Brontotheres (), also called titanotheres (), are an extinct group of perissodactyl (odd-toed ungulate) mammals belonging to the family Brontotheriidae. Brontotheres are traditionally interpreted as relatives of horses, though high-level perissodactyl relationships are not well understood. Brontotheres lived during the Eocene epoch and their fossils have mainly been found in Asia and North America, with some fossils also found in Eastern Europe. Early brontotheres were small animals, but later brontotheres grew to become among the largest land mammals of the Eocene.
The first brontothere fossils known to science were found in South Dakota in the mid-20th century. The name "brontothere" was derived from Lakota legends of wakíŋyaŋ, which can be translated to "thunder beasts". Over forty brontothere genera have since been recognized. The rapid diversification of the brontotheres, and their sudden extinction at the end of the Eocene after roughly twenty million years of existence, has been described as a "live fast, die young" pattern of evolutionary radiation. Over the course of their evolution, brontotheres experienced one of the most extreme increases in body size in mammalian evolutionary history, from early forms such as Eotitanops, weighing less than , to later four-tonne (4.4 short-tonne) animals, such as Megacerops and Embolotherium. At their peak in the Middle Eocene, brontotheres were the most diverse clade of large mammals in both Asia and North America.
Most brontotheres had relatively robust skeletons, especially later and larger forms. On the whole, the postcranial brontothere skeleton was conservative, lacking family-specific synapomorphic traits. Brontothere skulls were characterized by the eyes being positioned relatively far forward, resulting in short faces. Dentally, brontotheres were characterized by a unique combination of features in their upper molars.
Various taxonomic frameworks have been used for brontotheres historically, often diverging in what subgroups are recognized. Current brontothere taxonomy, supported by phylogenetic analyses, recognizes several increasingly derived (advanced) clades leading up to the three derived subtribes Rhadinorhinina, Telmatheriina, and Brontotheriina. Brontotheres are also sometimes informally divided into hornless brontotheres and the superficially rhinoceros-like horned brontotheres. The subtribe Brontotheriina contains all horned brontotheres, and about half of all known brontothere genera. Brontothere horns were made of bone and occurred in a variety of shapes and sizes.
Overview and characteristics
Brontotheres (family Brontotheriidae) are an extinct group of perissodactyls (odd-toed ungulates), the mammal order that includes modern-day equids, rhinoceroses, and tapirs. Fossils of brontotheres are known only from the Eocene epoch, and they are thus considered to have gone extinct during the Eocene–Oligocene extinction event. Some late brontotheres were previously treated as Oligocene animals but reassessments of the relevant fossil deposits since the 1990s have restricted the group to the Eocene.
Brontotheres are popularly known for their very large body sizes and the evolution of horns in various shapes. These features do not apply to all brontotheres, and are restricted to the derived horned brontotheres (the subtribe Brontotheriina). Most brontotheres are characterized by the orbits (eye sockets) being placed relatively far forward on the skull. Over the course of brontothere evolution, the face shortened further, and the postorbital (behind the eyes) part of the skull consequently became longer. Most brontotheres had comparatively robust skeletons, with later forms being more massive. All brontotheres had the primitive perissodactyl condition of four toes on their manus (front feet) and three on their pes (hind feet). In modern perissodactyls, this arrangement is retained only in the tapirs.
Dentally, brontotheres were characterized by bunoselenodont upper molars, meaning that the molars had inner cusps in the shape of blunt cones and outer cusps modified into crescents. The molars also had distinctive W-shaped ectolophs (shearing edges). The W-shaped ectolophs are shared with chalicotheres and some equoids, but unlike these groups, brontothere upper molars either lacked cross-lophs (transverse, i.e. widthwise, crests connecting the different cusps of the teeth), or had them only vestigially. Based on their dentition, it can be inferred that all brontotheres were browsers.
Research history
Early brontothere research
The first brontothere fossils known to science were found in the White River badlands of South Dakota. They were among the first fossils of mammals, and vertebrates generally, from the American West to be brought to scientific attention. White River brontothere fossils, including a jaw, were described for the first time in 1847, by Hiram A. Prout who believed them to represent remains of giant equoids. 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. The White River fossils are now recognized as remains of the genus Megacerops. Joseph Leidy was the first to provide more extensive scientific descriptions of brontothere fossils, though the fossils he had access to were mostly fragmentary and did not allow for detailed study of their anatomy or appearance. Leidy named both Megacerops and the much more primitive brontothere Palaeosyops, though close relations between the two were not recognized at the time.
The first paleontologists to study and describe complete brontothere skulls and skeletons were the rival researchers Edward Drinker Cope and Othniel Charles Marsh in the mid-to-late 19th century. Marsh's contributions were particularly important. In 1873, Marsh described the new genus Brontotherium (), which he named after Lakota legends of wakíŋyaŋ, which can be translated as "thunder beasts". It is believed that brontothere fossils, long before the time of paleontological research, were sometimes exposed by severe rainstorms and found by Native Americans of the Lakota Sioux and Pawnee peoples, who may have linked them to the wakíŋyaŋ. Modern Lakota legends link the abundant fossils in the White River bandlands to conflicts between the wakíŋyaŋ and monsters called uŋhcegila.
Marsh named the family Brontotheriidae in 1873 and was the first to outline the characteristics of the group. Marsh used the spelling "Brontotheridae", which was emended to Brontotheriidae by Oliver Perry Hay in 1902; the spelling Brontotheriidae has been used by all later researchers. Brontotherium is now considered a synonym of Megacerops but the name Brontotheriidae (and the informal term "brontotheres") remains the established name for the group.
Marsh conceptualized brontotheres as only the large and horned forms, and placed smaller and more primitive forms such as Palaeosyops in a separate family which he called "Limnohyidae". It was first with the description of the primitive horned form Diplacodon in 1875 that Marsh recognized a close connection between the "limnohyids" and brontotheres. By the turn of the century, a large number of North American brontothere taxa had been named, many of which has since been recognized as invalid and synonymous with others. In 1876, the first undisputed European brontothere fossils, found in Romania, were reported by and János Matyasovsky, and assigned to the new genus Brachydiastematherium. Brontothere fossils from Asia were first described in 1912, by Guy Ellcock Pilgrim and George de P. Cotter from the Pondaung Formation of Myanmar. These fossils were assigned to the now dubious genus Sivatitanops in 1925.
Osborn and post-Osborn research Henry Fairfield Osborn energetically pursued the study of brontotheres in the late 19th and early 20th century. In large part due to his efforts, the American Museum of Natural History (AMNH) amassed the largest collection of brontothere fossils in the world. In 1929, Osborn published an extensively illustrated nearly thousand-page monograph on brontotheres, The Titanotheres of Ancient Wyoming, Dakota, and Nebraska, which he believed would be the definitive work on the group. Osborn preferred the name "titanothere" over "brontothere"; "titanothere" is derived from the genus Titanotherium (), named by Leidy, now considered another synonym of Megacerops. The monograph was a comprehensive summary of North American brontothere research up until that time, including descriptions of every taxon recognized by Osborn, recountings of original descriptions, and extensive sections on brontothere anatomy.
Osborn included a preliminary summary of new Asian brontotheres in his monograph. Many brontothere fossils were at the time being found by the contemporary Central Asiatic Expeditions of the AMNH. These expeditions were often led by Roy Chapman Andrews and Walter W. Granger, though Osborn described many of the fossils. Through fossils found in the 1920s and 1930s, the diversity of brontotheres in Asia soon rivaled that of North America.
Osborn's brontothere taxonomy was deeply flawed, and influenced by his own discredited views on evolution (orthogenesis). There was a consensus among Osborn's peers, and among later paleontologists, that Osborn grossly oversplit brontothere taxa into an unrealistically high number, sometimes based on arbitrary distinctions. After Osborn, and for much of the 20th century, brontothere systematics and phylogeny were a neglected area of study. Because of the lack of detailed studies, Osborn's monograph long remained the primary reference work for the family, despite wide recognition of its flaws. The immense size of the monograph discouraged many paleontologists from pursuing the study of brontotheres since correcting the mistakes would be an enormous effort.
After Osborn's death, the taxonomy of the brontotheres known from Central Asia was extensively revised in a 1943 paper by Walter W. Granger and William King Gregory, A revision of the Mongolian titanotheres. North American brontotheres were subject to some minor attempts at taxonomic revision over the course of the 20th century, though no major work was done until revisions by Bryn J. Mader in the 1980s and 1990s. Attempts at further revision of Asian brontotheres were made by several Russian and Chinese paleontologists in the 20th century; most of these efforts were based on Granger and Gregory's work since the authors did not have access to the AMNH collections. Granger and Gregory's paper includes several misidentifications and misinterpretations, which impacted the reliability of the research based on it. In 2008, Matthew C. Mihlbachler published a new brontothere monograph, Species Taxonomy, Phylogeny, and Biogeography of the Brontotheriidae (Mammalia: Perissodactyla). Mihlbachler's monograph was the first species-level reexamination and revision since Osborn, featured both Asian and North American taxa, and was supported by a comprehensive phylogenetic analysis, which Mihlbachler used to establish the internal brontothere systematics that are in use today.
Description
Skull and dentition Brontothere skulls show a diversity of different morphologies. Some brontotheres had highly brachycephalic (short and broad) skulls, such as Palaeosyops, and some had highly dolichocephalic (long and narrow) skulls, such as Sphenocoelus. In most brontotheres, the face was shortened due to the orbits being placed far forward on the skull. In some derived (advanced) brontotheres, the facial region accounted for less than 40% of the total length of the skull. The facial regions of the very earliest brontotheres were probably not shortened; the basal (primitive) genus Eotitanops had a longer face similar to that of other primitive perissodactyls.