Hyracodon sp. - fossil mammal skull from the Oligocene of Wyoming, USA. (UW 15566, University of Wyoming Geological Museum, Laramie, Wyoming, USA)

From museum signage: Hyracodon Leidy, 1856

Geologic Range: 39.5-29.0 million years ago (late Middle Eocene-early Late Oligocene)

Geographic Range: Widespread in North America (Wyoming, Nebraska, South Dakota, North Dakota, Colorado, Montana, Utah, Texas, Saskatchewan, Mexico).

Adult Size: Weight up to 758 pounds (344.0 kilograms).

Habitat and Diet: Terrestrial. Browsing herbivore.

Characteristics: This skull and lower jaw of Hyracodon (UW 15566) was collected from rocks of the White River Formation near Lusk, Wyoming. Specialists classify Hyracodon in the Family Hyracodontidae, which is grouped within the Rhinocerotoidea (rhinoceroses and close relatives differ from "true" rhinoceroses (Family Rhinocerotidae) in the structure of their legs and feet. The bones in the limbs of Hyracodon and its close relatives are elongated and slender, indicating that these rhinoceroses evolved for fast running, hence the informal name "running rhinoceroses". The bones in the limbs of "true" rhinoceroses are short and strongly built and the bones in the feet are compact. This shows that the limbs of "true" rhinoceroses evolved to support a heavy body and did not evolve for fast running. Fossils of the Hyracodontidae are also known in Asia. One Asian genus, Paraceratherium, is the largest known land mammal.

Classification: Animalia, Chordata, Vertebrata, Mammalia, Placentalia, Perissodactyla, Hyracodontidae

Locality: unrecorded/undisclosed site near the town of Lusk, eastern Wyoming, USA

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