ASSEMBLAGES OF HERBIVOROUS DINOSAUR COPROLITES IN THE TWO MEDICINE FORMATION PROVIDE FOSSIL EVIDENCE OF ECOSYSTEM ENGINEERING AND OTHER ECOSYSTEM FUNCTIONS

Ecosystem engineers are organisms that physically modify the environments they inhabit; this activity often has positive or negative consequences for other members of the ecosystem. The Two Medicine Formation provides fossil evidence of ecosystem engineering in the Late Cretaceous, through preservation of suites of herbivorous dinosaur coprolites. These specimens demonstrate that habitat modification occurred on at least two spatial scales. Herbivorous dinosaurs harvested, comminuted, microbially enriched, and relocated vast stores of carbon resources, and the woody contents in the Two Medicine coprolites point to extensive disturbance of rotting logs and their detritivore communities. The 6-7 liter volumes of individual specimens represent single defecation events, indicating that the dinosaurs generated considerable dung each day. Net fecal accumulations in a given area would have been augmented by extended periods of occupation and by deposits from multiple dinosaurs. This suggests that the suite of Two Medicine coprolites represents a small subset of the dung originally deposited in the area. Ecosystem engineering on a smaller scale is demonstrated by evidence for coprophagy. Numerous back-filled burrows show considerable translocation of dung into the sediment. Coprophagy and burial of organic matter both facilitate biogeochemical transformations that enhance nutrient cycling and plant growth.

The Two Medicine coprolites also reveal other ecosystem functions that arose from large infusions of feces into the paleoenvironment. The presence of ~60 largely entire gastropods in some of the coprolites indicates post-depositional exploitation of dung by facultative coprophages. At the same time, characteristic back-filled burrows provide evidence for paracoprid dung beetle activity; this suggests that dependable supplies of dinosaur dung helped maintain a diversity of obligate coprophages. Although there is no evidence for it, dinosaurs likely benefited from their coprophagous symbionts through reductions in parasite populations. The inferred impacts of dinosaur feeding activities can be compared to those of extant megaherbivores that disrupt habitats while browsing and deposit copious quantities of dung. Such fecal windfalls initiate substantial bottom-up effects on food webs.