EXPLOITATION OF CRETACEOUS CRUSTACEANS: COPROLITIC EVIDENCE FOR OMNIVOROUS FEEDING BEHAVIOR IN LARGE, HERBIVOROUS DINOSAURS

Multiple coprolite deposits from the Kaiparowits Formation of southern Utah provide evidence that some large herbivorous dinosaurs were at least occasionally omnivorous. The large, calcareous coprolites were found as discrete, isolated deposits in fine-grained fluvial sediments, and are primarily comprised of comminuted conifer wood tissues. The fecal origin of the blocks is indicated by their comminuted contents, paucity of clastic grains, sedimentological context, and backfilled burrows characteristic of dung beetle activity. Associated fossils and the multi-liter coprolite volumes suggest that they were produced by large ornithischian dinosaurs. Histological analyses reveal that the woody tissues had been fungally degraded by selective delignification before ingestion. This indicates that the fecal producers had consumed decayed wood, likely in rotting logs or standing dead trees.

At least eleven of thirteen examined coprolite deposits contain calcareous, shell-like fragments up to 2 x 3 cm in area and ~0.5 to 0.9 mm thick. Much of this material is diagenetically altered, but well-preserved fragments display nodes, pores, and internal laminae. The thickness, microstructure, and chemical composition of these fragments are all consistent with crustacean cuticle, though there is insufficient morphology to identify the type of crustacean in the coprolites. Nevertheless, the robust cuticle thickness and large fragments suggest that these invertebrates were sizeable and were intentionally consumed. Ingestion of crustaceans would have provided a nutritional windfall in terms of both protein and calcium. Such resources would have been particularly beneficial during the breeding season when extra calcium was needed for eggshell production and higher protein levels were required to provision eggs. It is notable that many extant avian dinosaurs alter their dietary habits during breeding to accommodate elevated requirements for protein and calcium. Although it might have been challenging for megaherbivorous dinosaurs to capture small, isolated arthropods, aggregations of invertebrates are common in rotting logs. This new evidence for omnivorous feeding behavior compels us to reevaluate presumptions that large herbivorous dinosaurs fed exclusively on vegetation.