MICROTEXTURES OF DINOSAUR EGGSHELL FROM EBSD AND OPTICAL ORIENTATION IMAGING

Electron Backscatter Diffraction (EBSD) and optical orientation imaging by Computer Integrated Polarization microscopy (CIP) reveal details of both the primary structure and the diagenetic alteration of fossilized eggshell that are not apparent in standard thin-section analysis. EBSD and CIP facilitate routine quantitative analysis of crystal symmetry and orientation at the sub-micron-scale and have long been used for phase identification, microtextural analysis and deformation studies in metallography, material science and structural geology. Application of these techniques to studies of fossilized eggshells has the potential to provide new quantitative microstructural criteria for ootaxonomic classification, inference of behavior and assessment of diagenetic alteration. Preliminary analysis of theropod eggshells associated with Troodon formosus shows a range of microtextures that may be related to variable diagenetic alteration of the specimens. Troodon eggshells are approximately 1000 μm thick. Eggshell fragments that are interpreted to be relatively free form diagentic alteration display a prismatic structure with individual crystals (shell units) spanning nearly the entire radial thickness of the shell. Pole figures show a diffuse distribution about the radial direction. However, other cryptically altered specimens exhibit a finer-grained texture with interlocking 20-200-μm-long crystals in the prismatic layer and fanning 25-50-μm crystals in the mammillary layer. Pole figures from these shells show a small-circle distribution. Specimens of Spheroolithus also show a systematic range of distinct microtextures and pole figures that are interpreted to reflect diagenetic alteration.