NO OREODONT BODY SIZE CHANGE IN RESPONSE TO CLIMATIC CHANGE BETWEEN THE OLIGOCENE AND MIOCENE OF THE JOHN DAY BASIN

Oreodonts, part of the order Artiodactyla, are an abundant part of the paleofauna of the John Day Basin. Many morphological changes in oreodont evolution have been ascribed to selective pressures created by changing climate, and changes in body size distributions within communities have been taken to indicate climatic trends. Independent evidence indicates that the climate cooled between 18 and 33 Ma in the John Day Basin. In this case, changing climate should have pushed oreodonts towards larger body mass as in Bergmann’s Rule, which describes the tendency of mammalian body mass to increase at higher latitudes. We gathered data from 39 Oreodont specimens from the University of Oregon, as well as the collection in the John Day Fossil Beds National Monument. We measured first molar area from these specimens and applied a standard body mass regression to create a time series of oreodont body mass through John Day time. We then tested for a change in body mass in Oreodonts between 33 and 18 Ma. We found no significant change in body mass of Oreodonts through time, but this preliminary study is hampered by small sample size. Our results suggest that as climate cooled in the John Day Basin, Oreodont mass did not increase. If these results are supported with additional sampling, there may be no simple link between oreodont body size and temperature.