The brontothere genus Megacerops, 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 Hiram A. Prout, 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.
Early descriptions
Descriptions of Menodus and Titanotherium
The first brontothere fossil described was a fragment of a right jaw (USNM 21820), found in the White River badlands of South Dakota. This fossil was collected in 1843 by the fur trader Alexander Culbertson, of the American Fur Company. The fossil was transported to St. Louis, where it came into the hands of the physician Hiram A. Prout. Prout reported the fossil in 1846 as the "remains of a Palaeotherium", found in "the Tertiary near St. Louis". Palaeotherium, a fossil equoid, was otherwise known only from Europe. James Dwight Dana and Benjamin Silliman, of the American Journal of Science, noted that the complete jaw must have measured at least 30 inches (76 cm) long, far larger than any Palaeotherium known. In 1847, Prout illustrated and described the fossil, referring to it as the jaw of a "gigantic Palaeotherium". As one of the first mammal fossils from the American West brought to scientific attention, the publication of Prout's jaw captured the attention of the nascent paleontological community in the United States and set into motion the first wave of a "fossil rush" in the western parts of the country. Auguste Pomel concluded that Prout's jaw represented a new taxon. In 1849, Pomel named Menodus giganteus, with USNM 21820 as the type specimen. Pomel considered Menodus to be a subgenus of Palaeotherium. In 1873, Othniel Charles Marsh argued that Menodus was an invalid name, too similar to the reptile genus Menodon (=Nothosaurus) and thus preoccupied, but this is incorrect since the two names are spelled differently. In 1850, David Dale Owen, Joseph Granville Norwood, and John Evans recorded new brontothere fossils in a preliminary report on fossils collected by Evans. Owen, Norwood, and Evans described the new species Palaeotherium? proutii, named after Prout. The fossils were provisionally referred to Palaeotherium due to the difficulty in establishing genus-level characteristics for the fossils. The name was intended to cover the same taxon as Prout's original jaw (e.g. Pomel's Menodus giganteus): "These remarkable remains are thus named in compliment to Dr. Prout of St. Louis who first noticed them in the American Journal of Science and Arts". No type specimen was formally designated for P.? proutii, and it is not clear in the original description if the taxon was intended to be based on Prout's original jaw, or on new material collected by Evans. The fossils collected by Evans were turned over to Joseph Leidy, who studied and described them in greater detail. In 1852, Leidy wrote that he suspected that they "belong to a different genus from either Palaeotherium or Anchitherium, and should the suspicion prove correct, Titanotherium would be a good name for this animal, as expressive of its very great size". Leidy based the genus Titanotherium on several fossils, including Prout's original jaw, "Owen's specimen" (USNM 113, a partial left jaw found by Evans), and several fragmentary fossils. Leidy concluded, like Owen and colleagues, that USNM 21820 and USNM 113 were congeneric, and used the name Titanotherium proutii for the fossils.
Confusions of type specimens
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