Tinoceras

''Uintatherium'' ("Beast of the Uinta Mountains") is an extinct genus of herbivorous dinoceratan 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 brontothere. 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 ungulates 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 xenungulates, 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, ''Uintatherium'''s 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 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 proboscideans (elephants and relatives) and rhinocerotids.

''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 redwood 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 environment mixed with a semi-arid steppe.

Taxonomy

Early history

Fossils of ''Uintatherium'' were first discovered in the Bridger Basin near Fort Bridger by Lieutenant W. N. Wann in September 1870, and were later described as a new species of ''Titanotherium'', ''Titanotherium anceps'', by Othniel Charles Marsh in 1871. The specimen (YPM 11030) only consisted of several skull pieces, including the right parietal horn, and fragmentary postcrania. The following year, Marsh and Joseph Leidy collected in the Eocene Beds near Fort Bridger while Edward Drinker Cope, Marsh's competitor, excavated in the Washakie Basin. In August 1872, Leidy named ''Uintatherium robustum'' based on a posterior skull and partial mandibles (ANSP 12607). Another specimen discovered by Leidy's crews consisting of a canine was named ''Uintamastix atrox'' and was thought to have been a saber-toothed and carnivorous.

Eighteen days after the description of ''Uintatherium'', Cope and Marsh both named new genera of Uinta dinoceratans, Cope naming ''Loxolophodon'' in his "garbled" telegram and Marsh dubbed ''Tinoceras''. Due to ''Uintatherium'' being named first, Cope and Marsh's genera are synonymous with ''Uintatherium''. Cope described two genera in his telegram, ''Loxolophodon'' and ''Eobasileus''; the latter is currently considered separate from ''Uintatherium''. ''Tinoceras'' was a new genus made for ''Titanotherium anceps'' by Marsh. Several days later, Marsh erected the genus ''Dinoceras''. ''Dinoceras'' and ''Tinoceras'' would receive several additional species by Marsh throughout the 1870s and 1880s, many based on fragmentary material. Several complete skulls were found by Cope and Marsh crews, leading to theories like Cope's proboscidean assessment. 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, Hunterian Professor of Comparative Anatomy, 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 A cast of a ''Uintatherium'' skeleton is on display at the Utah Field House of Natural History State Park. A skeleton of ''Uintatherium'' is also on display at the Smithsonian National Museum of Natural History in Washington, DC.

Fossils assigned tentatively 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 canines) closely resembling those of the genus from Xintai, Shandong, China. In 1977, Gabounia reported fossils possibly referrable 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 of 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 Province, China), recovered an almost intact skull. Aside from damage to the nasal bone and zygomatic arches, it was essentially complete. The latter two 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''.

Classification

''Uintatherium'' was initially regarded by Marsh as a brontothere. However, similarities to proboscideans (relatives of elephants), noted by various authors,'''' lead Cope to classify it as a member of that group. While he acknowledged Marsh's reasoning, he nonetheless believed that it stemmed from "unusual sources", and that: "The absence of incisor teeth no more relates these animals to the Artiodactyla than it relates the sloth to the same order ..the presence of paired horns no more constitutes affinity to the ruminants than it does in the case of the ' horned-toad'." It has since been recognised that similarities to proboscideans are likely the product of convergent evolution. ''Uintatherium'' was reclassified by Henry Fairfield Osborn in 1881 as part of the order Dinocerata. At the time, dinocerates were believed to be part of Amblypoda, a group uniting an assortment of basal ungulates from the Palaeogene, and were sometimes referred to simply as "dinoceratous amblypods".

The group Amblypoda has since fallen out of use, and is generally regarded as polyphyletic, 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 xenungulates, have been suggested, 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. However, it has been stated that no strong evidence for this relationship exists, and that similarities may simply be the result of convergence. Prothero, Manning, and Fischer, in 1988, suggested that dinoceratans and pyrotheres were part of Paenungulata (now consisting solely of hyracoid and tethythere afrotheres), which by their definition also included perissodactyls. Bruce J. Shockey and Federico Anaya Daza, in 2003, rejected the use of the term Uintatheriamorpha, considering the supporting data too weak. Regardless, a phylogenetic analysis published in 2019 by Thomas Halliday et al. recovered ''Uintatherium'' (the only dinoceratan included in the dataset) within a clade consisting entirely of SANUs, as the most basal branch of a clade otherwise consisting of ''Astraponotus'', ''Carodnia'', ''Parastrapotherium'', and ''Pyrotherium

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