Nebraska is a fossil hunter's paradise. One of the finest fossil sites in Nebraska and all of North America is the Ashfall fossil beds in the uncrowded northern part of the state Ashfall Fossil Beds.
There is a hot spot in the earth's crust that the North American Plate slowly slides over as it moves from the northeast to the southwest. This hot spot currently sits near Yellowstone Park. Every 2-3 million years this hot spot explodes in a spectacular fashion. The next eruption is due relatively soon (within the next 100,000 years or so). About 12 million years ago the hot spot erupted in an area that is now Southwest Idaho. The debris cloud drifted east and left a distinctive layer of white powdered-glass ash that can be identified in Idaho, Wyoming, South Dakota, and Nebraska.
Most of the animals living in the savanna-like grasslands at the time survived the initial ashfall, but as they grazed across the ash covered plains, their lungs began to fill up with the abrasive powder and they began to die.
What makes Ashfall so unique (and heartbreaking) is that it is the largest collection of nearly fully intact animals. They are preserved in their death positions in a mass grave at an ancient watering hole, complete with evidence of their last meals, unborn animals, and with their last steps recorded in the stone. While this may have also occurred in other parts of Nebraska and other states, the glacial action to the east of Ashfall eliminated those fossil remains, while the erosion of the Rocky Mountains by the Platte River and other watersheds took care of any remains to the West. Ashfall is just in the sweet spot in between.
Even after 10 million years, there is only a thin layer about 8 feet deep that was deposited above the fossil layer. Amazingly, this fossil field, yielding more than 400 nearly complete skeletons, wasn't discovered until the 1970's.
Rhinos used to roam North America. These are the remains of a pregnant rhino.