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Stylemys (which means "pillar mouse") nebrascensis was the first fossil genus of dry land tortoise, which belonged to the order Testudines, to be discovered in the United States. It was first described by Dr. Joseph Leidy in 1851. Stylemys was common in the prehistoric Badlands of South Dakota and Nebraska. They lived in the temperate to subtropical areas of North America, Asia and Europe.

Stylemys ranged from the late Eocene to early Oligocene. The Oligocene was identified as a time of major changes in ecosystems. Tropical broad leaf forests regressed to the equatorial belt and were replaced by temperate deciduous woodlands. Open grasslands and deserts became more common.

During the Oligocene, the Stylemys shared its semi-arid home in the White River Badlands of South Dakota with a variety of endemic mammals. Included in these mammal groups were: Archaeotherium (giant pigs), Poebrotherium (camels), running rhinos, Mesohippus (three-toed horses), nimravids (saber-toothed cat-like mammals), protoceratids (deer), Hyaenodon (dogs), and Oreodonts (sheep-like mammals).

Stylemys had primitive jaw muscles and would have been herbivorous.

Kingdom: Animalia, Phylum: Chordata, Class: Reptilia, Superorder: Chelonia, Order: Testudines, Family: Testudinidae, Genus: Stylemys, Species: nebrascensis.