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Global warming 55 million years ago caused migration to North America

Monday, February 08, 1999

By Byron Spice, Science Editor, Post-Gazette

Palm trees in Alaska. Frost-free winters in Wyoming. Crocodiles in the Arctic. Is this what global warming has in store for us? Maybe. What is certain is that these all occurred during a period of global warming that occurred roughly 55 million years ago.

  K. Christopher Beard, right, and assistant Jay Norejko work in a Wyoming quarry, looking for evidence of global warming during the Paleocene period. (Bill Wade, Post-Gazette)

Scientists have been taking a closer look at this period, which came about 10 million years after the last dinosaur walked the Earth. "We have a perfect, natural experiment that occurred 55 million years ago," explained K. Christopher Beard of the Carnegie Museum of Natural History.

The results, still emerging from fossil beds and ocean sediments, may not necessarily foretell what effects industrialization will have on global climate, though they provide researchers with a way to judge the accuracy of the computer models they are devising to predict Earth's climatic future.

What is becoming clear, however, is that this warming, which began in the late Paleocene period and continued into the Eocene, brought on the dawn of the age of mammals, said Beard, the Carnegie's associate curator of vertebrate paleontology.

The defrosting of the normally frigid northern latitudes opened the way for mammals, including early primates accustomed to tropical and subtropical climes, to migrate out of Asia to North America and Europe, Beard said.

Evidence unearthed since 1992 in southwestern Wyoming by Beard and his colleagues suggests three successive waves of mammals crossed the Bering land bridge that then connected Siberia with Alaska. These ancestors of modern rodents, horses and cattle would become dominant, driving into extinction the mammal species that previously had inhabited the continent, Beard said.

"This is a new way of looking at the North American fossil record," he said. Before this finding, paleontologists believed that many modern mammals could trace their evolutionary origin to North America.

But the fossils he and his colleagues have excavated from Big Multi Quarry, a windy, desert site near Bitter Creek, Wyo., suggest otherwise. Prior to 57 million years ago, the mammals of North America were a motley collection of evolutionary dead ends. These included groups such as the multituberates, the so-called "rats of the dinosaur age," that have no modern descendants.

K. Christopher Beard. (Bill Wade, Post-Gazette) 

New species unlike these early North American species suddenly appeared in the Wyoming fossil record 57 million years ago. First came animals such as uintatheres, giant horned bunnies the size of small elephants that eventually went extinct, but may be a close cousin of today's rabbits. About 700,000 years later, another wave arrived, including the continent's first rodents.

Then, about 55.5 million years ago, a flood of species appear, including the Artiodactyls, ancestors of hoofed animals such as deer, pigs and camels, and the Perissodactyla, a group that includes rhinos, tapirs and the eohippus, or "dawn horse."

The sudden appearance of these species, combined with discoveries of early mammal fossils in China, strongly suggest that these new species originated in Asia, not North America, Beard said.

"Asia was this incredible Garden of Eden, or whatever you want to call it, for mammals," he said. By far the larger land mass, Asia was able to support a larger diversity of animals than North America and so animals that evolved there faced intense competition from other species. Asian mammals, as a group, thus may have been hardier than those that evolved in less competitive North America, giving the Asian animals an advantage that helped drive the North American native species to extinction.

The migration from Asia likely continued through North America to Europe. North America was connected to Europe then by a land mass stretching from Canada through Greenland to Scotland. Mary Dawson, the Carnegie's curator of vertebrate paleontology, has found evidence of rodents, apparently of Asian origin, on Ellesmere Island, just west of Greenland near the Arctic Circle.

The final, massive wave of migration coincided with a sudden spike in global temperatures. The climate had been warming for millions of years by this point, but deep ocean sediments from around the world show that there was a sudden shift in the relative abundance of carbon-12 at this time.

Scientists such as Jerry Dickens of James Cook University in Australia have suggested that the shift in carbon isotopes might be attributable to the sudden release of methane from the ocean floors, where massive quantities of the greenhouse gas are stored in the ice-like form of methane hydrate.

This warm period 55 million years ago has been obvious for more than 100 years, based on plant fossils from the high latitudes, Dawson said. "This evidence has been around for a while, but somehow, people don't seem to grasp it until animals are around." Though her field studies on Ellesmere Island ended in 1987, she only recently has published information about fossil species her team found there, including alligators, large tortoises and some lizards.

In Wyoming, the plant record shows that the climate 55 million years ago was markedly different from today, said Peter Wilf, a paleobiologist at the National Museum of Natural History in Washington, D.C. The Bitter Creek area today is arid, windy and often freezing cold in winter. But 55 million years ago, it was a humid, subtropical forest, with an average of 60 inches of precipitation annually and winter temperatures that never dipped below freezing.

"The plants and the animals are not in lockstep," said Wilf, who co-authored a paper with Beard on the Big Multi Quarry site that appeared late last year in the paleontology journal Palaios. Though a major wave of mammal immigration reached Wyoming 55 million years ago, the full effect of global warming was not reflected in plant species until 52 or 53 million years ago.

Before the warming, the area was covered with the same sort of deciduous forest that could be found in Europe and Russia - trees of the birch and walnut families, plus some conifers. Then, 55 million years ago, palm trees and guavas begin appearing. By 53 million years ago, tropical plants such as periwinkle and floating ferns had completely displaced the deciduous forest.

It's hard to say what lessons this ancient period of warming might have today, when scientists speculate that global warming will occur because of the accumulation of greenhouses gases in the atmosphere. Obviously, this earlier period was caused by natural forces, rather than from industrial pollution. But the time scales also are dramatically different, Wilf said, noting that the changes he has recorded stretched over millions of years.

But fossil data from the Paleocene and Eocene provide an interesting retrospective test of the computer models that provide the basis for many of today's climate predictions. "The climate models aren't working quite right," Wilf observed, noting that they consistently predict a cold winter climate for the Western United States during this ancient warm period. "The climate models can't predict a Rocky Mountain Eocene climate that matches what we see in the fossil record."

That's true, agreed Lisa Sloan, an authority on climate modeling at the University of California at Santa Cruz, at least in terms of regional predictions. The models make accurate global predictions, she maintained, but fall apart when looking at regional effects.

"The closer we look ... the more things we have to incorporate into the models," she said.

Some errors occur because the computers must make calculations for a broad area, using average elevations, for instance, that might not reflect elevations in a particular subregion, she added.

By using the fossil record, however, modelers have been able to make adjustments. By incorporating the presence of lakes, for instance, they have been able to correct some of the predictions for the Rocky Mountain region.

"We're using the fossils to improve our models," she said. "It's a very synergistic and symbiotic relationship."

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