"Tropical South America is the most biodiverse region today," says Dr. Peter Wilf, assistant professor of geosciences at Penn State. "However, this richness is usually considered to be geologically recent."
Typically, high neotropical diversity was thought to originate during the last two million years, when the ice ages may have shrunk the rainforests, to perhaps as long ago as 20 million years when the Andean range began to rise.
"There has been little evidence but much debate about the history of the exceptional plant diversity of tropical South America," says Wilf.
Working with N. Ruben Cuneo, director of Egidio Feruglio Paleontological Museum, Trelew, Argentina; Kirk R. Johnson, curator of paleontology and Jason F. Hicks, research associate, Denver Museum of Nature & Science; Scott L. Wing, curator of paleobotany, National Museum of Natural History, the Smithsonian Institution; and John D. Obradovich, geochronologist, U.S. Geological Survey, Denver, Wilf looked at fossils from Laguna del Hunco in Patagonia, Argentina.
"Adjusted for sample size, observed richness exceeds that of any other Eocene leaf flora, supporting an ancient history of high plant diversity in warm areas of South America," the researchers report in today's (April4) issue of the journal, Science.
Although the fossil site, 817 miles southwest of Buenos Aires, is currently in a temperate desert area, it had a warm, frost-free and moist climate with lush vegetation during the early Eocene, 52 million years ago. This period was the warmest of the past 70 million years of Earth history and predated the Andes mountain range, which currently blocks Pacific moisture from reaching the area.
"We can investigate the past biodiversity of South America not only in the modern tropics, but also in the past. During the warm Eocene, tropical vegetation grew at middle latitudes where we get abundant rock exposure today," says Wilf. He notes that often fossil hunters in the tropics encounter flooded areas, heavy forest cover and little, if no, exposed rock.
Many Eocene fossil sites in North America have been collected 100 years or more. Laguna del Hunco, though known for 80 years, is now the first of this age from South America to be heavily and quantitatively sampled. Quantitative sampling, where every specimen is tallied and identified, allows sample size to be taken into account when comparing recovered diversity. The age of the deposit was also not well constrained.
The researchers collected more than 1,500 fossils and identified more than 100 different fossil leaf species including dicots, monocots, conifers, ginkgophytes, cycads and ferns. They also identified a variety of seeds, fruit and flowers. In total, they more than tripled the known diversity of the site in two weeks. Using paleomagnetic dating, which uses the Earth's magnetic pole reversals, and argon argon dating, which compares the amounts of two isotopes of argon one of which is produced by the natural radioactive decay of potassium, the researchers dated the fossils to a half million year interval between 52 and 53 million years ago. These are the first high-precision ages for the deposit, which now can be correlated anywhere in the world.
"With good dating information, an adjusted sample size and a large number of species, we can say that South American plant diversity began very far in the past and continues to this day," said Wilf. "We can also compare the richness we found to that of diverse floras from the North American Eocene that also have quantitative data."
There are several Eocene sites in North America where a high diversity of plant species has been identified, but this results from years of collecting. At Laguna del Hunco, 1,300 dicot fossils yielded 90 species of dicots, while a North American fossil location in Wyoming showed only about 30 species from about 2,250 specimens. The more than 5,000 specimens collected at Chalk Bluffs, Calif., and 4,000 specimens from Florissant, Colo., produced fewer than 50 dicot species at each site.
Statistical projections show that the number of species that may be found at Laguna Del Hunco with further collecting is very high, a prediction that is borne out from follow-up field work the researchers conducted recently in the area. The original two-week collecting trip was supported by Andrew W. Mellon Foundation, University of Pennsylvania and American Chemical Society.
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