Megacerops

Early discoveries 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. Long before the time of scientific inquiry into the fossils, Megacerops remains were sometimes exposed by severe rainstorms and found by Native Americans of the Lakota Sioux and Pawnee peoples. The Lakota linked the great mammals to their legends of wakíŋyaŋ, 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. 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. In 1870, Leidy described the first known brontothere fossil to preserve most of the distinctive horns (ANSP 13362), Leidy suspected that ANSP 13362 could represent the same animal as Titanotherium, In 1873, Leidy speculated that Megacerops could belong to the same order as these animals. In honor of the legends, Marsh named the new genus Brontotherium ("thunder beast") in 1873. Marsh's 1873 description of Brontotherium gigas was the most important contribution to brontothere knowledge up until that time. Marsh continued to study brontothere fossils for the rest of his career, some collected by himself but most purchased from collectors "out West". 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. Most of the fossils worked on by Osborn were collected by Hatcher and originally intended to serve as material for Marsh's monograph. 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. Megacerops fossils have predominantly been recovered from the White River Group in the United States and the Cypress Hills Formation in Canada. In 1989, Bryn J. Mader published a proposed revision of the Brontotheriidae. 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. Mihlbachler's conclusions have been supported by researchers such as Donald Prothero, Parker D. Rhinehart, Alfred J. Mead and Dennis Parmley. Mader rejected the single-genus model, and continued to regard Menops, Brontops and Megacerops to be distinct genera, with several species. 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, 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. ==Description==

Megacerops was a huge and impressive animal, characterized by its large size and the two horns at the front of its skull. They were massively built and robust, (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. Beyond the similarities, several features separate Megacerops and other brontotheres from rhinoceroses. the primitive perissodactyl condition, Megacerops has well-developed canines and was closer in size to elephants, only shorter. 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. Large Megacerops would have reached about in length. to as high as or even . Skull The skull of Megacerops was massive The skull was long and broad, differences in dentition has historically been used to justify dividing the fossils into multiple genera. 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"). Megacerops had at most two pairs of incisors. Fossil evidence points to the incisors being of little use to Megacerops; they were reduced in size, fossils show very little wear, and Megacerops specimens of advanced age sometimes lost their incisors over the course of their lives. The lack of use for the incisors could suggest that brontotheres had a prehensile upper lip, similar to modern rhinoceroses. Postcranial skeleton The postcranial skeleton of Megacerops was massive, robust, and relatively short. 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. 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. The neck itself was stout and moderately long. The vertebrae of Megacerops were somewhat similar to those of modern rhinoceroses. The ribs were strong and massive. The pelvis of Megacerops was expanded transversely and wide, similar to elephant pelvises. The limbs of Megacerops were intermediate in proportion between those of modern rhinoceroses and elephants. 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. The forelimbs were especially robust and several adaptations, notably roughenings of the olecranon (the protruding part of the elbow) and the humerus, suggest great muscle power. Megacerops had four toes on its manus (front feet) and three on its pes (hind feet). The retention of the fourth digit on the manus is probably another feature that helped to support the animal's great weight. 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. == Classification ==

Evolution and relations brontothere Eotitanops (A), drawn to scale|upright=1.1Brontotheres composed the family Brontotheriidae, of which Megacerops was a derived member. in the different suborder Ceratomorpha as a sister group of rhinoceroses and tapirs, outside the clade that contains chalicotheres, horses, rhinoceroses, and tapirs, and as more closely related to chalicotheres, rhinoceroses, and tapirs than to horses. Brontotheres originated in the early Eocene, as part of the initial radition of the perissodactyls across the Holarctic realm. The brontotheres experienced rapid diversification in the Middle Eocene, becoming one of the most diverse large mammal groups in Asia and North America. The earliest brontotheres weighed around 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 . 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. 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). 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. 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. 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. Per Mihlbachler's 2008 revision, Megacerops is classified as part of the infratribe Brontotheriita. This group also includes the genera Dianotitan, Duchesneodus, Eubrontotherium, Notiotitanops, Parabrontops, Parvicornus, and Protitanops. The Brontotheriita were a sister group to the infratribe Embolotheriita, which includes genera such as Embolotherium, Metatitan and Gnathotitan. The cladograms below are the strict reduced consensus tree of brontotheres from Mihlbachler's 2008 analysis (collapsed to show only the Brontotheriita), and the strict consensus tree for the Brontotheriita from a 2021 study by Mihlbachler and Prothero on brontotheres from Texas. Some phylogenetic analyses recover Notiotitanops mississippiensis inside Megacerops, but the phylogenetic position of Notiotitanops is generally unstable between analyses. Mihlbachler, 2008 Mihlbachler & Prothero, 2021 == Paleobiology ==

Horns (1929), showing the variability in horn morphology|upright=1.1|left Many brontothere genera had horns or cranial domes. The horns of Megacerops were the most developed out of all North American brontotheres. The horns took the form of two bony protuberances above the nose, and were vaguely reminiscent of a slingshot in shape. Some brontothere horns show evidence of secondary bone growth, perhaps regrowth due to clashes with other brontotheres. Diet Brontotheres such as Megacerops had relatively low-crowned cheek teeth. This, and their general dental morphology, restricted them to a browsing diet; their teeth were used to shear or crush plants. Megacerops were folivores and/or frugivores, though their large size suggests that they must have been relatively non-selective when it came to food. The diet of Megacerops was probably similar to the diets of modern-day moose and black rhinoceros. Social behavior Megacerops fossils have been found in mass death assemblages. Mass deaths of several individuals together indicates that they were social animals, and that they may have traveled in herds. == Paleoecology ==

Megacerops lived in a warm temperate to subtropical environment, in forests and open woodlands. Oxygen isotope analyses of Megacerops tooth enamel from the White River Group has revealed low δ18O values and low δ13C values. 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). , about the size of a modern lion, was the largest predator contemporary with Megacerops'' A variety of predators coexisted with Megacerops, The largest contemporary predator was Hyaenodon megaloides, which weighed around , about the size of a modern lion. It is unlikely that any contemporary predator would have been able to hunt adult Megacerops on account of their great size. Even juvenile Megacerops were large compared to contemporary predators and would have been difficult to prey on. 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. == Extinction ==

The brontotheres went extinct at the end of the Eocene. No brontothere fossils are known from the Orellan in North America. Megacerops was the last brontothere on the continent, and the last living member of the Brontotheriita. 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. 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. Various explanations have been proposed for the relatively sudden disappearance of the brontotheres. Osborn believed that the brontotheres succumbed to overadaptation and "racial senescence". Another unsubstantiated hypothesis is that the brontotheres died out due to a trypanosomiasis epidemic, caused by ancestors of the modern tsetse fly. 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. Megacerops would have been drastically impacted by such an environmental change, since isotope analyses suggest that it preferred moister and denser habitats. 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. The ecological niche of the brontotheres would eventually be taken over by rhinocerotoids and elephants. == Notes ==