Dental Morphological Variation Research Articles

Variation in dental morphology and dietary breadth in primates and their kin

Abstract Sympatric animals may consume diets of differing breadths as a strategy to reduce competition. Studying patterns of dietary breadth in extinct taxa is difficult because available data are generally limited to morphology. Dental topographic analysis (DTA) is useful for comparing occlusal morphology and allows for examination of dietary adaptations in extinct taxa. What remains unknown is how dental morphology, quantified using DTA, covaries with dietary breadth. The niche variation hypothesis (NVH) posits that taxa with broader ecological niches will be characterized by greater variability in morphology relative to specialized taxa. Therefore, we predict that taxa with greater dietary breadth will have more varied dental morphology compared to specialists as a result of the molar morphology of specialists being under greater genetic control relative to generalists, with specialists requiring teeth specially adapted to efficiently process a smaller range of food sources. We measured curvature, complexity, and relief of the M2 of 3 pairs of closely related euarchontan mammals (primates and treeshrews), with each taxon within a pair categorized as a generalist or specialist. Our results indicate that generalists do not consistently show greater variability in dental morphology compared to specialists among primates, but that atelids and treeshrews do generally follow the predictions of the NVH, with the caveat that our treeshrew sample is small. This suggests that while dietary specialists may be under greater genetic constraint with respect to their molar topography, a link between dietary breadth and dental form is not clear. Our study demonstrates that variation in dental topography does not necessarily reflect dietary breadth and highlights the fact that it is difficult to categorize even the most specialized primates (i.e. bamboo lemurs) as “dietary specialists.”

Peopling of the Americas: A new approach to assessing dental morphological variation in Asian and Native American populations.

Through biodistance analyses, anthropologists have used dental morphology to elucidate how people moved into and throughout the Americas. Here, we apply a method that focuses on individuals rather than sample frequencies through the application rASUDAS2, based on a naïve Bayes' algorithm. Using the database of C.G. Turner II, we calculated the probability that an individual could be assigned to one of seven biogeographic groups (American Arctic, North & South America, East Asia, Southeast Asia & Polynesia, Australo-Melanesia, Western Eurasia, & Sub-Saharan Africa) through rASUDAS2. The frequency of classifications for each biogeographic group was determined for 1418 individuals from six regions across Asia and the Americas. Southeast Asians show mixed assignments but rarely to American Arctic or "American Indian." East Asians are assigned to East Asia half the time while 30% are assigned as Native American. People from the American Arctic and North & South America are assigned to Arctic America or non-Arctic America 75%-80% of the time, with 10%-15% classified as East Asian. All Native American groups have a similar degree of morphological affinity to East Asia, as 10%-15% are classified as East Asian. East Asians are classified as Native American in 30% of cases. Individuals in the Western Hemisphere are decreasingly classified as Arctic the farther south they are located. Equivalent levels of classification as East Asian across all Native American groups suggests one divergence between East Asians and the population ancestral to all Native Americans. Non-arctic Native American groups are derived from the Arctic population, which represents the Native American founder group.

Dental morphological variation in Chalcolithic and Bronze Age human populations from North-Eastern Romania

BackgroundThis study aims to evaluate the dental variability and the phenetic relationships among archaeological human populations of the Chalcolithic and Bronze Age in North-Eastern Romania. MethodsThe evaluation of phenotypic variability was performed using the 2D Geometric Morphometrics on the second molars (upper – M2, and lower – M2) belonging to adult skeletons discovered in archaeological sites of the Chalcolithic and Bronze Age in North-Eastern Romania. ResultsThe size (Centroid Size, CS) and shape analysis of the second molar highlighted specific characteristics for each prehistoric period. The CS doesn’t achieve any statistical significance for two of the comparison criterium used (i.e., period, archaeological site). Instead, statistically significant differences were recorded between males and females. The greater variability in the shape of M2 compared to M2 was noticed. The M2 model of the Bronze Age is flattened mesiodistal compared to the Chalcolithic one, while differences in M2 morphotypes are associated with the hypocone shape, particularly. ConclusionThis study of dental morphology provides new data that could contribute as evidence of interactions between biological human groups in different regional prehistoric contexts.

Range-wide variation in grey seal (Halichoerus grypus) skull morphology

The large interspecific variation in marine mammal skull and dental morphology reflects ecological specialisations to foraging and communication. At the intraspecific level, the drivers of skull shape variation are less well understood, having implications for identifying putative local foraging adaptations and delineating populations and subspecies for taxonomy, systematics, management and conservation. Here, we assess the range-wide intraspecific variation in 71 grey seal skulls by 3D surface scanning, collection of cranial landmarks and geometric morphometric analysis. We find that skull shape differs slightly between populations in the Northwest Atlantic, Northeast Atlantic and Baltic Sea. However, there was a large shape overlap between populations and variation was substantially larger among animals within populations than between. We hypothesize that this pattern of intraspecific variation in grey seal skull shape results from balancing selection or phenotypic plasticity allowing for a remarkably generalist foraging behaviour. Moreover, the large overlap in skull shape between populations implies that the separate subspecies status of Atlantic and Baltic Sea grey seals is questionable from a morphological point of view.

Diet drove brain and dental morphological coevolution in strepsirrhine primates.

The evolution of the remarkably complex primate brain has been a topic of great interest for decades. Multiple factors have been proposed to explain the comparatively larger primate brain (relative to body mass), with recent studies indicating diet has the greatest explanatory power. Dietary specialisations also correlate with dental adaptations, providing a potential evolutionary link between brain and dental morphological evolution. However, unambiguous evidence of association between brain and dental phenotypes in primates remains elusive. Here we investigate the effect of diet on variation in primate brain and dental morphology and test whether the two anatomical systems coevolved. We focused on the primate suborder Strepsirrhini, a living primate group that occupies a very wide range of dietary niches. By making use of both geometric morphometrics and dental topographic analysis, we extend the study of brain-dental ecomorphological evolution beyond measures of size. After controlling for allometry and evolutionary relatedness, differences in brain and dental morphology were found between dietary groups, and brain and dental morphologies were found to covary. Historical trajectories of morphological diversification revealed a strong integration in the rates of brain and dental evolution and similarities in their modes of evolution. Combined, our results reveal an interplay between brain and dental ecomorphological adaptations throughout strepsirrhine evolution that can be linked to diet.

Divergent morphological responses to millennia of climate change in two species of bats from Hall's Cave, Texas, USA.

How species will respond to ongoing and future climate change is one of the most important questions facing biodiversity scientists today. The fossil record provides unparalleled insight into past ecological and evolutionary responses to climate change, but the resource remains virtually untapped for many organisms. We use geometric morphometrics and a 25,000 year fossil record to quantify changes in body size and mandible shape through time and across climate regimes for two bat species present in Quaternary paleontological deposits of central Texas: Myotis velifer, a bat distributed throughout the Southwestern US and Mexico that is still found in central Texas today, and Eptesicus fuscus, a bat widely distributed throughout North America that has been extirpated in central Texas. Because of ecogeographic rules like Bergmann’s rule, which posits that endotherms are larger in colder environments, we hypothesized that both species were larger during cooler time intervals. Additionally, we hypothesized that both species would show variation in dental morphology across the studied sequence as a response to climate change. While we found a decrease in centroid size–a proxy for ­­body size–through time for both species, we could not establish a clear relationship between centroid size and temperature alone. However, we did find that specimens from drier environments were significantly larger than those from wetter ones. Furthermore, we found significant dental shape variation between environments reflecting different temperature levels for both species. Yet only M. velifer exhibited significant variation between environments of varying precipitation levels. This result was surprising because present-day populations of E. fuscus are highly variable across both temperature and precipitation gradients. We determined that the morphological change experienced by M. velifer through time, and between warmer and cooler temperatures, was associated with the coronoid process, condylar process, and the mandibular symphysis. These parts play a pivotal role in bite force, so changes in these features might relate to changes in diet. We show that long-term datasets derived from fossil material provide invaluable insight not only into the validity of ecogeographic rules, but also into the adaptive capacities of extant taxa when faced with environmental changes. Our results highlight diverging responses to a variety of climate factors that are relevant to consider in biodiversity research given ongoing global change.

Segmental Series and Size: Clade-Wide Investigation of Molar Proportions Reveals a Major Evolutionary Allometry in the Dentition of Placental Mammals.

Iterative segments such as teeth or limbs are a widespread characteristic of living organisms. While their proportions may be governed by similar developmental rules in vertebrates, there is no emerging pattern as regards their relation to size. Placental mammals span eight orders of magnitude in body size and show a wide spectrum of dietary habits associated with size and reflected in their dentitions, especially molars. Although variation in size constitutes an important determinant for variation in biological traits, few major allometric trends have been documented on placental molars so far. Molar proportions have been intensively explored in placentals in relation to developmental models, but often at a small phylogenetic scale. Here, we analyzed the diversity of upper molar proportions in relation to absolute size in a large sample of placental species ($n = 299$) encompassing most of the group's dental diversity. Our phylogenetically informed analyses revealed a 2-fold pattern of evolutionary integration among upper molars: while molars covary in size with each other, their proportions covary with the absolute size of the entire molar field. With increasing absolute size, posterior molars increase in size relative to anterior ones, meaning that large-sized species have relatively large rear molars while the opposite is true for small-sized species. The directionality of proportional increase in the molar row exhibits a previously unsuspected allometric patterning among placentals, showing how large-scale variations in size may have influenced variation in dental morphology. This finding provides new evidence that processes regulating the size of individual molars are integrated with overall patterns of growth and calls for further testing of allometric variation in the dentition and in other segmental series of the vertebrate body. [Dentition; evolution; model; phylogeny; segmentation; size.].

The relationship between molar morphology and ecology withinNeotoma

AbstractThe extensive diversity in dental form across mammals and its strong relationship with function provides insights into the diet, habitat, and behavior, of both extant and extinct taxa. Understanding the extent of variation in dental morphology across species allows for more accurate identification of fossils and a better ability to infer relationships between form and function and ecology. We examined variation in the size and shape of the first upper molar among the genus Neotoma. We employed elliptical Fourier analysis to quantify differences in the shape of 2D outlines for 23 populations and six species of Neotoma, varying in body size and habitat preference. As expected, molar length is a strong predictor of body size and is significantly and negatively correlated with temperature, particularly in species whose ranges span large latitudinal gradients. We found that differences in molar shape separate species into three general morpho-groups, with no evidence of a phylogenetic signal. While outline analysis could not robustly classify all molars to the species level, it did perform well for Neotoma cinerea, probably because of the greater degree of folding and more acute angling of molar lophs. In contrast, wider lophs with shallower enamel infolding was characteristic of species specializing on softer, more succulent resources (i.e., Neotoma albigula and Neotoma micropus). Neotoma floridana were inaccurately classified to species in the majority of cases, but were the only molars correctly identified to locality 100% of the time, suggesting that dietary specializations at a local level may drive morphological changes within the species as well as across the genus.

Dental Morphological Variation in Asian and Asian-Derived Populations

The use of dental morphology to estimate ancestry has become more prevalent within forensic anthropology, and several new methods have been published (e.g., Edgar 2005, 2013; Irish 2015; Scott et al. 2018). While these methods represent great advances, they are lacking in modern Asian reference samples. As such, they may be unable to adequately differentiate Asian individuals from those who express similar dental morphology (i.e., Hispanics and Native Americans). This study explores dental morphological population variation of various skeletal samples in an effort to suggest areas of improvement for methods of ancestry estimation.Biological distance analyses were conducted on dental morphological data from a Mexican skeletal collection housed at the Universidad Nacional Autonoma de Mexico in Mexico City (n = 88), Hispanic dental casts housed at the University of New Mexico in Albuquerque (n = 71), a Japanese skeletal sample at Chiba University in Chiba, Japan (n = 96), and a comparative global data set collected by T. Hanihara. Clear patterns emerge that align with the Sinodont/Sundadont dental variation, where Native American samples cluster with East Asian samples (Sinodont) and Southeast Asian groups cluster with Melanesia, Papua New Guinea, and Australia (Sundadont). The two Hispanic samples are found to be distinct from these Asian samples and cluster with Macro-European and West African samples. These results highlight the complexity of Asian and Asian-derived populations and underscore the need for appropriate references samples in method creation.

Metric variation of the tibia in the Mediterranean: Implications in forensic identification

Ancestry estimation from skeletal remains is a challenging task, but essential for the creation of a complete biological profile. As such, the study of human variation between populations is important for the fields of biological and forensic anthropology, as well as medicine. Cranial and dental morphological variation have traditionally been linked to geographic affinity resulting in several methods of ancestry estimation, while the postcranial skeleton has been systematically neglected. The current study explores metric variation of the tibia in six Mediterranean populations and its validity in estimating ancestry in the Mediterranean. The study sample includes 909 individuals (470 males and 439 females) from Cyprus, Greece, Italy, Spain, Portugal and Turkey. The sample was divided in two subsamples: a reference and a validation sample. Multinomial regression models were created based on the reference sample and then applied to validation sample. The first model used three variables and resulted in 57% and 56% accuracy for the two samples respectively, while the second model (6 variables) resulted in 80% and 74% respectively. Classification between groups ranged from 28% to 95% for the reference sample and from 15% to 91% for the validation sample. The highest classification accuracy was noted for the Greek sample (95% and 90% for the reference and validation sample respectively), followed by the Turkish sample (74% and 78% respectively). The Spanish, Portuguese and Italian samples presented greater morphological overlap which resulted in lower classification accuracies. The results indicate that although the tibia presents considerable variation amongst neighbour populations it is not suitable as a sole skeletal element to separate all groups successfully. A combination of different skeletal elements may be required in order to achieve the levels of reliability required for forensic applications.

Variation in dental morphology and inference of continental ancestry in admixed Latin Americans.

To investigate the variation in dental nonmetric traits and to evaluate the utility of this variation for inferring genetic ancestry proportions in a sample of admixed Latin Americans. We characterized a sample from Colombia (N = 477) for 34 dental traits and obtained estimates of individual Native American, European, and African ancestry using genome-wide SNP data. We tested for correlation between dental traits, genetic ancestry, age, and sex. We carried out a biodistance analysis between the Colombian sample and reference continental population samples using the mean measure of divergence statistic calculated from dental trait frequencies. We evaluated the inference of genetic ancestry from dental traits using a regression approach (with 10-fold cross-validation) as well as by testing the correlation between estimates of ancestry obtained from genetic and dental data. Latin Americans show intermediate dental trait frequencies when compared to Native Americans, Europeans, and Africans. Significant correlations were observed for several dental traits, genetic ancestry, age, and sex. The biodistance analysis displayed a closer relationship of Colombians to Europeans than to Native Americans and Africans. Mean ancestry estimates obtained from the dental data are similar to the genetic estimates (Native American: 32% vs. 28%, European: 59% vs. 63%, and African: 9% vs. 9%, respectively). However, dental features provided low predictive power for genetic ancestry of individuals in both approaches tested (R2 < 5% for all genetic ancestries across methods). The frequency of dental traits in Latin Americans reflects their admixed Native American, European and African ancestry and can provide reasonable average estimates of genetic ancestry. However, the accuracy of individual genetic ancestry estimates is relatively low, probably influenced by the continental differentiation of dental traits, their genetic architecture, and the distribution of genetic ancestry in the individuals examined.

Serial Homology and Correlated Characters in Morphological Phylogenetics: Modeling the Evolution of Dental Crests in Placentals.

Accurate modeling of the complexity of morphological evolution is crucial for morphological phylogenetics and for performing tests on a wide variety of evolutionary scenarios. In this context, morphological integration and the problem of correlated categorical characters represent a major challenge. In particular, the magnitude and implications of correlations among serially homologous structures such as teeth have been much debated but were never tested statistically within a broad phylogenetic context. Here, we present a large-scale empirical study analyzing the serial variation of cingular crests on successive molars (M1, M2, and M3) of 274 placental species in a phylogenetic context. Both likelihood analyses and analysis of phylogenetic co-distributions demonstrated highly correlated evolution in the entire sample and thus the non-independence of these serial features at a macroevolutionary scale. Likelihood analyses show that their serial variation should be better scored within a single composite character model with constrained paths for transitions enabling simultaneous changes on all three molars, which suggests a strong developmental or genetic integration. These results are congruent with current genetic and developmental knowledge related to dental morphological variation and call into question the frequent use of separate characters scored on serially homologous structures of the dentition in phylogenetic analyses. Overall, they provide long-overdue and clear empirical evidence that in-depth studies of patterns of integration constitute an essential step toward more realistic character construction and modeling. This approach is critical for more accurate morphological phylogenetics and, more generally, for testing macroevolutionary scenarios on groups of correlated characters.

2012 Dahlberg Award Winner Dental morphological variation among six Pre-Hispanic South American populations with implications for the peopling of the New World

The analysis of the human dentition pro-vides important information on the origins and dispersals of the first American inhabitants. However, most of this work has focused on North America, whereas less research has been devoted to variation within Central and South America. This study examines the permanent dentitions of 340 individuals from six pre-Hispanic South American populations and places them in the broader context of the peopling of the New World. Non-metric dental data were collected using the Arizona State University Dental Anthropology System (ASUDAS). Intra- and inter-regional comparisons were assessed using the Mean Measure of Divergence statistical program. All samples are characterized by relatively high frequencies of UM1 enamel extension and LM1 deflecting wrinkle and low frequencies of UM1 cusp 5 and LM2 Y-groove pattern. Although preliminary, results indicate that populations from Chile, Venezuela and Peru-Northern Coast are dentally similar and follow the Sinodont dental pattern. The Peru-Southern Highlands sample is the most divergent of the South American groups examined, showing the closest affinities with Sundadonts. Finally, no clear pattern was found for Bolivia and Peru-Amazonian Andes, as most of their trait frequencies fall within the range of overlap between Sinodont and Sundadont populations.

Heritability and genetic integration of anterior tooth crown variants in the South Carolina Gullah.

This article presents estimates of narrow-sense heritability and bivariate genetic correlation for a series of morphological crown variants of the anterior dentition. These results provide insight into the value of dental phenotypes as evolutionary proxies, as well as the development of tooth crowns as integrated or modular structures. African American dental casts from the Menegaz-Bock collection were scored for a standard set of dental morphological variables using the Arizona State Dental Anthropology System. Estimates of narrow-sense heritability and genetic correlations were generated using SOLAR v. 8.1.1, controlling for the covariates of age, sex, and birth year. Analyses were run using ordinal/continuous scale variables that were then dichotomized at various breakpoints, consistent with standard practices in dental anthropology. Heritability estimates were low to moderate for most traits, and lower in magnitude than those reported for odontometric data from the same study sample. Only winging, canine shoveling, and canine double shoveling returned narrow-sense heritabilities that did not differ significantly from zero. Genetic correlations were high among antimeres and metameres and low for different traits scored on the same tooth crown. These results affirm standard data cleaning practices in dental biodistance. Double shoveling was atypical in returning strong negative correlations with other traits, shoveling in particular. Additive genetic variation contributes to dental morphological variation, although the estimates are uniformly lower than those observed for odontometrics. Patterns of genetic correlation affirm most standard practices in dental biodistance. Patterns of negative pleiotropy involving lingual and labial crown features suggest a genetic architecture and developmental complex that differentially constrain morphological variation of distinct surfaces of the same tooth crown. These patterns warrant greater consideration and cross-population validation.

Coordinating bracket torque and incisor inclination : Part1: The development of widely applicable equations.

The objective of this investigation was to develop equations to describe the geometric relations among "targeted incisor inclinations" (tU1, tL1) accommodating different cephalometric norms (Ricketts, Bergen, etc.) with the "expected inclinations" (eU1, eL1), as they can be generated by bracket torque values according to Andrews, Roth, Ricketts, and MBT. In its key parts, the present study is a theoretical work in which structural relationships are described using standard mathematical and geometric methodologies. The "targeted norm-inclinations" (tU1, tL1) were calculated relative to a single reference plane (BOP according to Downs), thus, allowing for a direct comparison of different cephalometric values. Referring to the "expected inclinations" (eU1, eL1), it was found that in addition to bracket torque (BT) morphological and structural parameters also have to be taken into account. These are the "torque coordination angle" (TCA) representing the variation in dental morphology and, the correction angles between BOP and the upper (uOP) (α1) or the lower (lOP) occlusal plane (β1). Moreover, the angles α2 between an upper (uBPP) and β2 between a lower bracket positioning plane (lBPP) and the occlusal planes (uOP, lOP) have to be considered. As a consequence, suitable equations were developed (eU1(BOP) = 90° - BT(U1) - TCA(U1) + α1 - α2, and eL1(BOP) = 90° - BT(L1) - TCA(L1) + β1 - β2), allowing the calculation of expected torque-dependent inclinations (eU1, eL1) and representing the prerequisite for a comparison with the cephalometric targeted values (tU1, tL1). By developing suitable equations, it became possible to name and quantify those parameters that are responsible for incisor inclinations and enable a comparison with targeted cephalometric values.

The oldest Archaeopteryx (Theropoda: Avialiae): a new specimen from the Kimmeridgian/Tithonian boundary of Schamhaupten, Bavaria.

The iconic primeval bird Archaeopteryx was so far mainly known from the Altmühltal Formation (early Tithonian) of Bavaria, southern Germany, with one specimen having been found in the overlying Mörnsheim Formation. A new specimen (the 12th skeletal specimen) from the earliest Tithonian Painten Formation of Schamhaupten (Bavaria) represents the so far oldest representative of the genus. The new specimen shows several interesting anatomical details, including the presence of a postorbital in contact with the jugal, the presence of a separate prefrontal and coronoid, and opisthocoelous mid-cervical vertebrae. Based on observations on the new specimen, we discuss several problematic issues concerning Archaeopteryx, including the monophyly and diagnosis of the genus, the absence/presence of the sternum, the position of the gastralia, and variation in morphometrics and dental morphology in that genus. Based on a new diagnosis for the genus Archaeopteryx, the Berlin, Eichstätt, Solnhofen, Munich, Daiting, Thermopolis, 11th, and 12th specimens can be referred to this genus with high certainty. The Maxberg specimen is very probably also an Archaeopteryx, based on overall similarity, although none of the diagnostic characters can be evaluated with certainty. The ninth specimen (‘chicken wing’) might be Archaeopteryx, but cannot be referred to the genus with any certainty. In comparison with other paravians, the presence of distally thickened anterior pectoral ribs indicates that a rather large cartilagenous sternum was present in this taxon. In contrast to non-opisthopubic theropods, opisthopubic taxa, such as Archaeopteryx and many other paravians, have the posterior end of the gastral basket preserved at about half-length of the pubis, which might reflect the post-mortem collapse of enlarged abdominal air sacs in these taxa. Specimens that can be referred to Archaeopteryx show a high amount of variation, both in the morphometrics of the limb bones as well as in the dentition. In respect to the latter aspect, variation is found in tooth number, spacing, orientation, and morphology, with no two specimens showing the exact same pattern. The significance of this variation is unclear, and possible explanations reach from high intraspecific (and possibly ontogenetic and/or sexual dimorphic) variation to the possibility that the known specimens represent a ‘species flock’ of Archaeopteryx, possibly due to island speciation after the initial dispersal of the genus into the Solnhofen Archipelago.

Taxonomic Implications of a Large Sample of Tremacyllus (Hegetotheriidae: Pachyrukhinae) from the Late Miocene Cerro Azul Formation of La Pampa, Argentina

More than 40 remains of Tremacyllus Ameghino (Notoungulata, Hegetotheriidae, Pachyrukhinae) from the Province of La Pampa (Argentina) are analyzed. They were recovered from different outcrops of the late Miocene Cerro Azul Formation at Laguna Chillhue, Quehue, Telen, Salinas Grandes de Hidalgo, Estancia Re, Bajo Giuliani, Barrancas Coloradas and Caleufu. During the late Miocene, pachyrukhines are represented by two genera, Tremacyllus and Paedotherium Burmeister. The present sample of Tremacyllus allows for the dental morphological variation to be evaluated, especially in lower and upper premolars. This study shows high variability in the diagnostic characters of two recognized species, T. impressus Ameghino (Pliocene–Pleistocene of the Pampean Region) and T. incipiens Rovereto (late Miocene of Catamarca). We interpret these differences to actually correspond to intraspecific variation and, consequently, propose their synonymy. T. impressus has priority over T. incipiens, and an emended diagnosis is provided. This nomenclatural action implies, in turn, the extension of the biochron of Tremacyllus impressus from the late Miocene to the Pleistocene. T. impressus differs from Paedotherium in its smaller size, post-incisive impressions extended beyond the level of P2 (even reaching the anterior margin of P3), M3 shorter or equal to M2, relatively shorter mandibular symphysis, and lower premolars not molariform and more imbricated. Within the studied sample, we identify several juvenile specimens that confirm the dental eruption pattern previously proposed for Tremacyllus based on fewer remains.

Reconstructing human population history from dental phenotypes

Dental phenotypic data are often used to reconstruct biological relatedness among past human groups. Teeth are an important data source because they are generally well preserved in the archaeological and fossil record, even when associated skeletal and DNA preservation is poor. Furthermore, tooth form is considered to be highly heritable and selectively neutral; thus, teeth are assumed to be an excellent proxy for neutral genetic data when none are available. However, to our knowledge, no study to date has systematically tested the assumption of genetic neutrality of dental morphological features on a global scale. Therefore, for the first time, this study quantifies the correlation of biological affinities between worldwide modern human populations, derived independently from dental phenotypes and neutral genetic markers. We show that population relationship measures based on dental morphology are significantly correlated with those based on neutral genetic data (on average r = 0.574, p < 0.001). This relatively strong correlation validates tooth form as a proxy for neutral genomic markers. Nonetheless, we suggest caution in reconstructions of population affinities based on dental data alone because only part of the dental morphological variation among populations can be explained in terms of neutral genetic differences.

Identification of novel Fgf enhancers and their role in dental evolution.

Mammalian dental morphology is under strong evolutionary pressure because of its importance for mastication and diet. While the mechanisms underlying tooth development have been widely studied in model organisms, the role of genetic regulatory elements in patterning the different elements of the occlusal surface and crown height across species is not well understood. Previous studies showed that Fibroblast Growth Factor (FGF) genes are important regulators of tooth development that influence morphological variation. We hypothesized that inter-specific variation in rodent dental morphology could be governed by nucleotide variation in genetic regulatory elements that modulate the spatial and temporal expression of the genes encoding FGF signaling molecules. In this study, we compared the variation in dental morphology across nine taxa of rodents to the variation in sequences of non-coding evolutionary conserved regions (ECRs) of Fgf3, 4, 8, 9, and 10. We correlated the variation in molar tooth cusp shape and the evolution of high molar crowns (hypsodonty) to the patterns of sequence variation in two ECRs, Fgf10ECR3, and Fgf9ECR1, respectively. By conducting luciferase and electrophoretic mobility shift assays, we determined that these ECRs could function as enhancers. These data suggest that emergence of hypsodonty and occlusal cusp patterning may have happened through the evolutionary changes in enhancers, such as Fgf9ECR1 and Fgf10ECR3, which affected the expression of major signaling molecules involved in tooth development.

Making teeth to order: conserved genes reveal an ancient molecular pattern in paddlefish (Actinopterygii).

Ray-finned fishes (Actinopterygii) are the dominant vertebrate group today (+30 000 species, predominantly teleosts), with great morphological diversity, including their dentitions. How dental morphological variation evolved is best addressed by considering a range of taxa across actinopterygian phylogeny; here we examine the dentition of Polyodon spathula (American paddlefish), assigned to the basal group Acipenseriformes. Although teeth are present and functional in young individuals of Polyodon, they are completely absent in adults. Our current understanding of developmental genes operating in the dentition is primarily restricted to teleosts; we show that shh and bmp4, as highly conserved epithelial and mesenchymal genes for gnathostome tooth development, are similarly expressed at Polyodon tooth loci, thus extending this conserved developmental pattern within the Actinopterygii. These genes map spatio-temporal tooth initiation in Polyodon larvae and provide new data in both oral and pharyngeal tooth sites. Variation in cellular intensity of shh maps timing of tooth morphogenesis, revealing a second odontogenic wave as alternate sites within tooth rows, a dental pattern also present in more derived actinopterygians. Developmental timing for each tooth field in Polyodon follows a gradient, from rostral to caudal and ventral to dorsal, repeated during subsequent loss of teeth. The transitory Polyodon dentition is modified by cessation of tooth addition and loss. As such, Polyodon represents a basal actinopterygian model for the evolution of developmental novelty: initial conservation, followed by tooth loss, accommodating the adult trophic modification to filter-feeding.