Molar Size Research Articles

Interspecific and intraspecific variation in grasshopper (Orthoptera: Acrididea) molar form: implications for dietary ecology.

Like many mammals, grasshoppers (infraorder Acrididea) chew using molariform structures. Despite decades of research on mammals, little is known about grasshopper molar form and how it relates to grasshopper feeding biomechanics, diet, dietary ecology and evolution. Here, we develop a method for quantifying molar form and apply it to two species of distantly related grasshoppers with different diets (Phymateus saxosus, seven females; Valanga nigricornis, seven females, 11 males). We show that there are quantifiable differences in molar form, potentially related to diet. There are some differences in molar shape between left and right molars in both species and sexes, and significant differences in molar size, potentially due to scaling. Like in mammals, molar wear can cause large differences in molar shape. Species differences in molar shape did not match what was expected based on mammalian molar functional morphology. Dental topographic analysis is a promising new avenue for quantifying molar form in grasshoppers and a distinct advantage over traditional two-dimensional microscopy methods, and promises to reveal much about the biology, biomechanics and evolution of Acrididea.

Comparison of dental topography of marmosets and tamarins (Callitrichidae) to other platyrrhine primates using a novel freeware pipeline

Dental topographic metrics (DTMs), which quantify different aspects of the shape of teeth, are powerful tools for studying dietary adaptation and evolution in mammals. Current DTM protocols usually rely on proprietary software, which may be unavailable to researchers for reasons of cost. We address this issue in the context of a DTM analysis of the primate clade Platyrrhini (“New World monkeys”) by: 1) presenting a large comparative sample of scanned second lower molars (m2s) of callitrichids (marmosets and tamarins), previously underrepresented in publicly available datasets; and 2) giving full details of an entirely freeware pipeline for DTM analysis and its validation. We also present an updated dietary classification scheme for extant platyrrhines, based on cluster analysis of dietary data extracted from 98 primary studies. Our freeware pipeline performs equally well in dietary classification accuracy of an existing sample of platyrrhine m2s (excluding callitrichids) as a published protocol that uses proprietary software when multiple DTMs are combined. Individual DTMs, however, sometimes showed very different results in classification accuracies between protocols, most likely due to differences in smoothing functions. The addition of callitrichids resulted in high classification accuracy in predicting diet with combined DTMs, although accuracy was considerably higher when molar size was included (90%) than excluded (73%). We conclude that our new freeware DTM pipeline is capable of accurately predicting diet in platyrrhines based on tooth shape and size, and so is suitable for inferring probable diet of taxa for which direct dietary information is unavailable, such as fossil species.

Size does matter: metric variability of the upper second molars and differentiation of the Middle and Late Pleistocene Homo in Eurasia

Dental data are one of the most important sources of information on the evolution of ancient hominids. It preserves information about both the early stages of ontogenesis and the postnatal period of life. It is generally accepted that one of the main evolutionary trend within the genus Homo over the past two million years was reduction of teeth size. This trend substantially accelerated in modern Homo sapiens during the Upper Paleolithic and Neolithic. It is assumed that a number of factors could influence the teeth reduction process. However, no detailed understanding of its patterns, limits and chronological and territorial variability has been revealed so far. In this study, we analyze the variability of the upper second molar sizes in Neanderthals, Denisovans, Homo antecessor, Middle Pleistocene Homo from Sierra de Atapuerca (Gran Dolina), Homo erectus, Middle and Late Pleistocene Homo from China other than H. erectus, and compare them with the characteristics of the modern H. sapiens. The main goal of our analysis was to identify local chronological and geographical patterns in the evolution of the size of the second upper molars and assess their compliance with the general epochal trend. The results of the analysis showed that before the appearance of the Upper Paleolithic H. sapiens, two distinct local trends existed on the territory of Eurasia. The first trend, which can be detected in the European pre-sapiens humans, was a decrease in the molar size of the upper second molars. The second one, which got spread in the Middle Pleistocene in Asia, on the contrary, consisted of a sharp increase of the molar size, which formed the morphological specificity of the Denisovan lineage. No stable geographic differentiations of the size of the upper second molars are observed within H. sapiens sample.

MORPHOLOGICAL, MECHANICAL, ORGANIC VAPOUR PERMEATION PROPERTIES AND BIODEGRADABILITY OF POLY(ETHYLENE-CO-VINYL ACETATE)/WASTE PISTACHIO SHELL-DERIVED CELLULOSE COMPOSITE MEMBRANES

Cellulose fibers have attracted interest as a suitable candidate for manufacturing composites. In the present work, cellulose derived from waste pistachio shell was used to prepare composite membranes with poly(ethylene-co-vinyl acetate (EVA) via solution casting, and their morphological, mechanical, organic vapour permeation characteristics and biodegradability were evaluated. Scanning electron micrographs showed that the voids in the EVA polymer were filled effectively by cellulose fibers. The mechanical testing of the composites revealed an improvement in Young’s modulus with the increase in cellulose loading. The permeation of polar and non-polar solvents through the membranes was studied and explained by molar size, molecular mass and polarity of the solvents. Nielson’s permeability equation as modified by Baradwaj was used to analyse the relative permeability of the membranes. Soil burial degradation experiments of the composite membranes showed a decrease in weight and tensile strength, revealing the biodegradability of the membranes.

Towards a better knowledge of the molar morphology and ecology of extant and fossil grass rats (Muridae:ArvicanthisLesson, 1842)

AbstractAfrican rodents of the genusArvicanthisare presently restricted to sub-Saharan savannas and to the Nile Valley. In contrast, their distribution during the Quaternary included most of northern Africa, leading to the emergence of local fossil species. To date, there have been no comprehensive studies ofArvicanthispopulations in northern Africa, neither to clarify their taxonomy nor their paleoecology. The present study aims to explore both morphology and diet of modern and fossilArvicanthisspecies using geometric morphometric and dental microwear analyses on first upper molars. The geometric morphometric analysis efficiently discriminates the studied extant and fossilArvicanthisspecies and allowed for the identification of probable geographical variations within theA.niloticusgroup. Although all extant species of the genusArvicanthisare predominantly grass-eaters, microwear analyses also highlighted diet differences in various modern populations ofA.niloticus, as well as paleodiet inferences in theA.arambourgifossil species, but no clear link between molar size or shape and diet can be established. This work helps set the stage for a complete revision of the fossil remains ofArvicanthisfrom northern African Quaternary deposits, and for a better understanding of the geographical and temporal morphological variability of this genus in Africa.

Intraspecific craniomandibular and dental analysis of Pseudotypotherium exiguum (Mesotheriidae, Notoungulata) from Monte Hermoso, late Neogene, Buenos Aires Province, Argentina

ABSTRACT Mesotheriid taxonomy for Monte Hermoso, Buenos Aires province, Argentina is extremely complex, with much of the controversy centered on the Late Miocene–Pliocene mesotheriine notoungulate type species Pseudotypotherium pulchrum, which is a subjective junior synonym of P. exiguum. Confusion around P. exiguum intraspecific variation is exacerbated by specimens of the Pleistocene taxon Mesotherium cristatum originally defined in Toscas del Río de La Plata. As a result, remains of Monte Hermoso mesotheres have been referred to six species and four genera (Typotherium, Pseudotypotherium, Xenotherium, and Typotheriodon) during the past 100 + years. The current consensus is that Pseudotypotherium is the correct genus for Monte Hermoso mesotheres, but detailed analyses of variation, especially ontogenetic, have been lacking. We rectify that deficiency in this work through bibliographic and firsthand study of type materials and comparison of molar size (PCA of linear dimensions) and i1 ontogenetic variation (via CT) using a broad ontogenetic series. We conclude that the variation observed in Monte Hermoso mesotheres is consistent with ontogenetic and individual variation within a single species. As previously suggested, Pseudotypotherium (=Typotherium) exiguum is the valid senior synonym for Monte Hermoso mesotheres. We provide a detailed diagnosis and description of P. exiguum, including a rather complete ontogenetic series, the first for any species of Pseudotypotherium.

A technique for immediate implant placement in mandibular multirooted teeth by the aid of a dynamic guidance system and preparing implant bed before tooth extraction

Due to site-specific anatomical challenges, traditional immediate dental implant placement for mandibular multirooted molar sizes has many limitations. This technical report describes a modified approach for immediate dental implant placement in molar sites allowing for precise three-dimensional (3D) implant position and favourable clinical effect with the aid of a dynamic guidance system and flapless pre-extractive preparation of the implant bed.

Mammoth evolution in the late Middle Pleistocene: The Mammuthus trogontherii-primigenius transition in Europe

The late Middle Pleistocene (Marine Isotope Stages 11-6, ca. 425-125 ka) saw the transition from the so-called steppe mammoth (Mammuthus trogontherii) to the woolly mammoth (M. primigenius) in Europe. A metrical study of molariform teeth from more than 40 localities in Britain and continental Europe allows this transition to be traced in detail. The morphological trend began as early as MIS 10, with the final transition to M. primigenius around the MIS 7/6 boundary where several samples (Marsworth, Crayford, La Cotte) span both species' morphology. The ‘small mammoth’ of MIS 7 is not homogeneous, as those from Stanton Harcourt (Oxfordshire, UK) are smaller and more advanced than those from Ilford (Essex, UK), likely corresponding to their younger age within MIS 7. Contrary to expectation, the thinning of molar enamel and increase in lamellar packing from MIS 16 to MIS 7 appear not to have been driven by increase in lamellar number since they precede it, nor is lamellar number primarily correlated to molar size. The adaptive significance of size changes is unclear, but they affected molar function via their effects on lamellar spacing. This study underlines the importance of adequate sample sizes and bivariate analysis of metric variables to take account of size change. The implications of the study for taxonomy, biostratigraphy and the effects of insularity are discussed.

Keeping 21st Century Paleontology Grounded: Quantitative Genetic Analyses and Ancestral State Reconstruction Re-Emphasize the Essentiality of Fossils.

Simple SummaryOver the last two decades of biological research, our understanding of how genes determine dental development and variation has expanded greatly. Here, we explore how this new knowledge can be applied to the fossil record of cercopithecid monkeys. We compare a traditional paleontological method for assessing dental size variation with measurement approaches derived from quantitative genetics and developmental biology. We find that these new methods for assessing dental variation provide novel insight to the evolution of the cercopithecid monkey dentition, different from the insight provided by traditional size measurements. When we explore the variation of these traits in the cercopithecid fossil record, we find that the variation is outside the range predicted based on extant variation alone. Our 21st century biological approach to paleontology reveals that we have even more to learn from fossils than previously recognized.Advances in genetics and developmental biology are revealing the relationship between genotype and dental phenotype (G:P), providing new approaches for how paleontologists assess dental variation in the fossil record. Our aim was to understand how the method of trait definition influences the ability to reconstruct phylogenetic relationships and evolutionary history in the Cercopithecidae, the Linnaean Family of monkeys currently living in Africa and Asia. We compared the two-dimensional assessment of molar size (calculated as the mesiodistal length of the crown multiplied by the buccolingual breadth) to a trait that reflects developmental influences on molar development (the inhibitory cascade, IC) and two traits that reflect the genetic architecture of postcanine tooth size variation (defined through quantitative genetic analyses: MMC and PMM). All traits were significantly influenced by the additive effects of genes and had similarly high heritability estimates. The proportion of covariate effects was greater for two-dimensional size compared to the G:P-defined traits. IC and MMC both showed evidence of selection, suggesting that they result from the same genetic architecture. When compared to the fossil record, Ancestral State Reconstruction using extant taxa consistently underestimated MMC and PMM values, highlighting the necessity of fossil data for understanding evolutionary patterns in these traits. Given that G:P-defined dental traits may provide insight to biological mechanisms that reach far beyond the dentition, this new approach to fossil morphology has the potential to open an entirely new window onto extinct paleobiologies. Without the fossil record, we would not be able to grasp the full range of variation in those biological mechanisms that have existed throughout evolution.

Geometric morphometrics of the second molar teeth within the human population from the late medieval city of Iași, Romania

AbstractAlthough variability and sexual dimorphism (SD) have been intensively studied as regards molar size, patterns of SD on molar shape are not an equally approached topic. Therefore, this study uses size and shape variables of the second permanent molar (M2) from two‐dimensional geometric morphometrics to assess intrasample variability and shape patterns in SD within a human group from the late medieval city of Iași in Romania. Our findings are partially consistent with the view that phenotypic variability in M2 teeth is correlated with their developmental stages. No intra‐group dental models were highlighted, except for those determined by SD. The upper M2 tooth is a clearer sexual dimorphic than the lower M2. How different regions of the tooth are associated and how patterns of M2 shape variation are defined in the two sexes are also topics addressed in this paper.

Human Premolar Crown Sizes are Highly Variable Even in the Same Mouth, with Strong Crown Size Correlations among Premolars and All Molars Except the Third Molar

The Inhibitory Cascade Model (ICM) predicts in mice that a larger permanent first molar (M1) results in a smaller second molar (M2) and an even smaller third molar (M3). Counter to the ICM, our lab found in contemporary humans that instead of M1 size, only M2 size predicts M3 size. Our lab also identified in humans 13 different molar size ratio patterns instead of one classic ICM pattern (M1>M2>M3). Further, other work in nonhuman primates reported a Premolar Molar Module (PMM) where sizes of the fourth premolar (P4) and all three molars are highly correlated. However, premolar:molar size correlation in contemporary humans is yet to be explored. This exploration can help define how the development of these two different classes of teeth influence each other’s sizes. In humans, M1 initiates near birth, followed ~24 months later by P3 and P4 (almost simultaneously with each other), then ~12 months later by M2, and ~6 years later by M3. Our first aim was to define variation in premolar size ratio pattern. We hypothesized that all three mathematically possible premolar size ratio patterns (P3=P4, P3>P4, P3<P4) will be observed in our sample. Our second aim was to test whether premolar crown size and size ratio predict molar absolute sizes and molar size ratio patterns (e.g., M1>M2>M3). We hypothesized that absolute sizes as well as size ratios of earlier initiating teeth will predict the absolute sizes and size ratios of later initiating teeth. Maximum mesiodistal lengths of P3, P4, M1, M2, and M3 crowns were measured in millimeters to represent absolute tooth sizes using retrospective Cone Beam Computed Tomography scans of 91 dental patients (54 female, 37 male) aged 13‐23 years. A frequency analysis revealed all three mathematically possible premolar size ratio patterns in our sample (P3>P4: 49.83%, P3<P4: 34.13%, and P3=P4: 16.04%, n=293 oral quadrants). Also, 57% (n=49) of the subjects with data collected from all four quadrants showed all three premolar size ratio patterns within the same mouth. A linear mixed model analysis (95% confidence level, alpha = 0.05) showed that P3 and P4 predicted each other’s crown size, as well as that of M2, but not M3. M1 size predicts P4 size only. In sum, we found that premolar size ratio patterns are highly variable even within the same oral cavity. We also found strong correlations among P3, P4 and M2 sizes, and between M1 and P4 sizes. Our results found variability in premolar size ratio patterns that is similar to the variability that we previously found in molar size ratio patterns. Our study in contemporary humans does not support the ICM prediction of a single classic tooth size ratio pattern. However, our study outcomes supported the PMM findings with the exception that P4 was not correlated with M3.

Dental Tissue Volumes in Mice with Osteogenesis Imperfecta

Osteogenesis imperfecta (OI), commonly known as brittle bone disease, has long been known to cause significant dental problems that affect patients over the course of their lives, including increased dental fractures, discolored teeth, and malocclusion.At the molecular level, OI is a disorder of the type I collagen protein. Insufficient amounts or misshapen forms of this protein lead to disruptions in the microstructure of bone and teeth tissues.Dentin, the hard tissue which comprises the bulk of the tooth and absorbs shock forces during chewing, develops on a matrix of type I collagen. Thus, collagen abnormalities in OI lead to less organized and less stable dentin.Further, the teeth in people with OI tend to exhibit dentin hypertrophy, in which increased amounts of dentin are deposited at the interior of the tooth. In humans with OI, dentin hypertrophy tends to develop shortly after the eruption of teeth into the oral cavity.Enamel, unlike dentin, develops on a matrix of non‐collagenous proteins, and is thought to develop normally in OI. However, abnormalities in the underlying dentin in OI can frequently lead to enamel fractures. Finally, the teeth in people with OI have often been noted to be smaller than those of people without OI.Mice are the most commonly used animal model for dental development. However, differences in dental tissue volumes in OI have not yet been studied in mice.The objective of this study is to compare dental tissue volumes at different developmental stages in teeth from mice with and without OI. We hypothesize that both juvenile and young adult mice with OI will have smaller dental tissue volumes than mice without OI.To test this hypothesis, three‐dimensional models of teeth from OI mice (n = 10) and wild type mice (n = 10) were created from microCT scans taken at weaning age (week 4 after birth) and young adulthood (week 16 after birth). Dental tissue volumes were measured using 3D Slicer.Mann‐Whitney U tests showed that at week 16, mice with OI had significantly lower dental volumes compared to mice without OI for upper molars, lower molars, upper incisors, and lower incisors (p < 0.05). Additionally, at week 4, mice with OI had significantly smaller lower molars, upper incisors, and lower incisors (p < 0.05). There was no significant difference between the two groups in upper molar size among the week 4 mice.At week 16, the OI mice had significantly lower volumes of dentin compared to the mice without OI in the upper and lower molars (p < 0.05), but not in the upper or lower incisors. At week 4, the OI mice had significantly lower volumes of dentin compared to the mice without OI in the upper and lower incisors and lower molars (p <0.05), but not the upper molars.There were no significant differences between the two groups in enamel volume at either timepoint.These results demonstrate that mice with OI have smaller overall dental volumes compared to mice without OI, consistent with the human OI phenotype of smaller teeth.This research will provide important insights into the development of the dental phenotype in OI. The use of a mouse model to investigate dental tissue volumes represents a novel direction for OI research. Establishing the validity of the mouse model will allow future researchers to further characterize the effects of this disease on early dental development using methods, such as CT scans, that cannot be practically or ethically performed in young human patients.

Sorting the flock: Quantitative identification of sheep and goat from isolated third lower molars and mandibles through geometric morphometrics

Sheep and goat are often herded together and show morphological similarities in their skeleton. Being able to identify archaeological remains of these two taxa to species level is particularly important for understanding and characterising past herding practices. Discrete criteria are now available to identify a large number of their bones and teeth, and quantitative approaches have been developed for post-cranial elements but not for mandible and isolated teeth. In this paper we explore the discriminating potential of geometric morphometrics to identify modern sheep and goat third lower molar and mandible and its application on archaeological specimens. The size and shape of the mandible and the third lower molar of 143 modern specimens (101 sheep and 42 goats) were quantified using 2D-landmark and sliding semi-landmarks geometric morphometric approaches. The results show that sheep and goat differ in terms of the size, shape, and form (i.e. size and shape together) in both studied elements. Classification accuracy of the two species reaches 93.3% (CI: 90.0–95.7%) for third lower molar shape, 62.7% (CI 57.1–68.6%) for third lower molar size, 95.2% (CI: 92.0–97.4%) for mandible shape and 84.0% (CI 81.6–86.8%) for mandible size. Form does not provide better classification than shape alone. Sex and age appear to have little impact on the ability to differentiate between sheep and goat, despite the two species displaying distinct sexual dimorphism and changes through age. The same methodology was then applied on 32 Middle Ages third lower molars from Missignac-Saint Gilles le Vieux, Aimargues, France. The identifications obtained through geometric morphometrics were only partially congruent with the identifications based on visual observations calling for caution in the interpretation and further investigations. Further research should include molecular identification of the archaeological specimens to assess whether the geometric morphometric identification can be made with confidence for all periods and all geographic areas. Nevertheless, the results obtained with the newly developed geometric morphometric protocols represent an important contribution toward a better understanding of past livestock husbandry practices.

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 M components (cusps), we aim to assess the area of the protoconid (Prd), the phylogenetically oldest cusp of the lower Ms. Subject and methods We measured the Prd and the total crown area in the scaled photographs of a recent modern human sample of lower Ms (76 males and 39 females). The values were statistically analysed. Results The absolute size of the Prd 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 Prd forms in response to signals from the primary enamel knot, likely contributing to its stability. Inhibitory signals emitted during the Prd 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 M morphogenetic process.

Characterization of the molecular properties and allergenicity (IgE-binding capacity) of β-lactoglobulin by heat treatment using asymmetric-flow field-flow fractionation and ultra-performance liquid chromatography quadrupole time of flight mass chromatography

In this study, the heat product (90 °C, 10 min) of β-lactoglobulin (β-LG) was analyzed by asymmetric-flow field-flow fractionation (AF4) to observe the effect of heat treatment. The changes in molar mass (M) and molar size induced by heat treatment were characterized by AF4, and changes in molar shape were observed by transmission electron microscopy (TEM). The results showed that β-LG dissociated and aggregated into four fractions with different M values, sizes, and shapes after heat treatment. The vast aggregations with the highest allergenicity (IgE-binding capacity) might enhance the allergenicity of β-LG. However, the number of characterized epitope peptides was decreased due to heat treatment. The above results provide some references for related studies of β-LG and its allergenicity. Further separation and characterization of the high-allergenicity fractions and peptides will help to eliminate allergens in dairy products and reduce the occurrence of allergic reactions.

Unexpected variation of human molar size patterns

A tenet of mammalian, including primate dental evolution, is the Inhibitory Cascade Model, where first molar (M1) size predicts in a linear cline the size and onset time of the second (M2) and third (M3) molars: a larger M1 portends a progressively smaller and later-developing M2 and M3. In contemporary modern Homo sapiens, later-developing M3s are less likely to erupt properly. The Inhibitory Cascade Model is also used to predict molar sizes of extinct taxa, including fossil Homo. The extent to which Inhibitory Cascade Model predictions hold in contemporary H. sapiens molars is unclear, including whether this tenet informs about molar initiation, development, and eruption. We tested these questions here. In our radiographic sample of 323 oral quadrants and molar rows from contemporary humans based on mesiodistal crown lengths, we observed the distribution of molar proportions with a central tendency around parity (M1 = M2 = M3) that parsed into 13 distinct molar size ratio patterns. These patterns presented at different frequencies (e.g., M1 > M2 > M3 in about one-third of cases) that reflected whether the molar row was located in the maxilla or mandible and included both linear (e.g., M1 < M2 < M3) and nonlinear molar size ratio progressions (e.g., M1 > M2 < M3). Up to four patterns were found in the same subject’s mouth. Lastly, M1 size alone does not predict M3 size, developmental timing, or eruption; rather, M2 size is integral to predicting M3 size. Our study indicates that human molar size is genetically ‘softwired’ and sensitive to factors local to the human upper jaw vs. lower jaw. The lack of a single stereotypical molar size ratio for contemporary H. sapiens suggests that predictions of fossil H. sapiens molar sizes using the Inhibitory Cascade Model must be made with caution.

The cranial biomechanics and feeding performance of Homo floresiensis.

Homo floresiensis is a small-bodied hominin from Flores, Indonesia, that exhibits plesiomorphic dentognathic features, including large premolars and a robust mandible, aspects of which have been considered australopith-like. However, relative to australopith species, H. floresiensis exhibits reduced molar size and a cranium with diminutive midfacial dimensions similar to those of later Homo, suggesting a reduction in the frequency of forceful biting behaviours. Our study uses finite-element analysis to examine the feeding biomechanics of the H. floresiensis cranium. We simulate premolar (P3) and molar (M2) biting in a finite-element model (FEM) of the H. floresiensis holotype cranium (LB1) and compare the mechanical results with FEMs of chimpanzees, modern humans and a sample of australopiths (MH1, Sts 5, OH5). With few exceptions, strain magnitudes in LB1 resemble elevated levels observed in modern Homo. Our analysis of LB1 suggests that H. floresiensis could produce bite forces with high mechanical efficiency, but was subject to tensile jaw joint reaction forces during molar biting, which perhaps constrained maximum postcanine bite force production. The inferred feeding biomechanics of H. floresiensis closely resemble modern humans, suggesting that this pattern may have been present in the last common ancestor of Homo sapiens and H. floresiensis.

Human Molar Size Variation: Wisdom Teeth Just Don't Care?

The inhibitory cascade model (ICM) is a tenet of mammalian molar morphogenesis that predicts that a larger developing first molar (M1) results in a smaller and later-forming adjacent second (M2) and third (M3) molar. Recently, we have shown that late-developing M3s are also at higher risk of impaction, a common dental health concern worldwide. The mechanisms delaying M3 onset time are unclear.Here we tested the hypothesis that M1 crown size predicts M3 developmental timing and M3 eruption, based on a sample of young Western Canadian dental patients. We also tested the extent to which contemporary modern human molars conform to the ICM. We sampled retrospective cone beam computed tomography scans and panoramic radiographs taken for clinical diagnostic purposes of multi-ethnic patients aged 13-24 years residing in four Western Canadian municipalities. We studied 99 patients, sampling 327 quadrants from 57 girls and 42 boys, at two distinct times that allowed us to know if an M3 had properly erupted. Using Horos and Xoran i-Cat software, we scored permanent molar development using a 9-stage classification after Demirjian's system. We measured mesiodistal lengths of M1, M2 and M3 crowns. In SPSS v.25, we calculated a Generalized Estimating Equation model through a multivariable linear regression to test if M1 crown length (“size”) predicted M3 developmental timing and eruption status. We also tested other variables including jaw type (maxillary; mandibular). Significance was set to p<0.05. Unexpectedly, we identified 13 molar size ratio patterns occurring at different frequencies (e.g., M1>M2>M3 in only 31.6% of cases) that reflected whether molars were located in the maxilla versus the mandible. This pattern diversity included up to four patterns in the same person's mouth. Further, M3 size was predicted not by M1 size (Wald Chi-Square [Wald X2]=0.701, p=0.40) but by M2/M1 ratio and absolute M2 size (Wald X2=14.04, p=0.000). Relative to M1 crown length, if M2 crown length increased by 10%, the adjacent M3 crown lengthened by 7% (±3%), indicating that M2 increases “faster” than M3. Crown length ratios (M3:M1, M2:M1) did not predict M3 impaction (Wald X2=0.124, p=0.73) or M3 developmental stage (M2:M1, Wald X2=0.228, p=0.63; M3:M1, Wald X2=3.253, p=0.07). We conclude that M1 crown length does not predict early versus late M3 development or M3 eruption status. Thus, the ICM does not appear to be useful for predicting M3 impaction risk. Contemporary modern humans present several distinct molar size ratio patterns, indicating that Homo sapiens molar size is subject to oral quadrant-specific and jaw type-specific effects beyond inhibition from a developing M1.

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.].

Evolutionary Trends of Protypotherium (Interatheriidae, Notoungulata) Lineage throughout the Miocene of South America

Protypotherium (Mammalia, Notoungulata, lnteratheriinae) is a well-known and very diverse genus of extinct native ungulates of South America, widely distributed from southern to middle latitudes of Argentina, Chile, and Bolivia. This genus exhibits distinctive species throughout the Miocene to the beginning of the Pliocene that display an interesting size pattern. The large sample of specimens studied during several years of research allows us to analyze the shape and size of upper and lower molars for nine species of Protypotherium, in order to test the hypothesis of reduction of size ranges preserving a general tooth morphology as a response to climate deterioration, documented in South America by a marked reduction of temperature that occurred during Miocene times. Elliptic Fourier analyses were used to capture the shape of the occlusal morphology and the centroid size was also retained for subsequent analyses. Our results demonstrate that: 1) a similar morphological tooth pattern is observed among all species; 2) tooth shape variation is not associated with a change in size; and 3) there is a tendency to increase the size and decrease the number of species. This striking pattern could be correlated with a global trend to lower temperatures, which indicates a deterioration of paleoenvironmental conditions. A conservative tooth pattern, together with an increase of size and a reduction in number of species were the main evolutionary and ecological tendencies accounted in Protypotherium from the early to late Miocene-beginning of the Pliocene, which were accompanied by a latitudinal displacement from Patagonia to northern areas at the end of the Miocene.