Dinosaurs died out in the Cretaceous period about 65 million years ago. For this reason, there are few dinosaur fossils in Nebraska. An exception is the dinosaur tracks found in Jefferson County on the southeast part of the state. Much of Nebraska was under an inland sea during the Cretaceous period. Dinosaurs died 66 million years ago at the end of the Cretaceous period. At that time, the sea produced marine fossils. The Cenezoic period, the "Age of Mammals," followed the Cretaceous period. When the inland sea dried the plains and became a habitat for mammals.
Nebraska marine fossils include corals, shark teeth, fish, plants and other fossils associated with marine life. In other places, some of these fossils weren't preserved intact, but some of the Nebraska fossils have been found unbroken and complete. Scientists estimate that the end-Permian Mass Extinction killed approximately 95 percent of marine life. This extinction affected shallow marine environments the most.
In 1967 Nebraska named the mammoth as the state fossil. Several types of animals with trunks lived in Nebraska during the Cenozoic era. Nebraska contains many fossils of mammothlike animals, and herds of other mammals. Many of these became extinct in what is now America, but still live in Africa. Rhinoceros, elephants and camels are examples. The Nebraska plains also supported deer, doglike animals, large pigs and other animals that are now extinct.
Ashfall Fossil Beds is a state park that opened in 1991. It is located in northeastern Nebraska and the site of well-preserved fossils of camels, birds, horses and rhinos. The fossils have been exposed and left at the site. A structure covers the area to protect it from the elements. A watering hole filled with ash after a volcano eruption. A large number of mammals gathered there and died. Volcanic ash covered the animals and they became fossils. The ash buried smaller animals alive. Larger ones died over time by inhaling ash. The skeletons are intact and unbroken. Ashfall is a unique site that gives an overview of animals present in one place at one time.