Horned brontotheres (subtribe Brontotheriina) were a group, or clade, of derived (advanced) brontotheres characterized by large body sizes and horns. Horned brontotheres are known from the Middle to Late Eocene, and achieved a nearly holarctic distribution, ranging across North America, Asia, and eastern Europe. They were a diverse and dominant group of large herbivores, and were among the largest land mammals of the Eocene.
Horned brontotheres have no true modern-day analogues but were superficially similar to rhinoceroses, an example of convergent evolution; they differ in several major and minor features, for example in that brontothere horns were composed of bone, not keratin. The life appearance and function of the horns is uncertain, but common hypotheses include them being used for display and in intraspecific combat. Horns came in a variety of shapes and sizes, from small bony protuberances to elaborate structures with shapes reminiscent of slingshots and battering rams.
Horned brontotheres have only recently been recognized as a monophyletic group; Asian and North American brontotheres were previously believed to have evolved horns and large sizes independently. It has proven difficult to reconstruct the paleobiogeography of the horned brontotheres, but they are believed to have originated in Asia and to have undergone evolutionary radiation in both Asia and North America, alongside several migration events between the two continents. Advanced horned brontotheres are divided into two subgroups, the predominantly Asian Embolotheriita (or "embolotheres") and the predominantly North American Brontotheriita (or "eubrontotheres"). Both embolotheres and eubrontotheres reached very large body sizes in the Late Eocene, exemplified by the Asian Embolotherium and the North American Megacerops.
Description
The defining characteristic of the horned brontotheres is the presence of bony protuberances (horns) on their nasal or frontal bones, a trait that distinguishes them from all other brontotheres. Other apomorphies (defining shared traits) of the horned brontotheres include their distinctly saddle-shaped skulls, and their relatively straight zygomatic arches. Most horned brontotheres are also characterized by massive bodies and robust limbs. Like all brontotheres, they are also characterized by a relatively short face, and the skull being elongated behind the eyes.
All brontotheres had a distinctive dentition, characterized by molars with W-shaped ectolophs (outer shearing blades). Their teeth appear to have been adapted to both cutting and shearing, and have traditionally been interpreted as indicating a selective frugivorous (fruit-eating) and/or folivorous (leaf-eating) diet. The large size of many horned brontotheres means that they must have been relatively non-selective feeders. Their diet may have been similar to that of modern moose and black rhinoceroses.
Brontotheres have no true modern-day analogues. Horned brontotheres were superficially similar to modern rhinoceroses; this is an example of convergent evolution since they may not be closely related and horned brontotheres evolved from much smaller, hornless ancestors such as Eotitanops. Several major and minor features distinguish the horned brontotheres from rhinoceroses. Among the more obvious differences are that brontothere horns were composed of bone (not keratin, as in rhinoceroses), subtle differences in limb proportions, and that brontotheres had four toes on their manus (front feet) whereas rhinoceroses have three.
Classification
History Fossils of horned brontotheres were the first brontothere fossils to be discovered, and were among the first vertebrate fossils known from the western United States. The first fossils, found in the White River Formation, are now recognizable as Megacerops and were described in detail in the mid-to-late 19th century by American paleontologists such as Joseph Leidy, Othniel Charles Marsh, an Edward Drinker Cope. The first remains of horned brontotheres in Europe, belonging to the genus Brachydiastematherium, were reported by the Hungarian paleontologists and János Matyasovsky in 1876. In Asia, brontotheres were first reported in the early 20th century. Expeditions from the American Museum of Natural History in the 1920s and 1930s to East Asia recovered a large number of fossils, and the diversity of Asian horned brontotheres quickly rivaled that known from North America.
Brontothere classification and systematics have varied considerably over time. In 1929, Henry Fairfield Osborn divided horned brontotheres into a number of different subfamilies. Osborn believed that the Asian embolotheres represented a highly distinct lineage, and erected the subfamily Embolotheriinae to contain them. In a 1943 revision of Mongolian brontotheres, Walter W. Granger and William King Gregory followed Osborn in suggesting that the embolotheres formed a separate lineage from North American horned brontotheres, sharing a last common ancestor as far back as Palaeosyops. Granger and Gregory's work remained the definitive work on Asian brontotheres for much of the 20th century.
Understanding of brontotheres as a group was long hampered by the lack of reevaluations of the Asian fossil material. Asian brontotheres continued to be named over the course of the 20th century, and the taxonomy was partially revised by Russian and Chinese paleontologists. Much of this work built on Granger and Gregory's 1943 revision, without new comparisons to the large collection of Asian fossils in American museums. The collection of Asian brontothere fossils at the American Museum of Natural History is the largest collection of Asian brontothere fossils in the world and contains the majority of the holotype specimens. In the early 21st century, this collection began to be reexamined and reevaluated by the American paleontologist Matthew Mihlbachler. Mihlbachler's studies of the material, supported by phylogenetic analyses, led to large-scale taxonomic revisions and the recognition of horned brontotheres as a monophyletic group.
Systematics and phylogenetics Internal
Horned brontotheres are classified as the subtribe Brontotheriina. There are two clearly defined clades within the subtribe, the infratribes Brontotheriita and Embolotheriita, as well as several basal (primitive) genera. The taxonomic authorities (scientists who first validly published the names) for Brontotheriina and Brontotheriita is formally Othniel Charles Marsh, 1873, due to having named the family Brontotheriidae. Osborn, 1929, is the authority for Embolotheriita, due to having named the now deprecated subfamily Embolotheriinae.
The cladograms below show the primary strict reduced consensus tree from Mihlbachler's 2008 analysis, and the reduced strict consensus tree from a 2021 analysis by Mihlbachler and Donald Prothero:
Topology A: Mihlbachler (2008)
Topology B: Mihlbachler & Prothero (2021)
Both Brontotheriita and Embolotheriita are defined by their own sets of apomorphies. Embolotheriita are characterized by an elevated nasal process with thin and deep lateral (side) walls, as well as narrowly spaced or fused horns. Brontotheriita are characterized by an unelevated nasal process and several features of the teeth, including straight rows of small lower and upper incisors, upper incisors that were globular in shape, and weak labial premolar ribs (ridges on the premolars, facing the cheek).
External
Brontotheriina is classified within the tribe Brontotheriini, which also contains several brontothere groups and genera where the configuration of the frontal and nasal bones appear to foreshadow the evolution of horns in Brontotheriina, such as the closely related subtribe Telmatheriina (containing brontotheres such as Telmatherium and Metatelmatherium).
The cladograms below are again adapted from Mihlbachler's 2008 analysis, and Mihlbachler and Prothero's 2021 analysis:
Topology A: Mihlbachler (2008)
Topology B: Mihlbachler & Prothero (2021)
Etymology The names Brontotheriina and Brontotheriita, and "brontothere" itself, are derived from the genus Brontotherium (now considered a synonym of Megacerops). Brontotherium means "thunder beast", from the Greek βροντή (brontḗ, "thunder") and θηρίον (theríon, "beast"). The name Brontotherium was coined by Marsh in 1873 in honor of Lakota legends of wakíŋyaŋ ("thunder beasts"). The name Embolotheriita is derived from the genus Embolotherium. Embolotherium, coined by Osborn, means "battering ram beast", from the Greek έμβολή (emvolí, glossed as "battering ram" by Osborn) and θηρίον (theríon, "beast"), in reference to the unique fused horns of the genus.
Some basal horned brontotheres, such as Diplacodon and Protitanotherium, are sometimes informally grouped together as "diplacodonts", but this is not a formal taxon. Members of the Embolotheriita are informally referred to as "embolotheres". Prior to the formal recognition of infratribes, the clade corresponding to Brontotheriita was sometimes referred to as "eubrontotheres" (meaning "true brontotheres"), a term that has seen limited continued use as an informal term.
Paleobiology
Body size
Over the course of their relatively rapid evolution, the brontotheres experienced one of the most extreme size increases in all of mammalian evolutionary history. Stephen L. Brusatte has described the horned brontotheres as "the first mammals that truly tried to imitate the colossal dinosaurs of ages past".
Horned brontotheres were among the largest land mammals of the Eocene. Before to the brontotheres, the largest land mammals had been the dinoceratans, a group that went extinct in the Middle Eocene. The largest dinoceratan was Eobasileus, about at the shoulder. The largest of the horned brontotheres (such as Megacerops) were larger, at tall at the shoulder or more. Horned brontotheres were far larger than other contemporary perissodactyls. With the exception of the largest amynodonts, perissodactyls were otherwise mostly small in the Eocene. After their extinction, horned brontotheres were surpassed in size in the Oligocene by the gigantic paraceratheres (such as Paraceratherium). Recent size estimates of the paracerathere Urtinotherium suggest that paraceratheres may have equalled or surpassed the largest brontotheres in size already in the latest Eocene. The largest brontotheres were larger than all perissodactyls living today, including rhinoceroses.
The largest well-known horned brontotheres (and largest brontotheres overall) are the eubrontothere Megacerops and the embolothere Embolotherium. Several other genera may have rivaled them in size, such as Arctotitan, Gnathotitan, Nasamplus, Supplemental Material (Data S1, Sheet 6) and Titanodectes, but they are known from relatively poor fossil material. The jaws of Gnathotitan are the largest and deepest brontothere jaws known, but next to nothing is known of the rest of its body.
Horns