Megacerops

Megacerops Temporal range: Late Eocene (Chadronian), Da. S T Ypr. Lut. B Pr. Rup. Ch.
Mounted skeleton of M. coloradensis, Natural History Museum of Los Angeles County
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Perissodactyla
Family:	†Brontotheriidae
Subfamily:	†Brontotheriinae
Tribe:	†Brontotheriini
Subtribe:	†Brontotheriina
Infratribe:	†Brontotheriita
Genus:	†Megacerops Leidy, 1870
Type species
†Megacerops coloradensis Leidy, 1870
Other species
†M. kuwagatarhinus Mader & Alexander, 1995
Synonyms
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 ("great horned face"^[a]) is an extinct genus of brontothere, an extinct group of rhinoceros-like browsers traditionally classified as relatives of horses. Megacerops was endemic to North America during the Late Eocene,^[b] during 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. It was superficially similar to modern rhinoceros, but closer to elephants in size. The largest known Megacerops may have been over 2.5 meters (8 ft 2 in) tall at the shoulder and could have weighed up to five tonnes. They were the largest animals in their environment and 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. Several historical generic names now considered synonyms of Megacerops remain common in popular culture, such as Brontotherium, Brontops and Titanotherium.

Fossils of Megacerops were among the first mammal fossils from the American West to be brought to scientific attention.^[3] Long before the time of scientific inquiry into the fossils, Megacerops remains were sometimes exposed by severe rainstorms^[4] and found by Native Americans of the Lakota Sioux and Pawnee peoples.^[5] The Lakota linked the great mammals to their legends of wakíŋyaŋ,^[5] translated as "thunder beasts".^[4][5][6]

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.^[3] 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).^[3] 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.^[7] 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.^[3] 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.^[3]

The fossils collected by Evans were soon acquired by Joseph Leidy, who examined and described the material in greater detail in 1852 and 1853.^[3] Leidy suspected that the animal was not an equoid, and suggested the name Titanotherium, "as expressive of its very great size".^[3] Leidy used the name Titanotherium proutii for all of the fossils, including Prout's jaw.^[3] 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.^[8]

In 1870, Leidy described the first known brontothere fossil to preserve most of the distinctive horns (ANSP 13362),^[3] found in Colorado.^[1] Leidy suspected that ANSP 13362 could represent the same animal as Titanotherium,^[1][9] but struggled with identifying the fossil, speculating that it could perhaps represent a camelid or a North American representative of the giraffid Sivatherium.^[1] Leidy provisionally referred the fossil to a new genus and species, Megacerops coloradensis.^[3]

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.^[5] 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.^[9] In 1873, Leidy speculated that Megacerops could belong to the same order as these animals.^[9] 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.^[9]

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^[8][10] (bottom)

Othniel Charles Marsh's contributions to brontothere research were highly significant.^[8] Marsh's studies of brontotheres began in 1870, when he led an expedition to northern Colorado on behalf of Yale College.^[8] The expedition collected a large number of brontothere fossils, including well-preserved and complete skulls and postcranial elements.^[8] During the expedition, Marsh's crew were shown a brontothere jaw by a group of Lakota, who told them of their legends of wakíŋyaŋ.^[11] In honor of the legends, Marsh named the new genus Brontotherium ("thunder beast") in 1873.^[12]

Marsh's 1873 description of Brontotherium gigas was the most important contribution to brontothere knowledge up until that time.^[1] 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.^[1] Marsh recognized Brontotherium as a "true perissodactyl with limb bones resembling those of Rhinoceros".^[8] 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.^[8] 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.^[10]

Marsh continued to study brontothere fossils for the rest of his career, some collected by himself but most purchased from collectors "out West".^[8] 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.^[3] 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.^[1]

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.^[13] 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.^[1] By the end of his expedition, Hatcher reported that he had collected "nearly 200 complete skulls and many more or less complete skeletons".^[1] 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.^[13]

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,^[5] increasing the already large brontothere fossil sample to encompass further skeletons and many additional skulls.^[3] Henry Fairfield Osborn energetically pursued further studies of brontothere fossils, making the study of the group one of his life's quests.^[14] Most of the fossils worked on by Osborn were collected by Hatcher and originally intended to serve as material for Marsh's monograph.^[1] In 1929, Osborn published a monograph on the brontotheres, The Titanotheres of Ancient Wyoming, Dakota and Nebraska.^[15] 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.^[15]

The large number of fossils collected has made Megacerops one of the best represented large herbivores in the fossil record of North American mammals.^[16] Megacerops fossils have predominantly been recovered from the White River Group in the United States and the Cypress Hills Formation in Canada.^[16] All fossils now attributed to Megacerops appear to be confined to the Chadronian land mammal age, which corresponds to the Late Eocene.^[16] Despite the large number of specimens, the evolution and paleobiology of Megacerops remain understudied, largely due to longstanding taxonomic confusion obstructing further research.^[16] 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.^[3]

By the time of Osborn's 1929 monograph, at least 47 species of Chadronian brontotheres had been named, many based on poor and fragmentary fossils.^[3] 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.^[3] 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.^[3][15] 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.^[3] 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.^[3]

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.^[3] This was based on the discovery of several associated horn cores with variable morphology at a single site. Finding it unlikely that they were from four different species, Clark, Beerbower and Kietzke referred all four specimens to Menodus giganteus, the oldest available name, and designated other names, such as Brontotherium and Titanotherium, as junior synonyms.^[17] In 1989, Bryn J. Mader published a proposed revision of the Brontotheriidae.^[14] 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.^[3] 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.^[3]

In a 2004 preliminary revision,^[18] 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.^[3] 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 may be just one diagnosable taxon in the fossil assemblage. Mihlbachler, Lucas, and Emry recognized a rare second species, represented by skulls with bifurcating horns, a clear differing trait seen in very few specimens. These fossils had been described as a new species by Mader and John P. Alexander in 1995, Megacerops kuwagatarhinus.^[3] 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.^[3] 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.^[18]

A detailed species-level revision of Megacerops remains an area in need of future research.^[16] 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 since 2008.^[2][19] Mihlbachler's conclusions have been supported by researchers such as Donald Prothero,^[19] Karen J. Lloyd, Jaelyn J. Eberle,^[20] Parker D. Rhinehart, Alfred J. Mead and Dennis Parmley.^[21] Bryn J. Mader rejected the single genus model, and continued to regard Menops, Brontops and Megacerops to be distinct genera, with several species.^[22][23] Mihlbachler responded by pointing out that Mader undertook no species-level revision of these genera and did not demonstrate them to be monophyletic via phylogenetic analysis,^[18] and that the diagnostic features proposed by Mader are continuous in the fossil material and can thus not indicate distinct taxa.^[24] 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.^[2][19]

Reconstruction of the life appearance and musculature of M. coloradensis by Erwin S. Christman (1916)

Although superficially similar to modern rhinoceros in appearance, an example of convergent evolution,^[25] Megacerops exceeded all living rhinoceros in size.^[26][27] Megacerops was closer in size to modern elephants, only shorter.^[26] They were among the largest brontotheres, rivaled in size only by a handful of other genera, such as Gnathotitan and Embolotherium.^[18]

The largest Megacerops are typically estimated to have been 2.5 meters (8 ft 2 in) tall at the shoulder.^[26][28][29] 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.^[10] The largest Megacerops specimens have sometimes historically been estimated to have reached 3 meters (10 ft) tall at the shoulder.^[30] Large Megacerops would have reached about 5 meters (16 ft) in length.^[11]

Various weight estimates of Megacerops have been published, ranging from as low as 2–3 tonnes (2.2–3.3 short tons)^[31] to as high as 5 tonnes (5.5 short tons).^[26][32]

Megacerops had a massive skull^[16] which somewhat resembled that of modern rhinoceros.^[33] The skull was long,^[25][33] broad^[25] and saddle-shaped.^[16]

Many brontothere genera had horns or cranial domes.^[34] Megacerops had the most developed horns out of all of the North American brontotheres, consisting of two bony protuberances above the nose, vaguely reminiscent of a slingshot in shape.^[35] In Megacerops, the horns had evolved to become the dominant and central feature of the skull.^[36] The shape, size and orientation of Megacerops horns varied greatly between individuals,^[3] as did the degree to which the horns impacted the rest of the skull anatomy.^[18] 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.^[18] It is possible that the system controlling horn growth as Megacerops matured was relatively indeterminate, with minor genetic differences or "accidents of developments" giving rise to great variations in shape.^[17] Based on sexual dimorphism in modern horned mammals, larger Megacerops with larger horns are assumed to have been males.^[3][27][35]

The horns were composed of non-deciduous bone and not elaborated with any additional structures.^[3] There were large air cavities in the base of the horns.^[33] Among modern mammals, the anatomical structure that most closely resembles brontothere horns are the ossicones of giraffes.^[3]

The life appearance of the horns is unknown. They have been described as analogous to the horns of deer and bovines^[34] and historical reconstructions have often depicted them as sheathed in keratinous horns.^[37] It is unlikely that the horns were sheathed in such a way in life, since they lack the vascular impressions seen in horn cores of animals such as cows and ceratopsian dinosaurs.^[37] Like giraffe ossicones, brontothere horns were most likely instead entirely covered in skin.^[3] This skin may have been keratinized, but there is no evidence for this.^[27] The distal (furthest point from the body) ends of the horns often have a roughened surface. In many skulls, this roughened surface also extends down to around the nose and the eye orbits.^[3] The roughened surface is similar to the roughened patches beneath horns on rhinoceros skulls.^[37] In 1905, Richard Swann Lull speculated that the roughened surfaces on brontothere horns could have supported two to four (one to two per horn) additional, smaller horns of keratin anatomically similar to rhinoceros horns.^[37]

Brontothere horns are believed to have been used for intraspecific combat.^[34][35] The shape of the horns suggest that rivals could have locked horns with each other and thus protected vulnerable areas, such as their sides. The horns were probably most well suited to wrestling in this fashion.^[34] The strong neck musculature of Megacerops also means that brontotheres would have been capable of toppling each other with upward thrusts, and headbutting.^[35] The sides of Megacerops was probably the main focus of attack. One fossil (AMNH 518) preserves a rib broken and healed during the life of the animal.^[6][34] It is unlikely that any other animal than another Megacerops could have inflicted such an injury.^[38] Some brontothere horns show evidence of secondary bone growth, perhaps regrowth due to clashes with other brontotheres.^[18]

The dentition of Megacerops, as currently recognized, is more variable than is normal within a single species.^[3] There is no consistent dental formula for the entire fossil assemblage, which has historically been interpreted as a genus-level differentiating trait.^[33] In 1876, Marsh described the dental formula of "Brontotherium" as 2.1.4.32.1.3.3,^[c] that of "Menodus" as 2.1.4.32.1.4?.3, that of Megacerops as 2.1.4.30.1.3.3, and that of "Diconodon" as 0.1.4.31.1.3.3.^[33] In 1889, Marsh's "Brontops" was described as distinct based on its dental formula, 2.1.4.31.1.4.3.^[39] Although these differences may lend credence to the idea that there were multiple different species of Chadronian brontotheres, as interpreted historically, it is not possible to separate the fossils into discrete groups based on dentition, just as with other traits.^[3]

Megacerops had at most two pairs of incisors.^[39] 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.^[36] The fossil evidence points to the incisors being of little use to Megacerops; they were reduced in size, fossils show very little wear,^[27] and presumed old Megacerops specimens sometimes lost their incisors over the course of their lives.^[36] The lack of use for the incisors could suggest that brontotheres had a prehensile upper lip, similar to modern rhinoceros.^[27]

Side, front, and back views of a M. coloradensis skeleton

The postcranial skeleton of Megacerops was massive, robust,^[27] and relatively short.^[37] 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.^[37] This aspect of Megacerops anatomy has been compared to the vertebrae and neck musculature of modern bison.^[37] The neck itself was stout and moderately long.^[33] The vertebrae of Megacerops were somewhat similar to those of modern rhinoceros.^[39] The ribs were strong and massive.^[39] The pelvis of Megacerops was expanded transversely and wide,^[33][39] similar to elephant pelvises.^[39]

The limbs of Megacerops were intermediate in proportion between those of modern rhinoceros and elephants.^[33] The limbs of show several adaptations to withstand the great weight of the animal. Compared to rhinoceros, Megacerops limbs are stouter, particularly at the ankles and wrists, and there is a lesser degree of angulations between the segments of the limbs.^[37] The forelimbs were especially robust^[37][39] and several adaptations, notably roughenings of the olecranon (the protruding part of the elbow) and the humerus, suggest great muscle power.^[37]

Megacerops had four toes on its front feet and three toes on its rear feet.^[33] The arrangement of their feet bones indicate that Megacerops feet had a pad of elastic tissue, similar to the feet of modern elephants and rhinoceros.^[27]

Brontotheres composed the family Brontotheriidae, of which Megacerops was a derived member.^[18] The Brontheriidae belonged to the order Perissodactyla (odd-toed ungulates), the group that includes modern horses, rhinoceros and tapirs.^[18] The position of the brontotheres in the perissodactyl family tree is contentious and results vary between studies.^[18] 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).^[18] Brontotheres have alternatively been classified as inside Palaeotheriidae,^[40] in the different suborder Ceratomorpha as a sister group of rhinoceros and tapirs,^[41] outside the clade that contains chalicotheres, horses, rhinoceros and tapirs,^[42] and as more closely related to chalicotheres, rhinoceros and tapirs than to horses.^[43]

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).^[18] 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.^[18] Osborn's taxonomy was flawed because of his division of the material into a vast number of species. Osborn's taxonomy also reflected his personal views on evolution, the obsolete hypothesis of orthogenesis, and is largely incompatible with the modern understanding of evolutionary theory.^[18] Despite these shortcomings, the 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.^[18]

Per Mihlbachler's 2008 revision, Megacerops is classified as part of the infratribe Brontotheriita, alongside the genera Dianotitan, Duchesneodus, Eubrontotherium, Notiotitanops, Parabrontops and Protitanops.^[18] The Brontotheriita were a sister group to the infratribe Embolotheriita, which includes genera such as Embolotherium, Metatitan and Gnathotitan.^[18]

The cladograms below are the strict reduced consensus tree of brontotheres from Mihlbachler's 2008 analysis (collapsed to show only the Brontotheriita),^[18] and the strict consensus tree for the Brontotheriita from a 2021 study by Mihlbachler and Prothero on brontheres from Texas.^[16]

Megacerops lived in a warm temperate to subtropical environment, in forests and open woodlands.^[27] Oxygen isotope analysis of Megacerops tooth enamel from the White River assemblage has found them to have low δ¹⁸O values, which likely reflects the preference of the genus for moist environments.^[44] Megacerops was the largest animal in its environment.^[6][27] They have been interpreted as obligatory browsers because of their low-crowned teeth, which would have been used to shear or crush plants.^[27] Their teeth indicate that brontotheres were folivores and/or frugivores, though their large size suggest that they must have been relatively non-selective when it came to food. Their diet was likely similar to modern-day moose and black rhinoceros.^[18] Like large animals of today, Megacerops would have played an important ecological role in shaping the environment that they inhabited.^[6]

A variety of predators coexisted with Megacerops, including several false saber-toothed cats, canids and creodonts.^[27] It is unlikely that any contemporary predator would have been able to hunt Megacerops on account of their size.^[6][27] Even juvenile Megacerops were large compared to contemporary predators and would have been difficult to prey on.^[27]

M. coloradensis fossils have been found in mass death assemblages,^[18] suggesting that they were social animals who may have traveled in herds.^[11]

The brontotheres went extinct at the end of the Eocene.^[15] Megacerops was the last brontothere in North America, and the last living member of the Brontotheriita.^[16] Judging by the size of Megacerops and its relatives, and the development of its horns, the brontotheres apparently died out when at the peak of their evolutionary development.^[15] Various explanations have been proposed for their sudden disappearance. Osborn believed that the brontotheres succumbed to overadaptation^[18] and "racial senescence".^[15][18] Another unsubstantiated hypothesis is that the brontotheres died out due to a trypanosomiasis epidemic, caused by ancestors of the modern tsetse fly.^[15]

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.^[15] The ecological niche of the brontotheres was later taken over by rhinoceros and elephants.^[15]

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• Fast evolution of Megacerops after the end-Cretaceous mass extinction, 12 May 2023, Live Science