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THE PALEOBIOLOGICAL IMPLICATIONS OF HERBIVOROUS DINOSAUR COPROLITES FROM THE UPPER CRETACEOUS TWO MEDICINE FORMATION OF MONTANA: WHY EAT WOOD?

¹ Geological Sciences and Museum of Natural History, University of Colorado at Boulder, UCB 265, Boulder, Colorado 80309, USA Karen.Chin{at}colorado.edu

Rare assemblages of woody coprolites from different strata of the Two Medicine Formation provide surprising perspectives on the feeding behavior of Late Cretaceous ornithischian dinosaurs. Most of the irregularly shaped, calcareous specimens are largely composed of fragmented conifer wood (13%–85%) and can be identified as coprolites by the presence of distinctive backfilled dung beetle burrows. The large size (up to 7 L in volume), fibrous contents, and associated bones and eggshell strongly suggest that the source animals at one site were Maiasaura hadrosaurs. The wood-bearing coprolites occur in strata ranging in age from 74–80 Ma, revealing a recurring (possibly seasonal) habit of wood ingestion. The preponderance of wood in the specimens and the absence of recognizable small-diameter twig fragments suggest that wood ingestion was intentional—that the coprolite producers had not merely consumed wood inadvertently when feeding on the leaves and bark of terminal branches. Because undegraded wood provides inconsequential nutritive value for vertebrates, it is unlikely that ornithischians would have expended the energy to masticate intact wood for little benefit. Furthermore, patterns of tissue damage in the fecal wood fragments suggest fungal degradation. Thus, the most parsimonious explanation for the high fecal wood content is that the coprolite producers consumed decomposing wood to capitalize on resources released by fungal attack, along with the tissues of the decomposers and associated invertebrate detritivores. These multiple coprolite deposits provide direct fossil evidence of recurring dinosaur diets and suggest that some ornithischians at least occasionally tapped detrital resources. Although such feeding behavior is rare in large extant herbivores, utilization of rotting wood would have augmented the resource options of Cretaceous ecosystems that lacked fodder provided by grasses and other derived angiosperms.

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