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The Cretaceous Two Medicine Formation of northwestern Montana has yielded blocky, calcareous coprolites that contain abundant fragments of conifer wood and were produced by large herbivorous dinosaurs. The coprolites are generally dark gray to black in color due to a dark substance confined chiefly within what originally were the capillaries of tracheid and ray cells of xylem. This substance is a kerogen which consists in part of thin-walled vesicles 0.1-1.3 µm in diameter. Pyrolysis products of this kerogen are diagnostic of a bacterial origin with a possible contribution from terrestrial plants. The vesicular component is interpreted as the residue of bacterial cells, whereas a second filamentous component, closely associated with the vesicles, may be the residue of an extracellular binding material, such as glycocalyx.

At least two episodes of calcification of the coprolite are recognized by manganous cathodoluminescence. The earlier of these infilled the capillary channels of the conifer fragments. Wood cell walls, voids, cracks, and small burrows were filled during the later episode. Microprobe data confirm these results and show that phosphate is sequestered in the capillaries. These observations suggest that bacteria within the capillaries induced initial mineralization of the coprolite, and, in so doing, created barriers that protected organic residues from subsequent destruction. Early onset of mineralization is consistent with the degree of preservation of woody xylem found in the coprolites.


Based on dark color, low H/C and O/C ratios, low chemical reactivity, infrared spectrum and reflectivity, coprolite OM might be interpreted as a thermally mature kerogen that arose through diagenetic processes within a warm, deep sedimentary basin. On the other hand, the high nitrogen content and remarkably good preservation of its biomarkers show that the OM is an immature kerogen that was formed in a cool, shallow basin. Indeed, geological evidence shows that present exposures of the Two Medicine Formation were never deeply buried or strongly heated. In agreement with this evidence are paleontological indicators, such as the retention of internal microstructure and original minerals in dinosaur bones and egg shells and lack of calcite recrystallization in coprolites. Thus, the OM is best interpreted as an aromatic kerogen of the inertinite class that formed in a shallow basin. It should be mentioned in this regard that the deficiency of C-H bonds in the OM and the release of C-H rich organics during pyrolysis are not mutually contradictory observations. The mass of pyrolyzate compounds detected by GC/MS represents only a small fraction of the mass of the sample. Thus, these low molecular weight organics need not closely reflect the bulk chemistry of the OM.

Evidence for fossilized bacteria in ancient coprolites is rare. After dissolving the phosphatic matrix with acid, minute particles of silica were recovered that had coccoid, ovoid, budding, and rod shapes. These were interpreted as the replacement casts of bacteria and other microorganisms but were not reported to have been encased in or associated otherwise with organic material. The results of our studies on coprolites from the Two Medicine Formation are unique in supporting the existence of organic remnants of bacterial colonies in coprolites of dinosaur origin and in inferring a role for these colonies in initiating fecal mineralization.



Published Citation

Hollocher T.C., Chin K., Hollocher K.T., Kruge M.A. (2001) Bacterial residues in coprolite of herbivorous dinosaurs: Role of bacteria in mineralization of feces. Palaios 16:547–565