Supplementary Data 1

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).补充数据1：原始化石数据集。馆藏机构与标本编号：分别标注标本的馆藏单位及标本登记号。分类层级：依次标注标本的科、亚科、族、属及物种分类信息。发现地与编号：标注标本发现地点的名称及该地点的编号。地层单位：标注标本产出的地质地层。生物地理区：标注标本的发现生物地理区（依据Tedford等2004年的研究）。年代划分：标注标本所属的北美陆地哺乳动物年代（North American Land Mammal Ages，NALMA）分区（依据Tedford等2004年的研究）。纬度：标注发现地的十进制度纬度坐标。数据来源：标注该标本的测量数据由本文作者实测，还是源自已发表文献；若源自文献，则附含对应测量数据的论文引文。后续列依次标注测量类型及以毫米为单位的牙齿测量数据：2LPL为下第二前臼齿长度，3LPL为下第三前臼齿长度，4LPL为下第四前臼齿长度，1LML为下第一臼齿长度，2LML为下第二臼齿长度，3LML为下第三臼齿长度，2UML为上第二臼齿长度。马科动物体重：标注以千克为单位的马科动物体重估算值（采用Janis 1990年的计算公式）。