Oligocene Squirrel-Like Mammal (Ischyromys) Jaw Section

This is a jaw section containing 3 teeth from the Oligocene extinct squirrel-like mammal, Ischyromys typus. It's from the Brule Formation in South Dakota, making is Oligocene in age, or approximately 32 million years old.

The 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)