Prehistoric rhinos lived in super-herds

Researchers studied the isotopes of rhino teeth found in what is now northeast Nebraska. Here, more than 100 rhinos at a single water hole died and were entombed in ash from an eruption of the Yellowstone supervolcano.

Since the discovery of rhinos at Nebraska's Ashfall Fossil Beds State Historical Park in 1971, researchers have wondered what drew so many animals together in the same place. Did they converge from far away, perhaps to seek shelter from the unfolding natural disaster of the volcanic eruption with its choking ash?

"We found they didn't move very much," lead author and UC graduate Clark Ward said. "We didn't find evidence for seasonal migration or any evidence of a response to the disaster."

The study was published in the Nature journal Scientific Reports.

Ward, who is now pursuing a doctorate at the University of Minnesota, used isotopic analysis of Miocene rhinos as part of a master's research project under the guidance of advisers and study co-authors Brooke Crowley at UC and University of Nebraska Professor Ross Secord.

Researchers examined ratios of isotopes of strontium, oxygen and carbon in fossil teeth to track the movements of the long-extinct animals across landscapes. Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.

Grass or leaves that rhinos and other animals eat contain similar ratios of isotopes as the soil and bedrock where plants grow that allow researchers to determine where the animals fed, sometimes with surprising precision depending on how variable the vegetation and geology are.

Scientists use this technique in wildlife conservation. For example, they can track the migrations of wide-ranging animals such as caribou or identify the habitat needs of elusive animals like jaguars.

"By studying carbon in the animal, we can reconstruct carbon in the environment to understand what kinds of vegetation lived there," Ward said.

Meanwhile, oxygen tells scientists about climate, particularly rainfall.

"We can use it to reconstruct how wet or dry the environment was," Ward said. "And strontium tells us where the animal was foraging because the ratio of isotopes is related to the soil and supporting bedrock."

Teleoceras major was a one-horned rhino with a barrel-shaped body and stubby legs like a hippo. Like hippos, they fed on grass. And like hippos, researchers think these rhinos spent a lot of time in and around water. Because of their vast size, they had few predators in the Miocene epoch, Ward said.

But their calves would have been vulnerable to hyena-like predators called bone-crushing dogs. Indeed, some of the specimens found at the Nebraska site bear evidence that scavengers removed portions of their carcasses after they died. And ancient tracks from the 100-pound dogs have been found there.

Yellowstone's enormous volcano has erupted many times over the past 12 million years. Ash from the eruption easily would have traveled 700 miles across what is now Nebraska where it piled up like snow nearly a foot deep in places. But windblown ash continued to fall on Nebraska long after the initial eruption, Ward said.

"That ash would have covered everything: the grass, leaves and water," Ward said. "The rhinos likely weren't killed immediately like the people of Pompeii. Instead, it was much slower. They were breathing in the ash. And they likely starved to death."

Rhino expert John Payne spent his career working with endangered Sumatran rhinos in Malaysia. Payne, who was not part of the study, said UC's research addresses scientific debate about the social structure of this ancient species of rhino.

"I am not surprised that the analyses very strongly suggest that Teleoceras major lived in herds given that this animal resembles modern hippopotamus in form and hippos live in herds of several tens of animals -- with several herds in one geographical area," he said.

Ward worked as an intern at the place he loved as a child, Ashfall Fossil Beds State Historical Park, answering visitors' questions about the fossils and participating in fossil excavations and preparation at the site.

"I'm honored and privileged to have my name in science attached to the site," Ward said. "As someone who used to go to Ashfall as a kid, it's come full circle."