The perissodactyl family Brontotheriidae is represented by large numbers of specimens from latest early and middle Eocene sediments of the Western Interior of North America. Species level taxonomy of brontotheres has been confusing, and the relationships of the earliest occurring species to those that occur later have not been well documented or understood. Evidence based on new specimens from the earliest Bridgerian from the Green River Basin, in particular from South Pass and Opal, coupled with a reanalysis of previously existing specimens has led to the following conclusions concerning the alpha taxonomy of Bridgerian North American brontotheres. Four genera (Eotitanops, Palaeosyops, Mesatirhinus, and Telmatherium) are recognized. Eotitanops is represented by two species, E. borealis and E. minimus, and Palaeosyops by five species, P. paludosus, P. fontinalis, P. laevidens, P. robustus, and P. laticeps.
A re-examination of the temporal distribution of brontotheres requires that the biochronologic framework of the latest early to earliest middle Eocene be modified. The earliest Bridgerian can be defined by the first appearance of Eotitanops borealis in the Wind River Basin and Wapiti Valley, both in Wyoming and at Huerfano Park, Colorado. This interval has been termed the Eotitanops borealis Assemblage Zone and is equivalent to the early Gardnerbuttean, Bridgerian Biochronologic Zone Br0. The first appearances of Palaeosyops fontinalis and Eotitanops minimus mark the onset of the second biochronologic interval of the Bridgerian, Br1, here referred to informally as the ”Palaeosyops fontinalis Assemblage Zone.” This interval includes the late Gardnerbuttean (Br1a) and Bridger A (Br1b). Biochronologic Zones Br0 and Br1 are now included in the latest early Eocene while Br2 coincides with the beginning of the middle Eocene.