Paleontologists have been baffled by a prehistoric rhinoceros mass grave for more than fifty years. Over one hundred rhino skeletons were discovered in the same location, having died together 12 million years ago.
A new study has revealed that this mass of animals that died together also lived in a single large herd. How do we know? Their teeth.
Rhinos buried in ash
The Ashfall Fossil Beds, located about 160 kilometers from Sioux City, Iowa, were created by the Yellowstone volcanic eruption 11.9 million years ago. When the volcano erupted, a dense layer of ash blanketed the entire region. Smaller animals died almost instantly, suffocated by the abrasive ash.
It was slower for larger, tougher animals like the Teleoceras major, or barrel-bellied rhino. On a microscopic level, volcanic ash is sharp, resembling tiny glass shards. As it entered their lungs, animals gradually became ill and died. They came to the watering hole to get some relief from the cool water. There they died, and the wind blew more ash on top of them. What killed them also preserved them perfectly.
In 1971, Michael and Jane Voorhies were walking down a gully in Northwestern Nebraska. Michael, a paleontologist at the University of Nebraska State Museum, hoped that nearby Verdigre Creek’s erosion would reveal fossils.
It did. A flash of white bone appeared from the side of a gully, suspended in ash. Michael had discovered the skull of a baby rhinoceros. Excitingly, the skull remained connected to the neck, and the neck to the body.
Six years later, Dr. Voorhies returned with a team from the university. The site is now part of a national park and remains a working dig site. The animals are suspended in layers indicating their order of death: small birds at the bottom, which died first, horses and camels next, and rhinos last. There are over 20 species in total, with hundreds of skeletons, the majority of which are rhinos.
Enamel revelations
Paleontologists were initially perplexed as to why so many rhinos had congregated. Were these separate individuals and small herds fleeing to the same hole? Or could they possibly be part of a single massive herd? Researchers at the University of Cincinnati set out to answer the question.
They collected samples from the tooth enamel of over a dozen people. They then examined the isotope ratios found in the enamel. Atoms of the same element can have varying numbers of neutrons, resulting in different “species” of that element.
Isotope analysis quantifies the relative abundances of these various elements. Because different isotopes occur in different environments and thus in different foods, isotope analysis allows scientists to determine what (and thus where) an animal ate.
Using this analysis method, they were able to determine where and why the people were moving. Had they traveled a long way to avoid destruction? Did they migrate seasonally or leave for new territory as adults?
As it turns out, the answers to those questions are all no. Everyone they sampled had grown up eating the same local food. When their isotopic signatures were compared to those of a sabre-toothed deer, it was discovered that they ate more aquatic foods. If T. Major was semi-aquatic, as modern rhinos are, this would have limited its mobility, explaining the lack of migration.
The hundred-strong rhino group at Ashfall had not come together by chance; they were all fleeing the same disaster. They were a large herd that had lived and died together.
A stroll through prehistoric Nebraska
Before the eruption, Mesozoic Nebraska was a vast savannah dotted with streams and watering holes. Grazing animals fed on the open grasslands. The long-necked Aepycamelus, a giant extinct camel, grazed on the treetops, while small three-toed horses such as Pseudhipparion gratum and Neohipparion affine ate the grass beneath them.
The smaller grazing animals, as well as the young of their larger cousins, had to be wary of a variety of canine predators. Epicyon, a massive “bone-crushing dog” weighing up to 170 kilograms, was the deadliest of them all.
Large masses of barrel-shaped T. Major moved calmly along riparian corridors. They could reach four meters in length and were built more like modern hippopotamuses. In massive herds of dozens of these fleshy, tusked tanks, they enjoyed their muddy wallows, unconcerned about the bone-crushing dogs.
The volcanic eruption did not mark the end of T. Major. The rhino species survived for another seven million years, until climate change froze its wet, temperate grasslands.