Agate Fossil Beds National Monument in northwestern Nebraska preserves a prairie landscape along the Niobrara River and one of North America’s notable Miocene mammal sites. Scientific work began in 1892 and expanded after 1904, when competing museum teams unearthed complete skeletons of extinct animals previously known mostly from fragments, including early horses, rhinoceroses, Moropus, Daeodon, and the land beaver Palaeocastor with its corkscrew burrows. The area also preserves the history of Agate Springs Ranch, bought by James and Kate Cook in 1887, including a large collection of Plains Indian artifacts linked to the Cook family’s ties with Native communities.