Uintatherium, (Beast of the Uinta Mountains) is an extinct genus of herbivorous mammal that lived during the Eocene epoch, which includes a single species currently recognized, U. anceps. They were similar to today's rhinoceros both in size and in shape, although they are not closely related. Uintatherium was a large browsing animal. Its most unusual feature was the skull, which is both large and strongly built, but simultaneously flat and concave: this feature is rare and not regularly characteristic of any other known mammal except in some brontotheres. This animal appeared during Cenozoic era, the most recent era of geologic time, from about 65 million years ago to the present. The Cenozoic Era is characterized by the formation of modern continents and the diversification of mammals and plants. Grasses also evolved during the Cenozoic. The climate was warm and tropical toward the beginning of the era and cooled significantly in the second half, leading to several ice ages. Humans first appeared near the end of this era.