Pompeii of the Great Plains: When ash fell over the Great Plains

Courtesy UNL

By Sabrina Brown, Great Plains Graduate Fellow, UNL Earth and Atmospheric Science

The spectacular features of Yellowstone National Park – hot springs, geysers, and colorful landscapes – result from superheated groundwater interacting with heat from a supervolcano deep below the ground. There’s recently been a lot of hype about when the Yellowstone supervolcano will erupt again, and for a good reason. When the Bruneau-Jarbidge Eruptive Center, an extinct supervolcano, erupted nearly 12 million years ago it caused extensive devastation, including to the residents of what would be known as Nebraska. The animals of the Great Plains, including horses, camels, rhinos, and birds alongside others, died and were buried in a foot or two of glassy ash particles at a watering hole located near present-day Royal, Neb. Scientific research on the site began in the 1970s and it opened as a state park in 1991. Here are three cool facts about the Pompeii of the Plains, Ashfall Fossil Beds State Historical Park:

1. Its discovery
The rock layer of preserved ash wasn’t exposed at the surface until 1971, after runoff from heavy rainfall formed a gully alongside Melvin Colson’s agricultural field. Nebraska’s Niobrara River valley is famous for its fossilized mammals preserved in the Niobrara formation, and fossils had been found on the property before. Thus, when paleontologist Mike Voorhies found the skull of a baby rhino protruding from the side of the gully later that year, it didn’t come as a surprise; but what was surprising was that the skull was attached to a complete skeleton – the first of more than 100 rhino skeletons excavated at Ashfall Fossil Beds.

2. Its fossil preservation is unique
The site is a paleontological “time capsule” of sorts. Twelve million years ago, Nebraska was covered by sub-tropical jungles and grasses. Species roaming the Great Plains then were similar to the large mammals on the African savanna today. While the Niobrara formation is famous for containing mammal fossils, the preservation at Ashfall Fossil Beds is unique. The majority of fossils uncovered in the Great Plains are isolated bones or pieces of bone because scavengers tend to scatter remains before preservation. The conditions at the watering hole near modern-day Royal, Neb., were just right to cause large groups of animals to perish and be buried quickly – so fast that some mothers have been found with their calves next to them and some animals with their last footprints or meals preserved. Further, many of the animals were not crushed, as is typical of fossilization, but rather the skeletons were preserved in three-dimensions. This type of preservation is extremely valuable in helping paleontologists understand the animals and the environment in which they lived.

3. It’s like a crime scene
Paleontologists can act like detectives at a crime scene. Using clues in the fossils and surrounding rocks, they can understand which animals survived longer. Most of the animals preserved at the Ashfall site likely survived the initial volcanic eruption and blanket of ash, but they continued to eat ash-covered plants. Ash is essentially glass, with sharp edges that are abrasive. This eventually mortally damaged their insides. The smallest animals, including birds, turtles, and deer, died first. Next came the larger animals – horses and camels – and then, finally, rhinos and a few giant tortoises. Evidence for large predators, like bite marks and coprolites (fossilized droppings), has been unearthed, but skeletal remains still haven’t been uncovered – yet!

Ashfall Fossil Beds State Historical Park is a part of the University of Nebraska State Museum system. See http://ashfall.unl.edu/index.html for more information or to plan your visit.