MAMMALIAN BODY MASS COMMUNITIES IN THE MIOCENE OF OREGON

The body size of an organism relates to a host of other characteristics such as diet, metabolism, and trophic level. Properties of an environment tend to dictate potential ecological niches available to mammals, which acts as a control on the shape of body mass distribution. Thus, mammalian communities tend to exhibit a common ratio of body masses across different biomes. There are many potential drivers for a community wide shift in body masses, but a change may be associated with shifting climates and environments. Previous research has examined how the patterns of mammalian body size at a community scale are shaped by the environments that the organisms inhabit. However, the fossil record of Eastern Oregon has never been investigated through that lens. The extensive fossil record and well-studied long-term environmental shifts in the middle Miocene of Eastern Oregon make it an ideal location to study the effects of environmental changes on mammalian body masses. Despite the extensive work on the effects that the contemporaneous emerging grasslands had on mammals of the Great Plains region, little focus has been given to results of the change from closed woodland to open grassland in Oregon. Using multiple body mass proxies, we estimated body mass for all Miocene mammals from both the Barstovian Mascall and Hemphillian Rattlesnake Formations of Oregon. We then organized the body masses into empirically determined, binned size categories and compared the changes in size structure from the Barstovian to Hemphillian. Additionally, we examined mammal body mass distributions for five modern environments across North America, Africa, and Australia to determine whether a consistent pattern of body mass distribution exists throughout similar biomes. We compared these modern distributions to our estimated body mass distributions from the Mascall and Rattlesnake. We find that the Mascall distribution is distinct from the modern forest body mass distributions, and the Rattlesnake formation is similar to the modern African grasslands, though with a greater proportion of large mammals.