A revision of the biostratigraphy of oreodonts (superfamily Merycoidodontoidea) in the John Day region of Oregon, USA

Abstract Oreodonts (superfamily Merycoidodontoidea) existed in the John Day region of Oregon from the Eocene through the Miocene, with considerable changes in their diversity over time. We present a variation-informed taxonomic revision of local oreodont species that preserves two species of Agriochoeridae, Diplobunops kardoula Emery, Davis, and Hopkins, 2016 and Agriochoerus guyotianus Cope, 1878a, and seven species of Merycoidodontidae, Eporeodon occidentalis Marsh, 1875, Hypsiops brachymelis Douglass, 1907, Merycochoerus magnus Loomis, 1924, Oreodontoides oregonensis Thorpe, 1921b, Paroreodon marshi Thorpe, 1921b, Promerycochoerus superbus Leidy, 1870, and Ticholeptus zygomaticus Cope, 1878b. We then applied this updated taxonomic revision to the stratigraphically controlled specimens at the John Day Fossil Beds National Monument to provide a detailed biostratigraphic revision. We found the Picture Gorge ignimbrite marked a major change in oreodont ecological role coinciding with changing environment. The arboreal, clawed Agriochoerus was extirpated at the Picture Gorge ignimbrite, and the large terrestrial Promerycochoerus arrived. Eporeodon remained but decreased in size. A maximum of five oreodont species were recorded at any one time, peaking in diversity in the Kimberly Member (25.486–26.597 Ma).