For more than 150 years the central and lower Niobrara River Valley has been an important source for later Cenozoic mammal, fish, and reptile fossils.
Fossil deposits found along the Niobrara River dating from the Miocene and Pliocene epochs are figured prominently in scientific studies of mammal evolution in North America. One particular site found within the Scenic River, containing no less than 146 species of vertebrates, is the most diverse single-site of Miocene fauna known in North America.
More than 160 mapped paleontology sites are present within the Niobrara Scenic River's reach. The Niobrara River is exceptionally rich in documented fossil sites, averaging ten times the number of sites per unit area when compared to the State of Nebraska as a whole.
Fifteen sites in the Niobrara River study area are of world class (international) significance, 46 are of national significance, and 106 of regional significance.
Eighty species of extinct vertebrates were first discovered in the Niobrara River valley: 56 mammals, 8 amphibians, 13 reptiles, 2 birds, and 1 fish.
Collections of fossils from the Niobrara River region are housed in 15 research institutions throughout the United States; some of the most notable are New York's American Museum of Natural History, Chicago's Field Museum of Natural History, and the Smithsonian's Museum of Natural History in Washington, D.C.
The largest Niobrara fossil collections are located at the University of Nebraska State Museum (Morrill Hall) in Lincoln, and the Frick Laboratory at the American Museum of Natural History.