Perfectly preserved in ash from a volcanic eruption some 12 million years ago in present day Idaho. It dropped about 1 foot of ash which suffocated the animals when it got into their lungs. The area where they found the fossils was most likely a watering hole where all of these species came to drink. After many years the skeletons became perfectly preserved. The volcanic eruption that caused it all was much larger than the one at Mount Saint Helens (compare them on the map below).
Due to shifting of the earth's crust Yellowstone National Park is now sitting on top of the "hot spot" that formed the volcano which covered Nebraska in a foot of ash.
A rhino head (below)
A small horse skeleton
They also had a couple camels but I couldn't find a photo of a complete skeleton on my camera. Outside of the building they are still finding fossils, but not large skeletons, just bone fragments.
After finding so many fossils it seems that there would be no more, but the excavation continues. They are finding even more rhinos in the barn and they expect to come across many more in the future. Since this is a birding blog I figure I better mention some of the birds that they found there. Exotic birds such as Gray-crowned Cranes, Secretary Bird were found there although I didn't get to see the fossils. To learn more about this site and to see hours and get directions go to: http://ashfall.unl.edu/. Another fossil bed I personally recommend is the Mammoth Site in western South Dakota's Black Hills. It is an amazing place with numerous mammoth tusks and bones displayed in the same manor. It's website is http://www.mammothsite.com/.