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Trove of leaf fossils raises new questions about ancient plant life

Excavation findings in Argentina may spell extinction for an old theory and underscores the rich diversity of species in South America

Monday, April 07, 2003

By Byron Spice, Post-Gazette Science Editor

Peter Wilf's work as a paleobotanist has taken him far and wide, from the Rocky Mountains to Pakistan, in search of fossil evidence of ancient plant life. But nothing prepared him for what he found during excavations in the Patagonian desert of Argentina in November 1999.

A fossil of a three-lobed Eocene leaf collecte at Laguna del Hunco. (Peter Wilf photos)

In two weeks of collecting at Laguna del Hunco, a site 800 miles southwest of Buenos Aires, Wilf and colleagues from Argentina and the United States gathered more than 1,500 fossils and identified more than 100 leaf species in a deposit dating back 52 million years, more than tripling the known diversity of the site.

"This is something that really overwhelms you when you're down there," said Wilf, an assistant professor of geosciences at Penn State University, noting two-thirds of the new species were found in just four holes. And subsequent analysis would confirm that the diversity of plant species found in those holes is far greater than has been found in similar excavations anywhere else in the world.

South America today is known to harbor more species of plants, animals and insects than any other region. Wilf's findings, published Friday in the journal Science, show that South America enjoyed that same distinction 52 million years ago.

That fact runs counter to the so-called "refuge theory," which for the past three decades has been the leading explanation for the extreme biological diversity of South America, and thus may force scientists to rethink the conditions believed to give rise to new species.

Laguna del Hunco in Patagonia, Argentina. The dark rock forming skyline at center is Piedra Alta, a volcanic neck. The white rocks in front of and behind Piedra Alta are Eocene lake beds that have produced hyperdiverse fossil floras. (Peter Wilf photo)

"This is a wonderful piece of work," said Paul Colinvaux, an ecologist and adjunct scientist at the Marine Biological Laboratory in Woods Hole, Mass., who has been a leading critic of the refuge theory.

Explaining how new species come about has long been a puzzle for scientists -- 19th century evolutionist Charles Darwin called it "the mystery of mysteries." As two British ecologists, Sandra Knapp and James Mallet, suggest in a commentary accompanying Wilf's article, that would make the forests of South and Central America, home to more plant species than Asia and Africa combined, "the most mysterious of all."

Many biologists, they noted, have long suspected that new species arise when separate populations of a species become geographically isolated. With no interbreeding between the populations, distinct genetic mutations can accumulate until the populations diverge into different species.

This was the idea behind the refuge theory, developed in the 1960s by German ornithologist Jurgen Haffer and British botanist Ghillean Prance. They tied the rise of so many species in South America to the Pleistocene Age, the period beginning about 2 million years ago when successive ice ages caused glaciers to advance down from the polar regions and then recede.

In South America, the advancing glaciers would have broken up the thick tropical forest, isolating populations in "refuges" between the glaciers and spurring speciation. This process would be repeated each time the glaciers expanded and then receded, becoming a "species pump."

The problem is that, despite being the so-called Crown Jewel of Evolution, South America has not been as intensively studied as North America and many other regions.

"It's a hell of a big piece of real estate," explained Colinvaux, an emeritus professor of zoology and anthropology at Ohio State University, who spent 20 years studying the Amazon "from end to end."

Moreover, it's tough to find fossil evidence in a rain forest, where the ground cover is thick and the hot, wet climate has weathered the soil to a depth of 50 feet, Wilf said.

Patagonia is a different story. During the Eocene Age, beginning 55 million years ago, the Earth was in a period of global warming, when palm trees could be found in Alaska and alligators swam in the Arctic. Patagonia at the time wasn't a tropical rain forest, but it was a humid, subtropical forest. The Andes Mountains, which turned Patagonia into a desert by shielding it from moist Pacific air, wouldn't rise for another 30 million years.

The Laguna del Hunco site was first discovered in the early 1920s by an oil company geologist and scientists have excavated it ever since. But until the 1999 effort, no one had done a thorough sampling of the site that accounted for every specimen removed. This sort of "quantitative sampling" allows scientists to compare the site's biological diversity with that of sites elsewhere on the globe.

Scientists from the Denver Museum of Nature & Science, the Smithsonian Institution, U.S. Geological Survey and the Edigio Feruglio Paleontological Museum in Argentina also participated in the study.

With funding from the National Geographic Society, Wilf returned to the site last December for another excavation, gathering three times as many specimens as in 1999. Though the identification of species is continuing, he said last week that preliminary analysis suggests the diversity of the new specimens is consistent with the earlier dig.

"This suggests that South America may have had extreme plant diversity long before the ice ages," Wilf said. And if the ice ages weren't necessary to create such diversity, where does that leave the refuge theory?

"The refuge theory is completely dead," Colinvaux said. His own studies of pollen samples obtained from dry lake sediments and the work of other researches suggest the Amazon was forested throughout the Pleistocene Age and that glaciers never divided it into refuges.

The existence of extreme diversity 50 million years before the ice ages only strengthens the argument against the refuge theory, Colinvaux said.

But the question of why South America harbors such diversity remains. No one yet knows how far back South America enjoyed its relative abundance of species, Wilf said, but perhaps part of the answer may have something to do with the massive asteroid collision that many scientists now suspect killed the dinosaurs 65 million years ago. That asteroid struck in North America and may have had less effect on South America, he suggested.

Another theory, Knapp and Mallet said, is that the very vastness of the Amazon rain forest might explain the high rate of speciation. The size of the forest means that different populations within it might never have contact with each other. The populations thus become isolated, not because of any geographic separation, but because they are just so darned far away from each other.

It's an explanation that actually dates back to 1859 and Darwin's "Origin of Species," they noted.

"Although I do not doubt that [geographic] isolation is of considerable importance in the production of new species, on the whole I am inclined to believe that the largeness of area is of more importance," he wrote.

And despite the mounting evidence against the refuge theory, Colinvaux acknowledged that many are slow to abandon it. He recalled a visit years ago from Ernst Mayr, a renowned evolutionary biologist at Harvard University and a refuge theory proponent. He and Mayr talked about Colinvaux's conviction that physical evidence would eventually disprove the theory.

After two hours, Mayr got up to leave. "He said, 'Paul, even if you do get this evidence and prove that the Amazon was never arid, no one will believe you because the refuge theory is so beautiful,' " Colinvaux recalled, chuckling.

Byron Spice can be reached at bspice@post-gazette.com or 412-263-1578.

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