Megacerops hatcheri was first described by Henry Fairfield Osborn of the American Museum of Natural History in 1908. It was collected in the upper Chadron formation of South Dakota by J. B. Hatcher (Osborn 1908). Osborn initially classified it as Brontotherium hatcheri, a new species within the genus Brontotherium as he thought it represented an “early phase in the evolution” of Brontotherium gigas (Osborn 1908).
Osborn, in his extensive monograph on the Titanotheres of Ancient Wyoming, Dakota and Nebraska, maintained this early classification of Megacerops and Brontotherium as two separate genera. He also sorted the Chadron brontotheres into 7 genera with 37 species in total (Osborn 1929). Osborn was known as a taxonomic splitter, but his classification was not revised or challenged until 1941. In that year, Scott and Jepsen published a monograph on the Mammalian Fauna of the White River Oligocene that contained new information that was conveyed to them by W. K. Gregory, Osborn’s collaborator on the monograph on brontotheres. In this monograph, they argue that all the Chadron brontotheres except for Teleodus could be condensed into a single genus. Despite making this argument, they do not collapse the Chadron brontotheres into only two genera at this time. They keep with the more accepted system of nomenclature and say that Brontops, Megacerops, Brontotherium and Menodus possess enough variation to classify them as separate genera. Much of this variation was seen in tooth morphology and in the cross-section of the horns.
In 1967, Clark et al. revised the systematics of the Chadron brontotheres and suggested that only one genus, Menodus was present in the formation (Clark et al. 1967). Bryn Mader, in 1989, concluded that Brontotherium and Megacerops are synonymous genera and since Megacerops was described first, it takes precedence (Mader and Alexander 1995). Therefore, Brontotherium hatcheri could now be classified as Megacerops hatcheri. Menodus and Allops were synonimized with the genus Menops, the latter genus taking taxonomic precedence. Mader (1989) noted that a revision of the various species from the Chadron Formation was required.
In 2004, Mihlbachler, Lucas and Emry described the holotype of the species Menodus giganteus and attempted to tackle Scott and Jepsen’s “insoluble problem” of Chadron brontothere taxonomy. They concluded that the variation seen in the horns of the Chadron brontotheres was due to sexual dimorphism, thus concluding that all the specimens with unbifurcated horns belonged to one species-Megacerops coloradensis-while the specimens with branched horns belonged to the species Megacerops kuwagatarhinus (Mader and Alexander 1995; Mihlbachler et al. 2004). However, since the holotype (first fossil the species was described from) of Menodus giganteus is only part of the mandible or lower jaw, nothing about the horns can be inferred and there is a possibility for a valid third species. More work is clearly needed to definitively classify the Chadron brontotheres (Mihlbachler 2008). While not explicitly stated, it can be inferred from these papers that M. hatcheri is synonymous with the species M. coloradensis.
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