Supplementary Data 1. Raw fossil data. Repository and Specimen indicate the collection in which specimen is held and its specimen number. Family, Subfamily, Tribe, Genus, and Species indicate taxonomy of the specimen. Site and Site # indicate name and locality number of site at which specimen was found. Formation indicates geologic formation in which specimen was found. Region and Subregion indicate the biological region in which specimen was found (after Tedford et al. 2004). Age indicates NALMA subdivision to which specimen dates (after Tedford et al. 2004). Latitude indicates decimal latitude of site. Source indicates whether specimen was measured by authors or collected from published literature, in which case citation of paper containing measurement is included. Following columns indicate type of measurement made and dental measurements in millimeters: 2LPL = 2nd lower premolar length, 3LPL = 3rd lower premolar length, 4LPL = 4th lower premolar length, 1LML = 1st lower molar length, 2LML = 2nd lower molar length, 3LML = 3rd lower molar length, 2UML = 2nd upper molar length. Equid Mass indicates estimated mass of equids in kilograms (using equations of Janis 1990).