The Polynesian chin and mandible

We noted the similarity in mandibular profile between a 27,000-year-old skull from China and a 160-year-old Polynesian skull. Both had a more vertical ramus, a curved inferior border, and no chin. The European mandible has a greater gonial angle, an antegonial notch, and a chin. Is the presence of a chin sufficient to distinguish European mandible from Polynesian? Can the gonial angle and ramus shape do so? A blind study of fifty mandibles of both groups and both sexes revealed that each feature alone can differentiate the sexes, but the combination is required, to differentiate European and Polynesian, and loosely group their provenances.     

The influence of age,sex, population group,and dentition on the mandibular angle as measured on a South African sample

The mandibular angle is measured in physical anthropological assessments of human remains to possibly assist with the determination of sex and population affinity. The purpose of this investigation was to establish how the mandibular angle changes with age and loss of teeth among the sexes in South African population groups. The angles of 653 dried adult mandibles from the Pretoria Bone Collection were measured with a mandibulometer. Males and females of both South African whites and blacks were included. To compensate for imbalances in numbers among subgroups, type IV ANOVA testing was applied. No association was found between age and angle within either of the populations, within sexes, or within dentition groups. The angle was the most obtuse in individuals without molars and with an uneven distribution of molars, and most acute in the group with an even distribution of molars on both sides. Statistically significant differences (P < 0.001) were found in the angle between the two population groups and sexes in the overall sample as well as in the subgroup with absent molar teeth (P = 0.003 for sex, males more acute angle, and P = 0.001 for population group, blacks more acute angle), although a very large overlap existed. No significant differences could be demonstrated between the sexes or populations within the subgroups with molars. We concluded that the loss of molars, especially if complete or uneven, has a considerable effect on the mandibular angle. In the assessment of human remains, the mandibular angle is not very usable in determining sex. Am J Phys Anthropol, 2009. © 2009 Wiley‐Liss, Inc.     

拔除下颌第三磨牙对下颌角区形态影响的CBCT 研究

目的:探讨下颌第三磨牙的拔除会否造成下颌角区骨骼形态的变化以及这种变化对面型的影响。方法:30例因矫治需要拔除下颌第三磨牙的成人正畸患者,在拔牙前和拔牙后6个月拍摄CBCT片,采用三维测量软件mimics10.01测量下颌第三磨牙所在的下颌角区牙槽骨骨质的宽度、高度和长度,利用SPSS18.0统计软件分析前后数值有无差异。结果:下颌第三磨牙拔除前后下颌角区骨质宽度和高度减小,差异有统计学意义(P<0.05),下颌角区骨质前后向长度未见明显差异。结论:拔除下颌第三磨牙能从一定程度上减小下颌角区骨质高度和宽度,但是前后向长度度基本没有变化。     

Mandibular ramus flexure: A new morphologic indicator of sexual dimorphism in the human skeleton

In the skeleton, male and female characteristics lie along a continuum of morphologic configurations and metric values. Size alone is not the best indicator of sex. In contrast, morphologic differences that arise from genetically sex-linked growth and development allow better separation of the sexes. This study presents a new morphologic indicator of sexual dimorphism in the human mandible. A sample of 300 mandibles from adults of known sex primarily from the Dart collection was analyzed. Of these, 100 were found to have obvious bony pathologies and/or excessive tooth loss (“pathologic” sample). Thus, the normative sample consisted of 200 individuals (116 males, 84 females). Examination of morphologic features led to the discovery of a distinct angulation of the posterior border of the mandibular ramus at the level of the occlusal surface of the molars in adult males. Flexure appears to be a male developmental trait because it is only manifest consistently after adolescence. In most females, the posterior border of the ramus retained the straight juvenile shape. If flexure was noted, it was found to occur either at a higher point near the neck of the condyle or lower in association with gonial prominence or eversion. In the normative sample, overall prediction accuracy from ramus shape was 99%. When the “pathologic” sample was analyzed separately, 91.0% were correctly diagnosed. Because the African samples were overwhelmingly black, this trait was also tested on American samples (N = 247) of whites (N = 85), Amerinds (N = 66), and blacks (N = 96) that included a mix of healthy individuals and those with extensive tooth loss and evidence of pathology. The results were nearly identical to those of the “pathologic” African sample, with accuracies ranging from about 91% in whites and blacks to over 92% in Amerinds. Total accuracy for all African and American samples combined (N = 547) is 94.2%. In conclusion, at 99%, sexing from the shape of the ramus of a healthy mandible is on a par with accuracy attainable from a complete pelvis. Moreover, there is no record that any other single morphologic or metric indicator of sex (that has been quantified from the adult skeleton) surpasses the overall accuracy attained from the more representative mixed sample produced by combining all groups assessed in this study. The usefulness of this trait is enhanced by the survivability of the mandible and the fact that preliminary investigations show that the trait is clearly evident in fossil hominids. © 1996 Wiley-Liss, Inc.     

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.     

A note on functional implications of morphological variation in the human mandible

This study was carried out on 56 mandibles belonging to skeletal remains recovered from archaeological excavations in Israel dated to 6.000 BP. or less, 2 Neandertal mandibles dating between 50.000–60.000 BP. and 2 early H. sapiens sapiens mandibles both dating to circa 92.000 yr BP. Mandibular body length, the distance from the anterior border of the symphysis to a line bisecting the first molar (distance 1), and the distance from the line bisecting the first molar to the mandibular angle (distance 2) were measured. Distance 1, showed little variation between specimens. However, distance 2 showed a significant difference between sexes and between early and late specimens. For all specimens examined there was a low nonsignificant correlation, between the length of the mandible and distance 1, while there was a high correlation between the length of the mandibular body and distance 2. There was little or no correlation between distance 1 and 2. We propose that the human mandible, as a lever arm, can be divided into two functional parts; an anterior part which shows little change over the last 90000 years, and a posterior part which differs in accordance with the length of the mandibular corpus. These changes in distance 2 appear to correlate to changes in body size and diet, suggesting that as proposed by Hylander (1988) chewing rather than incision has played the main role in evolutionary trends of the hominid mandible. This is also in accordance with mandibular growth during development where the lengthening of the jaw takes place mostly in the posterior part by remodeling in the ramus area (Enlow, 1990) both during individual development (ontogenesis) and through evolutionary changes (phylogenesis).     

Cusp size, sexual dimorphism, and heritability of cusp size in twins.

Overall measures of mandibular molars reflect the combined size contributions of the component cusps and ridges. Until now, the size hierarchy of primary and permanent mandibular molar cusps remained unclear. This paper utilizes the relative plane surface areas (basal area dimensions) of the individual molar cusps, as assays of cusp size to demonstrate cusp size variations within populations, antimere cuspal variations, sexual dimorphism, and, the heritability of cusp size. Duplicate dental casts from 199 pairs of like-sexed twins provide the raw dats. Defined anatomic landmarks on the occlusal surfaces were reduced to X-Y rectangular coordinates prior to the computation of the basal areas dimensions. The results establish a cusp size hierarchy specific for molar type, i.e., five-cusped molars with a distal fovea and distal marginal ridge (5fd), five-cusped molars without a distal fovea and without a distal marginal ridge (5o), and four-cusped molars (4c). Sexual dimorphism in cusp size is apparent in 5fd molar cusped but not in 5o molar cusps. However, males have a significantly higher frequency of 5fd molars. Females have a higher frequency of smaller 5o and 4c molars which have fewer crown components. Moreover, female 5o molars have cusps as large as or larger than 5o male molor cusps. Right-side-left-side differences exist between antimere cusps based on relatively low correlations. The mirroring of molor types occurs infrequently. When observed, most intrapair differences for cusp size, using F-ratios, indicate a low component of hereditary variability.     

Rocker jaws

Most adult Polynesian mandibles are of the rocker form. Polynesian crania possess a very open cranial base angle and a large upper facial height. The mandibular growth rotations necessary to maintain occlusion in the presence of this cranial morphology lead to development of an exceptionally closed ramus-body angle, with consequent loss of the antegonial notch and appearance of the rocker form.     

Disproportionate sexual dimorphism in the human face

Analysis of facial dimensions of 86 young adults and their 76 parents indicates that a disproportionate sexual dimorphism exists in the ramus of the mandible, demonstrating a regional difference in growth response. The male ramus is on the average 14% longer than the female ramus, whereas other facial dimensions approximate an 8% sex difference. The findings have relevance to the analysis of skeletal remains and suggest the desirability of age specific discriminant function analysis for the sexing of adult mandibles.     

Growth-related changes in prehistoric Jomon and modern Japanese mandibles with emphasis on cortical bone distribution

Cortical bone distribution of the anthropoid mandibular symphysis has been addressed in relation to mechanical stress generated by mastication. To examine whether or not bone mass and distribution patterns of the human mandibular symphysis could be interpreted as an example of functional adaptation, we compared the skeletal growth series of two populations, prehistoric Jomon, considered to represent a "robust" mandibular morphology associated with a presumed heavier masticatory load, and modern Japanese. Results showed that the adult Jomon symphysis possessed significantly greater bone mass and thicker cortical bone compared to the modern Japanese condition. However, the second moments of area did not differ significantly between the two, indicating comparable rigidity against bending. Furthermore, the Jomon mandibles of the infant to juvenile stages exhibited most of the adult characteristics, in both bone mass/distribution of the symphysis and in mandibular corpus/ramus morphologies. The present study also demonstrated the presence of a growth pattern of symphyseal cortical thickness, common to both the Jomon and the modern Japanese series. In both populations, subsequent to deciduous molar occlusion, cortical bone tends to be thickest at the inferolingual symphysis, at the location where the highest tensile stresses presumably occur during mastication. These findings suggest that the "robust" characteristics of the Jomon mandible are initially manifested early in development, and that the effect of mechanical stimulus to bone mass formation in the human symphysis is largely confined to a regulatory role during growth modeling.     

Does space in the jaw influence the timing of molar crown initiation? A model using baboons (Papio anubis) and great apes (Pan troglodytes, Pan paniscus)

Radiographic and histological studies of baboon (Papio hamadryas, P. anubis) and chimpanzee (Pan troglodytes) permanent tooth development have found that periods of molar crown mineralization overlap markedly in chimpanzees but are staggered in baboons. Here we test the hypothesis that these intertaxon differences in molar initiation are primarily due to the space available in the mandibles of each species for these teeth. This study includes radiographic, linear measurement, and three-dimensional (3D) coordinate landmark data taken from baboon (Papio anubis n=51) and great ape (Pan paniscus n=43, P. troglodytes n=60) mandibles and permanent molars across a broad developmental range for each taxon. Unexpectedly, 3D multivariate statistical shape analysis of the molar crypt, crown, and root data shows that all three species trajectories of molar row shape change are indistinguishable from each other. Qualitative analysis of these 3D data reveals subtle and inconclusive intergeneric differences in the space maintained between adjacent molars during growth. The space distal to each newly initiated molar is slightly greater in the baboon. Bivariate analyses comparing molar row and mandibular corpus proportions in Papio and Pan fail to show clear or consistent taxonomic differences in the ratio of space afforded developing molars in the alveolar bone. Thus, there is a poor correlation between mandibular proportion and both intermolar spacing and 3D molar development pattern. Contrary to earlier studies, these results suggest that pattern of molar crown initiation and temporal overlap of adjacent mineralizing crowns is not significantly different between Papio and Pan. This may be due in part to the inclusion here of not only 3D molar crown data but also 3D molar crypt data. This study strongly refutes the hypothesis that space available in the mandible directly underlies different times of permanent molar crown initiation between Papio and Pan.     

A unique form of dental bilophodonty and a functional interpretation of peculiarities in the masticatory system of Arsinoitherium (mammalia,Embrithopoda)

The highly autapomorphic upper molar bilophodonty of the Oligocene mammal, Arsinoitherium (Embrithopoda) is an extreme form of dilambdodonty effected by lingual positioning of normally buccally situated cusps with reduction of lingual cusps. This effectively limits the molar dentition to a single phase shearing occlusal motion. Molar and premolar morphology is very different, premolars exhibiting high longitudinal ectolophs and typical two phase occlusal morphology. A double faceted mandibular condyle and angular discontinuity between lower molar and premolar dentitions is interpreted as a means of separating premolar from molar occlusion. A bifunctional masticatory system is proposed whereby efficient premolar occlusion is achieved only after a repositioning of the temporomandibular joint. Loss of phase II occlusion in the molars is compensated by maintenance of a crushing/grinding mode in the premolars. This coupled with the ability to maintain high occlusal pressures along the length of the mandible explains the unbroken dental arcade. Arsinoitheres therefore possess an extremely specialised masticatory apparatus and are interpreted as highly selective browsing herbivores.     

Metric and morphological variability in the dentition of Colobine monkeys

A morphological study was performed using 97 Colobus polykomos and 41 Nasalis larvatus. The premolars and molars of these two taxa of leaf-eating monkeys were similar in that they possessed deeply grooved and highly pointed cusps. Of 7 dental traits investigated, 4 were significantly different between the 2 forms. Indeed, with 2 of these, the position of the lingual cusp on PM₄ and the number of cusps on PM³ displayed highly contrasting structures.Metric analysis was conducted using 47 C. polykomos and 37 N. larvatus specimens. Mesiodistal and buccolingual measurements were taken on the upper canine and all postcanine teeth. Sexual dimorphism was detected in most maxillary dental measurements and all mandibular estimates of both species. In all cases, males exceeded females in tooth diameter. Testing between species by sex revealed that C. polykomos was significantly larger in several canine and premolar dimensions while N. larvatus had significantly larger molar dimensions. An additional 73 measurements were selected to represent occluding dental structures and a morphological integration analysis was conducted. Results of this analysis indicated that C. polykomos possessed a C¹-PM₃ complex which was more highly intercorrelated whereas the broad lophed N. larvatus molars constituted a more highly correlated functional unit.It was suggested that the above differences between these two groups of leaf-eating monkeys probably resulted from inbreeding, isolation and drift.     

Changes in orientation of attritional wear facets with implications for jaw motion in a mixed longitudinal sample of Propithecus edwardsi from Ranomafana National Park, Madagascar

In many mammalian species, the progressive wearing down of the teeth that occurs over an individual's lifetime has the potential to change dental function, jaw movements, or even feeding habits. The orientation of phase-I wear facets on molars reveals the direction of jaw movement during the power stroke of mastication. We investigated if and how molar wear facets change with increasing wear and/or age by examining a mixed longitudinal dataset of mandibular tooth molds from wild Propithecus edwardsi (N = 32 individuals, 86 samples). Measurements of the verticality of wear facets were obtained from three-dimensional digital models generated from μCT scans. Results show that verticality decreases over the lifetime of P. edwardsi, a change that implies an increasingly lateral translation of the jaw as the teeth move into occlusion. A more transverse phase-I power stroke supports the hypothesis that these animals chew to maximize longevity and functionality of their teeth, minimizing the "waste" of enamel, while maintaining sharp shearing crests. Results of this study indicate that wear facet verticality is more closely correlated with age than overall amount of tooth wear, measured as area of exposed dentin, suggesting that age-related changes in cranial morphology may be more responsible for adjustments in jaw motion over the lifetimes of Propithecus than wear-related changes inthe shape of occluding teeth. Finally, the rate of decrease in wear facet verticality with age is greater in males than in females suggesting differences in development and/or access to resources between the sexes in this species.     

Developmental constraints revealed by co-variation within and among molar rows in two murine rodents

SUMMARY Morphological integration corresponds to interdependency between characters that can arise from several causes. Proximal causes of integration include that different phenotypic features may share common genetic sets and/or interact during their development. Ultimate causes may be the prolonged effect of selection favoring integration of functionally interacting characters, achieved by the molding of these proximal causes. Strong and direct interactions among successive teeth of a molar row are predicted by genetic and developmental evidences. Functional constraints related to occlusion, however, should have selected more strongly for a morphological integration of occluding teeth and a corresponding evolution of the underlying developmental and genetic pathways. To investigate how these predictions match the patterns of phenotypic integration, we studied the co‐variation among the six molars of the murine molar row, focusing on two populations of house mice (Mus musculus domesticus) and wood mice (Apodemus sylvaticus). The size and shape of the three upper and lower molars were quantified and compared. Our results evidenced similar patterns in both species, size being more integrated than shape among all the teeth, and both size and shape co‐varying strongly between adjacent teeth, but also between occluding teeth. Strong co‐variation within each molar row is in agreement with developmental models showing a cascade influence of the first molar on the subsequent molars. In contrast, the strong co‐variation between molars of the occluding tooth rows confirms that functional constraints molded patterns of integration and probably the underlying developmental pathways despite the low level of direct developmental interactions occurring among molar rows. These patterns of co‐variation are furthermore conserved between the house mouse and the wood mouse that diverged >10 Ma, suggesting that they may constitute long‐running constraints to the diversification of the murine rodent dentition.     

Phylogenetic and environmental components of morphological variation: skull, mandible, and molar shape in marmots (Marmota, Rodentia)

The phenotype is a product of its phylogenetic history and its recent adaptation to local environments, but the relative importance of the two factors is controversial. We assessed the effects of diet, habitat, elevation, temperature, precipitation, body size, and mtDNA genetic divergence on shape variation in skulls, mandibles, and molars, structures that differ in their genetic and functional control. We asked whether these structures have adapted to environment to the same extent and whether they retain the same amount of phylogenetic signal. We studied these traits in intra- and interspecific populations of Eurasian marmots whose last common ancestor lived 2-5 million years ago. Path Analysis revealed that body size explained 10% of variation in skulls, 7% in mandibles, and 15% in molars. Local vegetation explained 7% of variation in skulls, 11% in mandibles, and 12% in molars. Dietary category explained 25% of variation in skulls, 11% in mandibles, and 9% in molars. Cyt b mtDNA divergence (phylogeny) explained 15% of variation in skulls, 7% in mandibles, and 5% in molars. Despite the percentages of phylogenetic variance, maximum-likelihood trees based on molar and skull shape recovered most phylogenetic groupings correctly, but mandible shape did not. The good performance of molars and skulls was probably due to different factors. Skulls are genetically and functionally more complicated than teeth, and they had more mathematically independent components of variation (5-6-in skulls compared to 3-in molars). The high proportion of diet-related variance was not enough to mask the phylogenetic signal. Molars had fewer independent components, but they also have less ecophenotypic variation and evolve more slowly, giving each component a proportionally stronger phylogenetic signal. Molars require larger samples for each operational taxonomic unit than the other structures because the proportion of within-taxon to between-taxon variation was higher. Good phylogenetic signal in quantitative skeletal morphology is likely to be found only when the taxa have a common ancestry no older than hundreds of thousands or millions of years (1% to 10% mtDNA divergence)--under these conditions skulls and molars provide stronger signal than mandibles.     

Exaggerated asymmetric head morphology of female Doubledaya bucculenta (Coleoptera: Erotylidae: Languriinae) and ovipositional preference for bamboo internodes

The lizard beetle Doubledaya bucculenta (Coleoptera: Erotylidae: Languriinae) female has evident asymmetric head morphology. The females excavate small holes in host bamboo internodes for the deposition of eggs. To understand the asymmetry and allometry of male and female adults and larvae of D. bucculenta, mandibular length, genal and head width, and elytral length were measured, and the oviposition preference for different-sized internodes of the bamboo Pleioblastus simonii and the relationship between internode size and emerging adult size were examined. Larval mandibles exhibited no clear asymmetry pattern, and genae showed fluctuating asymmetry in length. Adult male mandibles showed left-directional asymmetry, but genae showed fluctuating asymmetry. Adult female mandibles and genae exhibited marked left-directional asymmetry. The degree of asymmetry of mandibles and genae remained constant regardless of body size. Large females tended to choose large-diameter internodes of P. simonii and to lay eggs successfully, whereas small females tended to choose intermediate-diameter internodes, but to fail in oviposition, suggesting that small females pay a high cost on oviposition. There was a positive correlation between internode size and emerging adult size. Marked directional asymmetry of female mandibles and genae are discussed in relation to greater frequency of cutting bamboo fibers compared with adult males, and the traits of bamboo internode.     

青海地区藏汉族人群下颌第二磨牙根管形态的锥形束CT研究

目的:采用锥形束CT(Cone Beam CT,CBCT)技术探讨青海地区藏汉族人群下颌第二磨牙根管形态、数量以及C型根管发生率和髓腔差值的异同。方法:从2016年5月到2018年12月青大附院口腔门诊牙齿CBCT扫描结果中选择藏、汉民下颌第二磨牙各300颗,其中汉族150例,藏族150例。按照Vertucci分类对根管进行分型,探究青海地区藏、汉族人群下颌第二磨牙根管形态特点,同时对牙根类型、C型根管数量和发生率进行统计研究。结果:青海地区藏汉族人群下颌第二磨牙牙根类型以双根牙为主,且其在藏族人群中的发生率高于汉族人群。青海地区藏汉族人群下颌第二磨牙的根管分型以Ⅰ型和C型根管所占比例最高。在青海地区汉族人群和藏族人群下颌第二磨牙中,C形根管所占比率分别为40.7%和27.0%,汉族人群C型根管发生率显著高于藏族人群(P0.05)。在青海地区藏汉人群下颌第二磨牙中,女性的发生率显著高于男性(P0.05)。青海地区藏族人群的MB-DB、DB-P平均值显著高于汉族人群(P0.05)。结论:青海地区藏汉族人群下颌第二磨牙根管形态基本类似,C型根管发生率汉族人群是远远大于藏族人群的,具有一定的种族差异性。但是C型根管发生率在青海地区藏汉人群中都是女性大于男性。同时藏族人群髓腔差值也具有一定的种族差异性。     

Ontogenetic variation in the mandibular ramus of great apes and humans

Considerable variation exists in mandibular ramus form among primates, particularly great apes and humans. Recent analyses of adult ramal morphology have suggested that features on the ramus, especially the coronoid process and sigmoid notch, can be treated as phylogenetic characters that can be used to reconstruct relationships among great ape and fossil hominin taxa. Others have contended that ramal morphology is more influenced by function than phylogeny. In addition, it remains unclear how ontogeny of the ramus contributes to adult variation in great apes and humans. Specifically, it is unclear whether differences among adults appear early and are maintained throughout ontogeny, or if these differences appear, or are enhanced, during later development. To address these questions, the present study examined a broad ontogenetic sample of great apes and humans using two‐dimensional geometric morphometric analysis. Variation within and among species was summarized using principal component and thin plate spline analyses, and Procrustes distances and discriminant function analyses were used to statistically compare species and age classes. Results suggest that morphological differences among species in ramal morphology appear early in ontogeny and persist into adulthood. Morphological differences among adults are particularly pronounced in the height and angulation of the coronoid process, the depth and anteroposterior length of the sigmoid notch, and the inclination of the ramus. In all taxa, the ascending ramus of the youngest specimens is more posteriorly inclined in relation to the occlusal plane, shifting to become more upright in adults. These results suggest that, although there are likely functional influences over the form of the coronoid process and ramus, the morphology of this region can be profitably used to differentiate among great apes, modern humans, and fossil hominid taxa. J. Morphol. 275:661–677, 2014. © 2013 Wiley Periodicals, Inc.