Morphology is not Destiny: Discrepancy between Form, Function and Dietary Adaptation in Bovid Cheek Teeth

Complex adaptive landscape for a "Simple" structure: The role of trade-offs in the evolutionary dynamics of mandibular shape in ground squirrels

Trade-offs are inherent features of many biomechanical systems and are often seen as evolutionary constraints. Structural decoupling may provide a way to escape those limits in some systems but not for structures that transmit large forces, such as mammalian mandibles. For such structures to evolve in multiple directions on a complex adaptive landscape, different regions must change shape while maintaining structural integrity. We evaluated the complexity of the adaptive landscape for mandibular shape in Marmotini, a lineage of ground squirrels that varies in the proportions of seeds and foliage in their diets, by comparing the fit of models based on traits that predict changes in mandibular loading. The adaptive landscape was more complex than predicted by a two-peak model with a single dietary shift. The large number of adaptive peaks reflects a high diversity of directions of shape evolution. The number of adaptive peaks also reflects a multiplicity of functional trade-offs posed by the conflicting demands of processing foods with various combinations of material properties. The ability to balance trade-offs for diets with different proportions of the same foods may account for diversification and disparity of lineages in heterogeneous environments. Rather than constraints, trade-offs may be the impetus of evolutionary change.

Experimental assessment of the relationship between diet and mandibular morphology using a pig model: new insights for paleodietary reconstructions

Dietary habits exert significant selective pressures on anatomical structures in animals, leading to substantial morphological adaptations. Yet, the relationships between the mandible and diet are still unclear, raising issues for paleodietary reconstructions notably. To assess the impact of food hardness and size on morphological structures, we used an experimental baseline using a model based on the domestic pig, an omnivorous mammal with bunodont, thick-enameled dentition, and chewing movements similar to hominids. We hypothesized that the consumption of different types of seeds would result in substantial differences in the morphology of the mandible despite similar overall diets. The experiment was conducted on four groups of juvenile pigs fed with mixed cereal and soy flours. The control group received only flours. We supplemented the four others with either 10 hazelnuts, 30 hazelnuts, 30% barley seeds or 20% corn kernels per day. We investigated the shape differences between the controlled-fed groups using three-dimensional geometric morphometrics. Our results provide strong evidence that the supplemental consumption of a significant amount of seeds for a short period (95 days) substantially modify the mandibular morphology of pigs. Our analyses suggest that this shape differentiation is due to the size of the seeds, requiring high and repeated bite force, rather than their hardness. These results provide new perspectives for the use of mandibular morphology as a proxy in paleodietary reconstructions complementing dental microwear textures analyses. This article is protected by copyright. All rights reserved.

The diet of early birds based on modern and fossil evidence and a new framework for its reconstruction

Birds are some of the most diverse organisms on Earth, with species inhabiting a wide variety of niches across every major biome. As such, birds are vital to our understanding of modern ecosystems. Unfortunately, our understanding of the evolutionary history of modern ecosystems is hampered by knowledge gaps in the origin of modern bird diversity and ecosystem ecology. A crucial part of addressing these shortcomings is improving our understanding of the earliest birds, the non-avian avialans (i.e. non-crown birds), particularly of their diet. The diet of non-avian avialans has been a matter of debate, in large part because of the ambiguous qualitative approaches that have been used to reconstruct it. Here we review methods for determining diet in modern and fossil avians (i.e. crown birds) as well as non-avian theropods, and comment on their usefulness when applied to non-avian avialans. We use this to propose a set of comparable, quantitative approaches to ascertain fossil bird diet and on this basis provide a consensus of what we currently know about fossil bird diet. While no single approach can precisely predict diet in birds, each can exclude some diets and narrow the dietary possibilities. We recommend combining (i) dental microwear, (ii) landmark-based muscular reconstruction, (iii) stable isotope geochemistry, (iv) body mass estimations, (v) traditional and/or geometric morphometric analysis, (vi) lever modelling, and (vii) finite element analysis to reconstruct fossil bird diet accurately. Our review provides specific methodologies to implement each approach and discusses complications future researchers should keep in mind. We note that current forms of assessment of dental mesowear, skull traditional morphometrics, geometric morphometrics, and certain stable isotope systems have yet to be proven effective at discerning fossil bird diet. On this basis we report the current state of knowledge of non-avian avialan diet which remains very incomplete. The ancestral dietary condition in non-avian avialans remains unclear due to scarce data and contradictory evidence in Archaeopteryx. Among early non-avian pygostylians, Confuciusornis has finite element analysis and mechanical advantage evidence pointing to herbivory, whilst Sapeornis only has mechanical advantage evidence indicating granivory, agreeing with fossilised ingested material known for this taxon. The enantiornithine ornithothoracine Shenqiornis has mechanical advantage and pedal morphometric evidence pointing to carnivory. In the hongshanornithid ornithuromorph Hongshanornis only mechanical advantage evidence indicates granivory, but this agrees with evidence of gastrolith ingestion in this taxon. Mechanical advantage and ingested fish support carnivory in the songlingornithid ornithuromorph Yanornis. Due to the sparsity of robust dietary assignments, no clear trends in non-avian avialan dietary evolution have yet emerged. Dietary diversity seems to increase through time, but this is a preservational bias associated with a predominance of data from the Early Cretaceous Jehol Lagerstätte. With this new framework and our synthesis of the current knowledge of non-avian avialan diet, we expect dietary knowledge and evolutionary trends to become much clearer in the coming years, especially as fossils from other locations and climates are found. This will allow for a deeper and more robust understanding of the role birds played in Mesozoic ecosystems and how this developed into their pivotal role in modern ecosystems.

Effects of different irrigation protocols on dentin surfaces as revealed through quantitative 3D surface texture analysis

The combination of ethylenediaminetetraacetic acid (EDTA) and sodium hypochlorite (NaOCl) has been advocated as an effective irrigation methodology to remove organic and inorganic matter in root canal therapy. Yet, it was suggested that both solutions might lead to structural changes of the dentinal wall surface, depending on the order of application which might affect sealer mechanical retention. This study aims to evaluate the effect of different irrigating protocols on dentin surface roughness using quantitative 3D surface texture analysis. Data stems from 150 human root dentin sections, divided into five groups, each prepared according to one of the following protocols: Negative control; 17% EDTA; 17% EDTA followed by 5.25% NaOCl; 5.25% NaOCl; and 5.25% NaOCl followed by 17% EDTA. Each dentin sample was examined for its three-dimensional surface texture using a high-resolution confocal disc-scanning measuring system. EDTA 17% and the combined EDTA 17% with NaOCl 5.25% showed considerably higher roughness properties compared to the control and to NaOCl 5.25% alone. However, the irrigation sequence did not affect the dentin roughness properties. Therefore, mechanical retention is probably not dependent upon the selection of irrigation protocol sequence.

The relationship between molar morphology and ecology within Neotoma

The 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.

Complementary approaches to tooth wear analysis in Tritylodontidae (Synapsida, Mammaliamorpha) reveal a generalist diet

Stereoscopic microwear and 3D surface texture analyses on the cheek teeth of ten Upper Triassic to Lower Cretaceous tritylodontid (Mammaliamorpha) taxa of small/medium to large body size suggest that all were generalist feeders and none was a dietary specialist adapted to herbivory. There was no correspondence between body size and food choice. Stereomicroscopic microwear analysis revealed predominantly fine wear features with numerous small pits and less abundant fine scratches as principal components. Almost all analyzed facets bear some coarser microwear features, such as coarse scratches, large pits, puncture pits and gouges pointing to episodic feeding on harder food items or exogenous effects (contamination of food with soil grit and/or dust), or both. 3D surface texture analysis indicates predominantly fine features with large void volume, low peak densities, and various stages of roundness of the peaks. We interpret these features to indicate consumption of food items with low to moderate intrinsic abrasiveness and can exclude regular rooting, digging or caching behavior. Possible food items include plant vegetative parts, plant reproductive structures (seeds and seed-bearing organs), and invertebrates (i.e., insects). Although the tritylodontid tooth morphology and auto-occlusion suggest plants as the primary food resource, our results imply a wider dietary range including animal matter.

Examination of the Interproximal Wear Mechanism: Facet Morphology and Surface Texture Analysis

Dentition is considered a dynamic system with forces that directly affect dental treatment stability and success. Understanding the biomechanical forces that influence tooth alignment is essential for both planning and performing dental treatments, as well as for anthropological and evolutionary studies. While there is currently an abundance of research on the mechanics of dental wear at the occlusal surface, the mechanics of interproximal dental wear is largely unexplored. The fretting mechanism, a wear process resulting from small-amplitude cyclic motion of 2 solid contacting surfaces, was refuted as a possible mechanism for occlusal wear but has never been considered for interproximal wear. Therefore, the aim of the current study was to reveal the biomechanical process of the interproximal wear and to explore whether the fretting mechanism could be associated with this process. Premolar teeth with interproximal wear facets were examined by 3-dimensional surface texture analysis using a high-resolution confocal disc-scanning measuring system. The unique texture topography of 3 areas in the proximal surface of each tooth was analyzed by applying 3D dental surface texture analysis. Each area showed unique texture characteristics, presenting statistically significant differences between the inner area of the facet and its margins or the surface outside the facets borders. Based on these results, we concluded that fretting is a key mechanism involved in interproximal wear.

Tooth wear as a means to quantify intra-specific variations in diet and chewing movements

In mammals, tooth function, and its efficiency, depends both on the mechanical properties of the food and on chewing dynamics. These aspects have rarely been studied in combination and/or at the intra-specific level. Here we applied 3D dental surface texture analysis to a sample of field voles (Microtus agrestis) trapped from Finnish Lapland at different seasons and localities to test for inter-population variations. We also explored intra-individual variation in chewing dynamics by analysing two facets on the second upper molars. Our results confirm that the two localities have similar environments and that the voles feed on the same items there. On the other hand, the texture data suggest that diets are seasonally variable, probably due to varying concentrations of abrasives. Lastly, the textures on the buccal facets are more isotropic and their direction deviates more from the mesial chewing direction than the lingual facets. We interpret these results as reflecting food, rather than chewing, movements, where food particles are more guided on the lingual side of the molars. This has implications for the application of dental microwear analysis to fossils: only homologous facets can be compared, even when the molar row seems to constitute a functional unit.