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- All Subjects: Morphology
- Creators: Schwartz, Gary T.
Description
Mammals with a habitually orthograde trunk posture possess a more anterior foramen magnum than mammals with non-orthograde trunk postures. Russo & Kirk (2013) also found that bipedal orthograde mammals possess a more anteriorly placed foramen magnum than those that are just habitually orthograde. This finding has allowed us to use foramen magnum position as a predictor of trunk posture in early hominins. This prompts more research of how the other landmarks on the cranial base move in relation to this shift in foramen magnum positioning. I collected landmark data on images of 125 mammalian basicrania spanning 41 species that differed in trunk posture. Using Procrustes and Principal Components Analysis (PCA), I attempted to evaluate the effects of trunk posture on basicranial morphology, primarily focusing on the placement of the carotid and jugular foramina. The results supported Russo and Kirk's finding of a more anterior foramen magnum placement in orthograde mammals; in addition, the results displayed correlations between foramen magnum position and carotid foramen position among primates and diprotodonts.
ContributorsPena, Angela (Author) / Kimbel, William (Thesis director) / Schwartz, Gary T. (Committee member) / Barrett, The Honors College (Contributor) / School of Human Evolution and Social Change (Contributor)
Created2015-05
Description
Early hominins present an unusual pattern of sexual dimorphism. On one hand, the canine teeth of these species are weakly size-dimorphic, vertically short, and nonhoning, suggesting a social system characterized by infrequent, low-intensity intermale competition and monogamous pair-bonding. On the other hand, marked size variation in skeletal remains attributed to species of Australopithecus is thought to reflect strong body-mass dimorphism, which is more consistent with intense intermale competition. Reconciling these conflicting signals and understanding their adaptive significance is a major goal of paleoanthropology. This dissertation research contributes to this objective by investigating factors that may constrain or reduce canine height in extant anthropoid primates. Two hypotheses regarding the relationship between canine height and other elements of the masticatory system were tested using phylogenetic comparative methods. According to the first hypothesis, canine reduction is a pleiotropic by-product of changes in the sizes of other components of the dentition. With respect to canine height, the results of this study fail to support this idea. There is limited evidence for a relationship between basal canine crown dimensions and incisor and postcanine size, but significant interspecific correlations between these variables are not strong and are restricted primarily to the female maxillary dentition. These results indicate that if pleiotropy influences canine size, then its effects are weak. The second hypothesis proposes that canine reduction is a consequence of selection for increased jaw-muscle leverage. This hypothesis receives some support: there is a clear inverse relationship between canine height and the leverage of the masseter muscle in male anthropoids. Females do not exhibit this association due to the fact that dimorphism in muscle leverage is weak or absent in most anthropoid species; in other words, female muscle leverage tracks male muscle leverage, which is linked to canine height. Leverage of the temporalis muscle is not correlated with canine height in either sex. Two specimens of the 3.0-3.7-million-year-old hominin Australopithecus afarensis fall at or beyond the upper end of the great ape range of variation in masseter leverage, which is consistent with the idea that hominin canine evolution was influenced by selection for increased jaw-muscle leverage.
ContributorsScott, Jeremiah Ezekiel (Author) / Kimbel, William H. (Thesis advisor) / Schwartz, Gary T. (Committee member) / Spencer, Mark A. (Committee member) / Arizona State University (Publisher)
Created2010