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Uintatherium (from Uinta Mountains, and Ancient Greek θηρίον ( thēríon), meaning "beast") is an extinct genus of herbivorous mammal that lived during the Eocene epoch. Two species are currently recognized: U. anceps from the United States during the Early to Middle Eocene (50.5-39.7 million years ago) and U. insperatus of Middle to Late Eocene (48–37 million years ago) China. The first fossils of Uintatherium were recovered in the Fort Bridger Basin, and were initially believed to belong to a new species of Brontotheriidae. Despite other generic names being assigned, such as Edward Drinker Cope's Loxolophodon and Othniel Charles Marsh's Tinoceras, and an assortment of attempts at naming new species, Uintatherium anceps has since come to encompass all of these.
The phylogeny of Uintatherium and other dinoceratans has long been debated. Originally, they were assigned to the now-invalid order Amblypoda, which united various basal from the Palaeogene. Ambylpoda has since fallen out of use. Since then, various hypotheses of dinoceratan phylogeny have been proposed. The most widespread is that they are related to the South American Xenungulata, together forming a mirorder called Uintatheriamorpha. If this is correct, dinoceratans, and thus Uintatherium, may not be ungulates at all. However, it has been noted that traits shared between the two groups may be the result of convergent evolution. Within Dinocerata itself, Uintatherium belongs to the family Uintatheriidae, and is one of two members of Uintatheriinae; the other two are Eobasileus and Tetheopsis.
Uintatherium was a very large animal, with U. anceps having a shoulder height of and a body mass of . The largest Uintatherium skulls known, originally assigned to Loxolophodon, measure in length. It is overall similar to the other two uintatheriine genera, though it had a broader skull. Like them, Uintatheriums skull bears a series of bony, skin-covered protrusions: one pair on the tip of the snout, one pair above the gap between the Canine tooth and cheek teeth, and one pair toward the back of the skull. Eobasileus skull was quite similar, though the middle pair of protrusions sat further back, directly above the cheek teeth. The canines of Uintatherium were very large, and were supported by a pair of bony flanges extending from the lower jaw. They were likely sexually dimorphic, and may have been used in display or for defense. Behind the skull, the skeleton of Uintatherium bears a combination of characteristics often associated with (elephants and relatives) and Rhinoceros.
Uintatherium evolved during the Paleocene-Eocene thermal maximum, a period which saw some of the highest global temperatures in Earth's history. Most of the North American continent was covered in closed-canopy forests, with the Bridger Formation, one of the localities U. anceps is best known from, consisting of an inland lake surrounded by birch, elm and Sequoioideae trees. The depositional environment of the later Uinta Formation was interspersed by open savannahs, resulting from a global cooling event which resulted in the gradual aridification of North America. The Chinese U. insperatus lived in a Brackish marsh environment mixed with a semi-arid steppe.
Eighteen days after the description of Uintatherium, Cope and Marsh both named new genera of Uinta Basin . Cope named Loxolophodon in a telegram, though it was "garbled" due to his handwriting, and was misrendered by an operator as "Lefalophodon". Alongside Loxolophodon, Cope also described another genus of dinoceratan Eobasileus; the latter is currently considered separate from Uintatherium. Marsh's taxon, meanwhile, was named Tinoceras, and was intended to include the original Titanotherium anceps specimen. Several days later, Marsh erected the genus Dinoceras. Dinoceras and Tinoceras would be given additional species by Marsh throughout the 1870s and 1880s, many based on fragmentary material. Several complete skulls were found by Cope and Marsh crews. Because of Cope and Marsh's rivalry, the two would often publish scathing criticisms of each other's work, stating their respective genera were valid. The trio would name 25 species now considered synonymous with Marsh's original species, Titanotherium anceps, which was placed in Leidy's genus, Uintatherium. In 1876, William Henry Flower wrote a letter in Nature wherein he formally suggested incorporating all of Cope's, Leidy's, and Marsh's taxa into Uintatherium, due to it being named first (which would make it a senior synonym), and a lack of convincing evidence for their separation.
Many additional discoveries of Uintatherium have since occurred, making it one of the best-known and popular American fossil mammals. Princeton University launched expeditions to the Eocene beds of Wyoming in the 1870s and 1880s, discovering several partial since skulls and naming several species of uintatheres that are now considered synonyms of U. anceps. Major reassessment came in the 1960s by Walter Wheeler, who synonymized and redescribed many of the Uintatherium fossils discovered during the 19th century]]Fossils compared to Uintatherium have been described from parts of Asia since 1962, when Zhou Mingzhen and Y. S. Zhou reported teeth (the third upper molar and two upper Canine tooth) closely resembling those of the genus from Xintai, Shandong, China. In 1977, Leo Gabounia reported fossils possibly assignable to Uintatherium had been recovered from Tschaibulak, near Zaisan, Kazakhstan. These were both referred to an indeterminate position within Uintatheriidae, not to Uintatherium itself, though the former was documented as cf. Uintatherium sp. In November 1978, the first unambiguous Asian specimen of Uintatherium was recovered. Wang Daning, Tong Shuisheng, and Wang Chuanqiao, working at strata from the lower part of the Lushi Formation (Henan, China), recovered a skull. Aside from damage to the nasal bone and , it was essentially complete. Tong Shuisheng and Wang Chuanqiao wrote that the skull likely belonged to an elderly individual due to the condition of the teeth, which were severely worn. In 1981, the specimen was described. It was assigned to a new species of Uintatherium, U. insperatus.
The group Amblypoda has since fallen out of use, and is generally regarded as Polyphyly, meaning that it was an unnatural group consisting of an assortment of distantly related clades. Dinocerata, however, has persisted, though the precise relationships of the order have been the subject of debate. Relationships with South American native ungulates (SANUs), specifically Xenungulata, have been suggested, due in part to perceived similarities to Carodnia, with Spencer G. Lucas and Robert M. Schoch in 1998 supporting the complete removal of both clades from Ungulata. If dinoceratans and xenungulates are indeed related, they may constitute the mirorder Uintatheriamorpha. Lucas and Schoch, in 1985, noted dental similarities between uintatheriamorphs and the "Anagaloidea" Pseudictopidae, which might in turn be related to modern Lagomorpha (Rabbit, Hare, and Pika), and would thus, as the same authors commented in 1998, be "... tantamount to identifying uintatheres as giant horned bunnies ...". It has since been asserted that no strong evidence for this relationship exists, and that similarities observed may simply be the result of convergence, in no small part because of how small and specialised anagalids are in relation. In 1997, Malcolm McKenna regarded Uintatheriamorpha as a synonym of Dinocerata, though did not elaborate.Malcolm C. McKenna (1997). 9780231110129, New York : Columbia University Press. . ISBN 9780231110129 Bruce J. Shockey and Federico Anaya Daza, in 2003, rejected the use of the term Uintatheriamorpha altogether, considering the supporting data too weak. A 2015 analysis by Benjamin R. Burger, presented to the Society of Vertebrate Palaeontology as a conference abstract, not only recovered a Monophyly Uintatheriamorpha (consisting of Carodnia + Dinocerata), but recovered them immediately basal to the artiodactyl/Perissodactyla split.
Donald Prothero and colleagues, in 1988, suggested that dinoceratans and Pyrotheria were part of Paenungulata (now consisting solely of Hyrax and Tethytheria afrotheres), which by their definition also included perissodactyls. Regardless, a phylogenetic analysis published in 2019 by Thomas Halliday and colleagues recovered Uintatherium (the only dinoceratan included in the dataset) as the most basal branch of a clade otherwise consisting of Astraponotus, Carodnia, Parastrapotherium, and Pyrotherium, thus placing it within the SANUs.
A cladogram showing the phylogenetic position of Uintatherium, after Halliday and colleagues (2019), is as follows: has historically been divided into two families, Prodinoceratidae and Uintatheriidae, though some authors use only one family. Assuming two families exist, Uintatheriidae consists of the majority of dinoceratans, and has itself been divided into Gobiatheriinae and Uintatheriinae; occasionally, the latter has been divided even further, down to tribe level (Bathyopsini and Uintatheriini). The most basal uintatheriid was Bathyopsis. Walter H. Wheeler suggested in 1961 that the taxa now classed as uintatheriines formed a primarily Anagenesis lineage (one in which taxa descended from another without diverging), and that Uintatherium was one of few divergent genera, possibly evolving from Bathyopsis (which he believed to be ancestral to both Uintatherium and later dinocerates). Robert M. Schoch and Spencer G. Lucas, in 1985 (and later in 1998), offered a Cladistics hypothesis of Dinocerata, in which Uintatherium is the Sister group to a clade consisting of Eobasileus and Tetheopsis, slightly more derived than Bathyopsis.
The following cladogram depicts the possible interrelationships of dinoceratans, based on a later work by Schoch and Lucas:William D. Turnbull suggested in 2002 that both Tetheopsis species could be lumped into Eobasileus, and that Uintatheriini might thus consist exclusively of Eobasileus and Uintatherium.
The of Uintatherium were very long, comprising roughly half of the total length of the skull. They projected far enough anteriorly (forwards) that they completely overhang the external nares. While an elephant-like trunk or proboscis was suggested early on, based on alleged affinities to proboscideans, the structure of the Nasal cavity and the structure of the suggest that no such appendage existed. In its place, there may have been a flexible upper lip, similar to that of modern Rhinoceros, and a long, muscular tongue. The were large, almost as wide as long, though considerably shorter than the nasals. Uintatherium had large , of which the maxilla comprised the anterior portion, similar to . Like other dinoceratans, the skull of Uintatherium lacked a postorbital process. At the back of Uintatheriums skull was a very large occipital crest, extending posteriorly (rearward) further than the occipital condyles. To either side of the occipital crest sat a pair of very large parasagittal crests, alternatively known as the posterior crests, or the parietal-occipital crests. The of Uintatherium, like its close relatives, were tightly fused, and they bore a distinct transverse ridge which provided structural support. The Occipital bone, overhung by a large occipital crest, was rectangular in outline (though was subject to individual variation), and bore deep concavities on its posterior surface where powerful neck muscles and would have attached.
Much like other dinoceratans, Uintatheriums skull was adorned with a series of well-developed outgrowths, sometimes called horns, three pairs in total. The first pair sits at the front of each nasal, and differs in form between specimens: in some, these protrusions are small and deflected upward and outward, while in others, they are larger and more horizontal. In U. insperatus, it is slightly longer and more triangular, and the portion of nasal anterior to it is slightly longer. The second pair, above the , sit directly above the diastema (gap) separating the Canine tooth and . The last pair, the so-called parietal horns, sit far anterior to (in front of) the occipital bone, on the parasagittal crests. This differs from the related Eobasileus and Tetheopsis, in which the parietal horns are closer to the occipital. Furthermore, in those two genera, the maxillary set of horns sits above the , meaning the portion of the snout anterior to the maxillary horns is fairly long, whereas in Uintatherium, the portion of the snout anterior to the maxillary horns is fairly short. In U. anceps, the maxillary and parietal horns projected outward slightly, while in U. insperatus, they were essentially erect. Despite their description as horns, it is unlikely that any of these outgrowths were Keratin (reinforced by keratin), as there is no evidence of the Vascularisation necessary for a keratinous covering. It is likely that they were covered only by skin. Nevertheless, damage to several Uintatherium horn cores, likely inflicted while the animals were still alive, suggests that they used their horns in agonistic behaviors. Like other animals with extensive cranial ornamentation, Uintatheriums skull was lightened by well-developed sinuses, though not to the same extent. Projecting from the anteroventral (towards the front and at the bottom) portion of Uintatheriums (lower jaws) are a pair of large . In most specimens, these would have provided support to the large upper canines, though specimens formerly referred to Loxolophodon had smaller flanges which did not extend as far. It has been suggested that the observed difference in flange size is the result of sexual dimorphism, with larger-flanged jaws belonging to males. Similar structures are observed in the related Bathyopsis. The mandibles of Uintatherium are otherwise fairly slender. Unlike most other ungulates, the condyles of the jaw joint are deflected posteriorly, likely to accommodate the large upper tusks; without such a modification, the jaws would be unable to fully open. This condition is otherwise only seen in some and members of the former order Insectivora. The mandible's coronoid process is large, curves posteriorly, and is pointed dorsally (at the top). The mandibular condyles are small and convex, and sit slightly above the level of the cheek teeth. Below the condyles, the posterior face of the mandible roughens, due to the attachment of the pterygoid muscles.
When Uintatherium first appeared in North America, most of the continent was covered primarily closed-canopy forests.This environment is exemplified by the Bridger Formation, which consisted of inland lakes surrounded by dense forests. This is inferred by the abundance of plant fossils and the presence of a great diversity of primate fossils, which are predominantly arboreal. Fossils of Sequoioideae, , and Betulaceae are known from throughout North America during this period, suggesting that the amount of precipitation did not vary considerably across latitudes. Most of North America was likely covered by and temperate rainforests. Even organisms more typically adapted to low-latitude environments, such as and have fossils preserved as far North as Alaska and Ellesmere Island, exemplifying the extreme climatic conditions of the early and middle Eocene.
By the time of the Uinta Formation, the landscape had changed considerably. The large lakes emblematic of the earlier Eocene had shrunk, and the majority of deposition was the product of low-volume streams. Insectivory and frugivory mammals (especially primates) declined in diversity alongside a rise of folivory , which is interpreted as reflecting an increase in more open habitats resulting in a gradual decline in tree cover. Considerable forests existed, likely alongside the numerous waterways, but these were probably interspersed by open savannah environments. This trend towards aridifcation was facilitated by a general decline in the amount of precipitation in North America while average annual temperatures remained high. It would not be until the later parts of the Eocene that the global cooling began to affect North American ecosystems, by which point, Uintatherium was already extinct.
U. inseparatus appeared in Asia during the middle part of the Eocene. Its fossils are known from the Lushi Basin in China, which consisted of large, deep lakes that preserve fossils of and . These lakes were surrounded by forests and swamps and were interspersed by semi-arid steppe. Variations in sea-levels and intermittent flooding at the time also produced brackish lakes and swamps. The inland lakes varied in size over the course of the middle Eocene before eventually disappearing completely and being replaced by rivers and .
In the Bridger Formation, U. anceps coexisted with a variety of primitive including Helohyidae, Homacodontidae, Brontotheriidae, Amynodontidae, and . The environment was also host to some of the ancestors of modern perissodactyl groups including Hyrachyus (a primitive relative of rhinos), Helaletes (an early relative of tapirs), and several species of Orohippus (a primitive horse). North America at the time also had a diverse assemblage of early primates including Microsyops, Notharctus, Smilodectes, and the members of Omomyidae (relatives of modern ). Mammalian predators of the region included like Mesonyx and Harpagolestes, like Limnocyon and Sinopa, like Patriofelis and Machaeroides, and early Carnivoramorpha like Miacis and Vulpavus. A variety of more enigmatic mammal forms were also present including members of Tillodontia, Stylinodontidae, and Pantolestidae and the small insectivorous Apatemys and Metacheiromys. Primitive Sciuromorpha rodents, , and coexisted with the Herpetotherium and Peradectes.
Reptiles were also abundant in this environment. Fossils from turtles including trionychia, tortoises, terrapins, and lived alongside , , Teiidae, and as well as crocodilians like Boverisuchus and Borealosuchus. Remains of primitive owls and cranes have also been found.J. A. Wilson. 1986. Stratigraphic Occurrence and Correlation of Early Tertiary Vertebrate Faunas, Trans-Pecos Texas: Agua Fria-Green Valley Areas. Journal of Vertebrate Paleontology 6(4):350-373 In the transition from the Bridgerian to the Uintan, several of these animals became extinct and new forms emerged. The oxyaenids and phenacodontids disappeared during this transition and new groups like the Oromerycidae and the earliest (the Eomoropidae). This transition is followed by the appearance of several medium and large ungulate genera including Protylopus, Amynodontidae, and Eobasileus. This faunal subinterval is represented by the Devil's Graveyard Formation and has been argued to be a distinct land mammal sub-age (the "Shoshonian" or "UI1b biochronological zone"), although this is not universally accepted. This transition also saw a marked decline in primate diversity in North America, which would continue throughout the Eocene until primates eventually became extinct in North America.P. C. Murphey, T. S. Kelly, K. R. Chamberlain, K. Tsukui, and W. C. Clyde. 2018. Mammals from the earliest Uintan (middle Eocene) Turtle Bluff Member, Bridger Formation, southwestern Wyoming, USA, Part 3: Marsupialia and a reevaluation of the Bridgerian-Uintan North American Land Mammal Age transition. Palaeontologia Electronica 21.2.25A:1-52
The middle-Uintan land mammal age (sometimes called "UI2" biochronological zone) is the most recent interval from which fossils of U. anceps are known. This corresponds to the eponymous Uinta Formation. This interval saw the diversification of brontotheres, helohyids, and as well as the emergence of the first Protoceratidae, Agriochoerus, and camelidae. It also saw the extinction of North American and leptictids as well as most of the remaining North American primates, with only the omomyids remaining extant. Primitive carnivoramorphs like Miocyon also emerged. The end of this interval saw the final extinction of Uintatherium in North America alongside other long-lived genera such as Mesonyx and Hyrachyus.
The composition of Asian land mammal assemblages was similar in several ways to the contemporary assemblages in North America, although the precise timing of faunal turnover is not as well studied with respect to Eocene ecosystems in Asia. The carnivorous mammals of the continent were generally similar, with , Haplodectidae, , and the Miacis being the most abundant predators. However, several endemic carnivores coexisted with these including Eusmilus (an early nimravid), Cynodictis (a primitive amphicyonid), and the controversial carnivorous ungulate Andrewsarchus. Prey for these animals included a diverse array of terrestrial ungulates including late surviving members of Paleocene lineages such as the coryphodontidae Eudinoceras, dichobunidae, , and . Ungulate groups common in North America were also represented, including Hyrachyus as well as the Helohyidae, Brontotheriidae, Helaletidae, and amynodontidae. They were accompanied by a diverse array of perissodactyls, which underwent a radiation in Asia during the Middle Eocene. These new groups included the paraceratheriids, Hyracodontidae, Chalicotheriidae, and Deperetellidae. The artiodactyl anthracotheres also first evolved in Asia during this period.M. Chow, C.h. Li, and Y. Chang. 1973. Late Eocene mammalian faunas of Honan and Shansi with notes on some vertebrate fossils collected therefrom. Vertebrata PalAsiatica 11( 2):165-181
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