryos is lost in all tooth germs, including the maxillary molars. These results strongly suggest that other transforming growth factor beta molecules are not stimulating the activin signalling pathway in the absence of activin. This was confirmed by a non-genetic approach using exogenous soluble receptors to inhibit all activin signalling in tooth development, which reproduced the genetic phenotypes. Activin, thus, has an essential role in early development of incisor and mandibular molar teeth but this pathway is not required for development of maxillary molars.     

Biomechanical behaviour of modern human molars: Implications for interpreting the fossil record

Finite-element models of 29 intact molars were created and subjected to cleavage-type loads in order to assess differences in the biomechanical behaviour of molars. A simulated food particle, which was one-third the size of the intercuspal distance and had the properties of a Mezzettia seed, was pushed onto the occlusal basin of these models at various angles, resulting in either both or one particular cusp being preferentially loaded. In all cases, the maximum tensile stresses occurred in enamel at the intercuspal fissure. With regard to first maxillary molars, supporting (functional) and guiding (nonfunctional) cusps apparently dissipate loads equally well, whereas, in second and third maxillary molars, the guiding cusps are better designed to resist loads. Overall, lingual cusps of maxillary posterior molars dissipate loads poorly. Conversely, loads exerted toward supporting cusps of mandibular molars are consistently well dissipated, regardless of position along the tooth row. Because the directions of loads to which these teeth are best adapted change along the tooth row, it seems reasonable to suggest that these may correlate with the well-documented structural and functional orofacial complex. This study indicates that the biomechanical behaviour of molars and the orofacial skeleton are likely to have undergone complementary directional changes during evolution. Consequently, caution must be exercised in making inferences about dietary adaptations of extinct species on the basis of isolated teeth or fragmentary gnathic remains without proper regard of the orofacial skeleton as a whole. Am J Phys Anthropol 106:467–482, 1998. © 1998 Wiley-Liss, Inc.     

Patterns of sexual dimorphism and of variability in the dentition ofOtolemur crassicaudatus

Normative odontometric data are presented on a sample of 66 adult thick-tailed bushbabies Otolemur crassicaudatus(34 male, 32 female). This species is characterized by low levels of sexual dimorphism, with univariate differences centered on the canines and the maxillary third molar. Multivariate canonical analysis isolates a third discriminator, the maxillary second molar. Stepwise discriminant analyses, performed after jackknifing, indicate high percentages of correct classification (males, 79.8–81.8%;females, 81–85.2%). When variability profiles consisting of arrays of CVs are compared, males and females are found to share similar patterns. Data for maxillary teeth offer support for Gingerich’s occlusal complexity model, while morphogenetic clusterings within regressions of variability on tooth size conform to those previously reported in other species. These relationships are lost in the mandibular dentition, suggesting an independence of upper from lower toothsize determination.     

Helicoidal plane of dental occlusion

A helicoidal plane of postcanine occlusion has been patchily reported in many recent and fossil dentitions of man, and has been suggested as a taxonomic marker distinguishing between the dentitions of Homo and Australopithecines. The present paper describes the helicoidal plane in 19 out of 23 modern human (probably Indian) worn dentitions, in both gracile and robust Australopithecines and in extant anthropoids. It is suggested that tooth wear converts the plane of occlusion present in little-worn teeth, the Monson curve, into a helicoidal plane when 1) the diet is more abrasive, 2) the enamel is thinner and less abrasion resistant, and 3) a longer time separates the eruption of the three molar teeth in a jaw quadrant. A model demonstrates that during the power stroke of a chewing cycle the working side molars move in much the same direction whether the molar occlusal plan follows a Monson curve or a helicoidal plane. The difference is that in the former case the three molars work at the same time while in the latter case they work in sequence from anterior to posterior, thereby concentrating force on one tooth at a time. Because the occlusal plane changes during the life of individuals consuming an abrasive diet, the condition of most anthropoids and hominids, it is argued that the Monson curve has functional significance not because of its influence on occlusal relations and/or jaw movement but because the molar teeth are embedded in bone roughly perpendicular to it, a direction which resists tilting of the teeth during mastication. It is concluded that the helicoidal plane probably has little if any value as a taxonomic marker.     

Stereophotogrammetric analysis of occlusal morphology of extant hominoid molars: phenetics and function

Because teeth are commonly preserved in the fossil record, dental remains have often been employed in estimating evolutionary relationships among fossil hominoids. This is appropriate, however, only to the extent that dental morphology is phylogenetically informative. I have used phenetic analytic techniques to assess whether hominoid molars are likely to be useful for phylogenetic inference. Thirty-four occlusal landmarks for first and second molars were chosen; seven on each maxillary and ten on each mandibular tooth. Three-dimensional locations of these points were determined from stereophotographs of dental arcades of more than 260 specimens from six taxa (gorilla, chimpanzee, human, orangutan, siamang, and gibbon). Analytic emphasis was on canonical variates analyses of landmark coordinates for mandibular and maxillary second molars, adjusted for intergroup size differences. There is little correspondence between the systematic implications of hominoid molar morphometrics and reliable estimates of evolutionary propinquity based on interhominoid biomolecular similarities. The former seem to have been determined largely by dietary constraints. Although this suggests the possibility of using the protocol employed here to infer diets of fossil hominoids, molar crown measurements seem unlikely to serve well as phylogenetic indicators in the Hominoidea.     

The ontogeny of premolar dental wear in Cercocebus albigena (cercopithecidae)

The orientation of striated wear facets on primate teeth serves as a useful guide for reconstructing jaw movements during mastication. Most wear facets on the molars are formed during one of the two well-documented movements, Phase I or Phase II, of the power stroke. Another jaw movement direction, “orthal retraction” (OR) has been proposed to account for a third set of facets occasionally present on the pointed tips of premolars and molars. Evidence advanced here indicates that OR facets on pointed anterior premolars (P₃) of cercopithecoids are actually Phase I facets that have become reoriented as a result of a rotation of this tooth during its eruption. “Orthal retraction” probably does not exist as a discrete masticatory phase.