The Brontotheriidae is an extinct family of Eocene perissodactyls known from North America, Asia, and, rarely, Eastern Europe. Brontotheres are widely recognized as having evolved very large body size and conspicuous frontonasal horns, although these traits do not characterize every species. Characters shared by all brontotheriids include an anteroposteriorly abbreviated face and an elongate postorbital cranium. Dentally, brontotheriids share bunoselenodont upper molars with a W-shaped ectoloph, isolated lingual cusps, and with paraconules, metaconules, and transverse molar crests that are either vestigial or absent. Early North American paleontologists such as Leidy, Cope, Marsh, and Osborn placed considerable emphasis on brontothere research, however, since Osborn's massive 1929 monograph on North American brontotheres, serious research on this diverse Eocene family has waned. Nonetheless, a great need for a revision of the Brontotheriidae has long been recognized because earlier works on brontothere taxonomy and systematics, particularly those of Osborn, are universally considered problematic due to their reliance on the discredited theory of orthogenesis.
The present study reevaluates the species taxonomy of brontotheres based on craniodental materials. All known taxa were considered for revision except North American representatives of Eotitanops, Palaeosyops, and Megacerops. A phylogenetic species concept, where species are defined as the smallest diagnosable clusters of specimens, was adopted for this study. Monospecific quarry samples (mass death assemblages) consistently suggest that certain characters, including canine size, horn size, and many premolar characters show intraspecific polymorphic tendencies; such characters were not generally used to delimit species. All characters found to be monomorphic within mass death assemblages were considered when delimiting taxa.
A total of 116 potential species were considered for revision. Among these species, 41 were found to be valid, 34 species were found to be invalid junior synonyms of valid species, 31 species were found to be nonima dubia, and three others were found to be problematic due to extremely fragmentary fossils, but not necessarily invalid. One new species, Wickia brevirhinus, is named and six other potential species are recognized but remain unnamed due to very poor fossil material.
Phylogenetic analysis of 47 brontotheriid taxa was undertaken with 227 states distributed among 87 characters. The outgroup method was employed using Hyracotherium, Pachynolophus, Danjiangia, and Lambdotherium. Two analyses were performed, one with ordered multistate characters and another with unordered multistate characters. Both analyses yielded large numbers of most parsimonious trees. A strict reduced consensus was used to identify and prune fragmentary wildcard taxa a posteriori. Not surprisingly, the results of the phylogenetic analysis differ substantially from prior orthogenetic hypotheses of brontothere phylogeny and a radical revision in the higher classification of brontotheres is presented. Eotitanops and Palaeosyops are the most basal members of the Brontotheriidae. All other brontotheres form a clade, Brontotheriinae, which is supported by numerous molar apomorphies suggesting increased functional emphasis on shearing on the outer wall of enamel of the upper molars. Many of the previously named brontothere subfamilies are clearly paraphyletic while others correspond to subclades of the Brontotheriinae and have been assigned new ranks, resultin