Relatively few studies of cranial endocasts document intraspecific variation. Here, discrete morphological and morphometric variation from a sample of more than 150 natural cranial endocasts from a single Eocene locality, Reeves Bonebed (hereafter ‘RBB’) from West Texas are documented. The first description of a digital endocast of an oreodont, Bathygenys reevesi, is given to provide comparative data for other oreodont endocasts. The RBB endocasts were originally assigned to B. reevesi, but virtually none has diagnostic craniodental material still attached, and isolated endocasts of other cetartiodactyl species are known from this locality. Consequently, an attempt was made to identify apomorphies on the RBB sample to diagnose specimens to the lowest possible taxonomic level. The RBB endocasts were diagnosed as oreodonts based on the presence of large, rounded casts of the tuberculum olfactorium, but no specimen can be diagnosed to a particular species. The range for most linear measurements for the RBB sample overlapped with measures from multiple species of oreodonts from RBB. A coefficient of variation analysis of linear measurements suggests that the RBB sample represents more than one species. Therefore, endocasts from RBB that lack associated craniodental material cannot be definitively assigned to a species, and the original assumption that all are B. reevesi is not supported. This illustrates the necessity of applying apomorphy-based identifications for isolated specimens, including endocasts. In addition, this study reveals some gaps in data from cetartiodactyl brains, which need to be filled for future studies on brain evolution in this clade.
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