Accuracy of dental microwear impressions by physical properties of silicone materials

Dental microwear analysis is an oft-used paleodietary estimation method, and the impression molds or resin casts are often analyzed rather than the original tooth surfaces. A choice of silicone products for dental impressions is crucial because the quality of microwear data is affected by the impression accuracy of the molds. For this reason, microwear researchers have heavily depended on a few commercial products such as "President" (Coltène/Whaledent AG, Switzerland) to avoid analytical errors caused using different silicone materials. Considering that the production business might be terminated, however, heavy reliance on specific products could be a potential weakness in the field. In this study, we aimed at identifying specific indexes of physical properties of silicone materials with satisfactory accuracy. For this purpose, we measured dynamic viscoelasticity and shrinkage rates of various silicone compounds, including the standard impression material President and other eight affordable dental silicones. We scanned both original tooth surface and dental impression molds with a confocal laser microscope and conducted dental microwear texture analysis (DMTA) to quantitatively compare the scanned surfaces. The results showed relationships between the material properties of silicones and impression accuracy, indicating that the materials that cured slowly and began to shrink relatively early in the hardening process were less accurate. Some of these dental impression molds showed blurred surfaces, implying that molds were peeled off from the tooth surface at the microscopic level, as the shrinkage speed might exceed the curing speed. The following indices provided in the product information were found to be helpful in the search for substitutes: (1) medium viscosity, (2) short curing time after mixing (5-6 min), and (3) delayed change in shrinkage

Microwear textures associated with experimental near-natural diets suggest that seeds and hard insect body parts cause high enamel surface complexity in small mammals

In mammals, complex dental microwear textures (DMT) representing differently sized and shaped enamel lesions overlaying each other have traditionally been associated with the seeds and kernels in frugivorous diets, as well as with sclerotized insect cuticles. Recently, this notion has been challenged by field observations as well as in vitro experimental data. It remains unclear to what extent each food item contributes to the complexity level and is reflected by the surface texture of the respective tooth position along the molar tooth row. To clarify the potential of seeds and other abrasive dietary items to cause complex microwear textures, we conducted a controlled feeding experiment with rats. Six individual rats each received either a vegetable mix, a fruit mix, a seed mix, whole crickets, whole black soldier fly larvae, or whole day-old-chicks. These diets were subjected to material testing to obtain mechanical properties, such as Young’s modulus, yield strength, and food hardness (as indicated by texture profile analysis [TPA] tests). Seeds and crickets caused the highest surface complexity. The fruit mix, seed mix, and crickets caused the deepest wear features. Moreover, several diets resulted in an increasing wear gradient from the first to the second molar, suggesting that increasing bite force along the tooth row affects dental wear in rats on these diets. Mechanical properties of the diets showed different correlations with DMT obtained for the first and second molars. The first molar wear was mostly correlated with maximum TPA hardness, while the second molar wear was strongly correlated with maximum yield stress, mean TPA hardness, and maximum TPA hardness. This indicates a complex relationship between chewing mechanics, food mechanical properties, and observed DMT. Our results show that, in rats, seeds are the main cause of complex microwear textures but that hard insect body parts can also cause high complexity. However, the similarity in parameter values of surface textures resulting from seed and cricket consumption did not allow differentiation between these two diets in our experimental approach

Variation and process of life history evolution in insular dwarfism as revealed by a natural experiment

Islands are a classic focus for evolutionary studies. One topic of great interest has been the evolution of “dwarfs,” significantly smaller island mammals relative to their continental counterparts. Although a consensus has been achieved regarding the multivariate ecological causes behind changes in body size, the processes involved remain largely unexplored. Life history variables, including age at first reproduction, growth rate, and longevity, are likely to be key to understanding the process of insular dwarfism. The Japanese archipelago, with its numerous islands, offers a unique natural experiment for the evolution of different sizes within the same group of organisms; namely, deer. Thus, we investigated eight deer populations with a total number of 52 individuals exhibiting body size variation, both extant and fossil, to clarify the effect of insularity on life history traits. We applied several methods to both extant and extinct populations to resolve life history changes among these deer populations. Skeletochronology, using lines of arrested growth formed in long bones (femur and tibia), successfully reconstructed body growth curves and revealed a gradual change in growth trajectories reflecting the degree of insularity. Slower growth rates with prolonged growth periods in more isolated deer populations were revealed. An extensive examination of bone microstructure further corroborated this finding, with much slower growth and later somatic maturity evident in fossil insular deer isolated for more than 1.5 Myr. Finally, mortality patterns assessed by demographic analysis revealed variation among deer populations, with a life history of insular populations shifting toward the “slow life.”Hayashi S., Kubo M.O., Sánchez-Villagra M.R., et al. Variation and process of life history evolution in insular dwarfism as revealed by a natural experiment. Frontiers in Earth Science 11, 1095903 (2023); https://doi.org/10.3389/feart.2023.1095903

Variation and process of life history evolution in insular dwarfism as revealed by a natural experiment

Islands are a classic focus for evolutionary studies. One topic of great interest has been the evolution of “dwarfs,” significantly smaller island mammals relative to their continental counterparts. Although a consensus has been achieved regarding the multivariate ecological causes behind changes in body size, the processes involved remain largely unexplored. Life history variables, including age at first reproduction, growth rate, and longevity, are likely to be key to understanding the process of insular dwarfism. The Japanese archipelago, with its numerous islands, offers a unique natural experiment for the evolution of different sizes within the same group of organisms; namely, deer. Thus, we investigated eight deer populations with a total number of 52 individuals exhibiting body size variation, both extant and fossil, to clarify the effect of insularity on life history traits. We applied several methods to both extant and extinct populations to resolve life history changes among these deer populations. Skeletochronology, using lines of arrested growth formed in long bones (femur and tibia), successfully reconstructed body growth curves and revealed a gradual change in growth trajectories reflecting the degree of insularity. Slower growth rates with prolonged growth periods in more isolated deer populations were revealed. An extensive examination of bone microstructure further corroborated this finding, with much slower growth and later somatic maturity evident in fossil insular deer isolated for more than 1.5 Myr. Finally, mortality patterns assessed by demographic analysis revealed variation among deer populations, with a life history of insular populations shifting toward the “slow life.

Data from: Associated evolution of bipedality and cursoriality among Triassic archosaurs: a phylogenetically controlled evaluation.

Bipedalism evolved more than twice among archosaurs, and it is a characteristic of basal dinosaurs and a prerequisite for avian flight. Nevertheless, the reasons for the evolution of bipedalism among archosaurs have barely been investigated. Comparative analysis using phylogenetically independent contrasts showed a significant correlation between bipedality (relative length of forelimb) and cursoriality (relative length of metatarsal III) among Triassic archosaurs. This result indicates that, among Triassic archosaurs, bipeds could run faster than quadrupeds. Bipedalism is probably an adaptation for cursoriality among archosaurs, which may explain why bipedalism evolved convergently in the crocodilian and bird lineages. This result also indicates that the means of acquiring cursoriality may differ between archosaurs and mammals

Appendix

Results of correlation tests between cursoriality index and body size, and between cursoriality index and bipedality index after the effect of body size was removed, among Triassic archosaurs. The default settings of the correlation tests were (1) a branch length of one set for all branches and (2) reconstructed phylogenetic relationships among Triassic archosaurs (Fig. 2). Additional sensitivity analyses with arbitrary branch length (Grafen, Nee, and Pagel), temporal duration as branch lengths, or polytomic relationship (Fig. 2B) did not alter the result significantly. The analysis was also conducted with restricted taxa, which are crurotarsans, ornithodirans, and ornithodirans without sauropodomorphs except Eoraptor

Nonplantigrade Foot Posture: A Constraint on Dinosaur Body Size

<div><p>Dinosaurs had functionally digitigrade or sub-unguligrade foot postures. With their immediate ancestors, dinosaurs were the only terrestrial nonplantigrades during the Mesozoic. Extant terrestrial mammals have different optimal body sizes according to their foot posture (plantigrade, digitigrade, and unguligrade), yet the relationship of nonplantigrade foot posture with dinosaur body size has never been investigated, even though the body size of dinosaurs has been studied intensively. According to a large dataset presented in this study, the body sizes of all nonplantigrades (including nonvolant dinosaurs, nonvolant terrestrial birds, extant mammals, and extinct Nearctic mammals) are above 500 g, except for macroscelid mammals (i.e., elephant shrew), a few alvarezsauroid dinosaurs, and nondinosaur ornithodirans (i.e., the immediate ancestors of dinosaurs). When nonplantigrade tetrapods evolved from plantigrade ancestors, lineages with nonplantigrade foot posture exhibited a steady increase in body size following Cope’s rule. In contrast, contemporaneous plantigrade lineages exhibited no trend in body size evolution and were largely constrained to small body sizes. This evolutionary pattern of body size specific to foot posture occurred repeatedly during both the Mesozoic and the Cenozoic eras. Although disturbed by the end-Cretaceous extinction, species of mid to large body size have predominantly been nonplantigrade animals from the Jurassic until the present; conversely, species with small body size have been exclusively composed of plantigrades in the nonvolant terrestrial tetrapod fauna.</p></div

Body mass distribution for terrestrial tetrapod groups.

<p>Histograms for extant mammals of all foot postures (A), extinct Nearctic nonplantigrade mammals (B), nonvolant terrestrial birds (C), and Mesozoic dinosaurs including avialans (D). The dotted line indicates 500 g. In the histogram of extant mammals (A), plantigrade species are colored in white, species of Macroscelididae (elephant shrew) are in gray, and other nonplantigrade species are in black. In the histogram of dinosaurs (D), volant dinosaurs are in white, theropods are hatched, ornithischians are in gray, and sauropodomorphs are in black. Representatives of each group are shown as silhouettes on histograms. Except for Macroscelididae and two dinosaur species, all terrestrial nonplantigrade species were above 500 g. Except for nonvolant dinosaurs, the distributions of terrestrial nonplantigrade groups are not significantly different from a normal distribution.</p

Masticatory jaw movement of Exaeretodon argentinus (Therapsida: Cynodontia) inferred from its dental microwear.

Dental microwear of four postcanine teeth of Exaeretodon argentinus was analyzed using both two dimensional (2D) and three dimensional (3D) methods to infer their masticatory jaw movements. Results of both methods were congruent, showing that linear microwear features (scratches) were well aligned and mostly directed to the antero-posterior direction in all four teeth examined. These findings support the palinal masticatory jaw movement, which was inferred in previous studies based on the observation of gross morphology of wear facets. In contrast, the lack of detection of lateral scratches confirmed the absence of the lateral jaw movement that was also proposed by a previous study. Considering previous microwear studies on cynodonts, palinal jaw movements observed in Exaeretodon evolved within cynognathian cynodonts from the fully orthal jaw movement of its basal member. Although there are currently only three studies of dental microwear of non-mammalian cynodonts including the present study, microwear analysis is a useful tool for the reconstruction of masticatory jaw movement and its future application to various cynodonts will shed light on the evolutionary process of jaw movement towards the mammalian condition in more detail

Femur length of terrestrial nonplantigrade and plantigrade lineages from the Middle Triassic to Middle Jurassic.

<p>Log femur length was plotted against the midpoint of the geological age estimated for each taxon. Mean and variance of femur length in each time bin are plotted. These values were used to fit evolutionary models (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0145716#pone.0145716.s010" target="_blank">S9 Table</a>). A trend toward longer femur length, reflecting a larger body size, was found in the nonplantigrade lineage (dinosauromorphs and <i>Scleromochlus</i>, blue triangle), whereas no directional trend was observed in plantigrade lineages (nonornithodiran archosauromorphs, green crosses; therapsids, gray diamonds).</p