Called the Rooftop of the World, the Tibetan Plateau perches high above the surrounding terrain, surpassed only by the Himalayan mountains that mark its southern border. But when and how the plateau reached its high elevation has remained unclear.
For one scientist and his team, tracing the evolution of the Tibetan Plateau now has a much-needed clue: vertebrate fossils—woolly rhinos, shovel-tusked elephants, and climbing perch, to be precise.
“Most modern vertebrate fauna of the Tibetan Plateau have long histories living in this plateau,” said Deng Tao, deputy director of the Chinese Academy of Sciences Institute of Vertebrate Paleontology and Paleoanthropology in Beijing and lead scientist on the study. What’s more, “vertebrates are very sensitive to climatic and environmental changes,” he said. Could the story of the plateau’s evolution lie in their bones?
Deng’s answer is yes. After examining fossils of animals that lived and died on the plateau over the past 34 million years, he and his team could trace when the plateau’s elevation was low enough to allow animals to migrate across it.
“The uplift of the Tibetan Plateau was an important factor of global climate change during the late Cenozoic [era],” Deng said, “and it strongly influenced the development of the Asian monsoon system.” Pinpointing the timing and speed of the plateau’s uplift will help scientists better understand the geophysical processes that shaped southern Asia’s landscape and the forces that influenced the paleoclimate in the region.
Their study, which Deng will present Thursday, 14 December, at the 2017 American Geophysical Union Fall Meeting in New Orleans, La., suggests that the region hit its last growth spurt approximately 20 million years ago. This early uplift time frame agrees well with most isotopic records.
Fossils as a Window into Uplift History
The Tibetan Plateau, the youngest and highest in the world, rises approximately 5,000 meters above sea level, nearly one third of the way to the tropopause. Its height and location shape atmospheric flow and climate in the region. At present, the Tibetan Plateau supports a diverse range of ecosystems, including deserts, forests, and tundra, all adapted to its high-altitude environment.
In the same way, past fauna would have reflected past environments, the researchers figured. To test this, Deng and his team examined newly excavated as well as archived fossils of ancient fish and mammals from the plateau that date to the late Oligocene (34–23 million years ago (Ma)) through the mid-Miocene (23–5.3 Ma) and Pliocene (5.3–2.6 Ma) epochs. They analyzed fossils of rodents, fish, Tibetan woolly rhinoceroses, shovel-tusked elephants, and ancestral relatives of arctic foxes, snow leopards, and bighorn sheep.
Although previously published results by the team parse out analyses of separate species, the results on display at the 2017 Fall Meeting aggregate these studies, other research, and new fossil analysis into one comprehensive view. And what they found painted a clear picture.
During the late Oligocene, large mammals like the Tibetan woolly rhinoceros and shovel-tusked elephant roamed from northwestern China to the Indian subcontinent, Deng explained. The distribution of their fossils north and south of Tibet during the Oligocene indicates that the plateau’s elevation was low enough to allow migration across the continent. In addition, the team found fossils suggesting that lowland species like the ancestors of climbing perch likewise made their homes in Tibet during the late Oligocene.
Adapt or Perish in High-Altitude Tibet
By the early Miocene, however, fossil records suggest that the plateau was elevated enough that large mammals who lived just north of Tibet had to curtail their southward migration, Deng explained. For example, although paleontologists have found shovel-tusked elephant fossils from the mid-Miocene northward of the plateau, none exist within or south of the plateau during that epoch.
Vertebrates that were trapped on the now-elevated Tibetan Plateau either evolved to survive in a high-altitude cryosphere or died out. Lowland-adapted species of fish and rodents perished, and their fossils disappeared.
For vertebrates that survived, including the ancestral forms of the woolly rhinoceros, snow leopard, arctic fox, and bighorn sheep, their Pliocene fossils show that the animals had long since adapted to freezing temperatures at high elevations, Deng said. The plateau must have uplifted a few million years prior to the most recent ice age at the end of the Pliocene, the researchers argue, for the animals to have been well adapted to the cold by then.
That uplift timing supports that theory the Tibetan Plateau’s rise to the top resulted from the collision of the Indian and Eurasian continental plates that occurred 40–50 Ma. The evolutionary history suggested by this timing also agrees with the past environmental conditions suggested by isotope ratios of oxygen, hydrogen, and calcium found in carbonate rocks that surround the fossils.
For Deng and his team, the geographic locations of the fossil deposits and their stratigraphic origins tell a single story: a low-elevation Tibet in the late Oligocene, a constant uplift in the early Miocene, and a modern-elevation Tibetan Plateau by the middle to late Miocene.
“Fossils are really valuable and probably an underused source of insights,” Katherine Freeman said about resolving the controversy surrounding the timing of the Tibetan Plateau uplift. Freeman is an isotope biogeochemist and a professor of geosciences at Pennsylvania State University in University Park. “Although their record can be discontinuous, they would be excellent for marking changes on broad time and spatial scales.” She added that she “certainly welcome[s] this additional perspective on a tricky problem.”
However, not all paleoelevation proxies agree with the early Miocene rise suggested by fossils and stable isotope measurements. For example, isotope studies of tooth enamel found in fossils from the high Himalayas dating to the late Miocene show signatures consistent with the animals chewing C4 grasses, plants species that thrive in warmer conditions. Those results indicate that Tibetan climate in the late Miocene might have been much warmer, suggesting that elevations were likely much lower than they are today and contradicting the recent fossil-based analysis.
How do you resolve this discrepancy? “We want to find more fossil records from Cenozoic basins in the Tibetan Plateau,” Deng said. More fossils, the team thinks, will clear up the fuzzier details of the plateau’s past.
—Kimberly M. S. Cartier (@AstroKimCartier), News Writing and Production Intern