More than 100 ancient rhinoceroses died in the same place, at the same time, around 11.8 million years ago. Their fossils, unearthed at the Ashfall Fossil Beds in Nebraska, are now reshaping how scientists understand the social lives and movement patterns of prehistoric megafauna.
According to a new study in Scientific Reports, this mass death event was triggered by an immense eruption from the Yellowstone hotspot, whose ashfall blanketed the region and led to the gradual, painful demise of these animals.
A Supervolcanic Catastrophe, Frozen in Time
The Ashfall site offers a near-perfect time capsule: a layer of fine volcanic ash nearly a meter thick preserved a catastrophic scene. When the Yellowstone hotspot erupted roughly 11.86 million years ago, it sent 650 cubic kilometers of ash into the atmosphere. Wind patterns carried this ash eastward, smothering ecosystems across the Midwest.
Unlike the victims of sudden eruptions like Pompeii, the animals at Ashfall did not die instantly. Instead, they endured a slow decline. The ash “projected in enormous quantities into the atmosphere” suffocated vegetation, blackened the sky, and filled waterholes with sediment.
Scientists describe this as a “slow agony,” where the rhinoceroses likely died over the course of days or weeks. Some may have starved, others succumbed to the effects of inhaling ash, including fever, limb swelling, and arthritis.
The site where they were found was once an ephemeral watering hole. As smaller animals died and were buried, larger animals continued to use the site, unaware of the danger. Eventually, Teleoceras individuals — the last to die — became encased in layers of wind-blown ash, their bodies preserved in situ.
Social Structure Unlike Modern Rhinos
In today’s world, rhinos are usually solitary, but the pattern at Ashfall tells a different story. Most of the skeletons were females, calves, and immature females, with very few dominant males and no subadult males present. This demographic breakdown suggests a polygynous mating system and supports the idea of male-biased dispersal, where dominant males control access to herds and younger males are driven away.
The new study used isotope analysis of molars to investigate this further. Scientists analyzed carbon (δ13C), oxygen (δ18O), and strontium (87Sr/86Sr) isotopes in tooth enamel to reconstruct diet, drinking water sources, and geographic movement. They found no significant differences in these markers between molars that formed earlier and later in life, suggesting no natal dispersal occurred during the animals’ development.
Additionally, seasonal migration could not be detected. Variations in δ18O values — which can indicate climate seasonality — were present but did not align with long-distance movement. Strontium ratios, which reflect the underlying geology of an animal’s foraging range, were surprisingly consistent.
Most values fell within a narrow window of 0.70863 to 0.70874 — nearly indistinguishable from local baseline values. This strongly suggests that the Teleoceras population was non-migratory, occupying the same small area for their entire lives.
Semiaquatic Lifestyle Shaped Behavior
The morphology of Teleoceras major supports this interpretation. With barrel-shaped bodies, short limbs, and high-crowned molars, they likely lived a semi-aquatic lifestyle, akin to modern hippopotamuses. These adaptations made them well-suited for wetland environments, where they could graze on grasses and leafy vegetation.
Previous findings at Ashfall included grass macrofossils inside the stomach cavities of Teleoceras skeletons, suggesting a diet rich in C3 grasses. Microwear on their teeth indicated mixed feeding, meaning they consumed both grass and leaves — potentially avoiding the need to move in search of seasonal food sources.
Unlike migratory grazers, Teleoceras appears to have relied on a stable water source and consistent vegetation. Their large body mass (males: 880–1110 kg; females: 785–840 kg) might suggest mobility, but their brachypody — short leg bones — indicates the opposite. This body plan would not favor long-distance travel, and it’s likely that the need for immersion in water further anchored them to specific habitats.
How a Fossil Site Challenges Ancient Animal Behavior Assumptions
To test whether the Ashfall rhinos might have migrated in response to the volcanic disaster, researchers serially sampled the enamel of their molars. These samples offered insights into changes over time in an individual’s diet and location. But again, no significant shift appeared.
The study also compared isotope values from Teleoceras to other species found at Ashfall, including horses, camels, and a small ruminant called Longirostromeryx. The rhinos showed lower δ18O and 87Sr/86Sr values, suggesting they favored wetter microhabitats compared to the drier foraging preferences of the horses.
Interestingly, the ruminant species, likely local due to its small body size, had similar isotope signatures to the rhinos. This further supports the idea that Teleoceras was local, not a migrant from afar.
Even more compelling is the notion that Teleoceras herds may have maintained genetic diversity not through physical dispersal, but via social dispersal — mingling between herds. Some modern species, like the white rhinoceros, show preferences for genetically dissimilar mates, and it’s possible that Teleoceras practiced a similar strategy.
