Ancient, water-loving rhinos gathered in big, hippolike herds

Millions of years ago in Nebraska, chunky, stumpy-legged rhinoceroses were party animals, crowding together in huge herds at watering holes and rivers.

Chemical signatures in the fossilized teeth of the extinct, corgi-shaped beasts suggest they didn’t roam widely, instead forming big, local herds unlike the more solitary rhinos of today, researchers report April 4 in Scientific Reports.

About 12 million years ago during the Miocene Epoch, the Yellowstone supervolcano erupted and covered much of North America in ash. Around a watering hole that eventually became Nebraska’s Ashfall Fossil Beds State Historical Park, about a foot of the debris fell on the landscape.

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

The ash slowly suffocated and starved the local fauna, with many perishing in the watering hole and fossilizing. Researchers have since uncovered about two hundred animal skeletons at the site, including over 100 mostly complete skeletons of Teleoceras, an ancient, barrel-bodied member of the rhinoceros family.

Because Teleoceras had the roly-poly proportions of a hippopotamus, the herbivores have been commonly envisioned as similarly semiaquatic. But recent research had called this amphibious lifestyle into question, Ward says. It also wasn’t clear if Teleoceras’ had rich social lives, or if they were mostly solitary like modern rhinos.

To help answer these questions, Ward — then at the University of Cincinnati — and his colleagues wanted to know if the rhinos and other animals that died at Ashfall were migrants that gathered there, or if they were locals. The team took samples of 13 fossil rhinos’ molars, chemically analyzing them and comparing ratios of different forms of oxygen, carbon and strontium. Carbon ratios indicate what types of grasses the ancient rhinos’ ate in life, and oxygen ratios can tell researchers about seasonal changes the animals experienced. Together these can reveal details about the animals’ diet and habitat.

Additionally, ratios of different forms of strontium are tied to specific geographic locations, allowing researchers to see if ancient animals traveled between regions, says Danielle Fraser, an evolutionary biologist at the Canadian Museum of Nature in Ottawa who was not involved with the research.

The team found very little variation in the carbon and strontium ratios, suggesting Teleoceras was mostly a homebody that wallowed and fed right around Ashfall. This may also mean the plentiful rhino remains at Ashfall are the result of lives lived in huge herds dozens of animals strong.

Ward was a bit surprised that there wasn’t evidence the rhinos were moving between different locations.

“I suspected young males to travel far distances in search of mates, likely having to try and try again,” he says. Mating only within their home herds may have raised the chances of inbreeding, so it’s possible the rhinos intermingled with the herds next door to avoid this risk.

The findings also provide insights into the ancient Ashfall ecosystem, Fraser says. If the rhinos didn’t have to migrate to other regions, it suggests they would have had access to enough food and water year-round.

Ward says he and his colleagues were puzzled by how many rhinos, horses and camels living in the same place could coexist without stripping the area bare of vegetation. It’s possible the warmer climate of the Miocene led to more productive plant life. Or, Ward says, the herbivores’ presence boosted plant growth as they cleared out older vegetation and deposited manure.