Uintatherium

Early history remains of Uintatherium anceps (originally Titanotherium anceps) In September 1870, a fragmentary skeleton (catalog number YPM 11030) of Uintatherium was unearthed by US army Lieutenant W. N. Wann in the Bridger Basin of Wyoming. These sediments come from the Eocene-aged Bridger Formation. The skeleton was sent to paleontologist Othniel Charles Marsh, who described it in 1871 as a new species of the brontothere Titanotherium, Titanotherium anceps. The name Uintatherium refers to the Uinta Mountains, which rise to the south of the Bridger Formation outcrops, while the Ancient Greek () means . Another specimen discovered by Leidy's crews consisting of a canine was named Uintamastix atrox and was thought to have been a saber-toothed carnivore. that Walter Hall Wheeler criticized as "garbled" due to Cope's handwriting; Alongside Loxolophodon, Cope also described another genus of dinoceratan, Eobasileus; the latter remains separate from Uintatherium. Several days later, Marsh erected the genus Dinoceras. Because of their rivalry, the two would often publish scathing criticisms of each other's work, asserting their respective genera as valid. They named a plethora of species in six genera, A major reassessment of the Uintatheriidae came almost a century later, in the form of a 1961 work by Walter Hall Wheeler. In this work, Wheeler redescribed many of the Uintatherium fossils discovered during the 19th century. In 1977, Leo Gabounia reported fossils possibly assignable to Uintatherium that had been recovered from Tschaibulak, near Zaisan, Kazakhstan. These were both considered as indeterminate uintatheriids, and have not been assigned to Uintatherium itself, though the Chinese form was labelled as cf. Uintatherium sp. and 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'." In 1881, Henry Fairfield Osborn assigned Uintatherium to the order Dinocerata. At the time, dinocerates were believed to be part of Amblypoda, a group uniting an assortment of basal (early-diverging) ungulates from the Paleogene, and were sometimes referred to as "dinoceratous amblypods". The group Amblypoda has since fallen out of use and is generally regarded as polyphyletic, meaning that it is 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, in part due to perceived similarities to Carodnia. due because of how small and specialized anagalids are and the unlikelihood of them evolving into something like dinoceratans. Bruce J. Shockey and Federico Anaya Daza, in 2003, rejected the use of the term Uintatheriamorpha, considering the supporting data too weak. In 1988, Donald Prothero and colleagues suggested that dinoceratans and pyrotheres were part of Paenungulata (now consisting solely of hyracoid and tethythere 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: Occasionally, Uintatheriinae is divided even further, down to tribe level (Bathyopsini and Uintatheriini). ==Description==

of U. anceps|left Uintatherium was a large, graviportal animal (one with adaptations for supporting a high body weight), with short, robust, and dense limb bones. Most U. anceps skulls range from in length, Uintatherium has large zygomatic arches, of which the maxilla comprises the anterior portion, similar to proboscideans. Like other dinoceratans, the skull of Uintatherium lacks a postorbital process. or the parietal-occipital crests. Othniel Charles Marsh stated that it "could apparently have been drawn through the neural canal of all the pre-sacral vertebrae [those preceding the sacrum]". The olfactory bulbs, the parts of the brain dedicated to processing scents, were mostly large, though their size was variable. Uintatheriids generally lack upper incisors, and the same is true of Uintatherium. George Gaylord Simpson suggested in 1941 that Uintatherium's canines might have been employed in aggressive downward stabs. The reduction or loss of the upper incisors, and the retention and growth of the upper canines, is a trend which has been observed throughout dinoceratans. of Uintatherium, from below (left) and left lateral view (right) |265x265px Between the canines and cheek teeth, there is a large gap, the diastema. The first upper premolar was lost, with only the occasional preservation of the alveolus (tooth socket); while in 2002, an average body length of , based on three mounted specimens, was provided by William D. Turnbull. Two of his Uintatherium estimates average , Using Jerison's methods and additional data provided by Damuth, a 2002 paper by Turnbull gave . He recovered larger masses in other analyses, but believed that these were overestimates due to the methodologies applied. Four years prior, Lucas and Shoch had provided even larger estimates of for U. anceps. The size of U. insperatus is not certain, though the holotype skull was smaller than U. anceps. ==Paleoecology==

Diet and lifestyle Like other uintatheriids, the cheek teeth of Uintatherium were bilophodont (two-ridged). Animals with this cheek tooth morphology are often browsers, However, in 2002, Turnbull suggested that it, and other late-stage dinoceratans, were more ecologically analogous to hippopotamuses, citing traits such as pachyostosis, short legs, and a barrel-shaped ribcage as supporting evidence. As C4 grasses, on which hippopotamuses often feed, became widespread only fairly recently, and dinoceratan teeth were not suited for grazing, he noted that they likely fed quite differently to hippopotamuses. Whereas most modern ungulates ferment plant matter in their foregut, Turnbull suggested based on pelvic anatomy that Uintatherium was instead a hindgut fermenter, similar to proboscideans and some perissodactyls. He further proposed that late-stage dinoceratans had digestive systems similar to those of sea cows. If this model is accurate, the processing of food would have occurred primarily in the hindgut, reducing demands on the cheek teeth and resulting in the "inadequate appearance" observed by Wood. Paleoenvironment Uintatherium evolved during a period in Earth's climatic history called the Paleocene-Eocene thermal maximum. This period saw some of the highest average temperatures in Earth's history with temperatures in Colorado (where Uintatherium fossils have been found) reaching an annual average of —much higher than today where the mean annual temperature in Colorado is only around . Although global average temperatures declined throughout the Eocene, the average temperatures in North America remained relatively constant during the first half of the period, and only cooled slightly towards the end of the Eocene. Even though temperatures remained relatively constant, the uplifting of the Rocky Mountains and their associated volcanism led to considerable drying in the North American interior. The arid scrublands which characterize the western United States today (as exemplified by Arizona, Nevada, and New Mexico) began to emerge during this period. This transition, at least directly around the Rocky Mountains, appears to have begun around 42 million years ago. When Uintatherium first appeared in North America, most of the continent was covered in 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. Even organisms more typically adapted to low-latitude environments, such as palm trees and crocodylians, are found 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. Insectivorous and frugivorous mammals (especially primates) declined in diversity alongside a rise of folivorous artiodactyls, which is interpreted as reflecting an increase in more open habitats resulting from 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 aridification was caused by a general decrease in 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. Contemporary fauna North America Uintatherium anceps is known from the Bridger, In the Bridger Formation, U. anceps coexisted with a variety of primitive ungulates including helohyids, homacodontids, brontotheriids, amynodontids, and hyopsodontids. 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 tarsiers). Mammalian predators of the region included mesonychids like Mesonyx and Harpagolestes, hyaenodontids like Limnocyon and Sinopa, oxyaenids like Patriofelis and Machaeroides, and early carnivoran-relatives 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 sciuromorph rodents, leptictids, and eulypotyphlans coexisted with the metatherians Herpetotherium and Peradectes. 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 oromerycids and the earliest chalicotheres (the eomoropids). This transition is followed by the appearance of several medium and large ungulate genera including Protylopus, Amynodon, 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. 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 rhinocerotoids as well as the emergence of the first protoceratids, agriochoerids, and camelids. It also saw the extinction of North American cimolestans 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. Asia The second species of Uintatherium, U. insperatus, lived in the Lushi Formation what is now Henan, China during the Middle Eocene. This corresponds to the Sharamurunian Asian land mammal age, which lasted for about the same length of time. Remains assigned to U. cf. insperatus have also been found in the similarly-aged Uqbulak Formation in the Junggar Basin. 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 mesonychids, haplodectids, hyaenodontids, and the carnivoramorphan 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 coryphodont Eudinoceras, dichobunids, tillodontians, and taeniodontans. Ungulate groups common in North America were also represented, including Hyrachyus as well as the helohyids, brontotheriids, helaletiids, and amynodontids. 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, hyracodontids, chalicotheriids, and deperetellids. The artiodactyl anthracotheres also first evolved in Asia during this period. == Notes ==