Matching Items (13)
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
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
Description
Cercopithecid primates today occupy the greatest geographic and climatic range of any non-human primate group. Pliocene and Pleistocene cercopithecids are often found together in fossil deposits across East and South Africa, raising the question of how these species co-occurred with one another and survived in increasingly arid and seasonal environments. Aspects of shearing ability, molar enamel thickness, and relative incisor, premolar, and molar proportions were analyzed in principal component analysis and used to generate six potential models of the cercopithecid dental morphological niche. Resulting principal component axes distinguish between taxa with varying proportions of leaves, fruit, insects, and seeds in the diet, but lose some clarity when variable subsets are used that exclude poorly-preserved or wear-restricted variables. Resampling was used to reconstruct the aggregate dental morphological niches of cercopithecid communities (taxocenes) from Africa and Asia today and from the African Pliocene and Pleistocene. Modern Asian cercopithecid taxocenes occupy a more restricted niche than their counterparts in Africa, but in both regions variation in taxocene structure is linked with past and current climate factors related to precipitation, temperature, and seasonality. Fossil cercopithecids from the Turkana Basin occupy an expanded niche in comparison to modern African and Asian taxocenes. In contrast, South African fossil taxocenes occupy a more distinct and restricted niche, which may reflect a mix of paleoenvironmental and taphonomic factors. Overall these results are consistent with existing research on modern African and Asian primate taxocene diversity and highlight the utility of a dental metric model for examining community evolution among Plio-Pleistocene cercopithecids in Africa. Evidence for a possible niche expansion during the early Pleistocene coincides with a period of well-documented hominin co-occurrence at the same fossil sites, suggesting that these two primate groups were diversifying in response to shared environmental stimuli.
ContributorsSmail, Irene (Author) / Reed, Kaye E (Thesis advisor) / Campisano, Christopher J (Committee member) / Gilbert, Christopher C (Committee member) / Arizona State University (Publisher)
Created2021