Taxonomy of Megacerops

The brontotheregenusMegacerops, known from the Chadronian (a North American Land Mammal Age), has a complex and extensive taxonomic history. Known from hundreds of well-preserved specimens, the fossils are highly similar in most of their anatomy but differ considerably in the size, shape, and orientation of the horns, in features of the nasal bones, and in the thickness of the zygomatic arches. These features have historically been used to name over fifty species, classified in over twenty genera. The abundance of fossil specimens, and poor stratigraphic data for most of them has further complicated the taxonomy.

The first Chadronian brontothere fossil to be scientifically described was a jaw fragment described in 1847 by , who believed it came from a "giant Palaeotherium". Over the course of the late 19th and early 20th century, a large number of taxa were named based on Chadronian brontothere fossils, especially during the Bone Wars by rival researchers Edward Drinker Cope and Othniel Charles Marsh. Several taxa were later named by Henry Fairfield Osborn, who in 1929 published an extensive monograph on brontotheres. Osborn's monograph recognized 37 species as valid, grouped into seven genera. Osborn's taxonomy was dismissed as unrealistic by both contemporaries and later researchers, since so many species could not have coexisted in the relatively brief Chadronian. For much of the 20th century, Chadronian brontothere taxonomy was regarded as an "insoluable problem". No formal species-level taxonomic revision has been conducted since Osborn's monograph but overviews by later researchers have proposed wildly different taxonomies, recognizing as many as six genera or as few as one genus.

In a 2004 preliminary revision, Matthew C. Mihlbachler, Spencer G. Lucas, and Robert J. Emry concluded that the variation in the fossil material was slightly higher than could be expected for a single sexually dimorphic mammal species, but that it was not possible to separate the fossils into discrete units (i.e. species) based on any particular feature. They recognized only two Chadronian brontothere species, Megacerops coloradensis and M. kuwagatarhinus, differentiated by whether the horns are bifurcating (M. kuwagatarhinus) or not (M. coloradensis). Although this model has become generally accepted since 2004, the taxonomy remains unresolved due to the continued lack of a detailed species-level revision.