Type of Document	Thesis
Author	Creech, Jennifer Erin
Author's Email Address	creecher4@hotmail.com
URN	etd-04062004-135645
Title	Phylogenetic Character Analysis of Crocodylian Enamel Microstructure and its Relevance to Biomechanical Performance
Degree	Master of Science
Department	Biological Science, Department of
Advisory Committee	Advisor Name Title Gregory M. Erickson Committee Chair Joseph Travis Committee Member Scott J. Steppan Committee Member
Keywords	Hardness Crocodylia Tooth Enamel Microstructure Young's Modulus Nanoindentation
Date of Defense	2004-03-29
Availability	unrestricted
Abstract Tooth enamel microstructure has been shown to vary among mammals. Such variation has a major bearing upon whole-tooth biomechanical function and may reflect gross-level phylogenetic signal. Although variation is substantial within reptilian lineages, comprehensive standardized sampling has not been done from which a similar understanding can be garnered. In the present study I sampled caniniform teeth from the 23 extant species of Crocodylians. The 3-dimensional enamel microstructure was characterized by examining the tooth enamel in longitudinal, transverse, and tangential views using scanning electron microscopy. The microstructural characters were subsequently mapped onto a robust tree (Gatesy et al. 2004) and the ancestral character states were reconstructed using parsimony. The results of this study showed no correlations between individual microstructural characters and phylogeny or with diet. There is a range and variety of combinations of enamel microstructural variation between species; however there were no distinct sets of characters correlated with each other or with the phylogeny. The distribution of the microstructural enamel characters was extremely random, indicating a great deal of lability in the formation of the enamel within Crocodylia. The microstructural arrangement of apatite crystals has been posited to affect wear-resistance, crack-propagation, and cusp-sharpness in mammals. It seems reasonable that differential crystalline arrangement in non-prismatic enamel may have a similar biomechanical function. In order to make correlations between microstructural variation and the biomechanical properties of enamel it is vital to first determine the effects, if any, of preservation technique on biomechanical properties. Biomechanical testing was conducted using nanoindentation in order to determine the effect of traditional museum preparation techniques (drying or storage in ethanol after fixation in formaldehyde) on the biomechanical properties of prismless enamel. The results of this study indicate that storage methods can affect the biomechanical properties of tooth enamel. Desiccation significantly increases the biomechanical properties (Hv and E) of tooth enamel. Fixation in formalin and subsequent short-term storage in ethanol prior to preparation and testing does not significantly alter Hv nor E values, therefore these specimens may be used in conjunction with fresh specimens in future studies.
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