Uintatherium

Uintatherium is an extinct genus of dinoceratan mammal that lived during the Eocene epoch. Two species are currently recognized: U. anceps is found in the United States and lived during the early to middle Eocene (50.5–37 million years ago), while U. insperatus is found in China and lived during the middle and late Eocene (48–37 million years ago). The first fossils of Uintatherium were recovered from the Fort Bridger Basin and were initially believed to belong to a new species of brontothere. Several previously recognized species, as well as genera such as Edward Drinker Cope's Loxolophodon and Othniel Charles Marsh's Tinoceras, are now assigned to Uintatherium anceps. The systematic position of Uintatherium and other dinoceratans has long been debated. Originally, they were assigned to the order Amblypoda, which united various early ungulates from the Paleogene. Amblypoda has since fallen out of use. The most widespread hypothesis is that dinoceratans are related to the South American xenungulates, together forming a mirorder called Uintatheriamorpha. If this is correct, dinoceratans, and thus Uintatherium, would not be ungulates at all. However, it is possible that the traits shared between the two groups evolved independently. 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 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. The skull bears a series of bony, skin-covered protrusions, sometimes referred to as horns: 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. The skull was similar to that of the other two uintatheriine genera, though broader, and in Eobasileus, the middle pair of protrusions sat further back, directly above the cheek teeth. The canines of Uintatherium were very large and supported by a pair of bony flanges that extended from the lower jaw. They were likely sexually dimorphic, and may have been used in display or for defense. 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 forests. The depositional environment of the later Uinta Formation also featured open savannahs that resulted from a global cooling and aridification trend in North America. The Chinese species, U. insperatus, lived in an environment of brackish marshes and semi-arid steppe.

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