Millions of years ago in what’s now the U.S. Midwest, watering holes and rivers were home to huge herds of stumpy-legged rhinos. While most rhinos today prefer a solo lifestyle, these ancient party animals formed big herds. They also stuck close to home.

These new insights come from chemical clues in fossil teeth. Researchers shared the findings April 4 in Scientific Reports.

An extinct corgi-shaped cousin of today’s rhinos — Teleoceras — left the fossilized remains. They came from animals that died some 12 million years ago when a huge Yellowstone volcano erupted in what is now Idaho.

That explosion blanketed much of North America in ash. And here, in what’s now Nebraska, about 30 centimeters (1 foot) of ash fell over a watering hole used by Teleoceras and its neighbors.

“It would have been like a bad dust storm,” says Clark Ward. A paleontologist, he led the research. Ward works at the University of Minnesota in Minneapolis. 

This ash slowly suffocated and starved hundreds of animals at the Nebraska watering hole. This ancient animal graveyard is now preserved as Ashfall Fossil Beds State Historical Park. Scientists have uncovered more than 100 Teleoceras skeletons here.

What their remains show

Fossils provide all types of clues about long-extinct animals. What they ate, for example. And how they behaved.

Ward’s team wanted to know two things. Did Teleoceras live both on land and in the water? (The species’ round, hippo-like body suggested they did.) And was this species mostly solitary, like modern rhinos? Or were they more social, like today’s hippos?

For answers, the team looked into whether the rhinos that died at Ashfall lived there full-time or had just been caught during a brief visit. So the researchers sampled 13 rhino molars — a type of tooth. They looked at the relative amounts, or ratios, of different isotopes (forms of various elements) in their teeth.

The carbon isotopes would point to what types of grasses the ancient animals ate. Ratios of oxygen isotopes should reflect seasonal changes in their diet. Together, these details could paint a picture of the animals’ habitat. Strontium isotopes in a tooth, meanwhile, could be tied to specific areas.

Measuring these isotope ratios allowed the researchers to see if ancient animals traveled between regions.

The carbon and strontium ratios varied little, the team found. Those data suggest Teleoceras didn’t wander far. It lounged in mud and fed on grasses right around Ashfall. With so many remains at Ashfall, the results also suggest these rhinos lived in huge herds.

Surprising rhinos

Ward was a bit surprised that the rhinos seemed such homebodies.

“I suspected young males to travel far distances in search of mates,” he says. Mating only within their herds could have raised the chances of inbreeding. This can raise the risk of gene mutations building up in a population. The result could be weak or sickly offspring.

To limit this risk, the rhinos may have mated with neighboring herds.

The findings also shed light on the ancient Ashfall ecosystem, says Danielle Fraser. An evolutionary biologist, she did not take part in this study. She works at the Canadian Museum of Nature in Ottawa. If the rhinos didn’t have to migrate, she says, they likely had plenty of food and water year-round.

But Ward and his team remain puzzled by one thing. How could so many rhinos, horses and camels live together in the same place? Wouldn’t they eat all of the plants and run out of food?

Perhaps Earth’s warmer climate back then led to a bigger plant bounty. In fact, Ward says, maybe those plant-eating animals boosted plant growth. They might have had this effect as they munched on older vegetation — dropping fertilizing poop along the way.