MAMMALIAN COMMUNITY STRUCTURE THROUGH TIME: OREGON MIOCENE COMMUNITY CHANGE

Food webs offer insights into ecological processes by describing ecosystem function and community energy flow. Identifying ancient trophic connections can be difficult, but well-collected fossil localities provide a unique opportunity to examine evolving community structure. Through the Geoscientists-in-the-Parks program, we reconstructed two Miocene food webs that document climatic and faunal changes in Oregon. Working at the John Day Fossil Beds National Monument we focused on the faunal assemblages from the Mascall (~16-14 Ma) and Rattlesnake Formations (~7 Ma). These formations were deposited during the transition from a more closed habitat to a more open grassland environment and record a number of prominent taxa that emigrated from Eurasia. Using body mass estimates from m1 area and diet information from published literature, we assigned species to trophic guilds and reconstructed inter-guild relationships for the two faunal assemblages. These food webs show a change in mammalian community structure between the middle and late Miocene in Oregon, with large browser diversity decreasing and the number of large mixed feeding herbivores increasing. The number of taxa present in the large carnivore guild did not change substantially but some of the genera occupying that trophic guild did change, with large amphycyonids being replaced by borophagine dogs, true cats, and bears. The presence of beavers and other browsers in both food webs suggest that during the Miocene Oregon stayed more forested than the Great Plains. Both the Mascall and Rattlesnake Formations most likely had heterogeneous landscapes of wooded and open areas.