RARE HERBIVOROUS DINOSAUR COPROLITES AND WHAT THEY CAN TELL US ABOUT THE ROLE OF BACTERIA IN FOSSILIZATION AND CARBONATE PRECIPITATION

Microbial effects on mineral precipitation have occurred widely in modern and ancient systems. However, because of the small size and poor preservation potential of bacteria, identifying their presence and influences on mineralization in the rock record is challenging. To address these issues, we have examined herbivorous dinosaur coprolites from the Upper Cretaceous Two Medicine Formation of Montana, using scanning electron microscopy (SEM) based-energy dispersive x-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) techniques. Coprolites offer good opportunities to address these questions as feces are full of bacteria which are likely involved in fossilization. Herbivore and carnivore feces differ in their microflora, amount of undigested material, heterogeneity, and chemical composition. These differences may affect fossilization and diagenesis. Moreover, herbivore coprolites are rare and less well studied than carnivore coprolites. The calcareous samples are comprised of permineralized woody tissue suspended in a micritic matrix and have undergone minimal compaction and recrystallization. The wood fragments in the coprolites have dark organics concentrated in the tracheid cell lumens and the organic-rich contents appear to be protected within the cell lumens from later stages of diagenesis. Interestingly, these dark colored cell lumens contain micron to submicron sized voids in the calcareous groundmass which may represent molds of the original bacterial structures. The higher organic content of the cell lumens correlates with greater variation in crystal orientation shown by EBSD data. Preliminary analyses reveal differences in crystal orientation between cell lumens and cell walls in coprolitic wood. Conversely, tracheid cells in non-coprolitic wood from the same location shows more homogenous crystal orientation in both cell walls and lumens, and across adjacent tracheids. We hypothesize that the dark color inside the coprolitic wood cells reflects complex organics derived from dietary residues and the gut microflora of the fecal producers. As such we infer that differences in the abundance and diversity of bacterial communities of the coprolitic versus non-coprolitic wood may be manifested in differences in the crystallization of the wood cells.