Eocene plant diversity at Laguna del Hunco and Río Pichileufú, Patagonia, Argentina

South American floras are famous for their high species diversity. However, because of the undersampled South American plant-fossil record, remarkably little is known about how long this floral richness has existed. The Patagonian region of Argentina is an ideal place to study the history of South American plant diversity: fifty million years ago, Patagonia had humid subtropical forests with luxuriant vegetation, and today some of these forests are beautifully preserved in desert rock exposures accessible to paleontologists.

For this study, which will appear in the June 2005 issue of The American Naturalist, researchers collected more than 4000 specimens of fossil leaves, fruits, seeds, and flowers at a site with outstanding fossil preservation, Laguna del Hunco. Adjusted for sampling effort, the Laguna del Hunco flora, which has produced 186 species, is more diverse than any comparable deposit known elsewhere in the world. Preliminary collections at a second site, Río Pichileufú, show comparable richness. Precise radioisotopic ages, including the first from the Río Pichileufú flora, show it be almost 5 million years younger (47.46 ± 0.05 myr) than Laguna del Hunco (51.91 ± 0.22 myr). Highly diverse floras were present in a warm area of South America by the Eocene and were long lived and areally extensive.