Uintatherium

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 1.5 m (4 ft 11 in) and a body mass of 3,000–4,500 kg (6,600–9,900 lb). The largest Uintatherium skulls known, originally assigned to Loxolophodon, measure 91 cm (36 in) 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.