Megacerops
Megacerops 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, during the Chadronian land mammal age.
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 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
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 referred to these great mammals as wakíŋyaŋ, translated as "thunder beasts".The first brontothere fossil to be scientifically described was a fragment of a right jaw, found in Chadronian deposits in the White River badlands of South Dakota. 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". 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. 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.Owen's fossils were soon acquired by Joseph Leidy, who examined and described the material in greater detail through 1852 and 1853. 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". After more extensive examination, Leidy placed P.? proutii in the new genus Titanotherium and designated a left jaw from Owen's material as the type specimen. Although recognized as large perissodactyls, early descriptions of brontothere fossils up to and including Leidy's work failed to recognize their distinctive nature.
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. 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.
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. 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. In 1873, Leidy speculated that his Megacerops may belong to the same order as these animals, and not the ruminants. Leidy further noted similarities between the Megacerops specimen and fragmentary fossil horn cores he had previously speculated to perhaps belong to Titanotherium.
Further discoveries
Othniel Charles Marsh is considered the most important figure in the early research on the brontotheres. His studies of brontotheres began in 1870, when Marsh led an expedition to northern Colorado on behalf of Yale College. During the expedition, Marsh's crew were shown a brontothere jaw by a group of Lakota, who told them of their legends of wakíŋyaŋ. The expedition collected a large number of brontothere fossils, including jaws, skulls, and postcranial elements. In honor of Lakota legends, Marsh named the new genus Brontotherium in 1873. Marsh's 1873 description of Brontotherium gigas was the most important contribution to brontothere knowledge up until his time. 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. Marsh recognized Brontotherium as a "true perissodactyl with limb bones resembling those of Rhinoceros". 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. In 1875, H. C. Clifford discovered and excavated a large and relatively complete brontothere skeleton near Chadron, Nebraska. This skeleton was described by Marsh in 1887 as the type specimen of the new genus and species Brontops robustus.Marsh continued to study brontothere fossils for the reminder of his career, some collected by himself but most purchased from collectors "out West". 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. 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. The brontothere collection assembled for Marsh's monograph eventually included several nearly complete skeletons and nearly two hundred complete skulls. 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.
File:Expedition Members Lifting Titanotherium Specimen.jpg|thumb|Excavation of a "Titanotherium" in Bighorn Basin, Wyoming, 1931
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, increasing the already large brontothere fossil sample to encompass further skeletons and hundreds of skulls. In 1929, Henry Fairfield Osborn published a monograph on the brontotheres, The Titanotheres of Ancient Wyoming, Dakota and Nebraska. Osborn had made study of brontotheres one of his life's quests 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.
The large number of fossils collected has made Megacerops one of the best represented large herbivores in the fossil record of North American mammals. Megacerops fossils have predominantly been recovered from the White River Group in the United States and the Cypress Hills Formation in Canada. All fossils now attributed to Megacerops appear to be confined to the Chadronian land mammal age, which corresponds to the Late Eocene. Most Megacerops specimens have poorly recorded stratigraphic data, limiting the degree to which variation in single contemporary populations can be studied. 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.
Taxonomy and species
20th-century developments
By the time of Osborn's monograph, at least 47 species of Chadronian brontotheres had been named, many based on poor and fragmentary fossils. There was a general consensus throughout the 20th century that the brontotheres were highly oversplit, divided into far too many species. 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. In several publications, both Leidy and Osborn sometimes confused USNM 113 with USNM 21820. 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.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. 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. Almost every well-preserved skull was designated as a separate species. 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.
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". Scott further stated that "the probably great effects of sex and age and fluctuating variability have not been sufficiently evaluated". 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.
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. 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. The only other Chadronian brontothere genus recognized as valid by Clark, Beerbower, and Kietzke was Teleodus. Teleodus has since been recognized as invalid, with its type specimen revealed to have been a 19th-century hoax in 2004.
In 1989, Bryn J. Mader published the first true revision of the Brontotheriidae since Osborn. 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. 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. Mader undertook no species-level revision of the three genera, and did not demonstrate them to be monophyletic via phylogenetic analysis.