Chadronian brontotheres are well represented in the fossil record, known from several skeletons and hundreds of complete skulls. Because the fossils vary considerably in the size and shape of the horns, close to fifty species of Chadronian brontotheres have been named historically. Poor stratigraphic data for the majority of the fossils has further complicated the taxonomy. Today, variations among the fossils are considered to be the result of sexual dimorphism and other individual variation. Only one Chadronian brontothere species is confidently considered to be valid, M. coloradensis. Rare fossils with bifurcating horns may represent a second species, M. kuwagatarhinus. Several historical generic names now considered synonyms of Megacerops remain common in popular culture, such as Brontotherium, Brontops, and Titanotherium.
Although similar to modern rhinoceros, Megacerops was closer to elephants in size. The largest known Megacerops may have been over 2.5 meters (8.2 feet) 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. 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.
History of research
Early discoveries
The holotype specimen of Menodus giganteus (USNM 21820), the first scientifically published fossil now recognizable as a brontothere, and Hiram A. Prout's original 1846 illustrations of the specimen
Fossils of Megacerops were among the first mammal fossils from the American West to be brought to scientific attention.[2] Long before the time of scientific inquiry into the fossils, Megacerops remains were sometimes exposed by severe rainstorms[3] and found by Native Americans of the Lakota Sioux and Pawnee peoples.[4] The Lakota referred to these great mammals as wakíŋyaŋ,[4] translated as "thunder beasts".[3][4][5]
The first brontothere fossil to be scientifically described was a fragment of a right jaw (USNM 21820), found in Chadronian deposits in the White River badlands of South Dakota.[2] The jaw fragment was described by Hiram A. Prout in 1846 and 1847. Although Prout correctly identified the fossil as belonging to a large perissodactyl, he believed that the jaw came from a "giant Palaeotherium" (an equoid).[2] In 1849, Auguste Pomel realized that the fossil did not belong to Palaeotherium and instead designated it as the type specimen of a new genus and species, Menodus giganteus.[2] In 1850, Richard Owen and colleagues recorded further brontothere teeth and jaws. Owen and colleagues believed these fossils represented the same species as Prout's jaw but noted that it was difficult to establish diagnostic features on the genus level. Apparently ignoring Pomel's name, Owen and colleagues named the new species Palaeotherium? proutii.[2]
M. coloradensis type specimen (ANSP 13362)
Owen's fossils were soon acquired by Joseph Leidy, who examined and described the material in greater detail through 1852 and 1853.[2] Already before any in-depth study, Leidy suspected that the fossils did not belong to any known equoid, such as Palaeotherium or Anchitherium, and noted that "should the suspicion prove correct, Titanotherium would be a good name for this animal, as expressive of its very great size".[2] After more extensive examination, Leidy placed P.? proutii in the new genus Titanotherium and designated a left jaw from Owen's material (USNM 113) as the type specimen.[2] Although recognized as large perissodactyls, early descriptions of brontothere fossils up to and including Leidy's work failed to recognize their distinctive nature.[6]
The first known brontothere fossil to preserve most of the animal's distinctive horns was described by Leidy in 1870 as the type specimen of the new genus and species Megacerops coloradensis.[2] Leidy obtained the fragmentary fossil from "Dr. Gehrung of Colorado City" and, lacking comparative brontothere material, incorrectly believed it to have belonged to a relative of the extinct giraffid Sivatherium.[7]
In the 1870s and 1880s, the northern Great Plains were one of the main regions to be extensively excavated in the Bone Wars, a period of intense 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.[4] In the 1870s, Marsh and Cope described several dinoceratans, an extinct group of large mammals, some of which had horns similar to Megacerops, such as Uintatherium and Eobasileus.[7] In 1873, Leidy speculated that his Megacerops may belong to the same order as these animals, and not the ruminants.[7] Leidy further noted similarities between the Megacerops specimen and fragmentary fossil horn cores he had previously speculated to perhaps belong to Titanotherium.[7]
Further discoveries
"Brontops robustus" skeleton restored by F. Berger under the direction of Othniel Charles Marsh (1889)
Othniel Charles Marsh is considered the most important figure in the early research on the brontotheres.[6] His studies of brontotheres began in 1870, when Marsh led an expedition to northern Colorado on behalf of Yale College.[6] During the expedition, Marsh's crew were shown a brontothere jaw by a group of Lakota, who told them of their legends of wakíŋyaŋ.[9] The expedition collected a large number of brontothere fossils, including jaws, skulls, and postcranial elements.[6] In honor of Lakota legends, Marsh named the new genus Brontotherium ("thunder beast") in 1873.[10] Marsh's 1873 description of Brontotherium gigas was the most important contribution to brontothere knowledge up until his time.[11] 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.[11] Marsh recognized Brontotherium as a "true perissodactyl with limb bones resembling those of Rhinoceros".[6] Marsh recognized that Brontotherium was related to the animal described as Titanotherium by Leidy, and erected the new family Brontotheriidae to contain the two genera.[6] In 1875, H. C. Clifford discovered and excavated a large and relatively 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.[8]
Marsh continued to study brontothere fossils for the reminder of his career, some collected by himself but most purchased from collectors "out West".[6] New specimens were also uncovered and studied by Leidy and Cope. Marsh, Leidy, and Cope named new species for close to every Chadronian brontothere specimen that came into their possession.[2] In cooperation with the United States Geological Survey, Marsh wrote a series of monographs on prehistoric animals from the modern United States. Paleontological field work was carried out at an unprecedented scale by the US Geological Survey to gather material for Marsh's monographs on ceratopsian dinosaurs and brontotheres.[12] The brontothere collection assembled for Marsh's monograph eventually included several nearly complete skeletons and nearly two hundred complete skulls.[13] 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.[12]
In the late 19th and early 20th centuries, most of the museums in the United States funded fossil collecting in the so-called "Titanotherium beds" of the Great Plains,[4] increasing the already large brontothere fossil sample to encompass further skeletons and hundreds of skulls.[2] In 1929, Henry Fairfield Osborn published a monograph on the brontotheres, The Titanotheres of Ancient Wyoming, Dakota and Nebraska.[14] Osborn had made study of brontotheres one of his life's quests[15] and believed his work, spanning 951 pages and two volumes, and illustrated with 795 figures and 236 plates, would be the definitive work on the animals.[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.[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] Most Megacerops specimens have poorly recorded stratigraphic data, limiting the degree to which variation in single contemporary populations can be studied.[2] The American Museum of Natural History has a large collection of brontothere specimens with well-recorded stratigraphic data, collected in the 1940s and 1950s. The specimens of this collection are largely unprepared and still encased in the plaster field jackets they were originally put in, and interest and funding to examine them has been limited.[15]
Taxonomy and species
20th-century developments
Three M. coloradensis skulls, showing the variability of horn shape and size. By Osborn's classification scheme, these represent three different species.
By the time of Osborn's monograph, at least 47 species of Chadronian brontotheres had been named, many based on poor and fragmentary fossils.[2] There was a general consensus throughout the 20th century that the brontotheres were highly oversplit, divided into far too many species.[2][14] Taxonomic problems were compounded by confusion around the type specimens and names of several of the species. Marsh incorrectly argued in 1873 that Menodus was preoccupied by the reptile Menodon (=Nothosaurus). In several publications, both Leidy and Osborn sometimes confused USNM 113 (type specimen of Titanotherium) with USNM 21820 (type specimen of Menodus). Osborn incorrectly believed that USNM 21820 had been destroyed in the St. Louis Fire of 1849 and proposed designating a brontothere skull in the American Museum of Natural History, AMNH 505, as a neotype specimen.[2]
Chadronian brontotheres are very similar in most features of the skeleton. Clear differences between specimens are for the most part seen only in the shape, orientation, and size of horns, nasal bones, and zygomatic arches.[2] Before his 1929 monograph, Osborn considered the possibility that all Chadronian brontotheres belonged to a single genus, for which he preferred the name Titanotherium. In the mongraph, Osborn nevertheless recognized 37 of the up until then named species as valid.[2] Almost every well-preserved skull was designated as a separate species.[15] Osborn attributed the variation seen in the fossil material to both individual variation due to sex and age, and to species-level differences, but did not demonstrate how a differing feature could be determined to be due to one factor or another.[2]
Osborn's conclusions faced criticism by other paleontologists. In 1941, William Berryman Scott stated that "it is hardly worthwhile even to list the species; that 37 species could not have co-existed within the relatively brief space of the Chadron, is obvious; how many did exist and what names should be given to them, are insoluable problems".[2] Scott further stated that "the probably great effects of sex and age and fluctuating variability have not been sufficiently evaluated".[2] Despite criticism, further revisions of brontothere material were not attempted for most of the 20th century due to the daunting size of Osborn's monograph and the sheer number of fossil specimens known, many of which were yet to be sufficiently prepared.[14]
In 1967, John Clark, James R. Beerbower, and Kennetk K. Kietzke were the first to suggest that all Chadronian brontotheres belonged to a single species that exhibited great individual variation.[2] This suggestion was based on the discovery of four horn cores with variable morphology, found at a single site within 30 feet of each other, in a layer only one foot thick. Finding it unlikely that these associated fossils were from four separate species, Clark, Beerbower, and Kietzke referred all four to Menodus giganteus, the oldest available Chadronian brontothere name, and designated all other generic names, such as Brontotherium and Titanotherium, as junior synonyms.[17] The only other Chadronian brontothere genus recognized as valid by Clark, Beerbower, and Kietzke was Teleodus.[17]Teleodus has since been recognized as invalid, with its type specimen revealed to have been a 19th-century hoax in 2004.[6]
In 1989, Bryn J. Mader published the first true revision of the Brontotheriidae since Osborn.[15] Mader recognized a number of Chadronian brontothere species as valid, divided into the three genera Menops, Brontops, and Megacerops. Mader's classification scheme was to a large extent based on the cross-section shapes of horns.[2] By this approach, the name Menodus giganteus is a nomen dubium, since the holotype fossil of that species contains no horn material and thus no diagnostic features.[2] Mader undertook no species-level revision of the three genera, and did not demonstrate them to be monophyletic via phylogenetic analysis.[18]
Recognition of only two species
Illustration of F:AM 128600, the holotype specimen of M. kuwagatarhinus
In a 2004 preliminary revision,[18] Matthew C. Mihlbacher, Spencer G. Lucas, and Robert J. Emry concluded that while the variation among Chadronian brontotheres was greater than what is generally accepted within a single species, the variability resembles that of modern sexually dimorphic mammal species.[2] Mihlbacher, Lucas, and Emry noted that if only a single Chadronian brontothere species is recognized, the name Menodus giganteus should be used as the oldest available name. The problem was complicated by rare Chadronian brontothere skulls with bifurcating horns, a trait not seen in the vast majority of specimens. These fossils had been described as a new species by Mader and John P. Alexander in 1995, Megacerops kuwagatarhinus. Mihlbacher, Lucas, and Emry considered it impossible to determine if M. kuwagatarhinus was an isolated breeding population (i.e. separate species) from the more common form with unbifurcating horns, or just very atypical individuals. The holotype skull of M. kuwagatarhinus (F:AM 128600) preserves a molar that exhibits some differences to typical brontothere molars, perhaps suggesting species-level distinction, but known M. kuwagatarhinus fossils do not otherwise preserve any teeth.[2] Since clear distinctions between the two forms can only be seen in the horns, Mihlbacher, Lucas, and Emry suggested that the name Megacerops coloradensis should be used for the common species with unbifurcating horns. The M. coloradensis holotype (ANSP 13362) is the earliest described brontothere holotype to preserve unbifurcating horns, and thus to securely belong to this species.[2] Due to the poor state of brontothere fossil stratigraphic data, Mihlbacher, Lucas, and Emry noted that they could not rule out the possibility of Megacerops undergoing anagenic evolution over the course of the Chadronian, nor the possibility that two or more anagenically evolving species that were distinct at any given moment would not be distinguishable in the available temporally averaged fossil assemblage.[2]
Further studies by Mihlbacher in 2004 and 2008 further supported that only two Chadronian brontothere species, M. coloradensis and M. kuwagatarhinus were diagnosable, with all other species either being synonyms of those two species or nomina dubia.[18] In a comprehensive 2008 phylogenetic analysis, Mihlbacher recovered Megacerops as a monophyletic genus, and noted that the present data supported only two species but that more could perhaps be indicated in future studies on the Megacerops material.[18] Although a number of genera were synonymized with Megacerops, Mihlbacher did not perform a detailed revision of the genus, which he considered outside the scope of the 2008 study.[18] A detailed revision of Megacerops remains an area in need of future research. In 2021, Mihlbacher and Donald Prothero noted that "the evolution and paleobiology of Megacerops and closely allied species are potentially rich areas of investigation, although ambiguity regarding the basic systematics of this group is an obstacle to further research".[16]
Since 2008, the status of Megacerops as the only valid Chadronian brontothere genus, containing only two species, has become widely accepted.[1][19] Mihlbacher'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 Mihlbacher's work, and continued to regard Menops, Brontops, and Megacerops to be distinct genera, with several species.[22][23] 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.[1][19]
In 2021, Rory E. Sweedler, Eberle, and Mihlbacher described a new brontothere jaw (UCM 109045) from a Chadronian locality in the Antero Formation in Colorado. UCM 109045 has a lower number of incisors than the jaws of M. coloradensis, and has a longer postcanine diastema (longer gap between the canines and the other teeth). The jaw was determined to probably belong to a species distinct from M. coloradensis, and was noted to be morphologically similar to jaws of the earlier brontothere Protitanops, and to fossils described as Megacerops primitivus by Lawrence Lambe in 1908, designated as a nomen dubium in Mihlbacher's previous work. Sweedler, Eberle, and Mihlbacher concluded that it was possible that UCM 109045 belonged to the same species as the fossils referred to M. kuwagatarhinus, though little comparison was possible. In this case, the appropriate name for the second Chadronian brontothere species might be M. primitivus, with M. kuwagatarhinus being a junior synonym of that species.[24]
Description
Size
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 (8,2 feet) 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.[8] The largest Megacerops specimens have sometimes historically been estimated to have reached 3 meters (10 feet) tall at the shoulder.[30] Large Megacerops would have reached about 5 meters in length (16 feet).[9]
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]
Skull
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]
Horns
Four different M. coloradensis heads, restored by Charles R. Knight (1929)
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,[2] 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.[2][27][35]
The horns were composed of non-deciduous bone and not elaborated with any additional structures.[2] 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.[2]
Restoration of M. coloradensis by Richard Swann Lull (1905), with speculative keratinous horns on top of the bony horns
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.[2] 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.[2] 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.[5][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]
Dentition
Underside of an M. coloradensis skull, showing the teeth
The dentition of Megacerops, as currently recognized, is more variable than is normal within a single species.[2] 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.[2]
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]
Postcranial skeleton
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 arrangment of their feet bones indicate that Megacerops feet had a pad of elastic tissue, similar to the feet of modern elephants and rhinoceros.[27]
Classification
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 Mihlbacher in the 2000s.[18]
The cladograms below are the strict reduced consensus tree of brontotheres from Mihlbacher's 2008 analysis (collapsed to show only the Brontotheriita),[18] and the strict consensus tree for the Brontotheriita from a 2021 study by Mihlbacher and Prothero on brontheres from Texas.[16]
M. coloradensis ("titanotheres") painted by Charles R. Knight (1931)
Megacerops lived in a warm temperate to subtropical environment, in forests and open woodlands.[27] Oxygen isotope analysis of Megaceropstooth 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.[5][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.[5]
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.[5][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.[9]
Extinction
The brontotheres went extinct at the end of the Eocene.[14]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.[14] Various explanations have been proposed for their sudden disappearance. Osborn believed that the brontotheres succumbed to overadaptation[18] and "racial senescence".[14][18] Another unsubstantiated hypothesis is that the brontotheres died out due to a trypanosomiasis epidemic, caused by ancestors of the modern tsetse fly.[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.[14] The ecological niche of the brontotheres was later taken over by rhinoceros and elephants.[14]
Notes
↑From Ancient Greekméga- ("large"), kéras ("horn"), and ōps ("face")
↑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.[1]
↑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. 2.1.4.32.1.3.3 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.[33]
123456789Lucas, Spencer G. (2004). "O. C. Marsh and the Eocene Brontothere Teleodus: A Paleontological Hoax". Paleogene Mammals: Bulletin 26. New Mexico Museum of Natural History and Science. p.119.
↑Lloyd, K. J.; Eberle, J. J. (2012). "A late Eocene (Chadronian) mammalian fauna from the White River Formation in Kings Canyon, northern Colorado". Rocky Mountain Geology. 47 (2): 113–132. Bibcode:2012RMGeo..47..113L. doi:10.2113/gsrocky.47.2.113.
↑Mader, Bryn J. (2009). "Details of the cranial anatomy of a primitive diplacodont brontothere, cf. Protitanotherium , from the Wiggins Formation of Wyoming (Mammalia, Perissodactyla, Brontotheriidae)". Journal of Vertebrate Paleontology. 29 (4): 1224–1232. Bibcode:2009JVPal..29.1224M. doi:10.1671/039.029.0414.
↑Sweedler, Rory E.; Eberle, Jaelyn J.; Mihlbachler, Matthew C. (2021). "A latest Eocene (Chadronian) brontothere (Mammalia, Perissodactyla) from the Antero Formation, South Park, Colorado". Rocky Mountain Geology. 56 (1): 37–50. Bibcode:2021RMGeo..56...37S. doi:10.24872/rmgjournal.56.1.37.
↑Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. pp.258–259. ISBN978-1-84028-152-1.
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