During the Eocene and Oligocene geologic epochs, Nebraska preserved a landscape where miniature rhinos, pig-like animals, giant tortoises, tiny horses, and other creatures came to hunt, rest, and drink along a wide shallow river, leaving prints in sandbars and along the shore. Toadstool Geologic Park preserves this Golden Age of Mammals, and the White River Group forms the type section, or geologic standard, for rocks from this time. Scientists from around the world study the fossils, trackways, and geologic history found in these ancient rocks. The area captures a time when Nebraska resembled the savannahs of Africa and supported abundant wildlife, and over time some of these animals became extinct while others evolved into modern species, including zebras, camels, horses, and rhinoceroses, after migrating to Africa. Many of the preserved tracks were made by the subhyracodon, and tracks of hyrachodons, entelodonts, stylemys, and some cats also appear here.