Brontotheriidae

Brontotheres evolved massive bodies, with some species standing over 2.5 meters (7 feet) tall, with body masses of over a tonne, perhaps exceeding , in large individuals of Megacerops, though some small species such as Nanotitanops did persist through the Eocene. The evolutionary history of this group is well known due to an excellent fossil record in North America. Some of the early forms, The earliest stem-brontotheres, such as Eotitanops and Palaeosyops, were rather small, no more than a meter in height, hornless and had an estimated body mass of only . Brontotheres had likely adapted to the warmer and more humid climates of the Eocene, and probably became extinct because they could not adapt to the drier conditions and more open landscapes of the Oligocene. The lower jaw was generally robustly built and had a rather short symphysis. The majority of brontotheres showed a complete dentition with three incisors, one canine, four premolars, and three molars per jaw quadrant. Only the evolutionarily youngest forms had a dentition reduced by one incisor anteriorly and sometimes a missing anterior premolar. The incisors in many representatives had a small, globular shape. The generally low-crowned molars were equipped with crescent-shaped (bunoselodont) enamel cusps on the occlusal surfaces. Their teeth are adapted to shearing (cutting) relatively nonabrasive vegetation. Their molars have a characteristic W-shaped ectoloph (outer shearing blade), which connected the two main cusps there (paracone and metacone). The two lingual main cusps (protocone and hypocone) stood isolated from it. The wear patterns observed on brontothere teeth suggests a folivorous diet. Early Brontotheres had brachydont teeth with thick enamel, while later forms evolved a more hypsodont style tooth with thinner enamel. Horns '' Unlike the horns of rhinoceroses, the horn formations of brontotheres did not consist of keratin but were outgrowths of the skull bones and were therefore preserved as fossils. They formed from the posterior part of the nasal bone and were overlain by the anterior portions of the frontal bone. The majority of horn-bearing brontotheres typically possessed two paired horns, frequently located above the orbit, where the width of the nasal bone increased considerably. Also differing from rhinoceroses, the horns did not end in a point but were rounded. At the base they had a round to oval cross-section and sometimes projected obliquely outward; at the tips the horns could also be slightly forked, as in Megacerops. In many cases the horn formations consisted only of larger or smaller bony swellings, which was especially typical of phylogenetically older forms. Some later brontotheres developed horn-like bony projections of the skull. The North American brontothere Megacerops, for example, evolved large sexually dimorphic paired horns above their noses. The sexually dimorphic horns, along with highly developed neck musculature, suggest that brontotheres were highly gregarious and males may have performed some sort of head-clashing behavior in competition for mates. Females had smaller appendages, which may have been used to ward off predators and protect young. In Asia, another species of brontothere, Embolotherium, evolved a similarly gigantic body size; however, instead of the slingshot-like horns of the Megacerops, they evolved a single elongated bony process that was composed of both nasal and frontal bones. Embolotherium may have used its large nasal cavity to make vocalizations in order to communicate with others of its species. In some Asian brontotheres the horns frequently stood very close together or were fused, forming a bony battering ram, most clearly seen in Embolotherium. Some genera, such as Dolichorhinus, evolved highly elongated skulls. Similar to giraffes, their horns were covered in skin and did not have grooves for nutrient blood vessels. There is some evidence of secondary bone growth, likely due to impact from head clashing. However, it has since been demonstrated for Embolotherium that the entire battering ram was incorporated into the soft tissue covering of the front of the head, giving it a markedly distinctive face. == Fossil record ==

in the northwestern United States, 1931 Brontothere fossils are numerous and come from North America and Eurasia. The quality of finds varies both spatially and temporally due to the nature of fossil sites and differing excavation and preparation methods, so some representatives are better known than others. Several areas stand out where brontothere fossil remains have been particularly numerous or of exceptional quality. A true "treasure trove" proved to be the central-western part of North America, mainly the northwestern United States with the High Plains and the Rocky Mountains of South Dakota, Nebraska, and Wyoming. To the north the find area extends to Saskatchewan and British Columbia in southwestern Canada. This general area is where the first fossils were found. In this context, the White River Badlands must be mentioned above all, where countless material from the Chadron Formation in the drainage basin of the White River originates. This rock formation is assigned to the Upper Eocene and contained mainly remains of Megacerops. The strata containing these fossils are still called Titanotherium beds, after the old, now obsolete synonym for this genus. Also notable is the Duchesne River Formation in Utah, assessed as somewhat older, with high-quality remains of Duchesneodus, a close relative of Megacerops. Furthermore, another outstanding bed is the Clarno Formation in Oregon with well-preserved fossil finds of Eubrontotherium. The northernmost finds in North America so far came to light in the Margaret Formation on Ellesmere Island in the Canadian Arctic Archipelago north of the Arctic Circle. They date to the transition from the Lower to the Middle Eocene and are largely assigned to Eotitanops and Palaeosyops. Outside North America, other fossil sites are found in East Asia, with the Gobi Desert being exceptional. From the various geological formations exposed there, such as the Irdin Manha Formation, the Shara Murun Formation, or the Ulan Gochu Formation of the Middle and Upper Eocene, originate numerous fossil remains of Gnathotitan and Rhinotitan, but also of the massive Embolotherium. The fossil history here began largely in the 1920s, with expeditions of the American Museum of Natural History, but many finds have also been made since that time. Also important are the exposures of Balochistan in South Asia, from which some of the oldest brontothere remains in Asia originate. Especially since the 1990s, the importance of Central Asia as a major region for research on this group of odd-toed ungulates has grown considerably; the exceptionally well-preserved fossil remains of Aktautitan from Kazakhstan are noteworthy in this context. == Paleobiology ==

Diet , Minnosota Science Museum The characteristic bunoselodont (crescent-shaped) enamel pattern on the occlusal surfaces of the molars and their low crown height are usually indicators that the animals fed on soft plant material (browsing). This is confirmed by a microscopic analyses of the dental wear surfaces of various brontothere genera such as Eotitanops, Telmatherium, Metarhinus, Duchesneodus, and Megacerops. The numerous narrow scratches and small pits found indicate a dietary specialization more strongly focused on foliage, with only occasional incorporation of fine-grained sediment particles from the ground. Only in the earliest forms might a higher proportion of fruits have played a role, similar to what was the case with the earliest horses. Mixed plant diet consisting of bark and branches or seeds can be largely excluded. Since modern herbivores predominantly show a different chewing pattern, researchers conclude that brontotheres were highly selective in their foraging behavior. It was also determined that especially later brontothere representatives from the Upper Eocene showed considerably more traces of small scratches on their dental wear surfaces, which could be related either to a shift in preferred diet or to general landscape changes. Furthermore, isotope analyses of the tooth enamel of the molars showed that the latest brontotheres such as Megacerops derived nearly 100% of their diet from leaves and also had a fairly high water dependency. The dependency on liquid in particular indicates a digestive system similar to that of modern odd-toed ungulates, in which a large part of food is broken down in the hind gut. Sexual dimorphism Within individual brontothere genera, differences in body structure can be identified that are frequently interpreted as sexual dimorphism. In many, especially hornless representatives, differences in dental morphology are noticeable; in some individuals this usually includes a larger canine tooth that greatly exceeds neighboring teeth, while in other individuals it is less distinctly developed. In association with large canine teeth, more robust skulls often appear to show more massive muscle attachment points for the chewing musculature especially at the zygomatic arches, which then manifests as large bony swellings. The combination of more robust skulls and large canine teeth is usually associated with male animals, where similar features can be observed in modern horses as well. A further likely sexual difference is apparent in horn size in some horn-bearing brontotheres, with animals having larger horns and sometimes also associated more robust nasal bones being regarded as male, while female animals show more gracile formations. This is known from forms such as Megacerops, Duchesneodus, and Embolotherium. Notable in this context is that some of the latest brontotheres no longer show any difference between the sexes in dental structure, so that the canine teeth are similarly large in all fossils found. This can be demonstrated especially in Embolotherium, which had a distinctive bony battering ram on the skull, but also from finds of Duchesneodus, which had two individual horns. Something similar can be assumed for Embolotherium, which had very massive long bones suggesting a rather ponderous gait. On the other hand, the phylogenetically older Rhinotitan had considerably more slender limbs, which may have allowed more agile movement. Biological functions '' In some forms such as Metarhinus, Sphenocoelus, and Telmatherium, the actual internal nasal passages at the base of the skull are closed and replaced by openings shifted further posteriorly toward the vomer. This directs inhaled air through the nose directly to the olfactory mucosa. In the region of the ethmoid bone, a kind of nasal septum is developed that divides the internal airways into two separate tubes. It therefore appears possible that brontotheres were obligate nasal breathers and did not breathe through the mouth. A similar feature is also seen in modern horses, which are considered the closest living relatives of brontotheres, as well as, but to a lesser extent, in rhinoceroses. Less likely, but possible, would be a function in a potential aquatic lifestyle. In this case, the rearward displacement of the internal airways would theoretically support air intake when the throat is filled with water, though this has not yet been anatomically confirmed. Paleoenvironment Based on the dietary habits of brontotheres, with their preferred food and required water, these animals likely lived largely in dense, closed forests interspersed with rivers and swamps under humid climatic conditions. This is suggested among other things by isotope analysis on teeth from the Chadron Formation in the drainage basin of the White River, which belongs to the Upper Eocene. These findings are supported by a large part of the geological and paleontological find circumstances. Analyses of the paleofloristics of the likewise Upper Eocene Australian Creek Formation in British Columbia, which yielded brontothere dental remains, allow the reconstruction of mixed conifer-deciduous forests under temperate climatic conditions. The mean annual temperature is estimated at about 13 °C with a mean for the coldest month of −4 °C. Mean annual precipitation was 115 cm. By contrast, brontotheres also showed a certain adaptability to extreme environmental conditions, as suggested by the remains from the Lower-Middle Eocene Margaret Formation on Ellesmere Island in the high north of Canada. The former environment can be assumed to have been comparable to that of the Australian Creek and Chadron formations, but was subject to the effects of the polar day and polar night with alternating months-long light and darkness. This probably led to regular seasonal shortages in food plants. To what extent the animals spent the entire year in the far north remains unclear, but juveniles are documented for some species, suggesting a permanent presence. == Systematics ==

External systematics }} }} }} }} Brontotheriidae are an extinct family from the order Perissodactyla (odd-toed ungulates). Perissodactyla are traditionally divided into two major suborder, the Hippomorpha (the horses) and the Ceratomorpha (the related group of rhinoceroses and tapirs), with the Ancylopoda (the extinct chalicotheres) introduced later as a third. These three groups are predominantly regarded as suborders. In this frequently held view, the superordinate taxon of Tapiromorpha in turn underscores the closer relationship of Ceratomorpha and Ancylopoda to each other. In this scheme, brontotheres are usually placed within Hippomorpha and are therefore, despite their divergent appearance in advanced forms, more closely related to horses than to rhinoceroses. The relationship with early horses is shown by numerous skull characteristics, such as the construction of the broad nasal bone or the short symphysis of the lower jaw, and further by dentition. The closest relative group is, however, the likewise extinct Lambdotheriidae, with which brontotheres together form the superfamily Brontotherioidea. There are also views that brontotheres together with the lambdotheres, occupy a very basal position within Perissodactyla, as they unite numerous very primitive characteristics of this mammalian order. In this case, brontotheres and their relatives are assigned to their own suborder Titanotheriomorpha. However, the exact relationships between horses and brontotheres and in turn with other odd-toed ungulates have not yet been precisely established. Internal systematics Originally, the individual brontothere genera were assigned to various subfamilies such as Brontotheriinae, Telmatheriinae, or Embolotheriinae. The internal systematics of the family were, however, largely well-studied only for North American representatives, Although these were also organized in individual subfamilies, After these two revisions, three subfamilies initially remained within brontotheres, with Brontotheriinae containing all more modern forms. Thereby all further higher taxa were shifted to the level of tribes, subtribes, and infratribes. Here, Brontotheriina represents all brontotheres with horn formations, while Telmatheriina shows only bony swellings and Rhadinorhinina shows none of these features. The infratribe Brontotheriita and Embolotheriita differ in horn structure: the former typically has two separate horns, the latter closely set or fused and sometimes battering-ram-like formations. The individual groups and lineages of brontotheres are not geographically bounded but show complex relationships across the different continents of their former distribution. This implies multiple faunal exchanges in different directions between the individual landmasses during the evolutionary history of brontotheres. Overview of genera }} In total, more than 40 genera from the family Brontotheriidae are known today; the last comprehensive revision of the family was carried out, as mentioned, in 2008 by Mihlbachler. New genera described after 2008 are marked with an asterisk and cited to their original description. Lambdotherium and Xenicohippus were previously included in Brontotheriidae but are no longer considered members of this family. Lambdotherium, though excluded, may be the closest known relative to brontotheres. Xenicohippus is now thought to be an early member of the horse family, Equidae. == Evolution ==

Adaptive radiation General evolutionary trends among brontotheres include an increase in body size and the development of bony horns at the nasal-frontal junction. The considerable increase in weight that brontotheres underwent during their evolutionary history did not proceed in a linear fashion following Cope's rule, but rather in an undirected manner. The reason is that brontotheres were able to occupy ecological niches that were largely unoccupied in the early development of mammals in the Paleogene, allowing larger species to arise more frequently. The undirected nature is evident from the fact that under certain external conditions, smaller representatives of brontotheres repeatedly emerged. According to a 2023 study, the driving factor was the saturation of an ecological niche. Accordingly, the rate of speciation was very high in densely occupied ecological niches, but forms also went extinct more frequently. In less densely occupied areas, brontotheres were subject to less competitive pressure. Fewer species formed there, but they persisted longer and thereby also reached larger body sizes. The skull underwent very marked changes. In the most primitive representatives it was still very flat or slightly domed. The progressive elongation of the occipital bone, with the resulting low head posture, additionally caused a sometimes markedly saddled profile of the forehead. Shortenings in the anterior snout region further led to changes in the position of the orbit relative to tooth position. Thus the early brontotheres still had the eye socket above or slightly behind the third molar, while later forms had it above the first or second molar. This also resulted in the modern representatives having a very expansive posterior skull. In the dentition, the premolars became increasingly molariform, and in some lineages a diastema developed. Due to the dietary shift of early representatives from a more fruit-based to a leaf-rich diet, bunodont molars evolved over the course of evolutionary history into distinctly lophodont (with transverse enamel ridges) to selenodont (with crescent-shaped enamel ridges) shaped posterior cheek teeth. The number of teeth changed little; only some late brontotheres had one fewer incisor in the anterior dentition. Faunal exchange most likely occurred via the high latitudes of the north. Origin and development (1857–1935), from 1929 The evolution of this group is well known due to the excellent fossil record from North America and East and Central Asia, though its origin has not yet been fully resolved. Some researchers propose a North American origin for brontotheres. There, the earliest forms appeared in the Lower Eocene around 53 million years ago (locally known as the Wasatchian North American land mammal age). The partially accepted possible ancestral form is Lambdotherium, which is sometimes regarded as one of the most basal forms of brontotheres, but is sometimes placed in its own family. Early finds consist of two partial skeletons from the Fossil Butte Member of the Green River Formation in the Green River Basin in the northwestern United States. These date to the local faunal zone Lostcabinian and are around 52 million years old. According to some other researchers, the origin of the group might also lie in Asia. There, Danjiangia from the Lingcha Formation in the Chinese province of Henan is recorded as a possible very early member, showing some similarities to Lambdotherium. However, it dates to the Paleocene–Eocene boundary around 56 million years ago (locally referred to as Bumbanian). The unambiguous early North American brontotheres were overall still relatively small forms such as Eotitanops or Palaeosyops, with shoulder heights of one meter or less and no horns; for the former a weight of around 140 kg is estimated. which at the time were still covered by swamp forests. By contrast, weight estimates for the roughly contemporaneous but phylogenetically more advanced forms Wickia and Metatelmatherium range from about 1.6 to 2.1 t. and Xylotitan. Overall, brontotheres were the most diverse group of large mammals in both North America and Asia during the Middle Eocene. Notably, no independent adaptive radiation took place on either continent; instead, the phylogenetic development is strongly interlinked. Some researchers assume up to a dozen independent intercontinental dispersal waves in both directions. In the Upper Eocene, 37 to 34 million years ago, the number of different brontothere forms gradually declined. About ten genera are known from this time, but most are to be placed in the early phase of the epoch. All documented brontotheres were horn-bearing and predominantly very large, like the possibly up to 3 t heavy Megacerops. Two distinct lineages emerged: those with two widely spaced horns closely related to Megacerops, and those with two closely set or fused horns more closely related to Embolotherium. Toward the end of the Eocene, climatic changes in the form of temperature decline occurred, intensifying through the Oligocene, during which the first grasses spread in association with open landscapes. Brontotheres may have been unable to adapt their highly specialized diet to these landscape changes in the mid and higher latitudes where they were mainly distributed, and were displaced by more competitive herbivores such as the ascending groups of rhinoceroses and chalicotheres. Most brontotheres had already disappeared before the end of the Eocene; only Megacerops persisted until about 34 million years ago. == Discovery ==

The study of brontotheres dates back to the 1840s and began in North America with the first discovery of remains of this odd-toed ungulate group. The first find, a lower jaw fragment with the three preserved posterior cheek teeth, is credited to Hiram A. Prout and comes from the White River Badlands of the White River in South Dakota. In a short publication from 1846, Prout identified the lower jaw find as belonging to a giant Palaeotherium, a primitive horse relative; a year later he described the lower jaw in more detail. David Dale Owen and Joseph Granville Norwood created the species designation Palaeotherium proutii in 1850 in Prout's honor, based on additional individual finds from the White River, though it remains unclear whether the authors incorporated the lower jaw. Additional finds from the same region, including the lower jaw presented and illustrated by Prout, were later (in 1852) referred by Joseph Leidy (1823–1891), who is considered one of the founders of paleontology in North America, to Titanotherium, though he mentioned the name rather incidentally. However, the French researcher Auguste Pomel (1821–1898) had already described this lower jaw find in 1849 under the name Menodus, predating Titanotherium. Pomel not only named the genus but, unlike Prout and later Leidy, attached to it the species Menodus giganteus. This thus constitutes the oldest binomial in research history assigned to a brontothere representative. Furthermore, the lower jaw and the associated species name represent the first scientific designation given to a fossil from the extremely fossil-rich White River Formation area. '') at the San Diego Natural History Museum Already in the 1850s, American geologist Ferdinand Hayden (1829–1887) found extraordinarily rich fossil material in the White River Formation, including numerous brontotheres, making an important contribution during this very early research period. In the 1870s to 1890s, brontothere research was conducted in the shadow of the Bone Wars, also called the "Cope-Marsh feud," carried out between the two American paleontologists Edward Drinker Cope (1840–1897) and Othniel Charles Marsh (1831–1899). It was Marsh who first used the name Brontotherium in 1873 and defined it based on three skeletons from the Peabody Museum of Natural History at Yale University. However, Brontotherium is considered only a synonym of Megacerops. This designation also originally came from Leidy, who published it in 1870, and was introduced by him based on a skull from Colorado, noting similarities to his previously named genus Titanotherium. Cope and Marsh collected vast amounts of fossil material in North America during this period, especially in both Dakota states and Nebraska; Marsh engaged John Bell Hatcher, who sent him 11,000 t of find material from 1886 to 1888 alone. Numerous genera were also named, some of which, as later proved, were identical to others. Cope united both families in 1879 but assigned their members to the Chalicotheriidae. He then separated the Menodontidae (based on Menodus) from these as a younger evolutionary branch of the brontotheres. Due to this practice, several names for the family Brontotheriidae were in use by the end of the 19th century. Of importance here is the designation Titanotheriidae, first coined in 1876 by the British scientist William Henry Flower. Independently from Flower and apparently unaware of his work, Henry Fairfield Osborn (1857–1935) re-established the family name Titanotheriidae in 1890 and used it throughout his life. He also corrected Cope's erroneous referral of some brontotheres to chalicotheres. The designation Titanotheriidae is today only rarely and largely informally in use, as it is a synonym of Brontotheriidae, but it derives from Leidy's Titanotherium. At the beginning of the 20th century and after Marsh's death, it was Osborn who substantially advanced the research. Osborn, working at the American Museum of Natural History, gained access to the records of Marsh and Cope and subsequently produced a monograph on brontotheres (which he generally called titanotheres). In the 1920s he organized expeditions to East Asia into the Gobi Desert, which took place between 1922 and 1930 and are referred to as the Central Asiatic Expeditions of the American Museum of Natural History. During these expeditions, usually led by Roy Chapman Andrews and Walter W. Granger, discovered countless remains of brontotheres with previously unknown forms, such as Embolotherium and Gnathotitan, in addition to dinosaurs and other fossil animals, thereby proving the previously only suspected distribution of this odd-toed ungulate group as far as Asia. Osborn worked up the material in numerous publications; in 1929 his more than 890-page monograph with additionally over 230 plates appeared under the title Titanotheres of ancient Wyoming, Dakota, and Nebraska covering the North American finds, but still containing a more than 40-page appendix on the East Asian finds. After Osborn's death in 1935, Walter W. Granger and William King Gregory continued his work. ==Notes==