Abstract

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.