On the groove configuration of mandibular molars: The unreliability of the “dryopithecus pattern” and a new method for classifying mandibular molars

The long-used qualitative nomenclature for the cusp-groove pattern of lower molars, and the definition applying to it, was investigated and found to be unsatisfactory for the repetitive classification of lower molars. Moreover, developmental grooves per se were found to represent the results of highly variable environmental phenomena and therefore their use cannot be justified as an evolutionary determinant. A simple, but effective classification for lower molars, based on cusp size, cusp number, distal fovea, and distal marginal ridge was presented.     

Genetic integration of molar cusp size variation in baboons

Many studies of primate diversity and evolution rely on dental morphology for insight into diet, behavior, and phylogenetic relationships. Consequently, variation in molar cusp size has increasingly become a phenotype of interest. In 2007 we published a quantitative genetic analysis of mandibular molar cusp size variation in baboons. Those results provided more questions than answers, as the pattern of genetic integration did not fit predictions from odontogenesis. To follow up, we expanded our study to include data from the maxillary molar cusps. Here we report on these later analyses, as well as inter‐arch comparisons with the mandibular data. We analyzed variation in two‐dimensional maxillary molar cusp size using data collected from a captive pedigreed breeding colony of baboons, Papio hamadryas, housed at the Southwest National Primate Research Center. These analyses show that variation in maxillary molar cusp size is heritable and sexually dimorphic. We also estimated additive genetic correlations between cusps on the same crown, homologous cusps along the tooth row, and maxillary and mandibular cusps. The pattern for maxillary molars yields genetic correlations of one between the paracone–metacone and protocone–hypocone. Bivariate analyses of cuspal homologues on adjacent teeth yield correlations that are high or not significantly different from one. Between dental arcades, the nonoccluding cusps consistently yield high genetic correlations, especially the metaconid–paracone and metaconid–metacone. This pattern of genetic correlation does not immediately accord with the pattern of development and/or calcification, however these results do follow predictions that can be made from the evolutionary history of the tribosphenic molar. Am J Phys Anthropol, 2010. © 2009 Wiley‐Liss, Inc.     

Associations between Carabelli trait and cusp areas in human permanent maxillary first molars

Few dental anthropological studies have investigated the associations between tooth crown size and crown traits in humans using quantitative methods. We tested several hypotheses about overall crown size, individual cusp areas, and expression of Carabelli cusps in human permanent first molars by obtaining data from standardized occlusal photographs of 308 Australians of European descent (171 males and 137 females). Specifically, we aimed to calculate the areas of the four main molar cusps, and also Carabelli cusp, and to compare the relative variability of cusp areas in relation to timing of development. We also aimed to compare cusp areas between males and females and to describe how Carabelli cusp interacted with other molar cusps. Measurements included maximum crown diameters (mesiodistal and buccolingual crown diameters), the areas of the four main cusps, and the area of Carabelli cusp. The pattern of relative variability in absolute areas of molar cusps corresponded with their order of formation, the first-forming paracone displaying the least variation, and the last-forming Carabelli cusp showing the greatest. Overall crown size and areas of individual cusps all showed sexual dimorphism, with values in males exceeding those in females. Sexual dimorphism was smallest for paracone area and greatest for Carabelli cusp area. Overall crown size and cusp areas were larger in individuals displaying a Carabelli cusp, especially the hypocone area. Although the combined area of the protocone and a Carabelli cusp was greater in cuspal forms than noncuspal forms, protocone area alone was significantly smaller in the former. Our findings lead us to propose that, in individuals with the genotype for Carabelli trait expression, larger molar crowns are more likely to display Carabelli cusps, whereas molars with smaller crowns are more likely to display reduced forms of expression of the trait. We suggest that the pattern of folding of the internal enamel epithelium in developing molar crowns, particularly in the protocone region, can be modified by a developing Carabelli cusp.     

Morphological features of Jat dentition

Observations on morphological characters of milk and permanent teeth, based on 648 pairs of dental casts of 356 male and 292 female Jat children of Haryana (India) are reported. Deciduous teeth show high frequencies of bilateral winging of maxillary central incisor, Carabelli's cusp of maxillary second molar, and deflecting wrinkle of mandibular second molar. Reduction of maxillary molar cusps is more marked in males than in females. Y pattern is very common in deciduous molars. Permanent teeth have high frequencies of grooved cingulum of incisors, cingular nodule of lateral incisors and canines, and distal accessary ridge of canines. Low frequencies of Carabelli's cusp and winging are also common. The tendency towards faintly developed shovelling in milk incisors occurs more often than in the permanent teeth.     

A quantitative investigation of irregular cuspules in human maxillary permanent molars

The frequency of occurrence of anomalous cusps or tubercles on human upper first molars was investigated in seven racial populations using moiré contourography, which permits the three-dimensional measurement of minute cusps. Tubercles on the mesial marginal ridge were more frequently found in Mongoloid populations (Japanese and Eskimo) than in others. The frequency of the protoconule was high in Eskimos and Negroids (Bantu and San). The lingual paracone tubercle (mesial cusp) showed a particularly high frequency in Australian aborigines. The metaconule was rare or absent in all of the populations. Caucasoid groups (Dutch White and Asiatic Indian) showed generally low frequencies of all these abnormal tubercles, especially the distal accessory cusp (C5). Racial differences in the frequencies of occurrence may offer a key to understanding the adaptive significance of these traits and human microevolution. Confusion in nomenclature for upper molar tubercles is also discussed.     

Sexual dimorphism of cusp dimensions in human maxillary molars

Cusp dimensions of human maxillary molars were compared between males and females to determine whether the later-developed, distal cusps displayed greater sexual dimorphism than the earlier-developed, mesial cusps, and whether the later-forming second molar displayed greater sexual dimorphism than the first molar. First and second permanent molar crowns (M1 and M2) were measured indirectly, using dental casts obtained from 117 Japanese (65 males and 52 females). Measurements included maximum mesiodistal and buccolingual crown diameters and the diameters of the four main cusps: the paracone, protocone, metacone, and hypocone. Mean values of crown dimensions were larger in males than in females for both M1 and M2, but the sexual difference in protocone diameter of M1 was not significant. The protocone in M1 showed the least amount of sexual dimorphism, followed by the metacone, hypocone, and paracone, while in M2, the percentage sexual dimorphism corresponded to the order of cusp formation: paracone, protocone, metacone, and hypocone. With the exception of the paracone diameter, M2 showed greater sexual dimorphism than M1. Sexual dimorphism was not always greater in the later-developed, distal cusps of M1 or M2, but the protocone, the most important cusp in terms of occlusal function, displayed the least dimorphism in M1.     

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.     

Geometric morphometric 3D shape analysis and covariation of human mandibular and maxillary first molars

Dental casts of 160 Greek subjects (80 males, 80 females) were scanned by a structured‐light scanner. The upper and lower right first molar occlusal surface 3D meshes were processed using geometric morphometric methods. A total of 265 and 274 curve and surface sliding semilandmarks were placed on the upper and lower molar surfaces, respectively. Principal component analysis and partial least square analysis were performed to assess shape parameters. Molars tended to vary between an elongated and a more square form. The first two principal components (PCs), comprising almost 1/3 of molar shape variation, were related to mesiodistal–buccolingual ratios and relative cusp position. Distal cusps displayed the greatest shape variability. Molars of males were larger than those of females (2.8 and 3.2% for upper and lower molars respectively), but no shape dimorphism was observed. Upper and lower molar sizes were significantly correlated (r² = 0.689). Allometry was observed for both teeth. Larger lower molars were associated with shorter cusps, expansion of the distal cusp, and constriction of the mesial cusps (predicted variance 3.25%). Upper molars displayed weaker allometry (predicted variance 1.59%). Upper and lower molar shape covariation proved significant (RV = 17.26%, P < 0.0001). The main parameter of molar covariation in partial least square axis 1, contributing to 30% of total covariation, was cusp height, in contrast to the primary variability traits exhibited by PC1 and PC2. The aim of this study was to evaluate shape variation and covariation, including allometry and sexual dimorphism, of maxillary and mandibular first permanent molar occlusal surfaces. Am J Phys Anthropol 152:186–196, 2013. © 2013 Wiley Periodicals, Inc.     

Study of the cuspal ridges of the upper first molars in a modern Japanese population

Materials used were dental casts of the upper first molars of modern Japanese subjects, comprising 29 males and 25 females. Their molar occlusal surfaces were photographed by moiré contourography using the standard trigonal plane. The ridges of a cusp, comprising a central ridge and mesial and distal accessory ridges, were identified from the patterns of the moiré fringes. The central ridge was observed in all cusps except for the hypocone in both sexes. Frequencies of the mesial and distal accessory ridges of trigonal cusps were over 90% except for the distal accessory ridge of the metacone, and those of the hypocone were under 25% in both sexes. These values were generally higher in males than in females, especially for the distal accessory ridge of the metacone. The running pattern of the cuspal ridges showed little difference between sexes. The oblique ridge which was higher than the central groove formed a saddle-like structure. This ridge was observed in all materials, but its heights and structural components varied remarkably. In this study, the distal accessory ridge of the metacone was found to be incorporated into the oblique ridge in about 13% of cases. Variability in the running pattern of the ridges within a single cusp was highest in the hypocone and lowest in both the paracone and protocone. The results obtained are considered to represent the stability or reductive tendency of cusps in the upper first molars.     

Genetic structure of a tribal population, the Yanomama Indians. XIII. Dental microdifferentiation.

Data are presented on the frequency of the following eight dental traits in 635 Yanomama and 65 Makiritare Indians: upper central incisor rotation or winging, shoveling of maxillary incisors, maxillary molar hypocone reduction, Carabelli's trait, mandibular molar cusp number, mandibular molar cusp pattern rotation of second lower premolar, and pattern of second lower premolar cusps. Yanomama dentition is unusual in the high frequency of six cusps on the mandibular molars. There is marked dental microdifferentiation between villages; significant agreement was observed between a matrix of pairwise "dental distances" based on six morphological traits and corresponding matrices based on 11 genetic systems and on geographic location.     

The allometry of relative cusp size in hominoid mandibular molars

The crown area (MCBA) and cusp areas of mandibular molars of Homo sapiens (M-1 = 131; M-2 = 71), Gorilla (M-1 = 25) and Pongo (M-1 = 24) were studied to determine whether the relative size of the mesial and distal cusps are related to overall crown size. Allometric trends were assessed by examining the correlation between relative cusp areas and MCBA and by calculating the slope of the regression line of log cusp area and log MCBA. With the exception of the metaconid in the Homo sapiens M-2S, the results of the intraspecific analyses provide little evidence of an allometric trend for relative reduction of the mesial cusps with increasing crown size. None of the samples provide consistent or reliable evidence of such a trend for the protoconid, nor do the M-1 samples provide evidence for such a trend for the metaconid. The evidence from the distal cusps is also mixed: positive allometry for the entoconid for the Homo sapiens M-2S and for the hypoconulid for the Homo sapiens M-1S, with no departure from isometry in either Gorilla or Pongo. The interspecific data provide no evidence of any trend for the mesial cusps to decrease or the distal cusps to increase in importance in larger teeth. If one accepts the proposition that the static allometric trends observed in this study are reasonable analogues for any allometric relationships within, or between, fossil hominid taxa, then the evidence presented above does not support the hypothesis that the reduction of the trigonid, which is observed in the "robust" australopithecines, is an allometric phenomenon.     

Expression of the entoconulid (sixth cusp) on mandibular molar teeth of an Australian aboriginal population

The expression and genetic basis of the entoconulid (sixth cusp) on mandibular molars were examined in a geographically isolated group of aboriginals from Yuendumu in the Northern Territory of Australia. Four grades of trait expression, ranging from trace to small, medium, and large cusps, were defined on dental casts of 399 subjects. Frequencies of occurrence were among the highest reported in human populations. Approximately 80% of dm2s showed the trait, whereas frequencies in the permanent dentition ranged from around 50% on M2 to 70% on M1 and 80% on M3. The degree of expression increased distally along the molar series, with only 3% of dm2s showing large cusps compared with 25% of M3s. Fluctuating asymmetry was highest for M2 and lowest for dm2. No strong evidence for sexual dimorphism in occurrence or degree of expression was found. Based on a quasi-continuous threshold model, a genetic contribution to entoconulid variability was observed that was strongest for M1. Significant associations were noted between entoconulid expression on mandibular molars and metaconule expression on maxillary molars, indicating that similar developmental mechanisms may influence these traits. The entoconulid and the metaconule both provide additional bulk on the distal occlusal surface of molar teeth, an area subjected to early wear during mastication in aboriginals.     

Carabelli's trait and tooth size of human maxillary first molars

Carabelli's trait is a morphological feature that can occur on the protocone of human maxillary molars. This study tests the hypothesis that Carabelli's trait is correlated statistically with the dimensions of the crown's four principal cusps or whether, as a cingular feature, the trait truly accretes onto an otherwise unaffected crown. Computer-assisted image analysis was used to measure the 6 intercusp distances and 12 angular relationships among cusp tips on the permanent first molar of 300 young adult American whites. Carabelli's complex was scored using an 8-grade ordinal scheme. Crown size was quantified in three ways, namely as 1) maximum mesiodistal and buccolingual diameters, 2) the 6 intercusp distances, and 3) the 12 angular cusp arrangements. There was no sex difference in the morphological expression of Carabelli's trait in this sample. Overall crown size and intercusp distances were significantly and progressively larger in molars with larger Carabelli's trait expressions. There are graded size responses between crown size (mesiodistal and buccolingual diameters), sizes of the four principal cusps, and morphological stage of Carabelli's complex, though the statistical relationships are appreciably stronger in males than females. Carabelli's trait occurs preferentially in larger molars. In contrast, angular (shape) relationships among cusp tips are not discernibly affected by trait size in either sex. There is the situation, then, that Carabelli's trait is developmentally correlated with crown size, but with no apparent alteration of cusp arrangements, suggesting that the increases are isometric across the occlusal table. Why the association is much weaker in females remains speculative, but these data provide yet another line of evidence that, within a population, tooth size is associated in a positive fashion with crown complexity.     

Molar development in common chimpanzees (Pan troglodytes)

Numerous studies have reported on enamel and dentine development in hominoid molars, although little is known about intraspecific incremental feature variation. Furthermore, a recent histological study suggested that there is little or no time between age at chimpanzee crown completion and age at molar eruption, which is unlikely given that root growth is necessary for tooth eruption. The study presented here redefines growth standards for chimpanzee molar teeth and examines variation in incremental features. The periodicity of Retzius lines in a relatively large sample was found to be 6 or 7 days. The number of Retzius lines and cuspal enamel thickness both vary within a cusp type, among cusps, and among molars, resulting in marked variation in formation time. Daily secretion rate is consistent within analogous cuspal zones (inner, middle, and outer enamel) within and among cusp types and among molar types. Significantly increasing trends are found from inner to outer cuspal enamel (3 to 5 microns/day). Cuspal initiation and completion sequences also vary, although sequences for mandibular molar cusps are more consistent. Cusp-specific formation time ranges from approximately 2 to 3 years, increasing from M1 to M2, and often decreasing from M2 to M3. These times are intermediate between radiographic studies and a previous histological study, although both formation time within cusps and overlap between molars vary considerably. Cusp-specific (coronal) extension rates range from approximately 4 to 9 microns/day, and root extension rates in the first 5 mm of roots range from 3 to 9 microns/day. These rates are greater in M1 than in M2 or M3, and they are greater in mandibular molars than in respective maxillary molars. This significant enlargement of comparative data on nonhuman primate incremental development demonstrates that developmental variation among cusp and molar types should be considered during interpretations and comparisons of small samples of fossil hominins and hominoids.     

Evolutionary dental changes.

In the evolution of primates there has been a tendency towards reduction in jaw length and prognathism, mandibular canine size and first molar cusp number, and third molar presence. These oral structures were contrasted, and compared with cranial size, body height and weight, and finger length in 118 males and 102 females of the Burlington Growth Centre. Body weight was significantly related to canine width and to jaw length and prognathism. These relationships were stronger in the males than in the females. The evolutionary reduction in these dental dimensions may result from an evolutionary reduction in genetically determined body size. In the males the number of molar cusps was related to finger length and cranial height. Agenesis of third molars was related to the length of the maxilla in both sexes. In the females, canine width was related to the number of cusps of the first molars, agenesis of third molars, and length of a finger. Simultaneous reductions in dental structures were more frequent in the females.     

Occlusal molar surfaces in females with Turner's syndrome

The aim of this study was to identify the molar occlusal features in 73 subjects with the Turner's syndrome (TS) and compared to a control group (CG) of 322 healthy females. The occlusal features were scored on dental plaster casts using the Scoring Procedures for Key Morphological Traits of the Permanent Dentition: The Arizona State University Dental Anthropology System (ASU). The results were analyzed through frequency, percentage and chi 2-test. TS subjects have more frequent reduction of cusp number, distolingual cusp on the upper molars and distal cusp on the lower molar, with the consequent reduction of the occlusal surface. Reduced size of occlusal surface and number cusps on upper molars resulted in the transformation of rhomboid occlusal shape into triangular, with the consequent loss of H-shaped groove system (in the upper right first molars H-shaped groove system was significantly less frequently found in TS (p < 0.05); in the upper left second molars H-shaped groove system was significantly less frequently found in TS (p < 0.01). The X-chromosome aneuploidy can cause a decrease in developmental homeostasis, which results in the alteration of apposition of the enamel and in consequently substantial changes of the molar occlusal morphological features.     

Morphologic characteristics of the Alaskan Eskimo dentition. III. Number of cusps on the upper permanent molars

A study of 35 coastal and 64 inland Alaskan Eskimos revealed a reduction in the number of cusps from the first to the third maxillary molar. While 97% of the first molars had four cusps, only 39.6% of the second molars and 15.2% of the third molars had that number. The reduction occurs through elimination of the hypocone. No statistically significant sex difference in the trend towards reduction in the cusp numbers was found. In the inland female group the occurrence of four cusps in the maxillary second molar was statistically higher than in the coastal female group. This may be due to a more pronounced racial admixture of white people along the coast. A similar difference, although not statistically significant, was found in the corresponding male groups. Alaskan Eskimos have a tendency towards a lower frequency of four cusps on all three maxillary molars than Aleuts. Only the second molar exhibited a statistically significant difference in this respect. A statistical evaluation revealed that in the Alaskan Eskimo maxillary first and third molars the reduction of cusps is independent of the size and form of the molars and of the suppression of the third molar. For the second molar, however, the groups with four well-developed cusps showed significantly larger buccolingual diameter.     

Metamerism,morphogenesis, and the expression of carabelli and other dental traits in humans

The patterning cascade model of tooth morphogenesis has emerged as a useful tool in explaining how tooth shape develops and how tooth evolution may occur. Enamel knots, specialized areas of dental epithelium where cusps initiate, act as signaling centers that direct the growth of surrounding tissues. For a new cusp to form, an enamel knot must form beyond the inhibition fields of other enamel knots. The model predicts that the number and size of cusps depends on the spacing between enamel knots, reflected in the spacing between cusps. Recently, work by our group demonstrated that the model predicted Carabelli trait expression in human first molars. Here we test whether differences in Carabelli trait expression along the molar row can also be predicted by the model. Crown areas and intercusp distances were measured from dental casts of 316 individuals with a digital microscope. Although absolute cusp spacing is similar in first and second molars, the smaller size and more triangular shape of second molars results in larger cusp spacing relative to size and, likely, less opportunity for the Carabelli trait to form. The presence and size of the hypocone (HY) and a range of small accessory cusps in a larger sample of 340 individuals were also found to covary with the Carabelli trait in a complex way. The results of this study lend further support to the view that the dentition develops, varies, and evolves as a single functional complex. Am J Phys Anthropol, 2013. © 2013 Wiley Periodicals, Inc.     

记江苏泗洪首次发现森林古猿类化石

本文记述了在江苏泗洪松林庄发现的一种古猿类化石,它以个体小、颊齿宽、有发达的齿带等特征有别于我国云南的腊玛古猿、西瓦古猿;它也有别于在同一地点、同一层位发现的双沟醉猿。其形态与非洲的Proconsul属接近,根据这些形态特点和它的地史分布,我们订立了一新属一新种:Platodontopithecus jianghuaiensis,地质时代为中新世。     

Three-dimensional measurement of the occlusal surfaces of lower first molars of Australian aboriginals

The occlusal surfaces of lower first molars of Australian Aboriginals were measured in three dimensions with the aid of Moiré contourography. Molar cuspal heights in this population were higher than in Japanese (Mongoloid) but lower than in Dutch (Caucasoid) populations. Intercuspal distances were considerably larger than those in the two other populations. Populational differences in occlusal features may influence both craniofacial structures and jaw movements in the three populations. Low correlations between the cuspal heights and the intercuspal distances in the other two populations were also found in this population, indicating that human molar cuspal height is independent of the transverse size of the crown. Mean values for the height of the three principal cusps in lower first molars were less than those in upper ones. However, the height of the hypoconid, which was the highest in the lower molar cusps, showed almost the same mean value as the height of the upper three principal cusps, indicating that the height of the main functional cusp, in both upper and lower first molars, was almost the same.