Molar Row Research Articles

Rethinking Wear Rate Analysis: a New Dentin Exposure Proxy and its Applications to Ancient Chinese Populations

Assessing age through dentin exposure often leads to underestimated age due to assumptions of constant molar wear rate. New methods for age-related dentin exposure accrual could facilitate cross-population comparisons independent of dietary habits and sociocultural strategies. We analyzed 3D dentin exposure surfaces in four Chinese archaeological samples to reveal variations in dentin exposure rates linked to socioeconomic practices. Linear regression models of dentin exposure areas across molar rows showed significant correlations, with the first molars exhibiting steeper slopes and smaller intercepts compared to the second molars, which had intermediate values, and the third molars showing the highest intercepts and lowest slopes. The first molar contributed most to overall dentin exposure in the molar quadrant, while the second molar wore faster post-eruption. Among populations, Banlashan, predominantly agriculturalist; Houtaomuga, focused on fishing; and Jiayi, a nomadic hunting society, displayed similar wear rate patterns. In contrast, Dunping, a Bronze Age nomadic settlement situated on a high-altitude plateau, exhibited distinctively lower wear rates. These observed dentin exposure rates aligned with ecological and dietary constraints, enabling interpopulation comparisons using the proposed 3D dentin exposure proxy. Moreover, the statistical model allows for comparing wear rates across populations relative to dietary habits and potentially estimating age at death for isolated archaeological specimens, whether humans or animals. The precision of this physiological age estimation depends on the regression models used, necessitating further research with specimens of known age at death.

Molar crown morphometrics in Paucituberculata (Mammalia: Marsupialia)

ABSTRACT The Paucituberculata are a group of marsupials endemic to South America, recorded from the Early Eocene to the present. In terms of taxonomy and qualitative morphology, paucituberculatans are a quite diverse and disparate group, especially considering their extinct representatives. This study aimed to investigate the variability in the quantitative morphology of paucituberculatans in the context of the largest morphometric study of the group performed to date. Due to the fragmentary fossil record, this work focused on lower dental morphology which allowed us to compare around 95% of paucituberculatan species through the measurement and analysis of almost 1500 specimens. Dental measurements of 27 variables were analysed by Principal Component Analysis and Partial Least Squares correlation as exploratory methods. As a result, it is shown that the continuous variability is expressed not only in size differences among species but also in morphological variability of individual teeth, and differences in size and proportions along the molar row. We conclude that paucituberculatans are a group of important morphological diversity regarding quantitative morphology. Many of the quantitative taxonomic features discussed in the literature were corroborated in this study. In addition, we discussed potential evolutionary and palaeoecological implications of this study, highlighting prospects for future studies.

Morphometric analysis of the skulls of a ram and ewe Romanov sheep (Ovis aries) with 3D modelling.

With the development of technology, 2D images have left their place for 3D models. The 3D modelling technique is widely used in plastic surgery, orthopaedic surgery, neurosurgery, traumatology, dentistry and medical education. The skull is important in terms of containing the starting parts of systems with vital functions. The aim of the study is to reveal the difference between male and female and other species by 3D (three-dimensional) modelling and craniometric measurements of Romanov heads. In our study, skulls of Romanov sheep (10 females and 10 males) older than 1-year-old were used. The heads of Romanov sheep were scanned with computed tomography and modelled in 3D. In the study, it was determined that there was a statistically significant difference between male and female sheep in terms of the largest nose length, facial width, molar row length, viscerocranium length, and foramen magnum height parameters (p < 0.05). It was determined that the greatest width of the foramen magnum measurement parameter and the skull index showed statistically significant differences between the genders at the p < 0.01 level. No statistically significant difference was found in other measurements (p > 0.05). The data obtained as a result of the study will help in the racial discrimination and classification of bones obtained from zoo archaeological excavations.

Foramen Magnum and Dental Osteometry of Balami, Uda, and Yankasa Breeds of Sheep

The increasing demand for animal protein, particularly meat, requires a thoughtful approach to selecting the type of meat-producing animal to achieve maximum yields. This is crucial to maintain food security. This study aimed to evaluate and compare some skull osteometries. A total of 60 each of Balami, Uda, and Yankasa sheep (30 adult males and 30 adult females) were used in this study. The skulls were macerated using a hot water technique. Then 7 parameters were measured using a digital Vernier caliper. The obtained data were analyzed using ANOVA (p &lt; 0.05). The result of this study was that the mean foramen magnum height (FMH) was highest, medium, and lowest in the Balami (2.1 ± 0.1 cm), Uda (1.9 ± 0.1 cm), and Yankasa (1.8 ± 0.0 cm) breeds, respectively, and vice versa in the foramen magnum index (FMI). The foramen magnum width (FMW) was similar in Balami and Uda but higher than in Yankasa (1.8 ± 0.1 cm). The FMH, FMW, and FMI were similar within sexes of the same breeds in Balami, Uda, and Yankasa. The mean dental length (DL), oral palatal length (OPL), and length of the upper molar row (LUM) were highest and lowest in Balami and Yankasa, respectively, while the mean length of the upper premolar row (LUP) was highest and lowest in Balami and Uda, respectively. The (DL), (OPL), (LUM), and (LUP) were higher in males than females in Balami and vice versa in Uda and Yankasa. In conclusion, craniometric values, including foramen magnum and dental osteometry, of indigenous Nigerian sheep breeds were used to compare them to other breeds and among themselves. It could also guide oral surgery and assist in identifying sheep breeds through comparative study.

Molar wear in house mice: insight into diet preferences at an ecological timescale?

Abstract In molars without permanent eruption, wear deeply modifies the geometry of the crown. To test for a signature of diet on wear dynamics, the molar geometry was compared between commensal house mice, relying on an omnivorous-granivorous diet, and Sub-Antarctic relatives, characterized by a switch towards a more ‘predatory’ behaviour. Laboratory-bred offspring of commensal mice served as a reference by providing mice of known age. Molar geometry was quantified using dense 3D semi-landmark based descriptors of the whole molar row and the upper molar only. Laboratory offspring displayed a decreased rate of wear compared to their commensal relatives, due to reduced mastication in mice fed ad libitum. Sub-Antarctic mice displayed a similarly decreased rate of molar wear, in agreement with an optimization towards incisor biting to seize prey. Laboratory offspring and Sub-Antarctic mice were further characterized by straight molar rows, whereas in commensal mice, the erupting third molar deviated away from the longitudinal alignment with the other molars, due to masticatory loadings. Quantifying changes in molar geometry could thus contribute to trace subtle diet variations, and provide a direct insight into the constraints during mastication, shedding light on the functional role of adaptive changes in molar geometry.

Two new species of ektopodontid marsupial from the lower deposits of the Etadunna Formation (latest Oligocene), South Australia and a phylogenetic hypothesis for the Ektopodontidae

Molar morphologies of the latest Oligocene genera, Chunia and Ektopodon, are easy to distinguish, but species determinations are more difficult. As more specimens have been collected, more maxillary fragments with molars and/or alveoli and a portion of the rostrum preserved are available. With a suite of maxillae now known, new ektopodontid features have revealed themselves, including the shortening of the rostrum compared to other families of contemporary possums. The facial angulation represented by the flexure of the rostrum at the boundary between P3 and the molar row results in the rostrum being angled differently than the molar row. Ektopodontids have a significantly greater angulation and thus shorter faces, compared to that seen in contemporary phalangeriform possums. Two new species of the genus Chunia, from the lower Etadunna Formation, South Australia, can be distinguished from the type species, Chunia illuminata, of the Ditjimanka Local Fauna. Differences in molar morphologies and the degree of facial angulation distinguish a new species of Chunia from the lowest faunal zone and the Minkina Local Fauna (facial angulation of 52°) from C. illuminata from the next higher faunal zone (Ditjimanka Local Fauna), which has a greater facial angulation of 63°, while a second new species of Chunia, also from the Dijimanka Local Fauna, has a facial angulation of 58° and intermediate molar traits. The degree of facial shortening is a valid discriminator between species of ektopodontid marsupials as it coincides with the species level differences in dental morphology. Additionally, C. omega from the Tarkarooloo Local Fauna is placed in new genus, Ngathachunia, as it is outside of the hypodigms for both of the previously described genera, Chunia and Ektopodon. A well-supported phylogenetic hypothesis for the family indicates the species of Ektopodon form a monophyletic clade, while the species of Chunia are paraphyletic.Judd A. Case [jcase@ewu.edu], Department of Biology, Eastern Washington University, Cheney, WA 99004, USA. Case, J.A., Month, 202X. Two new species of ektopodontid marsupial from the lower deposits of the Etadunna Formation (latest Oligocene), South Australia and a phylogenetic hypothesis for the Ektopodontidae. Alcheringa XXX, X–X. ISSN 0311-5518.

Identification of species of the genus Sylvaemus of northeastern Ukraine by metric characters

The reliable identification of representatives of the genus Sylvaemus is compli-cated by the huge coincidence of external and metrical characters, as well as by the fact that in most of their range, mice are in conditions of sympatry or even syntopy Moreover, identification is complicated by opposite clinal variability in size and geographic variability in fur colouration. The species are also similar in karyotypes, so their reliable identification is usually possible only based on biochemical or molecular analysis. The craniological collection of small mam-mals of O. V. Zorya and collection specimens of the Museum of Nature of V. N. Karazin Kharkiv National University were studied. In total, 436 speci-mens of three species of the genus Sylvaemus were analysed: Sylvaemus ura-lensis, Sylvaemus sylvaticus, and Sylvaemus tauricus. Analysed were 14 odon-tometric and 23 craniological characters. Among the metric characters, three odontometrical—width of first upper molar (WM1), length of second lower molar (LM2), length of lower molar row (LM123)—and seven craniometrical characters—length of first upper molar (LM1), length of foramen incisivum (LFI), length of upper molar row (LUM), width of choana (WCH), height of skull including bulla tympanica (HBCB), length of lower molar row (LLM), length of mandible (LM)—made the greatest contribution to the differentiation of mice of the genus Sylvaemus. Width of first upper molar (WM1), length of first lower molar (LM1), length of second lower molar (LM2), length of brain-case (LBC), least interorbital constriction (LIOC), width of braincase (WBC), distance between incisor and M3 (LIM3), length of upper molar row (LUM), condylobasal length of skull (CBL), palatal length (LPP), greatest length of skull (GLS), and length of lower molar row (LLM) are the least variable metric char-acters. Scatterplots linking LLM/CBL, LUM/CBL, and LUM/GLS appeared to be the most suitable for differentiating the specimens, with minimal overlap of species in the morphospace. In north-eastern Ukraine, the best distribution of mice of the genus Sylvaemus was obtained as a result of the analysis based on both odontometrical and craniological characters, when the correctness of the classification was 100%.

Dental topography and dietary specialization in Papionini primates

Our understanding of primate adaptive evolution depends on appreciating the way in which dental functional morphology affects food processing. The Papionini tribe ofCercopithecoideaprimates shows great dietary versatility and ecological adaptations to resource seasonality across the African and Asian ecosystems, however, there are few studies focusing on the occlusal topography of the bilophodont teeth and the effect of tooth wear in the crown shape. Here, we explore the relationship between wear-related dental functional morphology and dietary ecological constraints within the Papionini. Three-dimensional (3D) polygonal meshes of the upper permanent molar row (M1-3) were obtained in a large papionine sample (838 specimens) of known dietary preferences including species from six genera (Cercocebus,Lophocebus,Macaca,Mandrillus,Papio, andTheropithecus). All the sample was classified in four diet categories and four topographic metrics (orientation patch count rotated, OPCR; Dirichlet normal energy, DNE; occlusal relief, OR; and ambient occlusion, portion de ciel visible, PCV) were measured for each tooth-type according to wear stage (lightly and moderately worn) to determine diet-related interspecific morphological changes with long-term functionality. The results indicate that hard-object feeders (CercocebusandLophocebus) and grass eaters (Theropithecus gelada) exhibit a pattern of occlusal complexity (OPCR), surface curvature (DNE), relief (OR), and morphological wear resistance (PCV) that is significantly different from the omnivores and folivore-frugivore species (MandrillusandMacaca) despite the overall homogeneity of the bilophodont dentition. A multifactorial ANOVA showed that the topographic metrics were sensitive to tooth wear as expected. The results also indicate that the interspecific variability of dental topography of the upper molars reflects dietary specializations rather than phylogenetic proximity. These findings support the hypothesis that evolutionary convergence processes could have affected the Papionini, clustering the hard-object feeders (LophocebusandCercocebus)together in the morphospace, and clearly discriminating this group from the graminivorous and frugivores-folivores.

Cranial differences in three-toed jerboas (Dipodinae, Dipodidae, Rodentia) according to recent taxonomic revisions.

Recent phylogenetic studies amended the taxonomy of three-toed jerboas (subfamily Dipodinae), including raising subspecies to full species. Here, we use geometric morphometrics to compare scaled-shape differences in dipodine crania while considering their revised taxonomy. We sampled Dipus deasyi, D. sagitta halli, D. s. sowerbyi, Jaculus blanfordi blanfordi, J. hirtipes, J. jaculus, J. loftusi, J. orientalis gerboa, J. o. mauritanicus, and Stylodipus andrewsi. Crania were not sexually dimorphic. Common allometry explained some of the shape variation, for example, reduced braincases in larger specimens. Most operational taxonomic unit pairs differed in both size and shape. Dipus and Stylodipus clustered together based on their cranial shape. Jaculus differed from the aforementioned genera by its larger tympanic bulla, broader braincase, larger infraorbital foramen, along with reduced molars and rostra. Jaculus orientalis differed from other Jaculus by its broader face versus reduced cranial vault. Jaculus blanfordi (subgenus Haltomys) resembles members of the subgenus Jaculus more than its consubgener (J. orientalis). Jaculus loftusi, previously considered a synonym of J. jaculus, clearly differed from the latter by its shorter rostrum, smaller infraorbital foramen, and more caudolaterally expanded tympanic bulla. Jaculus hirtipes, another recent synonym of J. jaculus, resembled J. blanfordi more in scaled cranial shape than it did J. jaculus. Dipus sagitta halli and D. s. sowerbyi were indistinguishable, but they clearly differed from D. deasyi (recently raised to full species) with the latter having a larger molar row, more inflated tympanic bulla, and shorter, slenderer rostrum. Ecological explanations for detected cranial shape differences are considered, including diet and habitat (particularly substrate).

Comparative morphology of the dormouse skull and the influence of size and ecology.

Dormice are widely dispersed across various ecosystems in Eurasia and Africa and among the oldest extant rodent lineages. Despite distinct morphological variation to be present between groups, comprehensive morphometrical studies on the dormouse skull are limited. Here, the form of eight out of the nine extant dormouse genera was analysed using 3D geometric morphometrics and linear biomechanical measurements, providing a better understanding of the overall morphological variation present within Gliridae. Species-, genus- and family-specific morphological trends in both the size and shape of the cranium and mandible are linked with specific habitats and feeding strategies. Smaller dormice show adaptations to a more arboreal lifestyle such as a relatively enlarged braincase and an inferiorly reoriented foramen magnum. Larger dormice show cranial modifications, including clear flattening of the skull and a more posteriorly positioned foramen magnum, hinting towards a more rupicolous lifestyle. Furthermore, specimens inhabiting arid areas appear to have more inflated auditory bullae, whereas other variable features, such as the length of the incisive foramen, were not associated with either size changes or climatic variables. Lastly, more robust and horizontally orientated zygomatic arches as well as increased robusticity of the molar row appear to be linked with herbivory in dormice, whereas thinner arches and small concave molars are seen in more insectivorous species. This study reveals clear convergent adaptations between dormouse species and results in a better understanding of ecological drivers underpinning the morphological divergence present within Gliridae.

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.

3D models related to the publication: Wild versus lab house mice: Effects of age, diet, and genetics on molar geometry and topography.

This contribution contains 3D models of upper molar rows of house mice (Mus musculus domesticus). The erupted parts of the right row and of the first upper molar are presented for specimens belonging to four groups: wild-trapped mice, wild-derived lab offspring, a typical laboratory strain (Swiss) and hybrids between wild-derived and Swiss mice. These models are analyzed in the following publication: Savriama et al 2021: Wild versus lab house mice: Effects of age, diet, and genetics on molar geometry and topography

Wild versus lab house mice: Effects of age, diet, and genetics on molar geometry and topography.

Molar morphology is shaped by phylogenetic history and adaptive processes related to food processing. Topographic parameters of the occlusal surface, such as sharpness and relief, can be especially informative regarding diet preferences of a species. The occlusal surface can however be deeply modified by wear throughout an animal's life, potentially obliterating other signals. Age being difficult to assess in wild populations, especially small rodents, experimental studies of wear through age in laboratory populations may constitute a powerful way to assess its impact on molar geometry and topography, and to validate descriptors of molar morphology that could mitigate this issue. Molar morphology was therefore quantified using 3D geometric morphometrics and topographic estimates in four groups of house mice: wild-trapped mice, lab-bred offspring of these wild mice, typical laboratory mice, and their hybrids. Three descriptors of the molar morphology were considered: the surface of the whole molar row, the surface of the first upper molar, and a truncated template of the first upper molar mimicking advanced wear. Increasing wear with age was demonstrated in the different groups, with a more pronounced effect in the wild-trapped population. The geometry of the molar row is not only modified by wear, but also by the relative position of the late developing molars on the jaw due to loading during mastication. As a consequence, the alignment of the molars is modified in wild mice, showing a qualitative difference between wild animals and their lab-bred offspring. Results obtained from the lab should thus be transferred with caution to the interpretation of differences in wild populations. Topographic estimates computed for the first upper molar seems to provide more stable parameters than those based on the whole molar row, because issues related to non-planar occlusal surface along the molar row are discarded. The truncated template was proven efficient in discarding the wear effect to focus on genetic differences, allowing an efficient characterization of the hybridization signature between wild and lab mice. Dominance of the wild phenotype for the first molar shape supports that the lab strain evolved in a context of relaxation of the selective pressures related to nutrition.

Morphometric analysis of skull features and sexual size dimorphism in Microtus arvalis from Slovakia

The results of a morphometric analysis of the common vole, Microtus arvalis, were evaluated in this study. Twenty cranial variables were measured and evaluated on 105 skulls (34 males, 71 females) with respect to sex. The main aim of this study was to analyse the presence of sexual dimorphism on skull morphological variation. We showed that values for adult females of M. arvalis were higher than for males. Therefore, the results presented here reveal sexual differences in the measured cranial traits most expressed for the length of the mandible, the length of the lower molar row measured in alveoli and the thickness of the incisor.

A morphometric comparison of the cranial shapes of Asian dwarf hamsters (Phodopus, Cricetinae, Rodentia)

Phodopus is the smallest cricetine genus, consisting of the dwarf hamsters (Phodopus sungorus, Phodopus campbelli, and Phodopus roborovskii) of arid central and east Asia. Persistent taxonomic issues in this genus include (1) whether P. sungorus and P. campbelli are con- or heterospecific and (2) whether P. roborovskii is con- or heterogeneric with the aforementioned species. To shed new light on these questions, the crania of Phodopus species were compared to each other using geometric morphometrics. Furthermore, to determine what cranial traits are shared by Phodopus crania, they were compared to the more ‘typical’ small hamster Nothocricetulus migratorius. The three Phodopus species significantly differed in cranial size and shape from each other and from N. migratorius. A large part of shape variation was allometric with only weak evidence for unique allometries. All four species lacked sexual dimorphism in cranial size and shape. As a genus, Phodopus is characterized by small-sized crania, that are broad, short, and with flaring zygomatic arches. They also have reduced rostra and enlarged foramina magna. P. roborovskii was the most distinct Phodopus, characterized by its smaller-sized cranium, with greater breadth, a rounder braincase and orbit, a shorter incisive foramen and rostrum, a larger foramen magnum, and smaller bulla with a longer eustachian tube. The crania of P. sungorus and P. campbelli are very similar, but they do diverge in relatively minor (statistically nonsignificant) features, such as cranial roundness, as well as the size of the molar row, bulla, zygomatic arch, and incisive foramen. Shape differences conformed to phylogenetic distance and with habitat differences.

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.].

Mammalian molar complexity follows simple, predictable patterns

Identifying developmental explanations for the evolution of complex structures like mammalian molars is fundamental to studying phenotypic variation. Previous study showed that a "morphogenetic gradient" of molar proportions was explained by a balance between inhibiting/activating activity from earlier developing molars, termed the inhibitory cascade model (ICM). Although this model provides an explanation for variation in molar proportions, what remains poorly understood is if molar shape, or specifically complexity (i.e., the number of cusps, crests), can be explained by the same developmental model. Here, we show that molar complexity conforms to the ICM, following a linear, morphogenetic gradient along the molar row. Moreover, differing levels of inhibiting/activating activity produce contrasting patterns of molar complexity depending on diet. This study corroborates a model for the evolution of molar complexity that is developmentally simple, where only small-scale developmental changes need to occur to produce change across the entire molar row, with this process being mediated by an animal's ecology. The ICM therefore provides a developmental framework for explaining variation in molar complexity and a means for testing developmental hypotheses in the broader context of mammalian evolution.

Enamel thickness variation in the deciduous dentition of extant large-bodied hominoids.

Enamel thickness features prominently in hominoid evolutionary studies. To date, however, studies of enamel thickness in humans, great apes, and their fossil relatives have focused on the permanent molar row. Comparatively little research effort has been devoted to tissue proportions within deciduous teeth. Here we attempt to fill this gap by documenting enamel thickness variation in the deciduous dentition of extant large-bodied hominoids. We used microcomputed tomography to image dental tissues in 80 maxillary and 78 mandibular deciduous premolars of Homo sapiens, Pan troglodytes, Gorilla, and Pongo. Two-dimensional virtual sections were created from the image volumes to quantify average (AET) and relative (RET) enamel thickness, as well as its distribution across the crown. Our results reveal no significant differences in enamel thickness among the great apes. Unlike the pattern present in permanent molars, Pongo does not stand out as having relatively thicker-enameled deciduous premolars than P. troglodytes and Gorilla. Humans, on the other hand, possess significantly thicker deciduous premolar enamel in comparison to great apes. Following expectations from masticatory biomechanics, we also find that the "functional" side (protocone, protoconid) of deciduous premolars generally possesses thicker enamel than the "nonfunctional" side. Our study lends empirical support to anecdotal observations that patterns of AET and RET observed for permanent molars of large-bodied apes do not apply to deciduous premolars. By documenting enamel thickness variation in hominoid deciduous teeth, this study provides the comparative context to interpret rates and patterns of wear of deciduous teeth and their utility in life history reconstructions.

3D Morphometric Analysis Reveals Similar Ecomorphs for Early Kangaroos (Macropodidae) and Fanged Kangaroos (Balbaridae) from the Riversleigh World Heritage Area, Australia

Understanding feeding ecology of extinct kangaroos is fundamental to understanding the evolution of kangaroos and the Australia paleoenvironment during the Oligo-Miocene. Comparisons with extant species have suggested that the macropodiforms of the Oligo/Miocene (kangaroos and allies) from the Riversleigh World Heritage Area, northern Australia, were predominantly folivorous browsers or fungivores, unlike the majority of extant species. To further test this hypothesis, we investigate the relationship between variation in cranial and mandibular shape of extant and extinct macropodiforms and ecological factors such as diet, locomotion, and body mass using 3D geometric morphometric analysis of 42 living species and eight extinct species from two radiations (the extinct clade of Balbaridae and some early representatives of the extant Macropodidae. Dietary class (fungivore, browser, grazer, and mixed feeder) correlated strongly with variation in cranial shape (20–25% of variance explained). There was also significant association between cranial shape, and both locomotor mode and body mass. In a principal component analysis of shape variation for crania (including the shape of the molar row), Riversleigh macropodiforms cluster with extant folivorous browsers on principal components (PC) 1 and 3, providing support for previous interpretations of these species as browsing kangaroos. However, as a group and regardless of phylogenetic association, the shape centroid of extinct species differs significantly from that of extant species. Riversleigh macropodiforms cluster with regular hoppers or arboreal tree kangaroos, but this may be a result of the correlation between diet and locomotor mode in kangaroos. Their similarity to extant browsers supports previous interpretations of rainforest and woodland environments at Riversleigh during the early and middle Miocene, respectively. Procrustes ANOVA Analysis of the full shape dataset and diet also shows that diet accounts for a significant portion of variation; however, when phylogeny is taken into account these results become nonsignificant. In analyses of dentary shape, some balbarid species cluster with extant mixed feeders, although this may reflect phylogenetic differences rather than ecological signal.

The geography of speciation in dasyurid marsupials

AbstractAimTo determine the effects of competition and divergence time on morphological dissimilarity and geographical range overlap between dasyurid species at both regional and local scales. Our hypothesis is that speciation in this group has been largely allopatric at regional scale, but involved morphological divergence at local scale through sympatric character displacement.LocationAustralia, New Guinea and surrounding islands.TaxonDasyurid (Dasyuridae) marsupials, 67 species.MethodsGeographical range overlap was quantified using polygons representing the outer limits of species distributions. Local‐scale range overlap was quantified as the degree of co‐occurrence of two taxa across a set of ecological survey plots representing 83 sampled communities. Phylogenies were generated using a novel DNA dataset, with divergence times estimated via total‐evidence dating incorporating fossils. Morphological divergence was determined using body mass and lower molar row length as proxy traits for reconstructing niche exploitation.ResultsSister species pairs were found to be sympatric in 52% (11/21) of cases. Range overlap tended to increase with node age, which supports the hypothesis that mammalian speciation is routinely allopatric. We detected no evidence of character displacement with increasing range overlap between sister species pairs. However, a negative relationship was observed between morphological divergence in body mass and range overlap across all sampled taxa, suggesting that selection in sympatry is convergent, while divergent selection occurs in allopatry. Local‐scale co‐occurrences revealed no trace of species aversion, indicating that competition has not impacted on the spatial distribution of dasyurids.Main conclusionsDespite moderate levels of sympatry through time, our results evince low rates of spatial co‐occurrence between dasyurid species. Although this may be indicative competitive exclusion, the lack of character displacement suggests that biotic interactions have likely not acted as a dominant driver of phenotypic evolution in this radiation. We alternatively posit that abiotic factors including aridity and geographical connectivity have more feasibly propagated character convergence, and led to both niche conservatism and speciation in this ubiquitous australidelphian clade.