Geology & Geophysics News

Fossils Provide New Clues to Tibetan Plateau’s Evolution

The bones of ancient rhinos, elephants, and fish constrain when the Tibetan Plateau rose high enough to prevent migration, a move that forced animals to adapt to high-altitude conditions.

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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.

Modern elevation map of Tibetan Plateau
Modern topographical map of the Tibetan Plateau and the surrounding region showing areas of low (green) to high (red and white) elevation. Credit: Darekk2, GLOBE, and ETOPO1, CC BY-SA 4.0

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.

Fossil of climbing perch
Late Oligocene fossil of Eoanabas thibetana, an ancestor of the modern-day climbing perch, collected in central Tibet and used to constrain the latest period of a low-elevation Tibet. Credit: Wu et al., 2017, https://doi.org/10.1038/s41598-017-00928-9, CC BY 4.0

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.

Fossil skull and mandible of Tibetan woolly rhinoceros
(top) Skull and (bottom) mandible fossils from a Tibetan woolly rhinoceros (Coelodonta thibetana), a cold-adapted mammal from the Pliocene. These fossils were discovered in the Zanda Basin in the Tibetan Plateau. Credit: Deng and Ding, 2015, https://doi.org/10.1093/nsr/nwv062

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.

Lingering Questions

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

Citation: Cartier, K. M. S. (2017), Fossils provide new clues to Tibetan Plateau’s evolution, Eos, 98, https://doi.org/10.1029/2017EO088853. Published on 12 December 2017.
© 2017. The authors. CC BY-NC-ND 3.0
  • aedgeworth

    Those fossils were formed by sudden, deep, burial. That is the only way fossils form. But, just remember to start off your story: “Once upon a time, long ago and far away, millions of years ago,” and they will be sure to enjoy your made-up story. That seems to really get peoples’ imaginations going. Oh, and you might end your story: “Lullaby and good night.”

    • João Simões Lopes Filho

      How do you explain different strata of fossils? If all fossilized beings are modern species buried by a mythical deluge, why Noah didn’t save all the extinct animals? Why we never find trilobites and rabbits together? Why we didn’t find marine and terrestrial animals mixed in the same level? Why would marine animals be “buried” by a flood? All aquatic fossil beings should haver survived the Flood.

      • Bill Kolstad

        To be read with gentle effect; Fossil variation between layers results from the order (relative timing) of the oceanic transgression; initial transgressions bring first the loose sea floor muds up onto the continent while pushing the still floating land creatures further up and inland. Final recessions did leave some marine survivors large enough and strong enough to remain alive if they were left in interior basins. This is why the giant ichthyosaur fossils in America’s now driest state (Nevada) are found emerging from the surface layer! The 20th century notion of geologic time has been accepted by academic repetition since the 1940s when text books no longer mentioned it was a theoretical process. Most graduate level professors and researchers have never heard this, much less that it consistently fails when tested.

        Academia is way too invested in the 20th century paradigm to pivot now, but it is readily verified (debunked) today. Get a radon test kit from the local hardware store and test your own home. As the daughter element of radium with such a short half life, the radon shouldn’t be detectable yet it is the 2nd leading cause of lung cancer in America. No young researcher today has the standing to rankle the establishment which routinely denies “Noah’s flood”, but world wide stratigraphic evidence confirms the “waterboarding” of a prior supercontinent which is why those ichthyosaurs have been found up on all continents (except Antartica).

        Revisiting our 20th century perspective of slow and gradual processes doesn’t require a complete overhaul to explain the stratigraphy, only the absolute dates assigned. These discussions aught not be combative, but rather should inspire field trips with a new sense of wonder! Good scientific investigation entertains wildly competing theories and weighs them both on the neutral scale of raw field evidence. Remember, the theory of plate tectonics was repeatedly rejected in the face of massive stratigraphic evidence right up until the sea floor imagery “surfaced” in the 1980s! Now, of course, we’re “certain” the continents never moved at a much faster rate because we’ve spent the last 35 years correlating their movements to the absolute dates developed in the prior 35 years without any thought to the possibility that it was theoretical physics that fails when tested.

        To your question of the extinctions, consider a super continent resurfacing and what kind of species could survive the massive decaying biomasses on the land surface with active volcanism on a level we cannot even imagine. The second extinction wave is explained by the rapid movement of continents combined with equally rapid orogeny. This explains why over forty ichthyosaurs were found a couple years ago as the chilean glacier retreated. As the ichthyosaurs were being thrust up to their new mountain top perches, ice age mega fauna was simultaneously going extinct as their local climates changed so rapidly that mammoth in Siberia were flash frozen even as their cousins were left desiccating in the Nevada desert.

        I realize it boggles the mind to imagine rapidly waterboarding a supercontinent, but it was equally difficult to imagine a continent drifting around the planet 40 years ago. No one should be expected to radically change their understanding from a blog post, and no one ever will unless they do their own truly independent investigation in the field. This is the downside of the self protecting “peer review” process. Such field research takes years, but the big question of age versus speed will make the investigation truly fascinating, I promise. Is the Grand Canyon the result of a little river cutting through hard rock over millions of years, or a rapidly rising super continent draining massively across freshly emplaced layers of soft mud?