BIOSTRATIGRAPHY AND RELATIVE ABUNDANCE OF OREODONTS IN THE JOHN DAY FORMATION

Oreodonts (Merycoidodontoidea) are the most abundant and diverse large herbivores from the Oligocene and early Miocene of Oregon, but unresolved taxonomy has hampered study of the group’s ecology and evolution. We re-assessed the taxonomy and stratigraphic distribution of oreodonts in the John Day Formation of eastern Oregon then analyzed relative changes in their abundances. The range of size and character variation of Promerycochoerus in the John Day is consistent with the modern analog of Hylochoerus meinertzhageni, and we propose that there is a single, sexually dimorphic species of Promerycochoerus in Oregon. We found no discrete character differences between Eucrotaphus trigonocephalus and Eporeodon occidentalis; overlapping morphologies of the auditory meatus and paroccipital process suggest these characters are unsuitable for genus-level diagnosis. Pooled coefficients of variation (CV) for molar teeth in Eporeodon and Eucrotaphus ranged from 8 to 12%, slightly less than the sexually dimorphic CV values of Promerycochoerus (9-14%). The width of the zygomatic arches, depth of the malar, and dimensions of the canine all showed high variability as in Promerycochoerus. We recommend synonymizing Eporeodon and Eucrotaphus but cannot currently resolve whether these are a single sexually dimorphic or two sympatric species.

The relative abundance of mid-sized oreodonts decreases dramatically after the Picture Gorge Ignimbrite (PGI): Agriochoerus disappears, with only one possible occurrence after the PGI, while Eporeodon drops from 84% of identifiable material to 35%. Small-bodied oreodonts (i.e. Paroreodon) remain relatively rare throughout this time period, while the significantly larger Promerycochoerus begins to dominate relative abundance. Overall oreodont abundance does not significantly decrease across the PGI (p <0.01), and they remain the dominant large-bodied herbivore of the area. The extreme sexual dimorphism, larger body size and presumed herd structures of Promerycochoerus are consistent with animals of more open habitat. Considering the ongoing trends of cooling and drying that dominate the Oligocene, it is possible the change in relative abundance results from adaptation within Merycoidodontoidea to more open climates rather than a reaction to the PGI event itself.