Eubrontotherium () is an extinct genus of horned brontothere that lived in North America and East Asia during the Late Eocene. The genus contains a single species, E. clarnoensis, known from fossils found in the Clarno Formation of Oregon and in the Ergilin Dzo Formation of Mongolia. Eubrontotherium fossils in Mongolia are younger in age than those in the United States; it is unclear if this has paleobiogeographical implications since relatively few fossils of the genus are known.
Although it was not the largest animal in its environment on either continent, Eubrontotherium was a large and heavy brontothere, estimated to have weighed over two tonnes. Eubrontotherium had two short horns in front of its eyes, relatively smaller than the horns of its close relatives.
Research history
North America
Hancock Mammal Quarry fossils
North American fossils of Eubrontotherium come from the Hancock Mammal Quarry in the John Day Fossil Beds National Monument, Oregon. Geologically, these fossils are from the Clarno Formation. The quarry was first discovered and excavated by the amateur geologist Lon Hancock in 1956, who had discovered fossil teeth of Hyrachyus at the site. Decades of excavations in the quarry have revealed a diverse Eocene fauna, including several large mammals.
Among the fossils found in the Hancock Mammal Quarry were well-preserved fossils of a large horned brontothere, including two near-complete skulls (UCMP 126100 and UCMP 126101), a complete mandible (UCMP 126102), and several other cranial fossils. It took a long time before these fossils were scientifically described in detail and securely taxonomically identified. In the meantime, the brontothere taxon found in the quarry was informally referred to as the "Clarno brontothere" and was often identified as belonging to the genus Protitanops, although no formal identification as such was published for some time.
The Clarno brontothere was first mentioned in the scientific literature by Bryn J. Mader in 1989. Mader doubted that it belonged to Protitanops but did not explain his reasons for why it could not be referred to that genus. In 1996, C. Bruce Hanson designated the Clarno brontothere as Protitanops sp., citing "only minor differences" when compared to the Protitanops type specimen. Hanson did describe the fossils in detail, noting that a description was "in manuscript for publication elsewhere". No study on the brontothere was published by Hanson. In 2004, Spencer G. Lucas, Scott E. Foss, and Matthew C. Mihlbachler also concluded that the Clarno brontothere was Protitanops, preliminarily referring the fossils to the Protitanops type species, P. curryi. Lucas, Foss, and Mihlbachler noted that a detailed description of the fossils was still needed but identified only two features separating the P. curryi and the Clarno brontothere: the size of the horns and the position of the posterior nares, features that have been found to vary within other brontothere species.
In 2007, Mihlbachler published the first detailed description of the Clarno brontothere fossils and determined that they belonged to a new genus and species, which he named Eubrontotherium clarnoensis. The genus name Eubrontotherium was chosen to allude to the informal term "eubrontotheres", sometimes used for the horned brontothere clade to which Eubrontotherium belongs (Brontotheriita). Etymologically, Eubrontotherium means "true thunder beast", from the Latin prefix eu- ("true") and the Greek βροντή ("thunder") and θηρίον ("beast"). The species name clarnoensis derives from the Clarno Formation. The skull UCMP 126100 was chosen as the type specimen.
The naming of the new genus and species was justified through unique features in the Clarno fossils that had previously gone unnoticed. None of the E. clarnoensis skulls have large swellings on their zygomatic arches, a feature seen in Protitanops and several other horned brontotheres. The degree of swelling in other brontotheres may be variable within species, possibly due to sexual dimorphism. In E. clarnoensis, these swellings are absent in both robust and gracile specimens. The only other known "eubrontothere" to lack these swellings are Parabrontops and Parvicornus. Eubrontotherium differs from Parabrontops in having two (and not three) pairs of upper incisors. Eubrontotherium differs from Parvicornus in that its row of incisors is straighter and placed between the canines, whereas that of Parvicornus is arched towards the canines.
Possible additional fossils No securely identified Eubrontotherium fossils are known from North America other than the Hancock Mammal Quarry fossils. It is possible that the range of E. clarnoensis also reached California; a fossil palate (LACM/CIT 2143) from the Sespe Formation may belong to either E. clarnoensis or Protitanops curryi, though it does not preserve enough features to distinguish between the two. This palate was originally described as a specimen of Teleodus (Duchesneodus) by Chester Stock in 1938, and was classified as Duchesneodus uintensis by Spencer G. Lucas and Robert M. Schoch in 1989. In 2008, Mihlbachler noted that the presence of a diastema (gap between teeth) between the canines and premolars precludes the palate from being Duchesneodus.
Asia In 2008, Mihlbachler identified several brontothere fossils from the Ergilin Dzo Formation of Mongolia as conspecific with E. clarnoensis. These fossils included a flattened skull (PIN 3109-39) and a partial skeleton, including a partial skull and mandible (PIN 3109, with specimen numbers 90–104, 106, 111–116, and 142–232). These fossils were originally described in 1980 by Nataliia Mikhailovna Yanovskaya, who referred them to Metatitan relictus (PIN 3109-90 and 91) and Parabrontops gobiensis (PIN 3109-39). Mihlbachler noted that several cranial and dental features of these fossils precluded Yanovskaya's classifications, notably that all the skulls had two pairs of upper incisors, differing from the three pairs in both Metatitan and Parabrontops.
All of the aforementioned PIN (Paleontological Institute of the Russian Academy of Sciences) fossils were instead found to be consistent with E. clarnoensis, especially in that they shared E. clarnoensis's unique combination of two pairs of upper incisors and the lack of swellings on the zygomatic arches. Mihlbachler thus referred the Ergilin Dzo fossils to E. clarnoensis. Small differences between the Mongolian and North American fossils, such as the horns of the E. clarnoensis type skull appearing to be lower than those of the Mongolian skulls, were attributed to taphonomic effects, in this case due to the North American skull being slightly crushed.
Description
Eubrontotherium was a large brontothere. The type skull UCMP 126100 measures long and wide. The second skull referred to the species, UCMP 126101, is wide. In 2023, Oscar Sanisidro, Mihlbachler, and Juan L. Cantalapiedra estimated E. clarnoensis to have weighed . Supplemental Material (Data S1, Sheet 6)
Skull
Eubrontotherium had a shallow, saddle-shaped skull. The zygomatic arches varied from straight and thin to slightly curved and relatively thick. Regardless of the robustness of the zygomatic arches, no Eubrontotherium skull has any conspicuous swellings on the zygomatic arches, an otherwise common trait in large brontotheres. The nasal process of Eubrontotherium was broad and angled slightly downwards, with a strongly rounded end. The premaxillomaxillary rostrum (the part of the jaw extending below the nasal incision) is thinned towards its front end and curved slightly upwards. Depending on the specimen, the nasal process and the premaxillomaxillary rostrum were either about equally long, or the nasal process was slightly shorter.
Eubrontotherium had a pair of small horns, rising up from the skull slightly in front of the orbits. The horns were slightly elliptical in cross-section at their bases, but more rounded at their ends, and pointed slightly dorsolaterally (upwards and outwards) or anterodorsolaterally (upwards, outwards, and forwards), depending on the specimen. In the known fossils, the horns do not appear to be highly elevated on the skull, though this may in part be due to taphonomic crushing of the skulls. Even accounting for crushing, the horns or the nasal process were not as highly elevated as in some other genera, such as Diplacodon. Horn size varied slightly between individuals, but was similar to the range seen in Duchesneodus. The horns were smaller and less massive than the horns of genera such as Dianotitan, Parabrontops, and Protitanops.
The nasal incision of Eubrontotherium was relatively shallow, extending back to the anterior margin (frontmost part) of the fourth upper premolar. The anterior rim of the orbit (eye socket) was above the first or the second upper molar. The sides of the skull had prominent parasagittal ridges (ridges running lengthwise along the skull), which somewhat constricted the skull's upper surface towards the back. The occiput is not preserved in the type skull (UCMP 126100), and is only partially preserved in UCMP 126101 and PIN 3109-90. The nuchal crest (the posterior edge of the skull) was relatively thick, similar to robust specimens of Duchesneodus and Dianotitan, and thicker than in Protitanops.
Dentition E. clarnoensis had the dental formula . Eubrontotherium had two pairs of small and globular upper incisors, positioned in a straight row. The reduced number of upper incisors is a trait seen only in certain horned brontotheres, such as Duchesneodus and Megacerops, and distinguishes Eubrontotherium from some more basal genera, such as Parabrontops. Some E. clarnoensis skulls have a short diastema between the upper incisors and canines, while others lack this diastema. Like the incisors, the upper canines were relatively small, similar to their small size in Protitanops and Notiotitanops. There was a long diastema between the canines and upper premolars, about as long as the length of the second upper premolar. The first upper premolar had a complex morphology, and its morphology varies between specimens, a trait seen in several brontothere species. The variability seen in the premolars of E. clarnoensis means that its premolars are not clearly differentiable from those of other brontotheres. The second to fourth upper premolars were almost rectangular in shape and had distinct hypocones (one of the major cusps). The upper molars were typical of derived brontotheres, though have been noted to have rather tall ectolophs (one of the ridges across the tooth), with labial (towards the lips) walls angled towards the tongue.
There were three pairs of lower incisors, lined up in a nearly straight row between the lower canines. The second pair of lower incisors were larger than the first and third pair. Many of the lower incisors had small gaps between them. Like in the upper dentition, there was no diastema between the lower incisors and canines. Also similar to the upper dentition, there was a diastema between the lower canines and premolars slightly longer than the length of the second lower premolar. Of the premolars, only the fourth lower premolar had a metaconid (one of the cusps). The trigonid (shearing part) of the second lower premolar was slightly longer than the tooth's talonid (crushing part). The lower molars were typical of advanced brontotheres and not notably different from those of close relatives of Eubrontotherium. The lower molars had shallow basins, and the third lower molar was noticeably slender.
Classification