Embolotherium () is an extinct genus of horned brontothere that lived in East Asia during the Late Eocene, in the Ulangochuian and Ergilian land mammal ages. Fossils of Embolotherium have mainly been found in Inner Mongolia in China, and in Mongolia, particularly in the Ulan Gochu and Ergilin Dzo formations.
Embolotherium is distinguished from other brontotheres by its "ram", a highly unique and elongated structure at the front of the skull. The ram developed from the horns, frontal bones, and nasal bones of Embolotherium's ancestors. In the past, the ram was interpreted as functioning like a horn and to have been used for combat. The skeletal features of the ram, such as the nasal septum running along the entire length of the structure, instead indicate that the ram was incorporated into a large soft-tissue snout, and served to support a large nasal cavity. The orientation and position of the ram is among the features that serve to distinguish the two currently recognized species of Embolotherium. In E. andrewsi, the ram is relatively vertical and rises above the orbits (eye sockets), whereas in E. grangeri the ram is curved forwards and rises behind the orbits.
Embolotherium was a massive and robust animal. Although known mainly from skulls and jaws, the limited postcranial fossils and comparisons to other brontotheres indicate that Embolotherium was one of the largest representatives of the group, and similar in size to the better known North American Megacerops. Embolotherium is estimated to have stood about tall at the shoulder, and to have reached a body mass in excess of three tonnes. Embolotherium lived in a subtropical and humid environment that was undergoing aridification and gradually changing from dense forest to open steppe, though it had in Embolotherium's time not yet transitioned into forest steppe.
Research history
Discovery and initial descriptions
Embolotherium fossils were first discovered in 1925 in the Ulan Gochu Formation of Inner Mongolia, China by an expedition from the American Museum of Natural History, led by Walter W. Granger. Additional and better-preserved fossils were found in 1928 on the museum's Fifth Central Asiatic Expedition, led by Granger and Roy Chapman Andrews. Cranial elements from at least fourteen Embolotherium individuals were recovered by the expeditions, including several nearly complete skulls. In 1929, Henry Fairfield Osborn described the new genus Embolotherium based on these skulls. The generic name Embolotherium references the unique bony protuberance on the front of the animal's skull, and was derived from the Greek word 'έμβολή (sic)' (embolḗ), glossed by Osborn as "battering ram", and the common prehistoric mammal suffix -therium (from the Greek θηρίον, theríon, meaning "beast"). Osborn easily distinguished Embolotherium from the other brontotheres known at the time by its unique protuberance, which Osborn referred to as the "ram" and described as a "completely novel form".
Osborn named three species of Embolotherium based on the skulls. The type species E. andrewsi was based on the skull AMNH 26001 and named after Andrews, E. grangeri on the skull AMNH 26002 and named after Granger, and E. loucksii on the skull AMNH 26010 and named after Harold Loucks, who had discovered this skull. The main feature distinguishing the three species was the shape and position of the ram. In E. andrewsi, the ram rises up directly above the orbits (eye sockets) and curves slightly backwards, and in E. grangeri the ram rises from the middle portion of the skull, slightly behind the orbits, and curves forwards and downwards. E. loucksii was distinguished by the ram rising forward and upwards, without curving. Both E. grangeri and E. loucksii were noted to have an elongated premaxillary rostrum (the part of the jaw extending below the nasal incision), which Osborn did not observe in E. andrewsi.
In 1943, Granger and William King Gregory revised the brontotheres known from Mongolia and Inner Mongolia. Granger and Gregory amended the diagnoses of the genus Embolotherium and the diagnoses of the species described by Osborn, changed the species designation of some of the fossils, and referred several new fossils to E. andrewsi, including skulls and lower jaws, all from the Ulan Gochu Formation. Granger and Gregory also described the new species E. ultimum, based on the partial skull AMNH 21604, found in the slightly later Baron Sog Formation. Granger and Gregory distinguished their new species from the others mainly by its larger size, the relatively great size of the third molar, and the greatly expanded zygomatic arches. They also described a new genus of large brontothere from Inner Mongolia, Titanodectes, with two species, based on lower jaws. Granger and Gregory noted that the species T. ingens could represent lower jaws of E. grangeri but that actual comparison was impossible since T. ingens and E. grangeri had no known overlapping fossil material.
Osborn had believed that the ram of Embolotherium was a completely unique anatomical structure. Granger and Gregory instead demonstrated that the ram was a highly derived form of the horns seen in other horned brontotheres.
Further discoveries
Granger and Gregory's 1943 revision remained the standard work on Asian brontotheres for much of the 20th century. Additional paleontological work was carried out on the brontotheres of Mongolia and Inner Mongolia by Soviet and Chinese paleontologists, but these studies were done without new comparisons to the large collection of Asian fossils in American museums. This was problematic, since the American Museum of Natural History still holds the majority of brontothere holotypes from the region. Instead, Soviet and Chinese studies largely followed on from Granger and Gregory's assessments, sometimes faulty, which hindered the understanding of the true diversity and evolution of Asian brontotheres.
Adding to E. andrewsi, E. grangeri, E. loucksii, and E. ultimum, two further species of Embolotherium were described in the latter half of the 20th century. In 1975, Demberelyin Dashzeveg briefly described the new species E. ergilense, based on a skull from the Ergilin Dzo Formation (National Museum of Mongolia, item 10). The E. ergilense skull does not preserve the premaxillae, or most of the maxillary dentition, but appears to be larger and more robust than other Embolotherium fossils. Also in 1975, Qi Tao described a new skull of E. grangeri (IVPP V.3015) from Ningxia, extending the known range of Embolotherium further south.
In 1980, Nataliia Mikhailovna Yanovskaya described the new species E. insigne in the monograph Brontoterii Mongolii ("The Brontotheres of Mongolia"). This species was based on the partial skull PIN 3110-52, from Khoer Dzan in the Ergilin Dzo Formation, and was distinguished by the ram being shorter and appearing more straight than those of the other species, similar to E. loucksii. Yanovskaya was also the first to refer limited postcranial fossils to Embolotherium. In addition to these fossils, the Paleontological Institute, Russian Academy of Sciences (PIN) also holds a large and largely undescribed collection of E. grangeri fossils, including skulls from several individuals of different ontogenetic stages.
Revisions
In a major 2008 monograph on brontotheres. Matthew C. Mihlbachler only recognized two valid species of Embolotherium: E. andrewsi and E. grangeri. E. ultimum and E. ergilense were designated as junior synonyms of E. andrewsi. Mihlbachler noted that E. ultimum had been distinguished mainly by its large size, that the supposedly relatively larger third molar had its apparent size exaggerated by several cracks in the tooth, and that the skull otherwise shared several features with E. andrewsi. E. ergilense was likewise determined to be consistent with skulls of E. andrewsi, though larger and more robust than most other fossils. E. loucksii and E. insigne were designated as junior synonyms of E. grangeri, with Mihlbachler noting that both of their type skulls appeared to be from subadult individuals and that the ram curving downwards may have been a ontogenetic feature. Additionally, Mihlbachler pointed out that the ram of AMNH 26010 (type of E. loucksii) was in large part reconstructed with plaster, and that its present appearance was therefore not reliable.
Mihlbachler also designated Titanodectes as a junior synonym of Embolotherium, with both Titanodectes species (T. ingens and T. minor) designated as junior synonyms of E. grangeri. Mihlbachler pointed out several errors in Granger and Gregory's interpretations of the dentition of Embolotherium and Titanodectes, which removed the main distinguishing features between the two genera. Comparing the Titanodectes jaws to those of Embolotherium, including a juvenile jaw of E. grangeri, Mihlbachler proposed that they were merely the lower jaws of adult E. grangeri.
In 2018, Bin Bai and colleagues agreed with Mihlbachler's assessment that only E. andrewsi and E. grangeri were valid species of Embolotherium, but rejected synonymization with Titanodectes, which they treated as a valid genus. Bai and colleagues noted that no reliable lower dentition was known from adult E. grangeri, which makes actual comparison to the Titanodectes fossils impossible, and that Titanodectes and Embolotherium are known from different localities, and different horizons. Later papers by Bai and colleagues in 2020 and 2025 have maintained Titanodectes as a valid genus. In a 2022 review of the fauna of the Ergilin Dzo Formation, Takehisa Tsubamoto, Khisigjav Tsogtbaatar, Tsogtbaatar Chinzorig, and Naoko Egi agreed with Mihlbachler's assessment, and treated only E. andrewsi and E. grangeri as valid, with both species of Titanodectes considered synonyms of E. grangeri.
Description
Embolotherium was a very large brontothere, chiefly distinguished by its unique skull. Based on comparisons with more completely known brontotheres, Embolotherium was likely one of the largest brontotheres, alongside the North American genus Megacerops. Embolotherium is estimated to have reached a size similar to Megacerops, likely standing about tall at the shoulder. In 2023, Oscar Sanisidro, Mihlbachler, and Juan L. Cantalapiedra estimated the body mass of E. andrewsi at and that of E. grangeri at . Supplemental Material (Data S1, Sheet 6)
Skull
Embolotherium had a saddle-shaped skull. Due to the different positions of their rams, the skull of E. andrewsi was more deeply saddle-shaped than that of E. grangeri. Both species had robust zygomatic arches, with massive swellings. The zygomatic arches of E. andrewsi were weakly curved, whereas those of E. grangeri were strongly bowed laterally (sideways). Unlike most horned brontotheres (and most perissodactyls overall), Embolotherium completely lacked a horizontal nasal process. Instead, the horns, nasal bones, and frontal bones of Embolotherium's ancestors had developed into the large ram. The ram of E. andrewsi was more vertical than that of E. grangeri, and positioned above the orbits (eye sockets). The ram of E. grangeri was positioned behind the orbits and curved forwards, at about a 45° angle. The premaxillomaxillary rostrum of E. andrewsi was small and short, thickening slightly posteriorly (towards the back). The premaxillomaxillary rostrum of E. grangeri was much larger than that of E. grangeri, and deepened slightly proximally (towards its center and the rest of the skull).