Megacerops

{{Short description|Extinct perissodactyl ungulate genus from the Late Eocene epoch}} {{Automatic taxobox | fossil_range = Late Eocene (Chadronian), {{fossil range|37|33.7|ref=<ref name="Hodnett-2022">{{Cite book |last=Hodnett |first=John-Paul M. |url=https://books.google.com/books?id=YCKIEAAAQBAJ |title=FOSSIL RECORD 8 |last2=Welsh |first2=Edward T. |last3=Santucci |first3=Vincent L. |last4=Tweet |first4=Justin S. |date=2022 |publisher=New Mexico Museum of Natural History and Science |pages=211 |language=en |chapter=A Middle Eocene brontothere (Mammalia; Perissodactyla; Brontotheriidae) from Yellowstone National Park, Wyoming}}</ref>}}{{Period fossil range|Paleogene|37|33.7}} | image = Titanothere brontops (Thunderbeast).jpg | image_caption = Mounted skeleton of ''M. coloradensis'', Natural History Museum of Los Angeles County | image_upright = 1.1 | taxon = Megacerops | authority = Leidy, 1870<ref name="Leidy-1870">{{Cite journal |last=Leidy |first=Joseph |author-link=Joseph Leidy |date=1870 |title=Remarks on ''Megacerops coloradensis'' |journal=Proceedings of the Academy of Natural Sciences of Philadelphia |volume=22 |pages=1–2}}</ref> | display_parents = 4 | type_species = {{Extinct}}'''''Megacerops coloradensis''''' | type_species_authority = Leidy, 1870<ref name="Leidy-1870"/> | subdivision_ranks = Other species | subdivision = *†'''''M. kuwagatarhinus''''' Mader & Alexander, 1995<ref>{{Cite journal |last1=Mader |first1=Bryn J. |last2=Alexander |first2=John P. |date=1995 |title=''Megacerops kuwagatarhinus'' n. sp., an Unusual Brontothere (Mammalia, Perissodactyla) with Distally Forked Horns |url=https://www.jstor.org/stable/1306330 |journal=Journal of Paleontology |volume=69 |issue=3 |pages=581–587 |doi=10.1017/S0022336000034958 |issn=0022-3360 |jstor=1306330}}</ref> | synonyms = {{Collapsible list|bullets = true |title=Genus synonymy |''Allops'' Marsh, 1887 |''Ateleodon'' Schlaikjer, 1935 |''Brontotherium'' Marsh, 1873 |''Brontops'' Marsh, 1887 |''Diconodon'' Marsh, 1876 |''Diploclonus'' Marsh, 1890 |''Eotherium'' Leidy, 1853 |''Haplacodon'' Cope, 1889 |''Leidyotherium'' Prout 1860 |''Megaceratops'' Cope, 1873 |''Menodus'' Pomel, 1849 |''Menops'' Marsh, 1887 |''Miobasileus'' Cope, 1873 (''nomen nudum'') |''Symborodon'' Cope, 1873 |''Teleodus'' Marsh, 1890 |''Titanops'' Marsh, 1887 |''Titanotherium'' Leidy, 1852 }} }} '''''Megacerops''''' ({{Literal translation|great horned face}}{{Efn|From Ancient Greek μέγας (''méga'', "great"), κέρας (''kéras'', "horn"), and ὤψ (''ōps'', "face").<ref name=":17" />}}) is an extinct genus of horned brontothere that lived in North America during the Late Eocene,{{efn|The Chadronian was formerly correlated to the Oligocene, and ''Megacerops'' has thus historically often been treated as an Oligocene animal. Since the 1990s, new research has instead correlated the Chadronian to the Eocene, and there is no longer any support for large brontotheres in the Oligocene.<ref name="RhinocerosGiants">{{Cite book |last=Prothero |first=Donald R. |author-link=Donald Prothero |url=https://books.google.com/books?id=fbyn88ExO9IC |title=Rhinoceros Giants: The Paleobiology of Indricotheres |date=2013 |publisher=Indiana University Press |isbn=978-0-253-00819-0 |pages=41–43 |language=en}}</ref>}} in the Chadronian land mammal age. ''Megacerops'' is among the best represented large mammals in the fossil record, known from hundreds of complete skulls and several complete skeletons. As the classic brontothere, ''Megacerops'' has enjoyed enduring fame in popular culture, often under various historically used names, such as ''Brontotherium'', ''Brontops'', and ''Titanotherium''.

''Megacerops'' was superficially similar to modern rhinoceroses, but closer to elephants in size. The largest known ''Megacerops'' may have been over {{convert|2.5|m|sp=us}} tall at the shoulder and could have weighed up to four or five tonnes. They were among the largest mammals in the Eocene, rivaled only by other brontotheres, and were by far the largest animals in their environment. Adult ''Megacerops'' were likely too large to be preyed upon by any contemporary predator.

The horns of ''Megacerops'', its signature feature, were anatomically similar to the ossicones of modern giraffes and are believed to have been used in intraspecific combat. In some cases, ''Megacerops'' fossils have been found in mass death assemblages, which suggests that they were social animals that may have traveled herds. Paleoclimatological models of the Eocene and isotope analyses of ''Megacerops'' teeth suggest that they lived in warm temperate to subtropical forests and woodlands, and preferred moist environments.

Skulls of ''Megacerops'' are highly variable in some features, especially the size and shape of the horns. This was once interpreted as indicating different species and even genera, and has made the taxonomic history of ''Megacerops'' highly complex; over fifty species of Chadronian brontotheres have been named historically. The taxonomy is not entirely resolved. Today, variations among the fossils are interpreted largely as the result of sexual dimorphism and other individual variation. The genus contains at least one diagnosable species, the type species '''''M. coloradensis'''''. A rarer second species is also generally recognized, '''''M. kuwagatarhinus''''', distinguished from ''M. coloradensis'' by its bifurcating horns.

== Research history == {{Main|Taxonomy of Megacerops}}

=== Early discoveries === [[File:Menodus proutii (Owen et al., 1850).jpg|left|thumb|The holotype specimen of ''Menodus giganteus'' (USNM 21820), the first scientifically published brontothere fossil]] Fossils of ''Megacerops'' were among the first mammal fossils from the American West to be brought to scientific attention.<ref name=":0">{{cite journal |last1=Mihlbachler |first1=Matthew C. |last2=Lucas |first2=Spencer G. |last3=Emry |first3=Robert J. |year=2004 |title=The holotype specimen of ''Menodus giganteus'', and the "insoluble" problem of ''Chadronian brontothere'' taxonomy |url=https://books.google.com/books?id=fJivCQAAQBAJ&pg=PA129 |journal=New Mexico Museum of Natural History and Science Bulletin |volume=26 |pages=129–136}}</ref> Long before the time of scientific inquiry into the fossils, ''Megacerops'' remains were sometimes exposed by severe rainstorms<ref name=":3">{{Cite web |last=Mayor |first=Adrienne |title=Placenames Describing Fossils in Oral Traditions |url=https://web.stanford.edu/dept/HPS/MayorPlacenames.pdf |url-status=dead |archive-url=https://web.archive.org/web/20140904044500/https://web.stanford.edu/dept/HPS/MayorPlacenames.pdf |archive-date=2014-09-04 |access-date=2019-06-21}}</ref> and found by Native Americans of the Lakota Sioux and Pawnee peoples.<ref name=":4">{{Cite book |last1=Carrano |first1=Matthew T. |url=https://books.google.com/books?id=KnmQDwAAQBAJ |title=Visions of Lost Worlds: The Paleoart of Jay Matternes |last2=Johnson |first2=Kirk R. |date=2019 |publisher=Smithsonian Institution |isbn=978-1-58834-676-6 |language=en |chapter=Rocky Mountain Floodplain}}</ref> The Lakota linked the great mammals to their legends of ''wakíŋyaŋ'',<ref name=":4" /> translated as "thunder beasts".<ref name=":3" /><ref name=":4" /><ref name=":5">{{Cite book |last1=Collins |first1=Cindi Sirois |url=https://books.google.com/books?id=FdmuEAAAQBAJ |title=Dinosaurs and Other Ancient Animals of Big Bend |last2=Elbein |first2=Asher |date=2023 |publisher=University of Texas Press |isbn=978-1-4773-2463-9 |language=en}}</ref>

The first brontothere fossil to be scientifically described was a fragment of a right jaw (USNM 21820), found in the White River badlands of South Dakota.<ref name=":0" /> The fossil was described by Hiram A. Prout in 1847. Prout correctly identified the fossil as belonging to a large perissodactyl but believed that it was the jaw of a "giant ''Palaeotherium''" (an extinct equoid).<ref name=":0" /> The publication of Prout's jaw captured the attention of the nascent paleontological community in the United States<ref name=":26" /> and set into motion the first wave of a "fossil rush" in the western parts of the country.<ref name=":26">{{Cite journal |last=Santucci |first=Vincent L. |date=2017 |title=Preserving fossils in the national parks: a history |url=https://www.researchgate.net/publication/320637952_Preserving_fossils_in_the_national_parks_A_history |journal=Earth Sciences History |volume=36 |issue=2 |pages=245–285 |doi=10.17704/1944-6178-36.2.245}}</ref><ref name=":27">{{Cite web |title=Brontothere: Large beasts of the Badlands |url=https://www.nps.gov/articles/brontothere.htm |access-date=2026-05-01 |website=U.S. National Park Service |language=en}}</ref> In 1849, Auguste Pomel concluded that Prout's fossil did not belong to ''Palaeotherium'' and instead designated it as the type specimen of a new genus and species, ''Menodus giganteus''.<ref name=":0" /> In 1850, David Dale Owen, Joseph Granville Norwood, and John Evans recorded additional brontothere teeth and jaws, collected by Evans. Owen and colleagues believed these fossils represented the same species as Prout's jaw. Apparently ignoring Pomel's name, Owen and colleagues named the new species ''Palaeotherium''? ''proutii''.<ref name=":0" />

The fossils collected by Evans were soon acquired by Joseph Leidy, who examined and described the material in greater detail in 1852 and 1853.<ref name=":0" /> Leidy suspected that the animal was not an equoid, and suggested the name ''Titanotherium'', "as expressive of its very great size".<ref name=":0" /> Leidy used the name ''Titanotherium proutii'' for all of the fossils, including Prout's jaw.<ref name=":0" /> Although recognized as the remains of large perissodactyls, early descriptions of brontothere fossils up to and including Leidy's work failed to recognize the distinctive nature of the animals.<ref name=":2">{{Cite book |last=Lucas |first=Spencer G. |url=https://books.google.com/books?id=fJivCQAAQBAJ |title=Paleogene Mammals: Bulletin 26 |date=2004 |publisher=New Mexico Museum of Natural History and Science |pages=119 |language=en |chapter=O. C. Marsh and the Eocene Brontothere ''Teleodus'': A Paleontological Hoax}}</ref> thumb|''M. coloradensis'' type specimen (ANSP 13362)|upright=0.8 In 1870, Leidy described the first known brontothere fossil to preserve most of the distinctive horns (ANSP 13362),<ref name=":0" /> found in Colorado.<ref name=":17">{{cite book |last=Osborn |first=Henry F. |author-link=Henry Fairfield Osborn |url=https://archive.org/details/titanotheresofan01osbo/ |title=The Titanotheres of Ancient Wyoming, Dakota, and Nebraska, Volume 1 |date=1929 |publisher=Department of the Interior, U.S. Geological Survey |pages=144–147, 208–209, 219, 230 |chapter=Discovery of the Titanotheres and Original Descriptions}}</ref> Leidy suspected that ANSP 13362 could represent the same animal as ''Titanotherium'',<ref name=":17" /><ref name=":6" /> but struggled with identifying the fossil, speculating that it could perhaps represent a camelid or a North American representative of the giraffid ''Sivatherium''.<ref name=":17" /> Leidy provisionally referred the fossil to a new genus and species, ''Megacerops coloradensis''.<ref name=":0" />

In the 1870s and 1880s, the northern Great Plains saw intense paleontological excavations due to the Bone Wars, a period of competitive fossil hunting between the rival researchers Othniel Charles Marsh and Edward Drinker Cope. Both Marsh and Cope funded expeditions to uncover and describe new prehistoric mammals in the region.<ref name=":4" /> In the 1870s, Marsh and Cope described several dinoceratans, an extinct order of large mammals, some of which had horns similar to ''Megacerops'', such as ''Uintatherium'' and ''Eobasileus''.<ref name=":6">{{Cite book |last=Leidy |first=Joseph |url=https://archive.org/details/contributionstoe00leid/page/n21/mode/1up |title=Contributions to the Extinct Vertebrate Fauna of the Western Territories |publisher=Department of the Interior, United States Geological Survey of the Territories |publication-date=1873 |pages=231, 239–242|author-link=Joseph Leidy}}</ref> In 1873, Leidy speculated that ''Megacerops'' could belong to the same order as these animals.<ref name=":6" /> Leidy also alternatively suggested that ''Megacerops'' could have been a proboscid, the nasal bones perhaps having served as attachment points for a tapir-like movable snout or a proboscis.<ref name=":6" />

=== Further findings === {{Multiple image | direction = vertical | total_width = 260 | image1 = Plate XXXIII, Brontops robustus type.png | align = left | image2 = Large Californian Brontops.jpg | footer = YPM VP 12048, a nearly complete skeleton made the type specimen of ''Brontops robustus'' by Othniel Charles Marsh (top) and an illustration of the same skeleton by F. Berger, done under Marsh's direction in 1889<ref name=":2"/><ref name=":16"/> (bottom) }} Othniel Charles Marsh's contributions to brontothere research were highly significant.<ref name=":2" /> His studies of brontotheres began in 1870, when he led an expedition to northern Colorado on behalf of Yale College.<ref name=":2" /> This expedition collected a large number of brontothere fossils, including well-preserved and complete skulls and postcranial elements.<ref name=":2" /> During the expedition, Marsh's crew were shown a brontothere jaw by a group of Lakota, who told them of their legends of ''wakíŋyaŋ''.<ref name=":19">{{Cite book |last=Brusatte |first=Steve |author-link=Stephen L. Brusatte |url=https://books.google.com/books?id=CHtZEAAAQBAJ |title=The Rise and Reign of the Mammals: A New History, from the Shadow of the Dinosaurs to Us |date=2022 |publisher=Pan Macmillan |isbn=978-1-5290-3424-0 |language=en |chapter=Mammals Modernize}}</ref> In honor of the legends, Marsh named the new genus ''Brontotherium'' ("thunder beast") in 1873.<ref>{{Cite book |last=Mayor |first=Adrienne |url=https://books.google.com/books?id=9fuUEAAAQBAJ |title=Fossil Legends of the First Americans |date=2023 |publisher=Princeton University Press |isbn=978-0-691-24561-4 |pages=241 |language=en}}</ref>

Marsh's 1873 description of ''Brontotherium gigas'' was the most important contribution to brontothere knowledge up until that time.<ref name=":17" /> The holotype was designated as another lower jaw, but Marsh was able to correctly describe several characters of both the jaws and the rest of the skeleton.<ref name=":17" /> Marsh recognized ''Brontotherium'' as a "true perissodactyl with limb bones resembling those of ''Rhinoceros''".<ref name=":2" /> Recognizing that ''Brontotherium'' was related to the animal described as ''Titanotherium'' by Leidy, Marsh also erected the new family Brontotheriidae to contain the two genera.<ref name=":2" /> In 1875, H. C. Clifford discovered and excavated a large and nearly complete brontothere skeleton near Chadron, Nebraska. This skeleton (YPM VP 12048) was described by Marsh in 1887 as the type specimen of the new genus and species ''Brontops robustus''.<ref name=":16">{{cite book |last=Osborn |first=Henry F. |author-link=Henry Fairfield Osborn |url=https://archive.org/details/titanotheresofan01osbo/ |title=The Titanotheres of Ancient Wyoming, Dakota, and Nebraska, Volume 1 |date=1929 |publisher=Department of the Interior, U.S. Geological Survey |pages=666 |chapter=Evolution of the Skeleton of Eocene and Oligocene Titanotheres}}</ref>

Marsh continued to study brontothere fossils for the rest of his career, some collected by himself but most purchased from collectors "out West".<ref name=":2" /> Both Marsh and Cope named new species for close to every good brontothere specimen that came into their possession, often differentiated only by minor features.<ref name=":0" /> The first fossils from outside of the United States were reported by Cope in 1886, from the Swift Current Creek in Assiniboia (today Saskatchewan), Canada. Cope named the new species ''Menodus angustigenis'' to accommodate these fossils.<ref name=":17" />

In cooperation with the United States Geological Survey, Marsh wrote a series of monographs on prehistoric animals from the United States. Paleontological field work was carried out at an unprecedented scale by the U.S. Geological Survey to gather material for Marsh's monographs on both ceratopsian dinosaurs and brontotheres.<ref name=":13">{{Cite journal |last=Osborn |first=Henry Fairfield |author-link=Henry Fairfield Osborn |date=1929 |title=Paleontological Monographs of the National Geological Surveys |journal=Science |volume=70 |issue=1814 |pages=315–317 |bibcode=1929Sci....70..315F |doi=10.1126/science.70.1814.315 |jstor=1653153 |pmid=17738314}}</ref> Brontothere fossils were collected for the monograph in large part by John Bell Hatcher, who spent fifteen months in South Dakota and Nebraska in 1886–1888.<ref name=":17" /> By the end of his expedition, Hatcher reported that he had collected "nearly 200 complete skulls and many more or less complete skeletons".<ref name=":17" /> Marsh's brontothere monograph was not completed before his death in 1899, and he left no known manuscript for it, only pencil notes and unpublished figures.<ref name=":13" /> [[File:Menodus angustigenis, type.png|thumb|Fossils referred to ''Menodus angustigenis'' by Edward Drinker Cope, the first ''Megacerops'' fossils reported from Canada]] In the late 19th and early 20th century, most of the museums in the United States funded fossil collecting in the so-called "''Titanotherium'' beds" of the Great Plains,<ref name=":4" /> increasing the already large brontothere fossil sample to encompass further skeletons and many additional skulls.<ref name=":0" /> Henry Fairfield Osborn energetically pursued further studies of brontothere fossils, making the study of the group one of his life's quests.<ref name=":15">{{Cite book |last=Prothero |first=Donald R. |author-link=Donald Prothero |url=https://books.google.com/books?id=4py5xVak_yYC |title=Greenhouse of the Dinosaurs: Evolution, Extinction, and the Future of Our Planet |date=2009 |publisher=Columbia University Press |isbn=978-0-231-51832-1 |language=en |chapter=Badlands Bestiary}}</ref> Most of the fossils worked on by Osborn were collected by Hatcher and originally intended to serve as material for Marsh's monograph.<ref name=":17" /> In 1929, Osborn published a monograph on the brontotheres, ''The Titanotheres of Ancient Wyoming, Dakota and Nebraska''.<ref name=":14">{{Cite book |last=Prothero |first=Donald R. |author-link=Donald Prothero |url=https://books.google.com/books?id=kWpQX-sfsLgC |title=Horns, Tusks, and Flippers: The Evolution of Hoofed Mammals |last2=Schoch |first2=Robert M. |date=2002 |publisher=The Johns Hopkins University Press |isbn=978-0-8018-7135-1 |pages=234–235, 239 |language=en|author-link2=Robert M. Schoch}}</ref> The two-volume monograph spanned 951 pages and was illustrated with 795 figures and 236 plates. Osborn believed that it would be the definitive work on brontotheres.<ref name=":14" />

The large number of fossils collected has made ''Megacerops'' one of the best represented large herbivores in the fossil record of North American mammals.<ref name=":20">{{Cite journal |last1=Mihlbachler |first1=Matthew C. |last2=Prothero |first2=Donald R. |author-link2=Donald Prothero |date=2021 |title=Eocene (Duchesnean and earliest Chadronian) brontotheres (Brontotheriidae), Protitanops curryi and cf. Parvicornus occidentalis, from West Texas and Mexico |url=https://palaeo-electronica.org/content/2021/3479-texas-brontotheriidae |journal=Palaeontologia Electronica |volume=24 |issue=3 |pages=a35 |doi=10.26879/944|doi-access=free }}</ref> ''Megacerops'' fossils have predominantly been recovered from the White River Group in the United States and the Cypress Hills Formation in Canada.<ref name=":20" /> All fossils now attributed to ''Megacerops'' appear to be restricted to the Chadronian land mammal age, which corresponds to the Late Eocene.<ref name=":20" /> Despite the large number of specimens, the evolution and paleobiology of ''Megacerops'' remain understudied, largely due to longstanding taxonomic confusion obstructing further research.<ref name=":20" /> An additional problem is that most ''Megacerops'' fossils have poorly recorded stratigraphic data, which has limited the degree to which variation in single contemporary populations can be studied.<ref name=":0" />

=== Taxonomy and species === thumb|Illustration of F:AM 128600, the holotype specimen of ''M. kuwagatarhinus''|leftBy the time of Osborn's 1929 monograph, at least 47 species of Chadronian brontotheres had been named, many based on poor and fragmentary fossils.<ref name=":0" /> Fossils are very similar in most of the features of the skeletons. Clear differences are for the most part only found in the shape, orientation, and size of the horns, the prominence of the nasal bones, and the thickness of the zygomatic arches.<ref name=":0" /> Because of this, there was a general consensus in the 20th century that the brontotheres were highly oversplit, i.e. divided into far too many species.<ref name=":0" /><ref name=":14" /> In his monograph, Osborn recognized 37 valid species of Chadronian brontotheres; this number was criticized even by Osborn's contemporaries as far too high for the relatively brief timeframe of the Chadronian.<ref name=":0" /> Osborn variously attributed the variation seen in the fossils to sexual dimorphism, ontogenetic differences, and to species-level differences, but did not show how a feature could be determined to vary due to one factor or another.<ref name=":0" />

In 1967, John Clark, James R. Beerbower and Kenneth K. Kietzke were the first to suggest that all Chadronian brontotheres belonged to a single species that exhibited great individual variation.<ref name=":0" /> This was based on a fossil site that preserved several horn cores with variable morphology, from a group of contemporary individuals. Finding it unlikely that the horns were from four different species, Clark, Beerbower and Kietzke referred all four specimens to the single species ''Menodus giganteus'', the oldest available name, and designated all other names, such as ''Brontotherium'' and ''Titanotherium'', as junior synonyms.<ref name=":9">{{Cite book |last=Clark |first=John |url=https://archive.org/details/oligocenesedimen05clar/page/n11/mode/2up |title=Oligocene Sedimentation, Stratigraphy, Paleoecology and Paleoclimatology in the Big Badlands of South Dakota |last2=Beerbower |first2=James R. |last3=Kietzke |first3=Kenneth K. |date=1967 |publisher=Field Museum of Natural History |series=Fieldiana: Geology Memoirs |volume=5 |pages=50–51}}</ref> In 1989, Bryn J. Mader published a proposed revision of the Brontotheriidae.<ref name=":15" /> Mader recognized a number of Chadronian brontothere species as valid, divided into the three genera ''Menops'', ''Brontops'' and ''Megacerops''. Mader's classification was mostly based on the cross section shapes of horns.<ref name=":0" /> By this approach, the name ''Menodus giganteus'' is a ''nomen dubium'' since the holotype of that species contains no horn material and thus no diagnostic features.<ref name=":0" />

In a 2004 preliminary revision,<ref name=":1">{{cite journal |last=Mihlbachler |first=Matthew C. |date=2008 |title=Species Taxonomy, Phylogeny, and Biogeography of the Brontotheriidae (Mammalia: Perissodactyla) |url=https://bioone.org/journals/bulletin-of-the-american-museum-of-natural-history/volume-2008/issue-311/0003-0090(2008)501%5b1%3aSTPABO%5d2.0.CO%3b2/Species-Taxonomy-Phylogeny-and-Biogeography-of-the-Brontotheriidae-Mammalia/10.1206/0003-0090(2008)501[1:STPABO]2.0.CO;2.full |journal=Bulletin of the American Museum of Natural History |volume=501 |page= |doi=10.1206/0003-0090(2008)501[1:STPABO]2.0.CO;2}}</ref> Matthew C. Mihlbachler, Spencer G. Lucas and Robert J. Emry concluded that the variability among Chadronian brontothere fossils is slightly higher than that of modern sexually dimorphic mammal species.<ref name=":0" /> Using data from a large number of skulls, it was found to be impossible to divide the fossils into discrete units (i.e. separate species), and that there thus might be just one diagnosable species in the fossil assemblage.<ref name=":0" /> Mihlbachler, Lucas, and Emry chose to recognize two species, with the second species distinguished by rare skulls with bifurcating horns. These fossils had been described as a new species by Mader and John P. Alexander in 1995, ''Megacerops kuwagatarhinus''.<ref name=":0" /> Since distinctions between the two forms can only be seen in the horns, Mihlbachler, Lucas and Emry suggested that the name ''Megacerops coloradensis'' should be used for the common species with unbifurcating horns. ''M. coloradensis'' is the earliest available name with a holotype that preserves unbifurcating horns, and thus the earliest name to securely apply to this species.<ref name=":0" /> In a comprehensive 2008 phylogenetic analysis, Mihlbachler recovered ''Megacerops'' as a monophyletic genus, and noted that while the present data only supported two diagnosable species, more could perhaps be indicated in future studies on the ''Megacerops'' material.<ref name=":1" />

A detailed species-level revision of ''Megacerops'' is still required to fully resolve the taxonomy.<ref name=":20" /> Despite the lack of a detailed revision, the status of ''Megacerops'' as the only valid Chadronian brontothere genus, containing only two species, has become widely accepted.<ref name="RhinocerosGiants" /><ref name=":10">{{Cite book |last=Prothero |first=Donald R. |author-link=Donald Prothero |url=https://books.google.com/books?id=leX4DAAAQBAJ |title=Giants of the Lost World: Dinosaurs and Other Extinct Monsters of South America |date=2016 |publisher=Smithsonian Institution |isbn=978-1-58834-573-8 |pages=42 |language=en}}</ref> Mihlbachler's conclusions have been supported by researchers such as Donald Prothero,<ref name=":10" /> Karen J. Lloyd, Jaelyn J. Eberle,<ref>{{cite journal |last1=Lloyd |first1=K. J. |last2=Eberle |first2=J. J. |title=A late Eocene (Chadronian) mammalian fauna from the White River Formation in Kings Canyon, northern Colorado |journal=Rocky Mountain Geology |date=2012 |volume=47 |issue=2 |pages=113–132 |doi=10.2113/gsrocky.47.2.113 |bibcode=2012RMGeo..47..113L }}</ref> Parker D. Rhinehart, Alfred J. Mead and Dennis Parmley.<ref>{{Cite journal |last1=Rhinehart |first1=Parker |last2=Mead |first2=Alfred |last3=Parmley |first3=Dennis |date=2019 |title=Eocene Terrestrial Mammals From Central Georgia |url=https://digitalcommons.gaacademy.org/gjs/vol77/iss2/6 |journal=Georgia Journal of Science |volume=77 |issue=2 }}</ref> Mader rejected the single-genus model, and continued to regard ''Menops'', ''Brontops'' and ''Megacerops'' to be distinct genera, with several species.<ref>{{cite journal |last=Mader |first=Bryn J. |title=A species level revision of Bridgerian and Uintan brontotheres (Mammalia, Perissodactyla) exclusive of Palaeosyops |journal=Zootaxa |date=2008 |volume=1837 |doi=10.11646/zootaxa.1837.1.1 |bibcode=2008Zoot.18377.1.1M }}</ref><ref>{{cite journal |last=Mader |first=Bryn J. |title=Details of the cranial anatomy of a primitive diplacodont brontothere, cf. ''Protitanotherium'', from the Wiggins Formation of Wyoming (Mammalia, Perissodactyla, Brontotheriidae) |journal=Journal of Vertebrate Paleontology |date=2009 |volume=29 |issue=4 |pages=1224–1232 |doi=10.1671/039.029.0414 |bibcode=2009JVPal..29.1224M }}</ref> In response, Mihlbachler pointed out that Mader had not done a species-level revision of these genera, had not shown them to be monophyletic via phylogenetic analysis,<ref name=":1" /> and that the diagnostic features proposed by Mader are continuous in the fossil material and can thus not indicate distinct taxa (i.e. the fossils do not cluster together in three clear genera, as proposed).<ref>{{Cite book |last=Janis |first=Christine M. |author-link=Christine Janis |title=Evolution of Tertiary Mammals of North America: Volume 2: Small Mammals, Xenarthrans, and Marine Mammals |last2=Hulbert |first2=Richard C. |last3=Mihlbachler |first3=Matthew C. |date=2008 |publisher=Cambridge University Press |isbn=978-0-521-78117-6 |pages=670 |language=en |chapter=Addendum}}</ref> Although historical generic names such as ''Titanotherium'', ''Brontotherium'' and ''Brontops'' are generally not treated seriously by researchers today, these names continue to remain famous and appear in popular books, on websites, and as names for toys.<ref name="RhinocerosGiants" /><ref name=":10" />

==Description== {{Multiple image | image1 = Brontops robustus (Megacerops), life restoration.png | image2 = Brontops robustus (Megacerops), musculature.png | total_width = 450 | footer = Reconstruction of the life appearance and musculature of ''Megacerops'' by Erwin S. Christman (1916) | align = right }} ''Megacerops'' was a huge and impressive animal, characterized by its large size and the two horns at the front of its skull.<ref name=":14" /><ref>{{Cite book |last=Rasmussen |first=D. Tab |url=https://www.google.com/books/edition/Vertebrate_Paleontology_in_Utah/qeRM16ndBx4C |title=Vertebrate Paleontology in Utah |last2=Conroy |first2=Glenn C. |last3=Friscia |first3=Anthony R. |last4=Townsend |first4=Elizabeth |last5=Kinkel |first5=Mary D. |date=1999 |publisher=Utah Geological Survey |isbn=978-1-55791-634-1 |pages=414 |language=en |chapter=Mammals of the Middle Eocene Uinta Formation}}</ref> They were massively built and robust,<ref name=":28">{{Cite book |last=Rose |first=Kenneth D. |url=https://www.google.com/books/edition/The_Beginning_of_the_Age_of_Mammals/3bs0D5ix4VAC |title=The Beginning of the Age of Mammals |date=2006 |publisher=Johns Hopkins University Press |isbn=978-0-8018-8472-6 |pages=250–252 |language=en |chapter=Brontotheriidae}}</ref><ref name=":21" /> and adapted for strength rather than speed.<ref name=":21">{{Cite journal |last=Lull |first=Richard S. |author-link=Richard Swann Lull |date=1905 |title=Restoration of the Titanothere ''Megacerops'' |url=https://www.journals.uchicago.edu/doi/pdf/10.1086/278533 |journal=The American Naturalist |volume=XXXIX |issue=463 |pages=419–424 |bibcode=1905ANat...39..419L |doi=10.1086/278533}}</ref> [[File:Megacerops and modern perissodactyls.png|thumb|''Megacerops'' compared to modern perissodactyls (white rhinoceros, Przewalski's horse, and the South American tapir)|440px]] ''Megacerops'' was superficially similar to modern rhinoceroses in appearance, an example of convergent evolution.<ref name="Osborn 19292">{{cite book |last=Osborn |first=Henry F. |author-link=Henry Fairfield Osborn |url=https://archive.org/details/titanotheresofan01osbo/ |title=The Titanotheres of Ancient Wyoming, Dakota, and Nebraska, Volume 1 |date=1929 |publisher=Department of the Interior, U.S. Geological Survey |pages=14–15, 19, 23, 29 |chapter=Introduction to Mammalian Paleontology}}</ref> Beyond the similarities, several features separate ''Megacerops'' and other brontotheres from rhinoceroses.<ref name=":22" /> ''Megacerops'' was a more robust animal in general.<ref name=":21" /> ''Megacerops'' had four toes on its manus (front feet) and three on its pes (hind feet),<ref name=":22">{{Cite book |last=Marsh |first=Othniel Charles |author-link=Othniel Charles Marsh |url=https://www.google.com/books/edition/The_American_Naturalist_An_illustrated_M/PRBBEAAAQBAJ |title=The American Naturalist, An Illustrated MAgazine of Natural History: Volume VIII |date=1874 |publisher=Peabody Academy of Science |pages=79–82 |chapter=On the Structure and Affinities of the Brontotheridæ}}</ref> the primitive perissodactyl condition,<ref name=":28" /> whereas rhinoceroses have three toes on each.<ref name=":24">{{Cite book |last=Prothero |first=Donald R. |author-link=Donald Prothero |url=https://www.google.com/books/edition/Evolution_of_Tertiary_Mammals_of_North_A/I-RgojcDyWYC |title=Evolution of Tertiary Mammals of North America: Volume 1: Terrestrial Carnivores, Ungulates, and Ungulate Like Mammals |date=1998 |publisher=Cambridge University Press |isbn=978-0-521-35519-3 |pages=595–605 |language=en |chapter=Rhinocerotidae}}</ref> ''Megacerops'' has well-developed canines<ref name=":22" /> while the canines are typically lost in rhinoceroses,<ref name=":24" /> and molars more similar to those of chalicotheres than rhinoceroses.<ref name=":22" /> Rhinoceros horns are composed of agglutinated (tightly packed) keratin,<ref name=":24" /> whereas the horn cores of ''Megacerops'' were composed of bone.<ref name=":22" />

=== Size === ''Megacerops'' was among the largest brontotheres, rivaled in size only by a handful of other genera, such as ''Embolotherium''.<ref name=":1" /> The largest brontotheres were the largest mammals of the Eocene.<ref name=":19" /> ''Megacerops'' exceeded all modern rhinoceroses in size<ref name=":12">{{Cite book |last=Webb |first=Steve |url=https://books.google.com/books?id=C0blLvmdp04C |title=Corridors to Extinction and the Australian Megafauna |date=2013 |publisher=Elsevier |isbn=978-0-12-407840-6 |pages=49 |language=en}}</ref><ref name=":11">{{Cite book |last1=Benton |first1=Rachel C. |url=https://books.google.com/books?id=ZcFtCQAAQBAJ |title=The White River Badlands: Geology and Paleontology |last2=Terry |first2=Dennis O. |last3=Evanoff |first3=Emmett |last4=McDonald |first4=H. Gregory |date=2015 |publisher=Indiana University Press |isbn=978-0-253-01608-9 |pages=165–166 |language=en}}</ref> and was closer in size to elephants, only shorter.<ref name=":12" /> The largest ''Megacerops'' are typically estimated to have been {{convert|2.5|m|sp=us}} tall at the shoulder.<ref name=":12" /><ref>{{Cite book |last=Fariña |first=Richard A. |url=https://books.google.com/books?id=kUAKgNfiAvoC |title=Megafauna: Giant Beasts of Pleistocene South America |last2=Vizcaíno |first2=Sergio F. |last3=Iuliis |first3=Gerry De |date=2013 |publisher=Indiana University Press |isbn=978-0-253-00719-3 |pages=121 |language=en}}</ref><ref>{{Cite news |last=Bigalke |first=R. C. |author-link=Rudolph Carl Bigalke |title=Perissodactyl - Evolution and paleontology |url=https://www.britannica.com/animal/perissodactyl/Evolution-and-paleontology |archive-url=http://web.archive.org/web/20250422174155/https://www.britannica.com/animal/perissodactyl/Evolution-and-paleontology |archive-date=2025-04-22 |access-date=2025-12-30 |work=Encyclopedia Britannica |language=en}}</ref> This measurement derives in part from YPM VP 12048, the well-preserved skeleton once considered the type specimen of ''Brontops robustus''. When first mounted in 1916, this specimen was measured at 2.502 meters (8 feet 2½ inches) tall at the shoulder and 4.635 meters (15 feet 2½ inches) long, including the tail.<ref name=":16" /> The largest ''Megacerops'' specimens have sometimes historically been estimated to have reached {{convert|3|m|0|sp=us}} tall at the shoulder.<ref>{{Cite journal |last=Barbour |first=Erwin |author-link=Erwin Hinckley Barbour |date=1932 |title=The Articulated Skeleton of Titanotherium |url=https://digitalcommons.unl.edu/museumbulletin/46 |journal=Bulletin of the University of Nebraska State Museum (1924–2023)}}</ref> Large ''Megacerops'' would have reached about {{convert|5|m|0|sp=us}} in length.<ref name=":19" />

Various weight estimates of ''Megacerops'' have been published, ranging from as low as {{convert|2-3|t|ST|abbr=}}<ref>{{Cite book |last=Smith |first=Felisa A. |url=https://books.google.com/books?id=Y5g9EAAAQBAJ |title=Mammalian Paleoecology: Using the Past to Study the Present |date=2021 |publisher=The Johns Hopkins University Press |isbn=978-1-4214-4140-5 |pages=70 |language=en}}</ref> to as high as {{convert|4.18|t|ST}}<ref>{{Cite journal |last=Sanisidro |first=Oscar |last2=Mihlbachler |first2=Matthew C. |last3=Cantalapiedra |first3=Juan L. |date=2023 |title=A macroevolutionary pathway to megaherbivory |url=https://www.science.org/doi/10.1126/science.ade1833 |journal=Science |volume=380 |issue=6645 |pages=616–618 |doi=10.1126/science.ade1833|url-access=subscription }} [https://www.science.org/doi/suppl/10.1126/science.ade1833/suppl_file/science.ade1833_sm.pdf Supplementary material], p. 23.</ref> or even {{convert|5|t|ST}}.<ref name=":12" /><ref>{{Cite book |last=Linzey |first=Donald W. |url=https://books.google.com/books?id=70kWAQAAIAAJ |title=Vertebrate Biology |date=2001 |publisher=McGraw-Hill |isbn=978-0-697-36387-9 |pages=180 |language=en}}</ref>

=== Skull === thumb|Illustration of a ''Megacerops'' skull, including the upper dentition The skull of ''Megacerops'' was massive<ref name=":20" /> and somewhat rhinoceros-like.<ref name=":22" /><ref name="Marsh1876">{{Cite journal |last=Marsh |first=Othniel Charles |author-link=Othniel Charles Marsh |date=1876 |title=Principal characters of the Brontotheridae |url=https://ajsonline.org/article/63464-principal-characters-of-the-brontotheridae |journal=American Journal of Science |language=en |volume=s3-11 |issue=64 |pages=335–340 |bibcode=1876AmJS...11..335M |doi=10.2475/ajs.s3-11.64.335}}</ref> The skull was long and broad,<ref name="Osborn 19292" /> and vaguely saddle-shaped.<ref name=":20" /> Several features of the skull were greatly developed,<ref name=":21" /> such as the nasal bones, which were coossified (grown together through ossification),<ref name=":22" /> and the zygomatic arches.<ref name=":21" /> The occiput (back of the skull) was very high and broad.<ref name=":21" /> Most of these developments appear to have been correlated with the development of the horns of ''Megacerops''.<ref name=":21" />

There is no consistent dental formula for the entire ''Megacerops'' fossil assemblage. Mammal teeth are highly diagnostic;<ref>{{Cite book |last=Smith |first=Felisa A. |url=https://www.google.com/books/edition/Mammalian_Paleoecology/dpg9EAAAQBAJ |title=Mammalian Paleoecology: Using the Past to Study the Present |date=2021 |publisher=Johns Hopkins University Press |isbn=978-1-4214-4141-2 |pages=88 |language=en |chapter=Show Me Your Teeth, and I Will Tell You What You Are}}</ref> differences in dentition has historically been used to justify dividing the fossils into multiple genera.<ref name="Marsh1876" /> In 1876, Marsh described the dental formula of "''Brontotherium''" as {{DentalFormula|lower=2.1.3.3|upper=2.1.4.3}},{{efn|The order of numbers represent types of teeth—incisors, canines, premolars, and molars—and the bar separates the dentition in the upper and lower jaws. {{DentalFormula|lower=2.1.3.3|upper=2.1.4.3}} is read as two pairs of incisors in both the upper and lower jaw, one pair of canines in both the upper and lower jaw, four pairs of premolars in the upper jaw and three pairs of premolars in the lower jaw, and three pairs of molars in both the upper and lower jaw.<ref name="Marsh1876"/>}} that of "''Menodus''" as {{DentalFormula|lower=2.1.4?.3|upper=2.1.4.3}}, that of ''Megacerops'' as {{DentalFormula|lower=0.1.3.3|upper=2.1.4.3}}, and that of "''Diconodon''" as {{DentalFormula|lower=1.1.3.3|upper=0.1.4.3}}.<ref name="Marsh1876" /> In 1889, Marsh's "''Brontops''" was described as distinct based on its dental formula, {{DentalFormula|lower=1.1.4.3|upper=2.1.4.3}}.<ref name=":23">{{Cite journal |last=Marsh |first=O. C. |author-link=Othniel Charles Marsh |date=1889 |title=Appendix; Restoration of Brontops robustus, from the Miocene of America |url=https://ajsonline.org/article/62744-appendix-restoration-of-brontops-robustus-from-the-miocene-of-america |journal=American Journal of Science |language=en |volume=s3-37 |issue=218 |pages=163–166 |bibcode=1889AmJS...37..163M |doi=10.2475/ajs.s3-37.218.163 |url-access=subscription}}</ref> The number of different teeth was evidently not an important diagnostic feature in brontotheres. Some ''Megacerops'' skulls for instance preserve three lower premolars on one side and four on the other, meaning that the number of this tooth cannot have been a feature that separated genera (as suggested by Marsh for "''Brontotherium''" and "''Menodus''").<ref>{{Cite journal |last=Scott |first=William Berryman |author-link=William Berryman Scott |last2=Osborn |first2=Henry Fairfield |author-link2=Henry Fairfield Osborn |date=1887 |title=Preliminary Account of the Fossil Mammals from the White River Formation contained in the Museum of Comparative Zoölogy |url=https://rhinoresourcecenter.com/wp-content/uploads/2013/04/1563609050.pdf |journal=Bulletin of the Museum of Comparative Zoology at Harvard College |volume=13 |issue=5 |pages=151–171}}</ref>

''Megacerops'' had at most two pairs of incisors.<ref name=":23" /> More basal brontotheres, such as ''Diplacodon'' and ''Protitanotherium'', had additional pairs of incisors but experienced atrophy of some pairs and hypertrophy of others, perhaps a step towards the condition seen in ''Megacerops''.<ref name=":18">{{cite book |last=Osborn |first=Henry F. |author-link=Henry Fairfield Osborn |url=https://archive.org/details/titanotheresofan01osbo/ |title=The Titanotheres of Ancient Wyoming, Dakota, and Nebraska, Volume 1 |date=1929 |publisher=Department of the Interior, U.S. Geological Survey |pages=444–448, 456 |chapter=Evolution of the Skull and Dentition of Oligocene Titanotheres}}</ref> Fossil evidence points to the incisors being of little use to ''Megacerops''; they were reduced in size, fossils show very little wear,<ref name=":11" /> and ''Megacerops'' specimens of advanced age sometimes lost their incisors over the course of their lives.<ref name=":18" /> The lack of use for the incisors could suggest that brontotheres had a prehensile upper lip, similar to modern rhinoceroses.<ref name=":11" />

=== Postcranial skeleton === {{image frame|content={{Photo montage | photo1a = Brontotherium hatcheri (Megacerops) skeleton, side view.jpg | photo2a = Brontotherium hatcheri (Megacerops) skeleton, front view.jpg | photo2b = Brontotherium hatcheri (Megacerops) skeleton, back view.jpg | size = 270 | spacing = 3 | color = #FFFFFF }} | width = 280 | caption = Side, front, and back views of a ''Megacerops'' skeleton | align = left }} The postcranial skeleton of ''Megacerops'' was massive, robust,<ref name=":11" /> and relatively short.<ref name=":21" /> The most notable feature of the postcranial skeleton is the elongated spines of the dorsal vertebrae above the shoulders, an adaptation to support the huge neck muscles needed to carry the heavy skull.<ref name=":21" /> This aspect of ''Megacerops'' anatomy distinguishes it from both rhinoceroses and elephants, and has been compared to the vertebrae and neck musculature of modern bison.<ref name=":21" /> The neck itself was stout and moderately long.<ref name="Marsh1876" /> The vertebrae of ''Megacerops'' were somewhat similar to those of modern rhinoceroses.<ref name=":23" /> The ribs were strong and massive.<ref name=":23" /> The pelvis of ''Megacerops'' was expanded transversely and wide,<ref name="Marsh1876" /><ref name=":23" /> similar to elephant pelvises.<ref name=":23" />

The limbs of ''Megacerops'' were intermediate in proportion between those of modern rhinoceroses and elephants.<ref name=":22" /><ref name="Marsh1876" /> The limbs of show several adaptations to withstand the great weight of the animal. Compared to rhinoceroses, ''Megacerops'' limbs are stouter, particularly at the ankles and wrists, and there is a lesser degree of angulations between the segments of the limbs.<ref name=":21" /> The forelimbs were especially robust<ref name=":21" /><ref name=":23" /> and several adaptations, notably roughenings of the olecranon (the protruding part of the elbow) and the humerus, suggest great muscle power.<ref name=":21" />

''Megacerops'' had four toes on its manus (front feet) and three on its pes (hind feet).<ref name=":22" /><ref name="Marsh1876" /> The retention of the fourth digit on the manus is probably another feature that helped to support the animal's great weight.<ref name=":21" /> The arrangement of their feet bones indicate that ''Megacerops'' feet had a pad of elastic tissue, similar to the feet of modern elephants and rhinoceroses.<ref name=":11" />

== Classification ==

=== Evolution and relations === [[File:Megacerops and Eotitanops.png|thumb|''Megacerops'' (B) and the basal brontothere ''Eotitanops'' (A), drawn to scale|upright=1.1]]Brontotheres composed the family Brontotheriidae, of which ''Megacerops'' was a derived member.<ref name=":1" /> The brontotheres are classified in the order Perissodactyla (odd-toed ungulates), the group that includes modern horses, rhinoceroses, and tapirs.<ref name=":1" /> The position of the brontotheres in the perissodactyl family tree is contentious and results vary between studies.<ref name=":1" /> The brontotheres have traditionally been placed in the perissodactyl suborder Hippomorpha, a group that also includes horses, the extinct palaeotheres (Palaeotheriidae), and in some classification schemes the extinct chalicotheres (Chalicotheriidae).<ref name=":1" /> Brontotheres have alternatively been classified as inside Palaeotheriidae,<ref>{{cite journal |last=Froehlich |first=David J. |title=Phylogenetic systematics of basal perissodactyls |journal=Journal of Vertebrate Paleontology |date=1999 |volume=19 |issue=1 |pages=140–159 |doi=10.1080/02724634.1999.10011129 |bibcode=1999JVPal..19..140F }}</ref> in the different suborder Ceratomorpha as a sister group of rhinoceroses and tapirs,<ref>{{Cite book |last1=McKenna |first1=Malcolm C. |url=https://archive.org/details/classificationof0000mcke/page/472/mode/2up |title=Classification of Mammals: Above the Species Level |last2=Bell |first2=Susan K. |date=1997 |publisher=Columbia University Press |isbn=978-0-231-52853-5 |pages=473 |language=en}}</ref> outside the clade that contains chalicotheres, horses, rhinoceroses, and tapirs,<ref>{{cite journal |last1=Hooker |first1=J. J. |last2=Dashzeveg |first2=D. |title=The origin of chalicotheres (Perissodactyla, Mammalia) |journal=Palaeontology |date=2004 |volume=47 |issue=6 |pages=1363–1386 |doi=10.1111/j.0031-0239.2004.00421.x |bibcode=2004Palgy..47.1363H }}</ref> and as more closely related to chalicotheres, rhinoceroses, and tapirs than to horses.<ref>{{cite journal |last1=Bai |first1=Bin |last2=Wang |first2=Yuan-Qing |last3=Meng |first3=Jin |title=The divergence and dispersal of early perissodactyls as evidenced by early Eocene equids from Asia |journal=Communications Biology |date=2018 |volume=1 |article-number=115 |doi=10.1038/s42003-018-0116-5 |pmid=30271995 |pmc=6123789 }}</ref>

Brontotheres originated in the early Eocene, as part of the initial radition of the perissodactyls across the Holarctic realm.<ref name="MihlbachlerDemere">{{Cite journal |last=Mihlbachler|first=Matthew C.|last2=Deméré|first2=Thomas A.|date=2009|title=A new species of Brontotheriidae (Perissodactyla, Mammalia) from the Santiago Formation (Duchesnian, Middle Eocene) of Southern California|url=https://www.researchgate.net/publication/230881822_A_new_species_of_Brontotheriidae_Perissodactyla_Mammalia_from_the_Santiago_Formation_Duchesnian_Middle_Eocene_of_Southern_California|journal=Proceedings of the San Diego Society of Natural History|issue=41|pages=1–36}}</ref> The brontotheres experienced rapid diversification in the Middle Eocene, becoming one of the most diverse large mammal groups in Asia and North America.<ref name="MihlbachlerDemere" /> The group also experienced one of the most extreme size increases in all of mammalian evolution.<ref name=":25">{{Cite journal |last=Sanisidro |first=Oscar |last2=Mihlbachler |first2=Matthew C. |last3=Cantalapiedra |first3=Juan L. |date=2023 |title=A macroevolutionary pathway to megaherbivory |url=https://www.science.org/doi/10.1126/science.ade1833 |journal=Science |volume=380 |issue=6645 |pages=616–618 |doi=10.1126/science.ade1833|url-access=subscription }}</ref> The earliest brontotheres weighed around {{convert|18|kg|lb|abbr=}} based on dental measurements. Over the course of the roughly twenty million years of the Eocene, more than half of the known brontothere species grew to surpass {{convert|1000|kg|lb|abbr=}}.<ref name=":25" /> Since the brontotheres were apparently restricted to the roughly twenty million years of the Eocene, their rapid diversification was described by Matthew C. Mihlbachler and Thomas A. Deméré as adhering to a "live fast, die young" pattern of evolutionary radiation.<ref name="MihlbachlerDemere" />

Phylogenetic analyses indicate that horned brontotheres (the subtribe Brontotheriina) originated in Central Asia.<ref name=":110">{{Cite journal |last=Mihlbachler|first=Matthew C.|date=2011|title=A new uintan horned brontothere from Wyoming and the evolution of canine size and sexual dimorphism in the Brontotheriidae (Perissodactyla: Mammalia)|url=https://doi.org/10.1080/02724634.2011.539653|journal=Journal of Vertebrate Paleontology|volume=31|issue=1|pages=202–214|doi=10.1080/02724634.2011.539653|bibcode=2011JVPal..31..202M|issn=0272-4634|url-access=subscription}}</ref> Horned brontotheres first appeared in North America during the Uintan land mammal age, diversifying into several species and genera, out of which ''Diplacodon'' is the best represented in the fossil record.<ref name=":110" /> Early North American horned brontotheres were ancestral to the more derived infratribe Brontotheriita, which includes ''Megacerops''.<ref name=":110" /> It is not clear where exactly the Brontotheriita originated since basal members of the infratribe are known from both North America (''Protitanops'', ''Eubrontotherium'') and Asia (''Parabrontops''). It is thus possible that the ancestors and close relatives of ''Megacerops'' migrated in and out of Asia more than once.<ref>{{Cite journal |last=Mihlbachler |first=Matthew C. |date=2007 |title=''Eubrontotherium clarnoensis'', a new genus and species of brontothere (Brontotheriidae, Perissodactyla) from the Hancock Quarry, Clarno Formation, Wheeler County, Oregon |url=https://www.researchgate.net/publication/230881825_Mihlbachler_M_C_Eubrontotherium_clarnoensis_a_new_genus_and_species_of_brontothere_Brontotheriidae_Perissodactyla_from_the_Hancock_Quarry_Clarno_Formation_Wheeler_County_Oregon_Paleobios |journal=PaleoBios |volume=27 |issue=1 |pages=19–39}}</ref>

=== Systematics === The family Brontotheriidae was created by Marsh in 1873 to contain the horned brontotheres known at the time, ''Titanotherium'' and ''Brontotherium'' (both genera now considered synonyms of ''Megacerops'').<ref name=":1" /> Serious attempts to classify the large number of American brontothere fossils were undertaken by Osborn in the early 20th century. In his 1929 monograph, Osborn divided the Brontotheriidae into a number of different subfamilies, which he believed represented several polyphyletic and separately evolving lineages.<ref name=":1" /> Osborn's taxonomy had major shortcomings, notably his oversplitting of the fossils and the influence of his personal belief in the obsolete hypothesis of orthogenesis.<ref name=":1" /> Despite criticism, Osborn's taxonomy was mostly retained throughout the 20th century, though the subfamilies used could vary between studies. Detailed revisions to brontothere taxonomy were not published until work by Mader in the 1980s and 1990s, and Mihlbachler in the 2000s.<ref name=":1" />

Per Mihlbachler's 2008 revision, ''Megacerops'' is classified as part of the infratribe Brontotheriita.<ref name=":1" /> This group also includes the genera ''Dianotitan'', ''Duchesneodus'', ''Eubrontotherium'', ''Notiotitanops'', ''Parabrontops'',<ref name=":1" /> ''Parvicornus'',<ref name="MihlbachlerDemere" /> and ''Protitanops''.<ref name=":1" /> The Brontotheriita were a sister group to the infratribe Embolotheriita, which includes genera such as ''Embolotherium'', ''Metatitan'' and ''Gnathotitan''.<ref name=":1" />

The cladograms below are the strict reduced consensus tree of brontotheres from Mihlbachler's 2008 analysis (collapsed to show only the Brontotheriita),<ref name=":1" /> and the strict consensus tree for the Brontotheriita from a 2021 study by Mihlbachler and Prothero on brontotheres from Texas.<ref name=":20" /> Some phylogenetic analyses recover ''Notiotitanops mississippiensis'' inside ''Megacerops'', but the phylogenetic position of ''Notiotitanops'' is generally unstable between analyses.<ref name=":1" />

{{col-begin|width=50%}} {{col-2}}'''Mihlbachler, 2008'''<ref name=":1"/> {{clade|{{clade |1=''Parabrontops gobiensis'' |2={{clade |1=''Protitanops curryi'' |2={{clade |1=''Eubrontotherium clarnoensis'' |2=''Dianotitan lunanensis'' |3=''Duchesneodus uintensis'' |4={{clade |1='''''Megacerops coloradensis''''' |2='''''Megacerops kuwagatarhinus''''' }} }} }} }}|style=font-size:85%; line-height:85%|label1=Brontotheriita}} {{col-2}}'''Mihlbachler & Prothero, 2021'''<ref name=":20"/> {{clade|{{clade |1=''Parabrontops gobiensis'' |2={{clade |1=''Parvicornus occidentalis'' |2={{clade |1=''Eubrontotherium clarnoensis'' |2={{clade |1=''Protitanops curryi'' |2={{clade |1=''Dianotitan lunanensis'' |2={{clade |1=''Duchesneodus uintensis'' |2={{clade |1='''''Megacerops kuwagatarhinus''''' |2={{clade |1='''''Megacerops coloradensis''''' |2=''Notiotitanops mississippiensis'' }} }} }} }} }} }} }} }}|style=font-size:85%; line-height:85%|label1=Brontotheriita}} {{col-end}} == Paleobiology ==

=== Horns === [[File:Titanothere heads.png|thumb|Four ''M. coloradensis'' heads restored by Charles R. Knight (1929), showing the variability in horn morphology|upright=1.1|left]] Many brontothere genera had horns or cranial domes.<ref name=":7">{{Cite book |last=Mader |first=Bryn J. |url=https://books.google.com/books?id=I-RgojcDyWYC |title=Evolution of Tertiary Mammals of North America: Volume 1, Terrestrial Carnivores, Ungulates, and Ungulate Like Mammals |date=1998 |publisher=Cambridge University Press |isbn=978-0-521-35519-3 |pages=534 |language=en |chapter=Brontotheriidae}}</ref> The horns of ''Megacerops'' were the most developed out of all North American brontotheres.<ref name=":8">{{Cite web |title=Brontotheriidae |url=https://research.amnh.org/paleontology/perissodactyl/evolution/groups/brontotheriidae |access-date=2025-12-29 |website=American Museum of Natural History}}</ref> The horns took the form of two bony protuberances above the nose, and were vaguely reminiscent of a slingshot in shape.<ref name=":8" /> The horns were the dominant feature of the skull.<ref name=":18" /> The shape, size and orientation of ''Megacerops'' horns varied greatly between individuals,<ref name=":0" /> as did the degree to which the horns impacted the rest of the cranial anatomy.<ref name=":1" /> Smaller ''Megacerops'' with smaller horns had a typical and well-developed nasal process. In larger ''Megacerops'' with larger horns, the nasal process was largely absorbed by the horns and reduced to a small triangle-shaped remnant structure.<ref name=":1" /> In some rare specimens, the two horns have fused together completely to produce a plate or shield-like structure extending across the entire muzzle.<ref name=":17" /> thumb|Rare ''Megacerops'' specimen with the two horns fused into a shield-like structure|upright=0.75 The great variability in horn shape in ''Megacerops'' could have been caused by a relatively indeterminate system controlling horn growth, in which minor genetic differences of "accidents of development" could give rise to great variations in shape, size, and orientation.<ref name=":9" /> Based on patterns of sexual dimorphism in living horned mammals, larger ''Megacerops'' with larger and more elaborate horns are traditionally assumed to have been male specimens.<ref name=":0" /><ref name=":11" /><ref name=":8" />

''Megacerops'' horns were composed of non-deciduous bone and not elaborated with any additional bone structures or ornamentation.<ref name=":0" /> There were large air cavities in the base of the horns.<ref name="Marsh1876" /> Among modern mammals, the anatomical structure that most closely resembles brontothere horns are the ossicones of giraffes.<ref name=":0" />

The life appearance of ''Megacerops'' horns is unknown. The horns have historically been described as analogous to the horns of deer and bovines<ref name=":7" /> and traditional reconstructions often depict them as sheathed in keratinous horns.<ref name=":21" /> Such reconstructions probably do not reflect the actual appearance of the animal; it is unlikely that the horns of ''Megacerops'' were sheathed in keratin since they lack the vascular impressions seen in horn cores of animals such as cows and ceratopsian dinosaurs.<ref name=":21" /> Similar to giraffe ossicones, brontothere horns were most likely instead entirely covered in skin.<ref name=":0" /> The skin on the horns may have been keratinized, but there is no evidence for this.<ref name=":11" /> [[File:Megacerops, Richard Swann Lull.png|thumb|Restoration of ''M. coloradensis'' by Richard Swann Lull (1905), with speculative small keratinous horns on top of the bony horns|upright=0.75]]The distal (furthest point from the body) ends of ''Megacerops'' horns often have a roughened surface. In many skulls, this roughened surface also extends further down around the nose and the eye orbits.<ref name=":0" /> The surface is similar to roughened patches beneath the horns on modern rhinoceros skulls.<ref name=":21" /> In 1905, Richard Swann Lull speculated that the roughened surface on brontothere skulls, especially at the ends of the horns, could have supported additional, smaller horns of keratin that were anatomically similar to the horns of rhinoceroses.<ref name=":21" /> Lull proposed that the two bony horns could have supported two to four (one to two per horn) additional such structures on their distal ends.<ref name=":21" /> Brontothere horns are believed to have been used for intraspecific combat.<ref name=":7" /><ref name=":8" /> Based on the shape of the horns, rivals would have been able to lock horns with each other, and thus protect vulnerable areas, such as their sides, from strikes. The horns were probably most well suited to wrestling in this fashion,<ref name=":27" /><ref name=":7" /> as well as pushing, and side-to-side strikes.<ref name=":27" /> The base of ''Megacerops'' horns may have been too thin and spongy for the horns to be used in head-on collisions, like in modern bighorn sheep.<ref name=":27" /> ''Megacerops'' had very strong neck musculature, which means that rivals may also have been able to topple each other with upward thrusts, and headbutting.<ref name=":8" /> The sides of ''Megacerops'' was probably the main focus of attack. One ''Megacerops'' fossil (AMNH 518) preserves a rib that was broken and healed during the life of the animal.<ref name=":5" /><ref name=":7" /> It is unlikely that any other animal than another ''Megacerops'' could have inflicted such an injury.<ref name="EoDP">{{cite book |title=The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals |publisher=Marshall Editions |year=1999 |isbn=978-1-84028-152-1 |editor-last=Palmer |editor-first=D. |location=London |pages=258–259}}</ref> Some brontothere horns show evidence of secondary bone growth, perhaps regrowth due to clashes with other brontotheres.<ref name=":1" />

=== Diet === Brontotheres such as ''Megacerops'' had relatively low-crowned cheek teeth.<ref name=":28" /> This, and their general dental morphology, restricted them to a browsing diet;<ref name=":28" /> their teeth were used to shear or crush plants.<ref name=":11" /> ''Megacerops'' were folivores and/or frugivores, though their large size suggests that they must have been relatively non-selective when it came to food.<ref name=":1" /> The diet of ''Megacerops'' was probably similar to the diets of modern-day moose and black rhinoceros.<ref name=":1" />

=== Social behavior === ''Megacerops'' fossils have been found in mass death assemblages.<ref name=":1" /> Mass deaths of several individuals together indicates that they were social animals, and that they may have traveled in herds.<ref name=":19" />

== Paleoecology == {{See also|White River Fauna}}thumb|''Megacerops'' painted by Charles R. Knight (1931)|upright=1.9|left ''Megacerops'' lived in a warm temperate to subtropical environment, in forests and open woodlands.<ref name=":11"/> Paleoclimatological analyses suggest that Chadronian North America was home to woodland-savannas and woody scrublands, and that the presence of fully treeless grassland biomes is unlikely.<ref name=":29">{{Cite journal |last=Boardman |first=Grant S. |last2=Secord |first2=Ross |date=2013 |title=Stable isotope paleoecology of White River ungulates during the Eocene–Oligocene climate transition in northwestern Nebraska |url=https://www.sciencedirect.com/science/article/pii/S0031018213000862 |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=375 |pages=38–49 |doi=10.1016/j.palaeo.2013.02.010 |issn=0031-0182|url-access=subscription }}</ref> Oxygen isotope analyses of ''Megacerops'' tooth enamel from the White River Group has revealed low δ18O values<ref name=":30">{{Cite journal |last1=Zanazzi |first1=Alessandro |last2=Kohn |first2=Matthew J. |date=2008 |title=Ecology and physiology of White River mammals based on stable isotope ratios of teeth |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |language=en |volume=257 |issue=1–2 |pages=22–37 |doi=10.1016/j.palaeo.2007.08.007 |bibcode=2008PPP...257...22Z }}</ref> and low δ13C values.<ref name=":29" /> This suggests that ''Megacerops'' preferred to feed, and perhaps live, in wetter and denser parts of the ecosystem,<ref name=":29" /><ref name=":30" /> such as riparian areas.<ref name=":29" /> In 2013, Grant S. Boardman and Ross Secord accordingly interpreted ''Megacerops'' as a forest-dwelling animal.<ref name=":29" />

''Megacerops'' was by far the largest animal in its environment.<ref name=":5" /><ref name=":11" /><ref name=":29" /> Like large animals of today, ''Megacerops'' would have played an important ecological role in shaping the environment that they inhabited.<ref name=":5" /> Based on their body size, the approximate home range (the area in which an animal lives and moves on a periodic basis) of ''Megacerops'' has been estimated at {{convert|20|km2|acre|abbr=}}.<ref name=":29" />

The White River badlands preserve perhaps the best Late Eocene fossil record in North America, which means that the fauna contemporary with ''Megacerops'' is known in great detail.<ref name=":31">{{Cite book |last=Saarinen |first=Juha |url=https://www.google.com/books/edition/The_Ecology_of_Browsing_and_Grazing_II/Me69DwAAQBAJ |title=The Ecology of Browsing and Grazing II |date=2019 |publisher=Springer Nature |isbn=978-3-030-25865-8 |pages=20–21 |language=en |chapter=The Palaeontology of Browsing and Grazing}}</ref> The large herbivore fauna included representatives from a large number of different groups. Fellow perissodactyls included early anchitheriine equids (''Mesohippus'') and a variety of rhinoceratoids, including hippopotamus-like amynodontids (''Metamynodon''), hyracodontids (''Hyracodon''), and early types of true rhinoceroses (''Trigonias'' and ''Subhyracodon'').<ref name=":31" /> A multitude of artiodactyl groups were also present, anthracotheriid hippopotamoids (''Aepinacodon'' and ''Elomeryx''), entelodontids (''Archaeotherium''), merycoidodontoids (''Agriochoerus'', ''Bathygenys'', ''Merycoidodon'', and ''Oreonetes''), peccaries (''Perchoerus''), protoceratids (''Pseudoprotoceras''), early camels without humps (the oromerycid ''Eotylopus'' and the camelid ''Poebrotherium''), and early ruminants (''Hypertragulus'' and ''Leptomeryx'').<ref name=":31" /> The small mammals of the ecosystem included various rabbits and several rodent groups, including beavers, gophers, mice, and squirrels.<ref name=":32">{{Cite book |last=Prothero |first=Donald R. |author-link=Donald Prothero |url=https://www.google.com/books/edition/After_the_Dinosaurs/Qh82IW-HHWAC |title=After the Dinosaurs: The Age of Mammals |date=2006 |publisher=Indiana University Press |isbn=978-0-253-00055-2 |pages=133 |language=en}}</ref> [[File:Hyaenodon Heinrich Harder.jpeg|thumb|''Hyaenodon megaloides'', about the size of a modern lion, was the largest predator contemporary with ''Megacerops'']] A variety of predators coexisted with ''Megacerops'',<ref name=":11" /><ref name=":32" /> including amphicyonids ("bear-dogs"; ''Brachyrhynchocyon'', ''Daphoenictis'', and ''Daphoenus''), canids (''Hesperocyon''), feliforms (''Paleogale''), hyaenodonts (several species of ''Hyaenodon''), mustelids (''Mustelavus''), nimravids ("false saber-toothed cats"; ''Dinictis'' and ''Hoplophoneus''), and ursoids (''Parictis'').<ref name=":33">{{Cite journal |last=Van Valkenburgh |first=Blaire |author-link=Blaire Van Valkenburgh |date=1994 |title=Extinction and replacement among predatory mammals in the North American late Eocene and Oligocene: Tracking a paleoguild over twelve million years |url=https://doi.org/10.1080/10292389409380474 |journal=Historical Biology |volume=8 |issue=1-4 |pages=129–150 |doi=10.1080/10292389409380474 |issn=0891-2963|url-access=subscription }}</ref> The largest contemporary predator was ''Hyaenodon megaloides'', which weighed around {{convert|165|kg|lb|abbr=}}, about the size of a modern lion.<ref name=":33" /> It is unlikely that any contemporary predator would have been able to hunt adult ''Megacerops'' on account of their great size.<ref name=":5" /><ref name=":11" /><ref name=":33" /> Even juvenile ''Megacerops'' were large compared to contemporary predators and would have been difficult to prey on.<ref name=":11" /> Circumstantial evidence may suggest that ''Megacerops'' formed part of the diet of ''H. megaloides'' in some way; both ''Megacerops'' and ''H. megaloides'' went extinct at or before the end of the Chadronian, and no predator as large as ''H. megaloides'' is known from the Orellan, the North American land mammal age that succeeded the Chadronian.<ref name=":33" />

== Extinction == The brontotheres went extinct at the end of the Eocene.<ref name=":14"/><ref name=":28" /> No brontothere fossils are known from the Orellan in North America.<ref name=":29" /> ''Megacerops'' was the last brontothere on the continent, and the last living member of the Brontotheriita.<ref name=":20"/> Judging by the size of ''Megacerops'' and its relatives, and the development of its horns, the brontotheres apparently died out when they were at the peak of their evolutionary development.<ref name=":14"/> It is unlikely that ''Megacerops'' went extinct due to competition with other animals; no contemporary mammal approached it in size and no new mammals comparable to ''Megacerops'' in size are known from the Orellan.<ref name=":29" />

Various explanations have been proposed for the relatively sudden disappearance of the brontotheres. Osborn believed that the brontotheres succumbed to overadaptation<ref name=":1" /> and "racial senescence".<ref name=":14" /><ref name=":1" /> Another unsubstantiated hypothesis is that the brontotheres died out due to a trypanosomiasis epidemic, caused by ancestors of the modern tsetse fly.<ref name=":14" />

Donald Prothero has attributed brontothere extinction to the Eocene–Oligocene extinction event, when a period of glaciation coincided with extinctions in several different mammal groups. The temperature changes at the boundary between the Eocene and Oligocene dramatically impacted vegetation, leading to a large-scale replacement of Eocene forests, on which brontotheres depended, with savanna environments.<ref name=":14" /> ''Megacerops'' would have been drastically impacted by such an environmental change, since isotope analyses suggest that it preferred moister and denser habitats.<ref name=":29" /> The habitats preferred by ''Megacerops'' were still present to some extent in the Orellan, as indicated by rare fossil finds of the tapiroid ''Colodon'', but they were probably much reduced.<ref name=":29" /> The ecological niche of the brontotheres would eventually be taken over by rhinocerotoids and elephants.<ref name=":14" />

== Notes == {{Notelist}}

==References== {{Reflist}}

==External links== {{Commons category|Megacerops}} {{Wikispecies|Megacerops}}

*[https://www.livescience.com/animals/extinct-species/rhino-like-thunder-beasts-grew-massive-in-the-evolutionary-blink-of-an-eye-after-dinos-died-off Fast evolution of Megacerops after the end-Cretaceous mass extinction], 12 May 2023, Live Science {{Brontotheriidae}} {{Taxonbar|from=Q135590}}

Category:Brontotheres Category:White River Fauna Category:Eocene mammals of North America Category:Priabonian genera Category:Fossil taxa described in 1870 Category:Taxa named by Joseph Leidy