In 1971, palaeontologist Michael Voorhies discovered a baby rhinoceros skull eroding from volcanic ash in Nebraska. As he began excavating the site, he uncovered the entire skeleton – along with several more rhino specimens.
The discovery of so many rhinos in one place was almost unheard of, and in the ensuing years Voorhies directed a major excavation of the site – an ancient watering hole now known as Ashfall Fossil Beds State Historical Park – eventually unearthing more than 100 complete rhino skeletons.
The animals were buried in ash from a distant eruption of the Yellowstone supervolcano 12 million years ago, which spread fine volcanic dust hundreds of miles across the region.
But since the discovery, experts have puzzled over why so many rhinos died at this one watering hole. Some suspected they had travelled from far away to escape the deadly ash. But a recent study of the teeth of the rhinos paints a different picture.
The work, conducted by the University of Cincinnati and published in the journal Scientific Reports, reveals that herds of rhinos once roamed the plains of what is now North America. These prehistoric giants lived in large, stable groups – much like modern hippos.
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Using isotope analysis – a method that looks at the chemical makeup of fossilised teeth – researchers were able to determine where these rhinos (Teleoceras major) had lived and fed.
“We found they didn’t move very much,” says lead author Clark Ward. "We didn't find evidence for seasonal migration or any evidence of a response to the disaster."
By analysing carbon, oxygen and strontium isotopes, the team discovered not only where the rhinos lived, but also what they ate and what the climate was like at the time. Carbon tells scientists what kinds of plants were growing, oxygen gives clues about rainfall, and strontium reveals details about local geology.
Teleoceras major looked a bit like a modern hippo – with a barrel-shaped body, short legs and a single horn. They were likely water-loving grazers and may have spent much of their time lounging near rivers and lakes.
Unfortunately for the rhinos, when the ash from Yellowstone blanketed the region, the animals didn’t die instantly. "The rhinos likely weren't killed immediately like the people of Pompeii," Ward explains. "Instead, it was much slower. They were breathing in the ash. And they likely starved to death."
Ward adds that some of the fossils – the calves – show signs that predators scavenged the bodies after death. Tracks from fierce, 100-pound, bone-crushing dogs have also been found at the site.
Rhino expert John Payne, who wasn’t involved in the study, says the findings strongly suggest these ancient rhinos lived in herds – just like today’s hippos.
Find out more about the study: Enamel carbon, oxygen, and strontium isotopes reveal limited mobility in an extinct rhinoceros at Ashfall Fossil Beds, Nebraska, USA
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