Family studies of tooth size factors in the permanent dentition.

Correlations between dimensions of the permanent teeth in Australian Aboriginals were studied by factor analysis to disclose the main sources of shared variability. Findings indicated that in both males and females most of the common variability in the tooth dimensions could be accounted for by factors representing mesiodistal size of anterior teeth, buccolingual size of anterior teeth, generalized size of the premolars and generalized size of the molars. Factor scores derived from the analysis were used to calculate intraclass correlations among brothers and among sisters. These correlations tended to be higher for the factors contributing most to the common variability indicating that the factors might represent fields under direct genetic control. There was no trend for intraclass correlations among siblings derived from multivariate scores to be consistently higher than those based on observed tooth dimensions. The main advantage to the user of factor analysis is the ability to interpret associations between interrelated variables more objectively than is possible by conventional correlation methods.     

Tooth size and arcadal length correlates in man

Intra-arcadal mesiodistal and buccolingual tooth size correlations were evaluated in a sample of 125 caucasoids with ideal occlusion. Dental dimensions were corrected for arcade mength (as a measure of jaw size) by a series of regression analyses of each mesiodistal dimension on the sum of the mesiodistal dimensions within each arcade. Regression coefficients of tooth dimension on arcade length were calculated to gain an insight into the dimensional sensitivity of individual teeth to arcade length variation. The data presented here suggest a strong association between arcadal length (jaw size) dependence, and the dimensional stability of individual teeth. When corrected for arcade length, a definite pattern of tooth size correlation emerges: postcanine maxillary and mandibular teeth are negatively correlated to the anterior teeth and are positively correlated to one another. The hypothesis is developed that anterior and postcanine teeth should be viewed as two separate and negatively size-correlated units, beyond the boundaries of the four morphological tooth classes. Recognition of this basic dichotomous size arrangement within each jaw allows for a reassessment of some of the problems associated with hominid dental evolution.     

A factor analysis of morphogenetic fields in the human dentition

The traditional morphogenetic fields of the human dentition were evaluated by means of factor analysis of dental dimensions taken from a series of human crania. When crown length, crown width and crown index were considered separately, factors emerged which could be identified with the tooth group fields. But a combined crown length-crown width analysis generated factors which extended beyond the regional tooth groups. Crown width itself was revealed to be an important axis of morphologic intergration. It was concluded that univariate methods are not adequate for identifying morphogenetic fields; the teeth must be treated as multidimensional units where the correlation among dimensions is accounted for.     

Genetic correlations between mandibular molar cusp areas in baboons

Primate evolutionary studies rely significantly on dental variation given the large role that teeth play in how an organism interacts with its environment (animal and plant) and conspecifics. Variation in cusp size has been shown to vary among primate taxa, although most studies to date focused on extant and extinct hominoids. Here we test the assumed hypothesis that a significant proportion of this variation in baboons is due to the additive effects of genes. We perform quantitative genetic analyses on variation in two-dimensional (2-D) mandibular molar cusp size in a captive pedigreed breeding population of baboons (Papio hamadryas) from the Southwest National Primate Research Center. These analyses show that variation in cusp size is heritable and sexually dimorphic. Additionally, we tested for genetic correlations between cusps on the same crown, between morphological homologues along the tooth row, and between cusp area and crown buccolingual width. We find that four of the six cusp pairs on the first molar have a genetic correlation of one, save for the metaconid-hypoconid and entoconid-hypoconid, which are not statistically different from zero. The second and third molars have lower genetic correlations, although the metaconid-hypoconid correlation is similarly estimated at zero and the entoconid-protoconid correlation is estimated to be one. This cross pattern of genetic and no genetic correlation does not immediately accord with the known pattern of development and/or calcification. We propose two explanative hypotheses.     

The dentition of the Indian Knoll skeletal population: odontometrics and cusp number.

Data on the permanent dentition of 153 individuals from the well known Indian Knoll skeletal population are presented. Mesiodistal and buccolingual measurements were taken with a Helios dial caliper. Cusp number of maxillary and mandibular molars are recorded. The Indian Knoll dentition is larger than many modern groups but smaller than Australoid or Mesolithic groups. With the exception of maxillary 12, males have larger teeth than females in both dimensions. The lower canine is the most dimorphic tooth. Through rank order correlation, an association was shown between the sexual dimorphism of the mesiodistal and buccolingual dimensions. Compared to modern groups, the Indian Knoll population displays a moderate degree of sexual dimorphism in tooth size. In general, the coefficients of variation were greater for the more distal teeth within morphological classes. Amounts of size variability did not differ significantly between the sexes; moreover, rank order correlations indicated that patterns of variability in both dimensions were similar for males and females. The predominant cusp number pattern for upper molars is 4-3-3 and for lowers 5-5(4)-5. No sex differences were shown for cusp occurrence or bilateral asymmetry in cusp number.     

Dentition of moustached tamarins (Saguinus mystax mystax) from Padre Isla, Peru, part 1: quantitative variation

Analyses of dental variation in geographically restricted, wild populations of primates are extremely rare; however, such data form the best source for models of likely degrees of variation within and between fossil species. Data from dental casts of a geographically restricted population of moustached tamarins (Saguinus mystax mystax) from Padre Isla, Peru, document high levels of dental variability, as measured by coefficients of variation, in a nonsexually dimorphic species, despite its isolation and small population size. Like other primates, moustached tamarins show lower variability in the dimensions of the first molars and increased variability in the dimensions of the final molars in the toothrow. Moustached tamarins from Padre Isla have a distinctive pattern of variability in the remaining teeth, including more stable tooth lengths in the anterior and posterior portions of the toothrow, and more stable tooth widths in the midregion of the toothrow. High variability in incisor width may be due to age effects of a distinctive diet and pattern of dental wear.     

Tooth Size Variation in Prehistoric India

Permanent dental crown dimensions are largely unknown for living and prehistoric populations of South Asia. This paper describes permanent tooth crown measurements and indices for the Late Chalcolithic skeletal series from Inamgaon (1700-700 B.C.) in western India. These data are compared with tooth size data for prehistoric populations in India and both living and prehistoric populations outside the South Asian subcontinent. In summed cross-sectional area, the Inamgaon (1,218 mm²) permanent teeth are most similar to prehistoric skeletal series from Mahurjhari, India, Non Nok Tha, Thailand, and Bellan Bandi Palassa, Sri Lanka. The Inamgaon total crown area is only 3.1% smaller than the figure reported for early Neolithic skeletons from Mehr-garh, Pakistan, indicating the relatively large size of the Inamgaon dentition. Large tooth size at Inamgaon is interpreted as a biological adaptation to coarse dietary items, basic food preparation methods, and a mixed economy that included hunting wild game and collecting wild fruit. Dental indices of "ethnic" significance are perhaps better interpreted as indicators of masticatory stress and differential dental reduction. Maxillary and mandibular third molars show minimal reduction in crown size, but mandibular second molar teeth are distinctly reduced in size. The Incisor Breadth Index may indicate mixed genetic ancestry of the Inamgaon people or similar dietary stresses in genetically different populations.     

An odontometric analysis of the early Griqua dentition

The teeth of 28 Griqua skeletons were subjected to odontometric analysis. Significant sexual dimorphism in tooth size was demonstrated in canines and molars. Coefficients of variation were generally greater in the more distal tooth within morphological classes. Rank order correlations suggest similar patterns of variability in males and females. Comparisons of the Griqua dentition with similar populations were made.     

The relation of canine size to pattern and extent of attrition in baboons

For most genera of animals the association between dental morphology and diet is such that inferences concerning the diet can be made on the basis of the dentition alone. The canine is the one tooth that appears exempt from such generalisation and exhibits a wide range of variability of size and shape in all groups. In order to determine the effect of dimorphism of the canine on the dental apparatus, male and female specimens of Theropithecus and Papio baboons were examined. Occlusal relationships and dental pathology for 21 Theropithecus and 40 Papio skulls were recorded, and crown height measurements obtained for maxillary incisors, buccal and lingual molar cusps. Attrition was the most common and severe cause of abscess formation in older animals: the pattern of attrition differed in adult males and females, the latter showing more wear of the incisor and lingual molar cusps. Partial correlations for incisor, buccal and lingual molar cuspal crown height show a significant correlation between incisor and lingual cusp attrition in juveniles of both sexes and adult females (p < 0.01). Adult males show no correlation of attrition. It is proposed that the correlation of incisor and lingual molar cusp attrition is associated with anterior-posterior grinding movements, such as take place during incision, that the large canine present in the male limits such movements, the sharp blade-like canine being employed as a cutting tool. This use of the canine by reducing functional demands on other teeth, prolongs the utility of the dental apparatus, and hence the lifespan of its possessor.     

Dimensional and discrete dental trait asymmetry relationships

Inuit (Eskimos) from the Foxe Basin region of the Northwest Territories, Canada, were studied to ascertain the amount of dimensional and morphological asymmetry in their dentitions. The results indicate that dimensional asymmetry does not appear to be greater on either the maxillary or mandibular teeth. Both types of asymmetry show partial conformity to the model of tooth fields with an increasing amount of asymmetry as one goes distally in each tooth group. The morphological asymmetry exception, the mandibular incisors, follows Dahlberg's "Field Concept." Rank-order correlations between the amount of dimensional asymmetry and morphological asymmetry reveal no detectable patterns. There appear to be no associations between the presence or absence of morphological asymmetry and the size of the tooth. This lack of association might be explained by differences in developmental timing of tooth dimensions and morphological traits; however, such a hypothesis requires experimental testing. In this population and those for which published results are available, it is practically impossible to overcome the "noise" level and test recent hypotheses regarding random dental asymmetry.     

Variability in mammalian dentitions: size-related bias in the coefficient of variation

The coefficient of variation has been used in many evolutionary studies. However, a strong negative correlation between this index and size may artificially inflate the apparent variability of small traits. This is most pronounced when variables whose size differs by more than an order of magnitude are compared or when the index is applied to variables whose size is within an order of magnitude of their measurement error. When this is likely to affect conclusions, other indexes of variability should be considered. One alternative is to use the standard deviation of log-transformed data; however, this index is sometimes still correlated with variable size so care should be exercised in its use. Another alternative is to regress the standard deviation onto mean variable size; however, this method may also be misleading if variables are not randomly distributed about the regression line. As an example of the effect of bias in the coefficient of variation, previous studies of mammalian dental variability profiles were re-evaluated. It was found that variation among teeth is relatively homogenous, both within and among species. The exception is that the canines of some mammalian species have variability that is considerably higher than would be expected from tooth size. Previous explanations of variability patterns that invoked developmental fields are incompatible with the new data.     

Evidence of strong stabilizing effects on the evolution of boreoeutherian (Mammalia) dental proportions

The dentition is an extremely important organ in mammals with variation in timing and sequence of eruption, crown morphology, and tooth size enabling a range of behavioral, dietary, and functional adaptations across the class. Within this suite of variable mammalian dental phenotypes, relative sizes of teeth reflect variation in the underlying genetic and developmental mechanisms. Two ratios of postcanine tooth lengths capture the relative size of premolars to molars (premolar–molar module, PMM), and among the three molars (molar module component, MMC), and are known to be heritable, independent of body size, and to vary significantly across primates. Here, we explore how these dental traits vary across mammals more broadly, focusing on terrestrial taxa in the clade of Boreoeutheria (Euarchontoglires and Laurasiatheria). We measured the postcanine teeth of N = 1,523 boreoeutherian mammals spanning six orders, 14 families, 36 genera, and 49 species to test hypotheses about associations between dental proportions and phylogenetic relatedness, diet, and life history in mammals. Boreoeutherian postcanine dental proportions sampled in this study carry conserved phylogenetic signal and are not associated with variation in diet. The incorporation of paleontological data provides further evidence that dental proportions may be slower to change than is dietary specialization. These results have implications for our understanding of dental variation and dietary adaptation in mammals.     

Individual variation in enamel structure of human mandibular first premolars

Nine human mandibular first premolars were examined to assess variation in external morphology and enamel structural organization within a tooth type. The relationship of enamel ultrastructure to gross dental morphology was also studied. The teeth were cut in the mesiodistal direction just lingual to the buccal cusp, and etched. Montages were constructed of the cut enamel surface photographed in the scanning electron microscope at 100 X magnification. Parameters were measured and correlation coefficients were calculated for the comparison of various odontometric features. The mesiodistal and buccolingual dimensions were highly correlated and the occlusal thickness of enamel was significantly correlated to crown height but not crown width. Hunter-Schreger bands were less pronounced in fossa areas than at lateral aspects, cusps, or ridges; these bands were directly related to the geometry of the tooth. It was concluded that within this tooth type, there is a large amount of individual variation not only in gross morphology but also in enamel ultrastructure. This result underscores the fact that interspecific comparisons must be made with care.     

Factors affecting the distribution of enamel hypoplasias within the human permanent dentition

Frequencies and morphological and chronological distributions of enamel hypoplasias are presented by tooth type (permanent I1 to M2s), based on a sample of 30 prehistoric Amerindians with complete and unworn dentitions. There is nearly a tenfold variation in frequency of defects by tooth, ranging from 0.13 per mandibular second molar to 1.27 per maxillary central incisor. The six anterior teeth average between 0.70 and 1.27 defects/tooth, whereas the eight posterior teeth average between 0.43 and 0.13 defects/tooth. Earlier developing teeth, such as incisors, have earlier peak frequencies of defects (2.0-2.5 years), while later developing teeth, such as second molars, have subsequent peak frequencies (5.0-6.0 years). These variations are relevant when comparing hypoplasia data based on different teeth. Differences in hypoplasia frequencies among teeth are not solely due to variation in time of crown development, as is usually reported. Rather, there is evidence for biological gradients in susceptibility to ameloblastic disruption. Anterior teeth are more hypoplastic than posterior teeth. More developmentally stable "polar" teeth are more hypoplastic than surrounding teeth. Polar teeth may be more susceptible to hypoplasias because their developmental timing is less easily disrupted. In all teeth, hypoplasias are most common in the middle and cervical thirds. Crown development and morphological factors, such as enamel prism length and direction, may influence the development and expression of enamel surface defects.     

Development of the orangutan permanent dentition: Assessing patterns and variation in tooth development

This study examines dental formation and alveolar emergence in a large cross-sectional sample composed primarily of wild-reared orangutans (N = 89) in order to provide information on the development of the permanent dentition in this hominoid and to address questions of variation in individual tooth formation, between teeth and between individuals. All specimens have been radiographed in lateral aspect and stages of crown and root formation recorded for all teeth. The ranges of crown and root formation of I¹₁, C¹₁, P⁴₄, and M³₃ have been calculated relative to the stage of M₁¹ development within a specific tooth quadrant. Then, for each specimen, BMDP scatterplot and nonparametric statistics have been used to graph changes in stages of these teeth relative to M¹₁ stages and to examine relationships between pairs of upper and lower dental counterparts and between teeth of each jaw. Results indicate 1) high correlations between upper and lower tooth pairs and between many of the permanent teeth within individuals, 2) a relatively large range of variability in individual tooth development (multistage ranges relative to M¹₁), 3) greater variation in root development at emergence than earlier reports, and 4) evidence of variability within the sequence emergence pattern of the orangutan. © 1996 Wiley-Liss, Inc.     

Intercusp relationships of the permanent maxillary first and second molars in American whites

Much of a human molar's morphology is concentrated on its occlusal surface. In view of embryologists' recent attention on the determination of crown morphology by enamel knots that initiate cusp formation, we were interested in the arrangement of cusp apices in the definitive tooth. Computer-assisted image analysis was used to measure intercusp distances and angles on permanent maxillary M1 and M2 in a sample of 160 contemporary North American whites. The intent was to generate normative data and to compare the size and variability gradients from M1 to M2. There is little sexual dimorphism in intercusp distances or angles, even though the conventional mesiodistal (MD) and buccolingual (BL) crown size is 2.0% and 4.0% larger in males, respectively, in these same teeth. Dimensions decreased in size and increased in variability from M1 to M2, but differentially. Cusps of the trigon were more stable between teeth, especially the paracone-protocone relationship. Principal components analysis on the six M1 distances disclosed only one eigenvalue above 1.0, indicating that overall crown size itself is the paramount controlling factor in this tooth that almost invariably exhibits a hypocone. In contrast, four components were extracted from among the 12 angular cusp relationships in M1. These axes of variation may prove useful in studies of intergroup differences. A shape difference occurs in M2, depending on whether the hypocone is present; when absent, the metacone is moved lingually, creating more of an isosceles arrangement for the cusps of the trigon. Statistically, correlations are low between occlusal intercusp relationships and conventional crown diameters measured at the margins of the crowns that form later. Weak statistical dependence between cusp relationships and traditional MD and BL diameters suggest that separate stage- and location-specific molecular signals control these different parts (and different stages) of crown formation.     

Patterns of metric variability in the dentition of Papio ursinus

The present investigation assesses a number of explanations for the patterns of variability in dental dimensions. Coefficients of variation were calculated for mesiodistal and buccolingual diameters in a sample of 105 Papio ursinus crania (52 male, 53 female). Variability profiles consisting of arrays of values of coefficients of variation were evaluated by means of Friedman's two-way analysis of variance and Kendall's coefficient of concordance. Although molar teeth were found to be the most dimensionally stable, our results failed to support either the morphogenetic field theory or the occlusal complexity hypothesis. The data presented here are generally supportive of Pengilly's phenotypic complexity theory. However, speciesspecific clustering patterns found in our regressions of dimensional variability on mean tooth size suggest that differences in variability levels might be related to differences in selective pressures.     

Enamel hypoplasia in the deciduous teeth of great apes: variation in prevalence and timing of defects

The prevalence of enamel hypoplasia in the deciduous teeth of great apes has the potential to reveal episodes of physiological stress in early stages of ontogenetic development. However, little is known about enamel defects of deciduous teeth in great apes. Unresolved questions addressed in this study are: Do hypoplastic enamel defects occur with equal frequency in different groups of great apes? Are enamel hypoplasias more prevalent in the deciduous teeth of male or female apes? During what phase of dental development do enamel defects tend to form? And, what part of the dental crown is most commonly affected? To answer these questions, infant and juvenile skulls of two sympatric genera of great apes (Gorilla and Pan) were examined for dental enamel hypoplasias. Specimens from the Powell‐Cotton Museum (Quex Park, UK; n = 107) are reported here, and compared with prior findings based on my examination of juvenile apes at the Cleveland Museum of Natural History (Hamman‐Todd Collection; n = 100) and Smithsonian Institution (National Museum of Natural History; n = 36). All deciduous teeth were examined by the author with a ×10 hand lens, in oblique incandescent light. Defects were classified using Fédération Dentaire International (FDI)/Defects of Dental Enamel (DDE) standards; defect size and location on the tooth crown were measured and marked on dental outline charts. Enamel defects of ape deciduous teeth are most common on the labial surface of canine teeth. While deciduous incisor and molar teeth consistently exhibit similar defects with prevalences of ～10%, canines average between 70–75%. Position of enamel defects on the canine crown was analyzed by dividing it into three zones (apical, middle, and cervical) and calculating defect prevalence by zone. Among gorillas, enamel hypoplasia prevalence increases progressively from the apical zone (low) to the middle zone to the cervical zone (highest), in both maxillary and mandibular canine teeth. Results from all three study collections reveal that among the great apes, gorillas (87–92%) and orangutans (91%) have a significantly higher prevalence of canine enamel defects than chimpanzees (22–48%). Sex differences in canine enamel hypoplasia are small and not statistically significant in any great ape. Factors influencing intergroup variation in prevalence of enamel defects and their distribution on the canine crown, including physiological stress and interspecific dento‐gnathic morphological variation, are evaluated. Am J Phys Anthropol 116:199–208, 2001. © 2001 Wiley‐Liss, Inc.     

Posterior dental size reduction in hominids: The Atapuerca evidence

In order to reassess previous hypotheses concerning dental size reduction of the posterior teeth during Pleistocene human evolution, current fossil dental evidence is examined. This evidence includes the large sample of hominid teeth found in recent excavations (1984–1993) in the Sima de los Huesos Middle Pleistocene cave site of the Sierra de Atapuerca (Burgos, Spain). The lower fourth premolars and molars of the Atapuerca hominids, probably older than 300 Kyr, have dimensions similar to those of modern humans. Further, these hominids share the derived state of other features of the posterior teeth with modern humans, such as a similar relative molar size and frequent absence of the hypoconulid, thus suggesting a possible case of parallelism. We believe that dietary changes allowed size reduction of the posterior teeth during the Middle Pleistocene, and the present evidence suggests that the selective pressures that operated on the size variability of these teeth were less restrictive than what is assumed by previous models of dental reduction. Thus, the causal relationship between tooth size decrease and changes in food-preparation techniques during the Pleistocene should be reconsidered. Moreover, the present evidence indicates that the differential reduction of the molars cannot be explained in terms of restriction of available growth space. The molar crown area measurements of a modern human sample were also investigated. The results of this study, as well as previous similar analyses, suggest that a decrease of the rate of cell proliferation, which affected the later-forming crown regions to a greater extent, may be the biological process responsible for the general and differential dental size reduction that occurred during human evolution. © 1995 Wiley-Liss, Inc.