A character analysis of oreodonts (Superfamily Merycoidodontoidea) in the John Day region of Oregon, USA

Abstract The abundance and diversity of oreodonts in the John Day region of Oregon resulted in 34 different holotypes, but these may not all truly represent separate species. The diagnostic characters of these taxa have not previously been assessed for the effects of intraspecific variation. We analyzed both qualitative and quantitative variation of these characters in modern taxa, using close phylogenetic relatives ( Camelus , Lama , and Vicugna ), taxa with analogous morphology ( Muntiacus and Hylochoerus ), and taxa with analogous diversity ( Cephalophus ) as a comparison. We found that certain characters showed high intraspecific variation that overwhelms their utility in oreodont taxonomic definitions, including the direction of the paraoccipital processes, fine-scale size and shape characters of the auditory bullae, and certain enamel abnormalities. Several characters showed clear links with sexual dimorphism or, as in the case of the rugose texture on promerycochoerine zygomatic arches, pathology. We found that no combination of diagnostic characters could be used to reject a single-species hypothesis for either eporeodontine or promerycochoerine oreodonts. As such we recognize only a single species of each group and recommend the usage of Promerycochoerus superbus for all promerycochoerine and Eporeodon occidentalis for all eporeodontine oreodonts in the John Day region of Oregon.