Browsing by Author "Kay, Richard F"

In this dissertation I compile modern mammalian faunal lists, as well as ecomorphological measurements on living marsupials and rodents, to relate the diversity of small mammals, specifically the distributions of their dental topographies, to the climates in which they are found. The emphasis of this dissertation is to demonstrate the potential of distributions of dental topography metrics as proxies for the reconstruction of paleoenvironments in the Miocene of South America.

In Chapter 2, I compile complete, non-volant mammalian species lists for 85 localities across South America as well as 17 localities across Australia and New Guinea. Climatic and habitat variables were also recorded at each locality using GIS spatial data. Additionally, basic ecological data was collected for each species, including: diet, body size, and mode of locomotion. Niche indices that describe the relative numbers of different ecologies were calculated for each locality. These indices then served as the predictor values in a handful of regression models, including regression trees, random forests, and Gaussian process regression. The Australian/New Guinean localities were used as a geographically and phylogenetically independent for the purposes of testing the models derived from South America.

As for the dental ecomorphological analysis, I use three separate measures of dental topography, each of which measures a different component of dental topography; relief (the Relief Index, or RFI), complexity (orientation patch count rotated, OPCR), and sharpness (Dirichlet normal energy, DNE). Together, these metrics quantify the shape of the tooth surface without regard for tooth size. They also do not depend on homologous features on the tooth surface for comparative analysis, allowing a broad taxonomic sample as I present here. After a methodological study of DNE in Chapter 3, I present correlative studies of dental topography and dietary ecology in marsupials and rodents in Chapters 4 and 5, respectively. Finally, using the same localities from Chapter 2, I analyze the distributions of dental topography metrics as they relate to climate and habitat.

Results suggest that sharpness and relief are positively correlated with a higher amount of tough foods—such as leaves or insects—in the diet of marsupials, and that relief is positively correlated with grass-eating in rodents. The distributions of all three metrics show some utility when used as a proxy for climatic variables, though the distributions of RFI in marsupials and OPCR in rodents demonstrate the best correlations.

Overall, this dissertation suggests that dental topography can be used to discriminate dietary categories in a wide variety of mammalian groups, and that the distributions of dental ecometrics can be used as proxies for paleoenvironment reconstruction. This may eliminate the need to reconstruct behavior in individual taxa in order to construct ecological indices for fossil mammalian communities, thus offering a more direct avenue to reconstructing past environments.

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Fossil taxa are crucial to studies of brain evolution, as they allow us to identify evolutionary trends in relative brain size and brain shape that may not otherwise be identifiable in comparative studies using only extant taxa, owing to multiple events of parallel encephalization among primate clades. This thesis combines indirect and direct approaches to understanding primate evolution, by evaluating variation in the endocranial morphology of extant primates and their fossil representatives. I use a comparative approach to examine the relationships between interspecific adult endocranial volume and shape, and brain evolution and cranial form among extant primate clades and their fossil representatives. The associations are evaluated via phylogenetically informed statistics perfomed on volumetric measurements and three-dimensional geometric morphometric analyses of virtual endocasts constructed from micro-CT scans of primate crania. Fossil taxa included in these analyses are: 1) anthropoids Parapithecus, Aegyptopithecus (Early Oligocene, Egypt), Homunculus and Tremacebus (Early Miocene, Argentina), and 2) Eocene euprimates Adapis and Leptadapis (Eocene adapoids, France), and the Rooneyia (Eocene omomyoid, Texas).

The first part of this work (Chapter 2) explores variation in residual mass of brain components (taken from the literature) among primates, and evaluates the correlated evolution of encephalization and brain proportions with endocast shape, quantified via three-dimensional geometric morphometric techniques. Analyses reveal a broad range of variation in endocast shape among primates. Endocast shape is influenced by a complex array of factors, including phylogeny, body size, encephalization, and brain proportions (residual mass of brain components). The analysis supports previous research, which concludes that anthropoids and tarsiers (Haplorhini) share the enlargement of several key brain regions including the neocortex and visual systems, and a reduction of the olfactory system. Anthropoids further differ from strepsirrhines in endocranial features associated with encephalization--a more flexed brain base, an inferiorly deflected olfactory fossa--and those associated with brain proportions--a small olfactory fossa, and a more caudally extended cerebrum that extends posteriorly past the cerebellar poles. Tarsiers are unique in having a mediolaterally broad and rostro-caudally short endocast with an attenuated anterior and middle cranial fossae. This morphology is likely related to the extreme orbital enlargement in this taxon, which limits anterior expansion of the endocranium. Finally, despite the correlation between residual endocranial volume and endocast shape among modern primates, early anthropoid fossils demonstrate a disconnect between these factors in sharing key features of endocast shape with extant anthropoids at a relatively small brain size.

The second part of this thesis (Chapter 3) explores the relationship between craniofacial organization--cranial base angle, facial size, facial hafting--and encephalization via the lens of the Spatial Constraints and Facial Packing Hypotheses. These hypotheses predict that interspecific adult variation in encephalization correlates with endocranial shape such that a larger brain for a given body size will be more "globular" or spherical in shape. These hypotheses futher predict that basicranial angle covaries with encephalization and that the relative size of the endocranium and facial skeleton will have an antagonistic effect on basicranial angle and facial hafting. Results show that various measures of globularity have inconsistent and weak relationships to phylogeny, encephalization, and basicranial flexion, owing to a diversity of clade-specific scaling patterns between the maximum length, breadth, and width of the endocast. Among extant primates, there is weak but significant evidence to suggest that both facial size and encephalization influence variation in basicranial flexion. Considering the fossil specimens in isolation, their relative ranks in encephalization, basicranial flexion, and midline facial size and shape follow the pattern expected from the Spatial and Facial Packing Hypotheses outlined above; however, relative to modern species, the early fossil anthropoids have more flexed cranial bases and shorter facial skeletons at much smaller level of encephalization than seen in modern anthropoids.

Together, the extant data suggest a moderately conserved pattern of correlated evolution among endocranial size, endocranial shape, brain proportions, and craniofacial organization, which may explain differences in endocranial and facial shape between extant strepsirrhine and anthropoid primates; however, the fossil record for early anthropoid evolution demonstrates that a shift towards key anthropoid-like traits of the endocranium, basicranium, and facial skeleton were initiated early in anthropoid evolution, with subsequent encephalization occurring within and among members of this clade. Thus, these anthropoid cranial traits evolved in tandem with changes in the relative size of brain components, rather than absolute or relative brain size alone. Basicranial flexion, facial length and orientation are influenced by both: 1) shifts in endocranial shape associated with changes in brain proportion--accounting for the initiation of the anthropoid-like craniofacial plan early in the evolution of the clade--and 2) encephalization, which influenced subsequent morphological divergence among extant anthropoid groups.

Reconstructing locomotor patterns from fossils is crucial for understanding the origins of primates and important transitions in various primate clades. Recent studies suggest that the semicircular canals of the inner ear provide evidence about locomotion. The canals sense rotational head accelerations and drive reflexes essential for normal movement. Because bony aspects of canal morphology influence canal sensitivity, this system can be studied in osteologic specimens and fossils. Variation in canal morphology in living and, by inference, extinct primates has been attributed to interspecific differences in locomotor behavior. However, the manner in which movement selects for canal morphology is debated, alternative scenarios are plausible, and no relevant measurements are available documenting head movements in primates.

To refine proposed links between canal morphology and locomotor function, and to resolve conflicting functional interpretations, this study examines head rotations in lemurs and lorises exhibiting diverse locomotor behaviors. Three-dimensional kinematic analyses were used to characterize angular velocities of the head during locomotion. These data are used to test hypotheses concerning intraspecific, interspecific, and body-size dependent variation in head rotations. Cranial CT scans are used to model canal sensitivity to rotations in different directions. Observed patterns of head rotation are compared to predicted patterns of sensitivity to test hypotheses about the relationship between locomotor behavior and canal design.

Evaluation of existing locomotor inferences reveals that brain size exerts a significant effect on canal size and that the prevailing equations for predicting agility from body and canal size are highly inaccurate. Intraspecific comparisons between maps of observed angular velocity and predicted sensitivity allow identification of map types associated with different general locomotor modes and do not support existing hypotheses about the primary selective forces acting on canal morphology. The new data are used to formulate and test a novel "fast-accurate hypothesis" to explain why all vertebrates are more sensitive to rotations about some axes than others. The fast-accurate hypothesis stipulates that angular velocities presented about axes of mean sensitivity are most accurately interpreted by the brain, and that selection aligns axes of mean sensitivity with axes of habitually fast rotation because accurate perception of rapid rotations confers survival benefit. The fast-accurate hypothesis was used to predict which features of the canals should be correlated with high mean angular velocities of head movement. Novel equations that predict behavior from these newly identified canal morphologies were generated and found to outperform existing equations when tested on the original sample of 11 strepsirrhine species.

The semicircular canals of the vestibular system detect angular head rotations and play a fundamental role in guiding motor reflexes during locomotor behaviors. While extensive research has documented the relationship between the semicircular canal shape (i.e. radius of curvature and canal length) and locomotor behaviors, levels of intraspecific variation in primates are relatively unknown. Predictive models using these metrics to reconstruct locomotion in extinct animals are generally based on one individual per species. Furthermore, the influence of body size and to a lesser degree brain size heavily influences overall canal morphology.

This study documents intraspecific variation in the size, shape and orientation of the semicircular canals in relation to changes in function, brain size, and body size via analysis of high resolution CT scans of large samples of extant primate species. I test the hypothesis that the extent of intraspecific variation differs across a sample of primates, reflecting the intensity of selective pressure on canal shape in species that require agility during locomotion. I also examine whether spatial constraints resulting from the size of the skull (reflected by the size of the brain) affect canal radii of curvature and canal orthogonality more strongly than observed agility during locomotion.

To this end, data was gathered from high-resolution CT images of museum specimens. For the comparative analysis, 14-matched pairs of adult extant primate species were selected that contrast in agility and brain size in closely related genera. CT images of these specimens were used to measure functional measures of canal sensitivity (e.g., canal radii of curvature, orthogonality). This data was used to test hypotheses concerning intraspecific and interspecific variation in semicircular canal functional morphology. This data was then combined with a larger mammalian dataset culled from the literature, to further test hypotheses relating to body-size and brain size dependent variation in individual canal metrics.

Evaluation of levels of intraspecific variation support the hypothesis put forth by Billet et al. (2012), that selection on canal morphology is relaxed in animals with slow locomotor behaviors, who are observed to have higher levels of intraspecific variation. Analyses of interspecific variation provides tentative support for the use of canal orthogonality in reconstructive models, most especially in canal angles that seem least effected by other constraints—brain size, etc. However, locomotor signals are complex and brain/skull interactions can potentially produce misleading results when reconstructing locomotor behaviors. This work highlights the importance of critically assessing comparative groups used for inferring behaviors in both extinct and extant animals.