Ashfall Fossil Beds: Miocene Rhinos Buried by Volcanic Ash

The Ashfall Fossil Beds in north-eastern Nebraska preserve hundreds of complete skeletons of rhinos, horses, camels, birds and turtles that died around a waterhole about 12 million years ago. They were killed by fine volcanic ash from a massive eruption of the Yellowstone hotspot more than 1,500 kilometres away. Discovered by the palaeontologist Michael Voorhies in 1971, the site is often called a “prehistoric Pompeii”.

The Great Plains in the Miocene

During the Miocene Epoch, the region that is now Nebraska was a warm, open savanna of grasses and scattered trees, crossed by streams and dotted with seasonal ponds. It supported a rich community of large grazing and browsing mammals, comparable in some ways to the plains of East Africa today.

Rhinoceroses, which are now found only in Africa and Asia, were common in North America. Horses, many still with three toes, camels, and small hoofed animals shared the grasslands with predators such as bone-crushing dogs.

The eruption

About 12 million years ago, a series of enormous explosive eruptions occurred in the Bruneau-Jarbidge volcanic field of south-western Idaho. This field was an earlier centre of the same hotspot that today lies beneath Yellowstone National Park. As the North American plate moved south-west over the stationary hotspot, the volcanic activity appeared to migrate north-east over millions of years.

Winds carried the ash eastward over the Great Plains. In Nebraska, a layer roughly 30 centimetres thick settled across the landscape. Wind then drifted the powdery ash into low areas, and in the waterhole at Ashfall it piled up to depths of around 3 metres.

How the animals died

The animals were not killed instantly, as at the Roman city buried by the eruption of Vesuvius. Instead they inhaled the abrasive glass particles as they grazed and drank. The ash damaged their lungs, and many bones show abnormal growths typical of a condition called hypertrophic pulmonary osteodystrophy, sometimes known as Marie’s disease, which develops when the lungs fail.

Animals seem to have died in sequence over a period of weeks. Birds and small animals such as turtles, which are found lowest in the ash, died first. Horses and camels followed, and the large rhinos, which are found highest and show the most severe bone growths, survived longest. Many animals may have come to the waterhole looking for water as the surrounding land was smothered.

Because the ash buried the bodies quickly, many skeletons remain complete and in their original positions. Some female rhinos have unborn calves inside them, and some calves lie beside adults as if they had been nursing.

Discovery and excavation

In 1971 Michael Voorhies of the University of Nebraska State Museum was surveying in Antelope County when he noticed the skull of a young rhinoceros eroding from the side of a gully. Further digging revealed that it was part of a complete skeleton, and that many more lay nearby.

Large-scale excavations took place in the late 1970s, supported in part by the National Geographic Society. Voorhies’s article in National Geographic in 1981 brought the site wide public attention.

In 1991 the site opened to the public as Ashfall Fossil Beds State Historical Park, run jointly by the Nebraska Game and Parks Commission and the University of Nebraska State Museum. A building over the excavation, the Hubbard Rhino Barn, opened in 2009, allowing visitors to watch palaeontologists at work. The site was named a National Natural Landmark in 2006.

What the fossils show

The most common animal is the barrel-bodied rhinoceros Teleoceras major, a short-legged, heavy animal that may have lived partly like a hippopotamus. More than 100 skeletons have been found, making it possible to study a whole herd, including differences between males, females and young.

The site has also yielded several kinds of horses, including three-toed forms such as Cormohipparion and the one-toed Pliohippus, as well as camels such as Procamelus and the tall, long-necked Aepycamelus. Birds include a crowned crane closely related to those living in Africa today. Fossil seeds and grasses help reconstruct the environment.

Rhinos at Ashfall were much smaller than the giant Paraceratherium of Oligocene Asia, but they show how widespread and varied rhinoceroses once were.

Timeline

• c. 12 million years ago: An eruption in the Bruneau-Jarbidge field of Idaho spreads ash across the Great Plains.
• Weeks after the eruption: Animals at the Nebraska waterhole die and are buried in drifting ash.
• 1971: Michael Voorhies finds a young rhino skull in Antelope County.
• Late 1970s: Major excavations reveal dozens of complete skeletons.
• 1981, January: National Geographic publishes Voorhies’s account of the site.
• 1991: Ashfall Fossil Beds State Historical Park opens.
• 2006: The site is designated a National Natural Landmark.
• 2009: The Hubbard Rhino Barn opens over the excavation.

Why it matters

Ashfall records not just individual animals but a whole community caught in a single disaster, revealing herd structure, family groups and the effects of volcanic ash on living bodies. It also shows how far the effects of a large eruption can reach, a lesson echoed by modern events such as the eruption of Mount St. Helens, whose ash fell across several states.