Browsing by Subject "Skull"

Novelty presents a paradox in evolutionary biology. Novel features appear to be qualitatively unique to a specific type of organism, but historical continuity requires that some common form must have pre-existed them. This dissertation focuses on the morphology and development of three inter-related evolutionary novelties of cetacean skulls: telescoping, the blowhole, and extensive maxillo-frontal overlap. To investigate these novelties, museum specimens and skulls collected from stranded dolphins were studied using computed tomography, morphometrics, and histological methods. The first part of this dissertation sets an evolutionary-developmental framework for empirical study of skull telescoping, defined as extensive bone overlap and shortened maxillo-occipital distance (Chapter 2). The following two chapters (3 and 4) identify distinct ontogenetic changes that contribute to reorientation of the blowhole in a dolphin species and the fin whale. Lastly, the maxillo-frontal in the bottlenose dolphin was characterized using computed tomography and histological methods (Chapter 5).

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Primates span incredible behavioral and ecological diversity, and this diversity is reflected in the shape of the skull. This dissertation asks two questions surrounding the evolution of primate skull shape: 1) what are the macroevolutionary correlates of primate skull shape? And 2) what is the pattern of phenotypic integration in the primate skull at a macroevolutionary scale? To address these questions, I compiled a broad comparative dataset of anatomical landmarks identified from 3D scans of primate skulls and analyzed this data using statistical methods that combine geometric morphometrics and phylogenetic comparative methods. To investigate the macroevolutionary correlates of skull shape, I used multivariate phylogenetic generalized linear models to test for relationships between skull shape and several variables that are predicted to be correlated with skull shape: allometry, typical diet, tree gouging behavior, activity pattern, and sexual dimorphism. I found strong phylogenetic signal for primate skull shape, confirming the need for phylogenetic comparative methods. Allometry was a significant predictor of skull shape, with larger primates having relatively small, convergent orbits, and anteroposteriorly short skulls compared to small primates. Sexual dimorphism was associated with a dramatically lengthened rostrum, probably to facilitate a large gape in aggressive displays. Folivory was associated with deeper mandibles, which may improve mechanical advantage and strain resistance. To investigate patterns of phenotypic integration in the skull, I performed hierarchical clustering analyses on phylogenetically corrected inter-landmark covariance matrices. In contrast to previous research, I did not find evidence for distinct phenotypic modules in the primate skull, and I argue that this discrepancy is due to methodological shortcomings of past research that biased results towards identifying different anatomical regions as discrete modules. This dissertation represents one of the first investigations of primate skull shape at a macroevolutionary scale, and demonstrates that the combination of geometric morphometrics and phylogenetic comparative methods can yield novel insights into evolutionary morphology.

Child head injury is a very costly problem, both in terms of morbidity/mortality and direct medical costs. In fact, it is the leading cause of death and disability for those in the United States under age 18-years-old. Currently, head injury in children ages newborn to 19-years-old is responsible for 7500 deaths per year--30% of all childhood deaths in the United States. Given its importance and effect on the population, the study of pediatric head injury is greatly hindered by the lack of available pediatric post mortem human specimen (PMHS) data. As a substitute for PMHS testing, anthropometric test devices (ATDs) and finite element models (FEMs) have been developed to model the head. However, there is a dearth of data for the design and validation of these models.

The goal of this study was to use pediatric PMHSs to both advance the study of pediatric head injury and to provide validation data for ATD and finite element head models. 14 pediatric heads, 8 adult heads, and 6 ATD heads were studied to obtain geometrical, inertial, structural stiffness, and impact properties. The computational tomography (CT) method was used on pediatric heads to get inertial properties, and clinical CT scans were used to develop average head and skull contours for 12 different age groups. To obtain impact properties, the heads were dropped onto a rigid plate from 15cm and 30cm, and the acceleration-time pulses were analyzed to obtain acceleration HIC and other impact properties. The heads were then placed between two aluminum plates and compressed at four different rates to obtain structural stiffness values. Using the PMHS results, the ATD heads were compared against age-matched human heads, and the scaling rules used for ATD production were tested for accuracy.

The study found that between the ages of 5-months-old and 22-months-old, the human head was susceptible to fracture from drops as low as 15cm. The structural stiffness of the human head was shown to increase by three orders of magnitude from neonate to adult. For the impact properties, the human head's peak acceleration and head injury criteria increased with age, while the human head's pulse duration and coefficient of restitution decreased with age. The 50th percentile Hybrid III head was found to adequately model the response of the adult head for multiple head impact locations, while the 3-year-old Q3 child ATD was found to be too stiff during impact. Overall, this study provides novel data that can be directly applied to pediatric head injury curves, and pediatric ATD and finite element head models.