SIMILARITIES AND DIFFERENCES BETWEEN THE JOHN DAY AND CROOKED RIVER BASINS: OLIGOCENE LANDSCAPE HETEROGENEITY IN TERRESTRIAL FAUNAS

The Oligo-Miocene section from the John Day Basin of Oregon has been a key study area for biostratigraphy and paleoecology of North American terrestrial vertebrates since its discovery more than a century ago. The formations of the John Day Basin section (Clarno, John Day, Mascall, and Rattlesnake) also crop out in the Crooked River Basin, 40 miles to the southwest. While the occurrences from Crooked River have been included in some past studies of these faunas, there has not been a comprehensive effort to compare differences in the faunas between the two areas. After 6 years of fieldwork in the Crooked River Basin, we have established that there are some important differences between the John Day and Crooked River Basin faunas throughout the time spanned by these formations. Some of these differences can be explained by minor differences in the time of fossil preservation; the Mascall fauna is slightly older in the Crooked River Basin than it is in the John Day Basin, for example. Some differences also are explained by differences in preservational characteristics, in that the faunas of the Crooked River Basin preserve material that is more readily screenwashed for small mammal remains. However, there are clearly some real differences in habitat represented by the differences in ecology between the two areas. Our ongoing work in these areas aims to elucidate the role that ongoing tectonic activity played in shaping differences in these mammalian communities.