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All Subjects: Biomechanics

Biomechanical Constraints on Molar Emergence in Primates

Description

Across primates, molar-emergence age is strongly correlated to life-history variables, such as age-at-first-reproduction and longevity. This relationship allows for the reconstruction of life-history parameters in fossil primates. The mechanism responsible for modulating molar-emergence age is unknown, however. This dissertation uses a biomechanical model that accurately predicts the position of molars in adults to determine whether molar emergence is constrained by chewing biomechanics throughout ontogeny. A key aspect of chewing system configuration in adults is the position of molars: the distal-most molar is constrained to avoid tensile forces at the temporomandibular joint (TMJ). Using three-dimensional data from growth samples of 1258 skulls, representing 21 primate species, this research tested the hypothesis that the location and timing of molar emergence is constrained to avoid high and potentially dangerous tensile forces at the TMJ throughout growth. Results indicate that molars emerge in a predictable position to safeguard the TMJ during chewing. Factors related to the size of the buffer zone, a safety feature that creates greater stability at the TMJ during biting, account for a large portion of both ontogenetic and interspecific variation in the position of emergence. Furthermore, the rate at which space is made available in the jaws and the duration of jaw growth both determine the timing of molar emergence. Overall, this dissertation provides a mechanical and developmental model for explaining temporal and spatial variation in molar emergence and a framework for understanding how variation in the timing of molar emergence has evolved among primates. The findings suggest that life history is related to ages at molar emergence through its influence on the rate and duration of jaw growth. This dissertation provides support for the functionally integrated nature of craniofacial growth and has implications for the study of primate life history evolution and masticatory morphology in the fossil record.

ContributorsGlowacka, Halszka (Author) / Schwartz, Gary T (Thesis advisor) / Kimbel, William H. (Committee member) / Reed, Kaye E (Committee member) / Wright, Barth W (Committee member) / Arizona State University (Publisher)

Created2017

Locomotor function and the evolution of the primate pelvis

Description

The bony pelvis is a pivotal component of the locomotor system, as it links the hindlimb with the trunk and serves as anchorage for the primary propulsive musculature. Its shape is therefore expected to be adapted to the biomechanical demands of habitual locomotor behavior. However, because the relationship between locomotor mechanics and pelvic morphology is not well understood, the adaptive significance of particular pelvic traits and overall pelvic shape remains unclear. This study used an integrative, dual approach to elucidate the relationship between form and function in the primate pelvis. A biomechanical cylinder model of pelvic stress resistance was tested using in vitro strain analysis of monkey and ape cadaver specimens. These results were used to refine adaptive hypotheses relating pelvic form to locomotor mechanics. Hypotheses of adaptation were then tested via univariate and geometric morphometric methods using a taxonomically broad, comparative sample of 67 primate taxa. These results suggest that the pelvis exhibits some iliac and ischial adaptations to stress resistance that are associated with the biomechanical demands of habitual locomotor loading and of body size. The ilium and ischium exhibit relatively low levels of strain during experimental loading as well as adaptations that increase strength. The pubis exhibits relatively high strains during loading and does not vary as predicted with locomotion. This integrated study clarifies the relationship between strain and adaptation; these results support the hypothesis that bones adapted to stress resistance exhibit low strains during typical loading. In general, the cylinder model of pelvic biomechanics is unsupported. While the predictions of loading regimes were generally rejected, the inability of these methods to test the possible occurrence of overlapping loading regimes precludes outright rejection of the cylinder model. However, the lack of support for predicted global responses to applied loading regimes suggests that pelvic stress resistance may be better explained by a model that accounts for local, functional subunits of pelvic structure. The coalescence of a localized model of pelvic biomechanics and comparative morphometrics has great potential to shed light on the evolution of the complex, multi-functional structure of the pelvis.

ContributorsLewton, Kristi Lynn (Author) / Spencer, Mark A. (Thesis advisor) / Reed, Kaye E (Committee member) / Schwartz, Gary T (Committee member) / Ward, Carol V. (Committee member) / Arizona State University (Publisher)

Created2010

Premolar Molarization in Haplorrhine Primates

Description

Differences in the postcanine dentition of primates likely represent dietary adaptations given that the teeth interact directly with foods during mastication. Among early hominins, changes to both molar and premolar morphology are purported to indicate consumption of foods differing in their material properties. Some early hominins, such as the robust australopiths, possess premolars that resemble molars with enhancements to the distal part of the tooth (i.e., the talonid), including additional cusps and/or expanded basins. Such molarized premolars are thought to indicate that these hominins were processing mechanically challenging foods; that is, food items that were either hard or tough. Hypotheses tested in this study evaluated the link between the degree of premolar molarization and consumption of mechanically challenging foods in extant primates. Surface anatomy of the distal-most mandibular premolar (the P4) was quantified using a combination of 3D scans of postcanine dental casts and craniodental landmark data collected from 541 individuals, representing 22 extant primate taxa with well-studied diets and known food material properties. Taxa with more mechanically challenging diets were expected to have premolars with expanded talonids and enlarged P4s (and/or molar rows) relative to several mechanically-relevant size proxies. Taxa consuming high proportions of structural carbohydrates were also expected to have postcanine teeth with high occlusal relief (RFI), sharpness (DNE), and complexity (OPCR). Taxa consuming harder food items were expected to have lower relief and higher complexity, with sharpness determined by the proportion of structural carbohydrates included in their diet. The work presented in this dissertation supports most of these expectations, though talonid expansion per se was not clearly linked to the consumption of any particular diet. Overall, taxa with more mechanically challenging diets generally had relatively enlarged premolars when compared to taxa with softer diets and also differed predictably in their occlusal topography. The results of this dissertation support the functional significance of P4 crown size and measures of dental topography as they relate to diet and have implications for improving dietary inferences from the fossil record.

ContributorsDaly, Elizabeth Susanne (Author) / Schwartz, Gary T (Thesis advisor) / Delezene, Lucas K (Committee member) / Kimbel, William H (Committee member) / Arizona State University (Publisher)

Created2021