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
There has been considerable confusion in later literature over the type specimens of Menodus and Titanotherium. Owen, Norwood, and Evans, and later Leidy, considered USNM 21820 and USNM 113 to belong to the same taxon (Titanotherium proutii). The earlier described USNM 21820 was already designated as the type specimen of Menodus giganteus by Pomel in 1849. By the process of elimination, the type specimen of Titanotherium proutii is thus the next fossil included when the taxon was named, i.e. "Owen's specimen", the left jaw USNM 113.In 1853, Leidy mentioned that Prout had loaned him the original jaw fragment, with "another corresponding to the opposite side, apparently of the same individual". It is clear from Leidy's text and figures that he confused specimens. In a corresponding figure, Leidy illustrated what is most likely the opposite jaw of the same individual as USNM 113, not USNM 21820 (based on the size of the specimens). The jaw figured by Leidy is a specimen now designated as ANSP 12669, and modern examination has revealed that it is likely the right jaw of the same animal as USNM 113.
In a major 1929 monograph on brontotheres, Henry Fairfield Osborn reproduced Leidy's figure of the right jaw (e.g. ANSP 12669) as "Figure 159, Type of Menodus giganteus. Prout's original specimen", repeating Leidy's mistake. Osborn made this mistake despite also featuring Prout's original illustration of USNM 21820 on the preceding page (as Fig. 158). Leidy's left jaw, e.g. USNM 113, was reproduced by Osborn as "Figure 160, Owen's specimen of Palaeotherium? proutii".
In 1929, Osborn incorrectly stated that USNM 21820 had been destroyed and was known only from Leidy's figures and measurements, attributing its destruction to the St. Louis Fire of 1849. Osborn's assertion was accepted as correct for decades, until the fossil was found to be in the collections of the National Museum of Natural History, having been donated there in 1957 by Courtney Werner, Associate Professor of Geology at the Washington University in St. Louis.
Tooth taxa Leidy named several new species based on incomplete teeth. Palaeotherium maximum was named in 1852, based on parts of the ectolophs (a ridge on the upper molars of certain perissodactyls) of two molars from Owen's collection. Leidy believed the fossils to represent a "true species of Palaeotherium". Later in the same year, Leidy named the species Rhinoceros americanus based on two large premolar teeth he believed to belong to "a new species of Rhinoceros, or probably Aceratherium". These were the first proposed fossils of Rhinoceros in the Americas, and Leidy noted that the size of the teeth suggested a species larger than any modern Rhinoceros. The R. americanus fossils were not part of Owen's collection. In 1853, Leidy created the new genus Eotherium for the teeth previously referred to R. americanus, and renamed the species to Eotherium americanum. Leidy considered the genus Eotherium to combine characteristics of Rhinoceros and Palaeotherium. In 1854, Leidy named the new species Palaeotherium giganteum, based on parts of the ectolophs of five molars, from Owen and Culbertson's collections.
In later writings, Leidy treated both Eotherium americanum and Palaeotherium giganteum as synonyms of Titanotherium proutii. In 1929, Osborn considered all of Leidy's tooth taxa to be indeterminate (not diagnostic enough to assign to any taxon) brontothere fossils, and noted that the type specimens of P. maximum and E. americanum could no longer be located.
In 1860, Prout provisionally named a new genus, Leidyotherium, without designating any species. Leidyotherium was based on a "fragment of a large molar tooth" found near Abingdon, Virginia. Prout distinguished the tooth from those of Titanotherium based on "the great width and depth of the groove between the outer and what may have been the inner border of the tooth". Prout did not formally describe the genus or specimen, in the hope that further fossils would be discovered to more definitely establish the characteristics of the animal. Leidy later treated Leidyotherium as a synonym of Titanotherium, and Osborn (1929) considered the tooth to be an indeterminate brontothere fossil. The Leidyotherium tooth has since been lost.
Description of Megacerops coloradensis
In 1870, Leidy described the new genus and species Megacerops coloradensis. The new taxon was based on the type specimen ANSP 13362, consisting of a pair of horns and some of the nasal bones. ANSP 13362 was found in the so-called Titanotherium zone of the Chadron Formation in Colorado. ANSP 13362 is the earliest described Chadronian brontothere fossil to preserve the distinctive horns.
Leidy compared the horns of ANSP 13362 to those of the extinct giraffid Sivatherium, and struggled with identifying the animal. In his initial description, Leidy stated that "it is probable that the fossil may pertain to the same animal as the remains from the Mauvaises Terres of Nebraska, described under the name of Titanotherium" but since there was a "state of extreme uncertainty as to its collocation", the fossil was equally likely to belong to some other animal, such as the camelid Megalomeryx or an American species of Sivatherium, and that it could thus provisionally be referred to a new genus and species.
Bone Wars From 1873 to 1891, research on Chadronian brontotheres was dominated by contributions by the rival researchers Edward Drinker Cope and Othniel Charles Marsh. During the so-called Bone Wars, Cope and Marsh viciously competed to assemble great collections of fossils, and to name more species than the other. Collections across the United States were rapidly filled with fossils of dinosaurs and other prehistoric life.
Both Marsh and Cope's work on brontotheres was exclusively descriptive and systematic. Marsh named fourteen new species, in eight new genera, and Cope named twelve species, in three new genera. New taxa were named for nearly every brontothere fossil that came into the hands of either Marsh or Cope and little to no work was done on the interrelationships or validity of the animals.
Taxa named by Marsh