Objectives: Molar crenulation is defined as the accessory pattern of grooves that appears on the occlusal surface of many mammalian molars. Although frequently used in the characterization of species, this trait is often assessed qualitatively, which poses unavoidable subjective biases. The objective of this study is to quantitatively test the variability in the expression of molar crenulation in primates and its association with molar size and diet. Methods: The variability in the expression of molar crenulation in hominids (human, chimpanzee, gorilla, and orangutan) was assessed with fractal analysis using photographs of first, second and third upper and lower molars. After this, representative values for 29 primate species were used to evaluate the correlation between molar complexity, molar size, and diet using a phylogenetic generalized least squares regression. Results: Results show that there are statistically significant differences in fractal dimensions across hominid species in all molars, with orangutan molars presenting higher values of occlusal complexity. Our results indicate that there is no significant association between molar complexity and molar size or diet. Discussion: Our results show higher levels of occlusal complexity in orangutans, thus supporting previously published observations. Our analyses, however, do not indicate a clear association between molar complexity and molar size or diet, pointing to other factors as the major drivers of complexity. To our knowledge, our study is the first one to use fractal analysis to measure occlusal complexity in primates. Our results show that this approach is a rapid and cost-effective way to measure molar complexity.
A foundational idea of evo-devo is that morphological variation is not isotropic, that is, it does not occur in all directions. Instead, some directions of morphological variation are more likely than others from DNA-level variation and these largely depend on development. We argue that this evo-devo perspective should apply not only to morphology but to evolution at all phenotypic levels. At other phenotypic levels there is no development, but there are processes that can be seen, in analogy to development, as constructing the phenotype (e.g., protein folding, learning for behavior, etc.). We argue that to explain the direction of evolution two types of arguments need to be combined: generative arguments about which phenotypic variation arises in each generation and selective arguments about which of it passes to the next generation. We explain how a full consideration of the two types of arguments improves the explanatory power of evolutionary theory. Also see the video abstract here: https://youtu.be/Egbvma_uaKc
Objectives: The occlusal surface of many mammalian teeth has grooves that have been collectively called crenulations. The evolutionary significance of this trait is unknown, but it has been associated with a hard diet. It has not been explained, however, why crenulated molars may present an increased mechanical resistance. The objective of this study was to determine whether a crenulated surface dissipate mechanical stress more efficiently than a smooth one. Materials and methods:Using μCT scans we built 3D models of lower second molars from Homo, Pan, Gorilla, and Pongo. The crenulated models from Homo and Pongo were modified to remove crenulations. Finite element analysis was used to determine the distribution of mechanical stress in all the models when a vertical force was applied. Results:The results show that crenulated molars have a distinctive pattern of mechanical stress, namely the stress is higher in the valleys than in the crests of the crenulations. In non-crenulated molars, mechanical stress is more homogeneously distributed. Highly crenulated molars of orangutans show the smallest values of mean stress among the compared species. Artificially removing crenulations results in more homogeneous distribution of stresses and increased mean stress values.Conclusions: Molar crenulations may increase molar resistance by canalizing mechanical stress from the tip to the base of the cusps. The overall cusp shape also influences the distribution of stress. This mechanism may be a functional hypothesis to explain the association between crenulated molars and mechanically demanding diets.
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