Ischyromys typus is one of the oldest known rodents. It lived during the Oligocene (32-30 million years ago). It had the characteristic rodent incisors and resembled a mouse or squirrel, but was not related to either. Its hind legs were longer than its forelegs and it was likely arboreal.

Because of replacement minerals, many of the fossil mammal teeth from the White River Formation of Nebraska and South Dakota will fluoresce. Be sure to check them out in our fluorescent section under the Nebraska and South Dakota pages and then by the appropriate page for the species you are interested in. The item number will be the same as the item number in this section, but will be followed by an "F."

Ischyromys had four molars on either side, top and bottom. This specimen is a nearly complete upper skull with the maxilla and incisors showing at the front edge. The specimen is from the White River Formation, Brule Member.

Kingdom: Animalia, Phylum: Chordata, Class: Mammalia, Order: Rodentia, Sub-Order: Sciurognathi, Family: Ischyromyidae, Genus: Ischyromys, Species: typus.

Squirrel - Ischyromys typus
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Time: Early Eocene through Early Oligocene, approximately 40 - 30 million years ago. Location: White River Formation, Brule Member, Nebraska Badlands, USA Dimensions: 1-5/8" x 3/4" x 3/4"