Reconstructed functional diversity and food-web structure reveal mammalian community shifts through the Oligo-Miocene in Oregon

Abstract Understanding how ecological communities respond to environmental change over long timescales is essential for predicting modern ecosystem dynamics. Here, we reconstructed functional community composition and trophic network structure of seven mammal communities from the eastern region of Oregon. We focused on fossil localities that span the past 32 million years. We used diet and body mass to reconstruct community functional diversity and evaluated community differences using correspondence analysis and Fisher’s exact tests. To understand whether there were changes in community trophic structure through time, we also reconstructed potential trophic-species food webs using trait- and body-mass-derived dietary interactions and calculated network metrics that have been linked to ecosystem differences. Overall, functional diversity patterns mirror previously described North American continental trends, including the decline of browsing herbivores and small-bodied omnivores after the mid-Miocene Climatic Optimum. Immigration of novel taxa from the Eurasian continent did influence the functional diversity of these assemblages. Functional diversity also changed because of the late Cenozoic extinction of large-bodied taxa. The reconstructed food-web structure adds further resolution to these patterns. For example, reconstructed food webs for the Oligocene-aged lower Turtle Cove assemblage exhibit high connectance, long trophic chains, and a rich omnivore guild, features consistent with a structurally complex habitat. The Oregon fossil record suggests that as climate shifts, certain mammalian functional groups decrease in diversity, which can impact trophic network structure and overall community structure. These findings highlight how combining trait-based and network-based approaches deepens our understanding of ancient ecosystem change.