Model of tooth morphogenesis predicts carabelli cusp expression, size, and symmetry in humans

Sex-specific effects of environmental temperature during gestation on fluctuating asymmetry in deciduous teeth

OBJECTIVES

External environmental heat exposure during gestation impacts the physiology of human development in utero, but evidence for these impacts has not yet been explored in dentition. We examined deciduous teeth for fluctuating asymmetry (FA), a measure of developmental instability, together with gestational environmental temperature data drawn from historical weather statistics.

MATERIALS AND METHODS

We measured dental casts from the longitudinal Burlington Growth Study, representing 172 participants (ages 3-6 years) with health records. FA was calculated from crown dimensions and intercuspal distances that develop during gestation. Multiple regression separated by sex (nfemale = 81) examined the effects of mean temperatures in each trimester, controlling for birth year.

RESULTS

In females, increased temperatures during the first trimester are significantly associated with an increase in FA (p = 0.03), specifically during the second and third prenatal months (p = 0.03). There is no relationship between temperature and FA for either sex in the second or third trimesters, when enamel is formed.

DISCUSSION

Dental instability may be sensitive to temperature in the first trimester in females during the scaffolding of crown shape and size in the earliest stages of tooth formation. Sexual dimorphism in growth investment strategies may explain the differences in results between males and females. Using enduring dental characteristics, these results advance our understanding of the effects of temperature on fetal physiology within a discrete period.

Evaluating predictions of the patterning cascade model of crown morphogenesis in the human lower mixed and permanent dentition

OBJECTIVE

The patterning cascade model of crown morphogenesis has been studied extensively in a variety of organisms to elucidate the evolutionary history surrounding postcanine tooth form. The current research is the first to use a large modern human sample to examine whether the crown configuration of lower deciduous and permanent molars aligns with expectations derived from the model. This study has two main goals: 1) to determine if metameric and antimeric pairs significantly differ in size, accessory trait expression, and relative intercusp spacing, and 2) assess whether the relative distance among early-forming cusps accounts for observed variation in accessory cusp expression.

METHODS

Tooth size, intercusp distance, and morphological trait expression data were collected from 3D scans of mandibular dental casts representing participants of the Harvard Solomon Islands Project. Paired tests were utilized to compare tooth size, accessory trait expression, and relative intercusp distance between diphyodont metameres and permanent antimeres. Proportional odds logistic regression was implemented to investigate how the odds of greater accessory cusp expression vary as a function of the distance between early-developing cusps.

RESULTS/SIGNIFICANCE

Comparing paired molars, significant differences were identified for tooth size and cusp 5 expression. Several relative intercusp distances emerged as important predictors of cusp 6 expression, however, results for cusp 5 and cusp 7 did not match expected patterns. These findings support previous quantitative genetic results and suggest the development of neighboring crown structures represents a zero-sum partitioning of cellular territory and resources. As such, this study contributes to a better understanding of the foundations of deciduous and permanent molar crown variation in humans.

Nutritional supplementation, tooth crown size, and trait expression in individuals from Tezonteopan, Mexico

Understanding how epigenetic factors impact dental phenotypes can help refine the use of teeth for elucidating biological relationships among human populations. We explored relationships among crown size, principal cusp spacing, and accessory cusp expression in maxillary dental casts of nutritionally supplemented (n = 34) and non-supplemented (n = 39) individuals from Tezonteopan, Mexico. We hypothesized that the non-supplemented group would exhibit smaller molar crowns and reduced intercusp spacing. Since intercusp spacing is thought to be more sensitive to epigenetic influences than crown size, we predicted that the supplemented and non-supplemented groups would differ more in the former than the latter. Previous work suggests that molar accessory cusp expression may be elevated under conditions of stress. We therefore expected evidence of greater Carabelli and Cusp 5 trait expression in the non-supplemented group. We further hypothesized that anterior teeth would be affected by nutritional stress during development, with the non-supplemented group having smaller anterior tooth crowns and therefore limited space to form the tuberculum dentale. Finally, we tested whether the presence of molar accessory traits followed predictions of the Patterning Cascade Model of tooth morphogenesis in the entire sample. Our results supported the expectation that cusp spacing would differ more than molar crown size between the two groups. Carabelli trait showed little evidence of frequency differences between groups, but some evidence of greater trait scores in the non-supplemented group. The non-supplemented group also showed evidence of greater Cusp 5 frequency and expression. In the central incisors and canines, there was strong evidence for smaller crown sizes and reduced tuberculum dentale frequency in the non-supplemented group. With both groups pooled together, there was strong evidence of closer mesiodistal distances among principal cusps in molars with accessory cusps, a finding that is consistent with the PCM. Overall, our findings suggest that nutritional stress may affect accessory cusp expression.

Variability in molar crown morphology and cusp wear in two Western chimpanzee populations

OBJECTIVES

Chimpanzees (Pan troglodytes) possess a relatively generalized molar morphology allowing them to access a wide range of foods. Comparisons of crown and cusp morphology among the four subspecies have suggested relatively large intraspecific variability. Here, we compare molar crown traits and cusp wear of two geographically close populations of Western chimpanzees, P. t. verus, to provide further information on intraspecific dental variability.

MATERIALS AND METHODS

Micro-CT reconstructions of high-resolution replicas of first and second molars of two Western chimpanzee populations from Ivory Coast (Taï National Park) and Liberia, respectively were used for this study. First, we analyzed projected tooth and cusp 2D areas as well as the occurrence of cusp six (C6) on lower molars. Second, we quantified the molar cusp wear three-dimensionally to infer how the individual cusps alter with advancing wear.

RESULTS

Both populations are similar in their molar crown morphology, except for a higher appearance rate of a C6 in Taï chimpanzees. In Taï chimpanzees, lingual cusps of upper molars and buccal cusps of lower molars possess an advanced wear pattern compared to the remaining cusps, while in Liberian chimpanzees this wear gradient is less pronounced.

DISCUSSION

The similar crown morphology between both populations fits with previous descriptions for Western chimpanzees and provides additional data on dental variation within this subspecies. The wear pattern of the Taï chimpanzees are in concordance with their observed tool rather than tooth use to open nuts/seeds, while the Liberian chimpanzees may have consumed hard food items crushed between their molars.

Genetic Correlation, Pleiotropy, and Molar Morphology in a Longitudinal Sample of Australian Twins and Families

This study aims to expand our understanding of the genetic architecture of crown morphology in the human diphyodont dentition. Here, we present bivariate genetic correlation estimates for deciduous and permanent molar traits and evaluate the patterns of pleiotropy within (e.g., m1–m2) and between (e.g., m2–M1) dentitions. Morphology was observed and scored from dental models representing participants of an Australian twin and family study (deciduous n = 290, permanent n = 339). Data collection followed Arizona State University Dental Anthropology System standards. Genetic correlation estimates were generated using maximum likelihood variance components analysis in SOLAR v.8.1.1. Approximately 23% of deciduous variance components models and 30% of permanent variance components models yielded significant genetic correlation estimates. By comparison, over half (56%) of deciduous–permanent homologues (e.g., m2 hypocone–M1 hypocone) were significantly genetically correlated. It is generally assumed that the deciduous and permanent molars represent members of a meristic molar field emerging from the primary dental lamina. However, stronger genetic integration among m2–M1/M2 homologues than among paired deciduous traits suggests the m2 represents the anterior-most member of a “true” molar field. The results indicate genetic factors act at distinct points throughout development to generate homologous molar form, starting with the m2, which is later replaced by a permanent premolariform crown.

The protoconid: a key cusp in lower molars. Evidence from a recent modern human population

BACKGROUND

The molar (M) size sequence in the genus Homo is decreasing and the general pattern in Homo sapiens is M1 > M2 > M3.

AIM

To gain a better understanding of the reduction patterns of molar components (cusps), we aim to assess the area of the protoconid, the phylogenetically oldest cusp of the lower molars.

SUBJECT AND METHODS

We measured the protoconid and the total crown area in the scaled photographs of a recent modern human sample of lower molars (76 males and 39 females). The values were statistically analysed.

RESULTS

The absolute size of the protoconid increases significantly between M1 and M2/M3, whereas the relative size of this cusp increases significantly from M1 to M3. In the latter, reduction or disappearance of the cusps of the talonid is common.

CONCLUSIONS

The results can be explained in the framework of the patterning cascade model. As the first cusp to appear developmentally, the protoconid forms in response to signals from the primary enamel knot, likely contributing to its stability. Inhibitory signals emitted during the protoconid formation may lead to the reduction or disappearance of the talonid cusps, if these do not have enough time to form before the end of the molar morphogenetic process.

The genetic architecture of anterior tooth morphology in a longitudinal sample of Australian twins and families

OBJECTIVE

This study presents a quantitative genetic analysis of human anterior dental morphology in a longitudinal sample of known genealogy. The primary aim of this work is to generate a suite of genetic correlations within and between deciduous and permanent characters to access patterns of integration across the diphyodont dental complex.

DESIGN

Data were recorded from casted tooth crowns representing participants of a long-term Australian twin and family study (deciduous n = 290, permanent n = 339). Morphological trait expression was observed and scored following Arizona State University Dental Anthropology System standards. Bivariate genetic correlations were estimated using maximum likelihood variance decomposition models in SOLAR v.8.1.1.

RESULTS

Genetic correlation estimates indicate high levels of integration between antimeres but low to moderate levels among traits within a tooth row. Only 9% of deciduous model comparisons were significant, while pleiotropy was indicated for one third of permanent trait pairs. Canine characters stood out as strongly integrated, especially in the deciduous dentition. For homologous characters across dentitions (e.g., deciduous i¹ shoveling and permanent I¹ shoveling), ∼70% of model comparisons yielded significant genetic correlations.

CONCLUSIONS

Patterns of genetic correlation suggest a morphological canine module that spans the primary and secondary dentition. Results also point to the existence of a genetic mechanism conserving morphology across the diphyodont dental complex, such that paired deciduous and permanent traits are more strongly integrated than characters within individual tooth rows/teeth.

Accessory cusp expression at the enamel-dentine junction of hominin mandibular molars

Studies of hominin dental morphology frequently consider accessory cusps on the lower molars, in particular those on the distal margin of the tooth (C6 or distal accessory cusp) and the lingual margin of the tooth (C7 or lingual accessory cusp). They are often utilized in studies of hominin systematics, where their presence or absence is assessed at the outer enamel surface (OES). However, studies of the enamel-dentine junction (EDJ) suggest these traits may be more variable in development, morphology and position than previously thought. Building on these studies, we outline a scoring procedure for the EDJ expression of these accessory cusps that considers the relationship between these accessory cusps and the surrounding primary cusps. We apply this scoring system to a sample of Plio-Pleistocene hominin mandibular molars of Paranthropus robustus, Paranthropus boisei, Australopithecus afarensis, Australopithecus africanus, Homo sp., Homo habilis and Homo erectus from Africa and Asia (n = 132). We find that there are taxon-specific patterns in accessory cusp expression at the EDJ that are consistent with previous findings at the OES. For example, P. robustus M₁s and M₂s very often have a distal accessory cusp but no lingual accessory cusp, while H. habilis M₁s and M₂s show the opposite pattern. The EDJ also reveals a number of complicating factors; some apparent accessory cusps at the enamel surface are represented at the EDJ only by shouldering on the ridges associated with the main cusps, while other accessory cusps appear to have little or no EDJ expression at all. We also discuss the presence of double and triple accessory cusps, including the presence of a double lingual accessory cusp on the distal ridge of the metaconid in the type specimen of H. habilis (OH 7–M₁) that is not clear at the OES due to occlusal wear. Overall, our observations, as well as our understanding of the developmental underpinnings of cusp patterning, suggest that we should be cautious in our comparisons of accessory cusps for taxonomic interpretations.

Prevalence of talon cusp: Systematic literature review, meta-analysis and new scoring system

OBJECTIVES

Talon cusp is a developmental anomaly consisting of a vertical ridge or cusp projecting labially or lingually from an anterior permanent or primary tooth. A range of prevalence between 0.06 % and 40.8 % has been reported in the literature. Although many epidemiological studies have been conducted, no synthesis of these data has been performed to date. The aim of this paper was to determine the prevalence of talon cusp and to investigate the link between sex and talon cusp.

SOURCE

A systematic search was undertaken using the PubMed/Medline, Embase and Cochrane Library electronic databases for publications from 1981 to 2020 investigating the prevalence of talon cusp and the link between talon cusp and sex.

STUDY SELECTION

Three reviewers selected the studies independently, extracted the data in accordance with the PRISMA statement, and assessed the risk of bias via the GRADE & Cochrane approaches.

RESULTS

From 39 potentially eligible studies, 9 were selected for full text analysis and 8 were included in the meta-analysis, representing 35,224 participants. The meta-analyses were performed with a random model, calculating a weighted-mean prevalence (at least one talon cusp per individual) of 1.67 %. Sex and talon cusp were not statistically significantly associated in our study (OR = 1.10; 95 % CI [0.82-1.47]; p > 0.05).

DISCUSSION

We report the first prevalence of talon cusp provided by meta-analysis in a non-syndromic sample. A non-statistically significant association between sex and talon cusp is relevant in terms of genetic etiology. In order to improve the homogeneity and accuracy of the results of further studies, we propose a new universal scoring system for talon cusp.

CLINICAL SIGNIFICANCE

Talon cusp is a developmental anomaly with various clinical implications. Therapeutics may vary from simple monitoring to extraction of the supporting tooth. This condition occurs in approximately 1.67 % of the population.

Testing the inhibitory cascade model in the Middle Pleistocene Sima de los Huesos (Sierra de Atapuerca, Spain) hominin sample

The Middle Pleistocene Sima de los Huesos (SH) site has yielded more than 7.500 human fossil remains belonging to a minimum of 29 individuals. Most of these individuals preserve either the complete mandibular molar series or at least the first (M₁ ) and second (M₂ ) molars. The inhibitory cascade mathematical model was proposed by Kavanagh et al. (Nature, 449, 427-433 [2007]) after their experimental studies on the dental development of murine rodent species. The activator-inhibitor mechanism of this model has shown its ability for predicting evolutionary size patterns of mammalian teeth, including hominins. The main aim of this study is to test whether the size molar patterns observed in the SH hominins fit the inhibitory cascade model. With this purpose, we have measured the crown area of all SH molars in photographs, using a planimeter and following techniques used and well contrasted in previous works. Following one of the premises of the inhibitory cascade model, we expect that the central tooth (M₂ in our case) of a triplet would have the average size of the two outer teeth. The absolute difference between the observed and the expected values for the M₂ s ranges from 0.23 to 8.46 mm² in the SH sample. In terms of percentage, the difference ranges between 0.25% and 10.34%, although in most cases, it is below 5%. The plot of the estimated M₃ /M₁ and M₂ /M₁ size ratios obtained in the SH hominins occupies a small area of the theoretical developmental morphospace obtained for rodent species. In addition, the majority of the values are placed near the theoretical line which defines the relationship predicted by the inhibitory cascade model in these mammals. The values of the slope and intercept of the reduced major regression obtained for the SH individuals do not differ significantly from those obtained for rodent species, thus confirming that the size of the molars of the SH hominins fits the inhibitory cascade model. We discuss these results in terms of dental development. Despite the promising results in the SH sample, we draw the attention to the fact that most Early Pleistocene Homo specimens exhibit a pattern (M₁  < M₂  > M₃ ), which is outside the expected theoretical morphospace predicted by the inhibitory cascade model. The shift from the M1 < M2 < M3 size relationship observed in early hominins (including H. habilis) to the M1 > M2 > M3 size relationship, which is predominant in modern humans, includes sequences that depart from predictions of the inhibitory cascade model. Additional studies are required to understand these deviations.

Neutral evolution of human enamel-dentine junction morphology

Teeth have been studied for decades and continue to reveal information relevant to human evolution. Studies have shown that many traits of the outer enamel surface evolve neutrally and can be used to infer human population structure. However, many of these traits are unavailable in archaeological and fossil individuals due to processes of wear and taphonomy. Enamel-dentine junction (EDJ) morphology, the shape of the junction between the enamel and the dentine within a tooth, captures important information about tooth development and vertebrate evolution and is informative because it is subject to less wear and thus preserves more anatomy in worn or damaged specimens, particularly in mammals with relatively thick enamel like hominids. This study looks at the molar EDJ across a large sample of human populations. We assessed EDJ morphological variation in a sample of late Holocene modern humans (n = 161) from archaeological populations using μ-CT biomedical imaging and geometric morphometric analyses. Global variation in human EDJ morphology was compared to the statistical expectations of neutral evolution and "Out of Africa" dispersal modeling of trait evolution. Significant correlations between phenetic variation and neutral genetic variation indicate that EDJ morphology has evolved neutrally in humans. While EDJ morphology reflects population history, its global distribution does not follow expectations of the Out of Africa dispersal model. This study increases our knowledge of human dental variation and contributes to our understanding of dental development more broadly, with important applications to the investigation of population history and human genetic structure.

A functional analysis of Carabelli trait in Australian aboriginal dentition

OBJECTIVES

Carabelli is a nonmetric dental trait variably expressed as a small pit to a prominent cusp in the maxillary molars of modern humans. Investigations on the occurrence and expression rates of this trait have been conducted extensively, tracing its origin to genetic sources. However, there remains a lack of understanding about its potential role in chewing. In this study, we examine molar macrowear with the aim of reconstructing Carabelli trait occlusal dynamics occurring during chewing.

METHODS

We have examined 96 deciduous and permanent maxillary molars of children and young adults from Yuendumu, an Australian Aboriginal population that was at an early stage of transition from a nomadic and hunter-gatherer way of life to a more settled existence. We apply a well-established method, called Occlusal Fingerprint Analysis, which is a digital approach for analyzing dental macrowear allowing the reconstruction of jaw movements required to produce wear pattern specific to each tooth.

RESULTS

Carabelli trait slightly enlarges the surface functional area, especially in those molars where this feature is expressed in its cuspal form and it is closer to the occlusal plane. Moreover, the highly steep contact planes would also indicate that Carabelli wear areas contribute to increasing the shearing abilities of the occluded teeth, which are particularly important when processing fibrous and tough foods.

CONCLUSIONS

The macrowear analysis suggests that Carabelli trait in the Aboriginal people from Yuendumu slightly enhanced occlusion and probably played some functional role during mastication. Future biomechanical and microwear analyses could provide additional information on the mechanical adaptation of Carabelli trait in modern human dentition.

The functional role of the Carabelli trait in early and late hominins

The Carabelli trait is a dental feature that forms along the lingual margin of the protocone of deciduous and permanent maxillary molars. It is variably expressed, ranging from a small pit or furrow to a large cusp, and its development seems to be associated with crown size and molar cusp spatial configuration. The degree of expression of the Carabelli trait differs systematically between hominin taxa, and for this reason, it has been used extensively in the reconstruction of their phylogeny. However, the functional implications of having a large Carabelli trait remain unclear. In this study, we analyze the macrowear pattern of maxillary molars of early and late hominins using the occlusal fingerprint analysis method, an approach based on digital models of teeth that helps in reconstructing occlusal dynamics occurring during mastication. Tooth crowns with a small Carabelli cusp generally exhibit larger wear contact areas that extended cervically, while two additional new occlusal contact areas are common in teeth characterized by a large Carabelli cusp. These wear areas are created at the beginning of the chewing cycle, when occluding with the slopes of the lingual groove of the lower molars, between the metaconid and entoconid cusps. Advancing tooth wear leads to a slight enlargement of Carabelli occlusal contacts increasing their functional area. A steep inclination could be mechanically important in food reduction and in balancing the functional load distribution during mastication contacts. Steep wear areas are particularly developed in primates that process foods characterized by tough and fibrous textural properties. Future biomechanical and microwear texture analyses could provide additional information on the mechanical adaptation of this dental trait.

Influence of long and short arms of X chromosome on maxillary molar crown morphology

Although genes on the human X chromosome reportedly influence tooth crown morphology, little is known about X chromosome activation or inactivation systems relevant to morphological variations. We assessed the relationships between tooth crown size and crown morphological traits in females with Turner syndrome, the variants of which include complete absence of one X chromosome, lack of the short arm (Xp), or duplication of the long arms (Xq), and then estimated the functions of Xp and Xq in the process of unilateral X chromosome inactivation during tooth crown development. The mesiodistal and buccolingual diameters in the maxillary first (M1) and second (M2) permanent molars were compared among X chromosome karyotypes by multiple regression analyses, and their relationships with the development of Carabelli’s cusp and the distolingual cusp were analyzed using logistic regression analysis.

The crown sizes increased in the order of the 46,X,i(Xq) karyotype, 45,X and 45,X/46,XX karyotypes, and control group. A lower frequency of Carabelli’s cusp and higher frequency of a reduced distolingual cusp in M1 were characteristics of Turner syndrome. The overall M1 and M2 crown sizes differed among the X chromosome karyotypes, whereas a smaller crown size was associated with a reduced distolingual cusp but not with Carabelli’s cusp. Considering the differences in chromosome arrangement among females with Turner syndrome and the process of unilateral X chromosome inactivation, the observed results can be considered in terms of quantity or number of promoters/inhibitors during tooth crown development.

The X chromosome karyotypes have a strong influence on the overall crown sizes of the M1 and M2 molars because those karyotypes with variable numbers of active gene regions directly influence tooth germ development in an early stage of human odontogenesis. The later forming cusps, such as the distolingual cusp and Carabelli’s cusp, may be affected by this developmental prerequisite.

Illuminating the processes of microevolution: A bioarchaeological analysis of dental non-metric traits from Armenian Highland

Non-metric dental traits provide useful information for assessing temporal changes as well as for assessing biological relationships among living and ancient populations. Dental morphological traits were employed in this study as direct indicators of biological affinities among the populations that inhabited the Armenian Highland from the Late Chalcolithic-Early Bronze Age to Modern times. Sixteen morphological features in 2643 permanent adult teeth from 41 samples coming from 5 areas within the Armenian Highland were scored. Both Zubov's standard protocol and Turner's ASUDAS were employed. Given the paucity of odontological data for this area, this study contributes to the dental non-metric traits' history of Armenian Highland and is a summary compilation and comparison of previously conducted work where non-metric traits were used in relation to dental reductions within the ancient Caucasus and Near East. The chronological sequence is considerable and spans from the Late Chalcolithic-Bronze Age to the Modern Age. It is suggested that offspring of ancient inhabitants of Armenian Highland continued to inhabit this area during the Late Iron Age, Classical/Late Antiquity, Middle Age and Modern period. This scenario indicates genetic continuity and gene flow between populations. Such a perspective is supported by the archaeological and molecular findings.

Genetic Evidence Supporting the Role of the Calcium Channel, CACNA1S, in Tooth Cusp and Root Patterning

In this study, we report a unique dominantly inherited disorganized supernumerary cusp and single root phenotype presented by 11 affected individuals belonging to 5 north-eastern Thai families. Using whole exome sequencing (WES) we identified a common single missense mutation that segregates with the phenotype in exon 6 of CACNA1S (Ca_v1.1) (NM_000069.2: c.[865A > G];[=] p.[Ile289Val];[=]), the Calcium Channel, Voltage-Dependent, L Type, Alpha-1s Subunit, OMIM ^∗ 114208), affecting a highly conserved amino-acid isoleucine residue within the pore forming subdomain of CACNA1S protein. This is a strong genetic evidence that a voltage-dependent calcium ion channel is likely to play a role in influencing tooth morphogenesis and patterning.

Evo-devo models of tooth development and the origin of hominoid molar diversity

The detailed anatomical features that characterize fossil hominin molars figure prominently in the reconstruction of their taxonomy, phylogeny, and paleobiology. Despite the prominence of molar form in human origins research, the underlying developmental mechanisms generating the diversity of tooth crown features remain poorly understood. A model of tooth morphogenesis-the patterning cascade model (PCM)-provides a developmental framework to explore how and why the varying molar morphologies arose throughout human evolution. We generated virtual maps of the inner enamel epithelium-an indelibly preserved record of enamel knot arrangement-in 17 living and fossil hominoid species to investigate whether the PCM explains the expression of all major accessory cusps. We found that most of the variation and evolutionary changes in hominoid molar morphology followed the general developmental rule shared by all mammals, outlined by the PCM. Our results have implications for the accurate interpretation of molar crown configuration in hominoid systematics.

Homology, homoplasy and cusp variability at the enamel-dentine junction of hominoid molars

Evolutionary studies of mammalian teeth have generally concentrated on the adaptive and functional significance of dental features, whereas the role of development on phenotypic generation and as a source of variation has received comparatively little attention. The present study combines an evolutionary biological framework with state-of-the-art imaging techniques to examine the developmental basis of variation of accessory cusps. Scholars have long used the position and relatedness of cusps to other crown structures as a criterion for differentiating between developmentally homologous and homoplastic features, which can be evaluated with greater accuracy at the enamel-dentine junction (EDJ). Following this approach, we collected digital models of the EDJ and outer enamel surface of more than 1000 hominoid teeth to examine whether cusp 5 of the upper molars (UM C5) and cusps 6 and 7 of the lower molars (LM C6 and LM C7) were associated each with a common developmental origin across species. Results revealed that each of these cusps can develop in a variety of ways, in association with different dental tissues (i.e. oral epithelium, enamel matrix) and dental structures (i.e. from different cusps, crests and cingula). Both within and between species variability in cusp origin was highest in UM C5, followed by LM C7, and finally LM C6. The lack of any species-specific patterns suggests that accessory cusps in hominoids are developmentally homoplastic and that they may not be useful for identifying phylogenetic homology. An important and unanticipated finding of this study was the identification of a new taxonomically informative feature at the EDJ of the upper molars, namely the post-paracone tubercle (PPT). We found that the PPT was nearly ubiquitous in H. neanderthalensis and the small sample of Middle Pleistocene African and European humans (MPAE) examined, differing significantly from the low frequencies observed in all other hominoids, including Pleistocene and recent H. sapiens. We emphasize the utility of the EDJ for human evolutionary studies and demonstrate how features that look similar at the external surface may be the product of different developmental patterns. This study also highlights the importance of incorporating both developmental and morphological data into evolutionary studies in order to gain a better understanding of the evolutionary significance of dental and skeletal features.

Internal Tooth Structure and Burial Practices: Insights into the Neolithic Necropolis of Gurgy (France, 5100-4000 cal. BC)

Variations in the dental crown form are widely studied to interpret evolutionary changes in primates as well as to assess affinities among human archeological populations. Compared to external metrics of dental crown size and shape, variables including the internal structures such as enamel thickness, tissue proportions, and the three-dimensional shape of enamel-dentin junction (EDJ), have been described as powerful measurements to study taxonomy, phylogenetic relationships, dietary, and/or developmental patterns. In addition to providing good estimate of phenotypic distances within/across archeological samples, these internal tooth variables may help to understand phylogenetic, functional, and developmental underlying causes of variation. In this study, a high resolution microtomographic-based record of upper permanent second molars from 20 Neolithic individuals of the necropolis of Gurgy (France) was applied to evaluate the intrasite phenotypic variation in crown tissue proportions, thickness and distribution of enamel, and EDJ shape. The study aims to compare interindividual dental variations with burial practices and chronocultural parameters, and suggest underlying causes of these dental variations. From the non-invasive characterization of internal tooth structure, differences have been found between individuals buried in pits with alcove and those buried in pits with container and pits with wattling. Additionally, individuals from early and recent phases of the necropolis have been distinguished from those of the principal phase from their crown tissue proportions and EDJ shape. The results suggest that the internal tooth structure may be a reliable proxy to track groups sharing similar chronocultural and burial practices. In particular, from the EDJ shape analysis, individuals buried in an alcove shared a reduction of the distolingual dentin horn tip (corresponding to the hypocone). Environmental, developmental and/or functional underlying causes might be suggested for the origin of phenotypic differences shared by these individuals buried in alcoves.

Prevalence of a few variant dental features in children aged 11-16 years in Davangere, a city in Karnataka

CONTEXT

Variations in morphology of shape of teeth have always been of interest to dentists from ancient times. But to our surprise, till date, no studies related to the prevalence of dental features have been conducted in any part of the world.

AIMS

To evaluate the prevalence of a few variant dental features in a group of children aged from 11 to 16 years in the city of Davangere that belongs to the state of Karnataka, India.

MATERIALS AND METHODS

A cross-sectional survey was conducted where children aged 11-16 years were selected (both girls and boys) and type III clinical examination was done. They were checked for the following features - Carabelli's cusp, 3-cusped maxillary 2(nd) molar, 5-cusped maxillary 1(st) molar, 4-cusped mandibular 1(st) molar, 5-cusped mandibular 2(nd) molar, cusp 6 present in mandibular 1(st) molar, and 7-cusped mandibular 1(st) molar.

STATISTICAL ANALYSIS USED

The Chi-square test was used to analyze the categorical data. P value of 0.05 or less was considered for statistical significance.

RESULTS

Around 99.3% of the school children examined had at least one of the dental variations that were examined in relation to the shape of teeth.

CONCLUSIONS

This study definitely provides us with baseline data, but further epidemiological studies are required to determine the prevalence of the above mentioned dental anomalies.

Size and Shape Variability in Human Molars during Odontogenesis

Under the patterning cascade model (PCM) of cusp development inspired by developmental genetic studies, it is predicted that the location and the size of later-forming cusps are more variable than those of earlier-forming ones. Here we assessed whether differences in the variability among cusps in total and each particular crown component (enamel-dentin junction [EDJ], outer enamel surface [OES], and cement-enamel junction [CEJ]) could be explained by the PCM, using human maxillary permanent first molars (UM1) and second deciduous molars (um2). Specimens were µCT-scanned, and 3D models of EDJ and OES were reconstructed. Based on these models, landmark-based 3D geometric morphometric analyses were conducted. Size variability in both tooth types was generally consistent with the above prediction, and the differences in size variation among cusps were smaller for the crown components completed in later stages of odontogenesis. With a few exceptions, however, the prediction was unsupported regarding shape variability, and UM1 and um2 showed different patterns. Our findings suggested that the pattern of size variability would be caused by temporal factors such as the order of cusp initiation and the duration from the beginning of mineralization to the completion of crown formation, whereas shape variability may be affected by both topographic and temporal factors.

Carabelli's trait in Croatian populations over 1800 years

Examination and comparison of the morphological features of tooth crown in archaeological and recent samples can be difficult due to the different levels of tooth wear seen both within and between populations. These differences make the comparison of frequency data for Carabelli trait problematic. The aim of the present study is to detect the frequency and degree of expression of Carabelli's trait in Croatian populations from late antiquity to recent times and to use these data as supplementary evidence of complex population migration. A total of 1287 individuals from the late antiquity, medieval, early modern and modern periods were examined. Correlation between the presence of Carabelli's trait and tooth crown size was tested. The results of our analyses show that the frequency of Carabelli's trait is significantly greater in the early modern period (51.3%) and in the 21st century (43.1%) than in the late antiquity (20.4%) and medieval periods (23.4%). These results are consistent with historical evidence of migration and population change in the territory of present-day Croatia throughout the almost 1800 years covered by this study. The results also provide additional evidence for the complex nature of population change in the transition from the late antiquity to the early medieval period.

Dental occlusion in a split Amazon indigenous population: genetics prevails over environment

Background

Studies examining human and nonhuman primates have supported the hypothesis that the recent increase in the occurrence of misalignment of teeth and/or incorrect relation of dental arches, named dental malocclusion, is mainly attributed to the availability of a more processed diet and the reduced need for powerful masticatory action. For the first time on live human populations, genetic and tooth wear influences on occlusal variation were examined in a split indigenous population. The Arara-Iriri people are descendants of a single couple expelled from a larger village. In the resultant village, expansion occurred through the mating of close relatives, resulting in marked genetic cohesion with substantial genetic differences.

Methodology/Principal Findings

Dental malocclusion, tooth wear and inbreeding coefficient were evaluated. The sample examined was composed of 176 individuals from both villages. Prevalence Ratio and descriptive differences in the outcomes frequency for each developmental stage of the dentition were considered. Statistical differences between the villages were examined using the chi-square test or Fisher's exact statistic. Tooth wear and the inbreeding coefficient (F) between the villages was tested with Mann-Whitney statistics. All the statistics were performed using two-tailed distribution at p≤0.05. The coefficient inbreeding (F) confirmed the frequent incestuous unions among the Arara-Iriri indigenous group. Despite the tooth wear similarities, we found a striking difference in occlusal patterns between the two Arara villages. In the original village, dental malocclusion was present in about one third of the population; whilst in the resultant village, the occurrence was almost doubled. Furthermore, the morphological characteristics of malocclusion were strongly different between the groups.

Conclusions/Significance

Our findings downplay the widespread influence of tooth wear, a direct evidence of what an individual ate in the past, on occlusal variation of living human populations. They also suggest that genetics plays the most important role on dental malocclusion etiology.