DINOSAUR COPROLITES FROM THE UPPER CRETACEOUS KAIPAROWITS FORMATION IN SOUTHERN UTAH SUPPORT AN UPDATED INTERPRETATION OF HERBIVOROUS DINOSAUR FEEDING HABITS

An assemblage of dinosaur coprolites from the Campanian Kaiparowits Formation in Grand Staircase-Escalante National Monument provides new evidence that herbivorous dinosaur diets were more complex than has been traditionally assumed. More than 17 fossilized fecal masses were discovered as discrete deposits in fine-grained fluvial sediments that host fossils from several large ornithischian taxa. The fractured and irregularly shaped calcareous coprolites are perforated by back-filled burrows characteristic of fossilized dung beetle activity. Dimensions of large blocks suggest that the original fecal masses were likely a few decimeters on a side, with larger deposits representing multiple defecation events. Thin sections of the specimens reveal non-clastic ground-masses dominated by comminuted conifer wood fragments and disaggregated tracheids. The micromorphology of the wood indicates delignification by white rot fungi prior to ingestion.

These specimens are nearly identical in composition and types of bioturbation to coeval Campanian coprolites from the Two Medicine Formation of Montana, though the Kaiparowits coprolites show more evidence of ingested invertebrate tissues. Both coprolite assemblages signify intervals of concentrated feeding on rotting conifer wood—a resource that would have provided nourishment in the form of decay-released cellulose, fungal tissues, and associated fauna. The Utah specimens demonstrate that this habit was more widespread than previously documented, and that different ornithischian taxa engaged in the same feeding strategy despite occupying different habitats. The sites are separated by roughly 1,000 km and 10 degrees in latitude, and represent paleoenvironments interpreted as wet and subtropical (Kaiparowits Formation) versus more temperate (Two Medicine Formation).

It is unlikely that rotted wood served as a primary food source year round, because wood grows and accumulates more slowly than herbaceous tissues. However, it is striking that this feeding strategy reflects exploitation of a food source that is rarely utilized by extant mammalian megaherbivores. Although we don’t know which dinosaurs produced the Utah coprolites, these fossils reveal intriguing changes in patterns of resource utilization by large vertebrates over geologic time.