2.95" Fossil Squirrel-Like Mammal (Ischyromys) Skull - Nebraska

This is a 2.95" long skull of Ischyromys sp., a squirrel-like mammal. It was collected from the the Brule Formation in Nebraska and would be approximately 30-34 million years old. The jaw is about 85% restored, though some of the teeth on the right side of the jaw are natural. Both zygomatic arches are restored and have been stabilized with a rock-like material. Most of the upper skull is natural, along with a left incisor still in place. The restoration here is primarily in the form of gap fill where fragments of the bone weren't recovered.

This 60 cm (2 ft) long creature is one of the oldest rodents known. It resembled a mouse and already had characteristic rodent incisors. Ischyromys's hind legs were longer than the forelegs, which could be used for other means than walking. Unlike most other mammals of its time, Ischyromys was probably arboreal (lived in trees).