Out one end or the other, analysis of novel terrestrial feeding traces from Egg Mountain

Abstract

The Egg Mountain locality within the Upper Cretaceous Two Medicine Formation of the Willow Creek anticline is thought to represent well-drained soils, with evidence drawn from the fossil record. It has many fossil discoveries, such as the nests and egg clutches of Troodon formosus and trace fossils, including invertebrate coprolites and pupal cases. Three unique specimens found at the site, MOR 10878-9, MOR 10878-4A, and MOR 10878-4B, are ovoid dark masses with matrix, and range in size from 68 mm to 17 mm in length. They were examined to determine if their initial identification as coprolites was correct, how and why they were preserved, and what this could tell us about the producer's diet, ecology, and environment. To determine the overall composition of the specimens, pXRF and EDS elemental analysis were performed on MOR 10878-9 and thin sections of MOR 10878-4B. 3D Computed Tomography and synchrotron scans were conducted to identify inclusions and understand the internal structure of each specimen. The EDS data reveal that the matrix has low levels of phosphorus and calcium, suggesting a composition similar to the host sediment rather than a phosphatic coprolite. Synchrotron data and thin sections show that the dark mass of the specimen resembles trabecular bone in cross-section while the cavities have fill that is different from the matix though this is not clearly understood why. Fragments of both cortical and trabecular bone suggest that the producer had eaten material from a potentially juvenile dinosaur. Synchrotron scans also revealed potential invertebrate burrows and fecal pellets, suggesting that invertebrates may have fed on the remains, thereby degrading the soil-facing portion. These specimens may be coprolites that were degraded mainly by detritivores. However, the lack of a pervasive phosphatic ground mass might indicate that the specimens represent a regurgitalite instead. The producer was likely a small to medium-sized predator that may have fed on a younger dinosaur that came from one of the many nests in the area. Further analysis of the fragments within the specimens may provide insight into both predators and prey, as well as their roles in the ecosystem as a whole.