EOCENE PLANT DIVERSITY AT LAGUNA DEL HUNCO AND RÍO PICHILEUFÚ, PATAGONIA, ARGENTINA

The Eocene floras of Laguna del Hunco (LH) and Río Pichileufú (RP) in Patagonia, Argentina, are among the richest known from the paleobotanical record. They indicate ancient origins of high plant diversity in South America, which generally lacks reliable fossil evidence regarding its exceptional modern richness. The floras also provide paleoclimatic data and occurrence data relevant to the biogeographic history of Gondwanic and tropical plant lineages. However, the LH and RP floras are historically undersampled and lacking in stratigraphic context. We report on extensive, unbiased collections from LH and on results of preliminary sampling at RP.

At LH, from 4303 identified specimens at 25 quarries in a 170 m measured section, we recognize more than 190 species of plant organs (more than four times the total of historic accounts) and more than 155 species of leaves. Adjusted for sample size using rarefaction, the LH flora is significantly more diverse than any comparable Eocene flora known from other continents. Rarefaction of the RP flora, which is compositionally similar to LH, shows it to be as rich as the most diverse locality from LH. Three high-precision Ar-Ar ages, the first for the RP flora, all are near 47.5 Ma, about 5 Myr younger than Laguna del Hunco. Thus, diverse, stable floral associations existed over a broad area of Patagonia for several million years; this suggests the existence of extraordinary richness at low latitudes of South America via the latitudinal diversity gradient.

Paleobotanical data from LH indicate a mean annual temperature near 16°C, mean annual precipitation of more than 1 m, and a maritime climate with little seasonal variation. These results do not indicate a tropical rainforest biome as some have suggested, but they match coeval sea surface temperatures. The floras grew near the northern limit of a frost-free, humid biome that existed in South American middle latitudes during the warm Eocene. The lack of frost permitted the presence of diverse tropical lineages along with taxa now confined to temperate and/or tropical montane forests. Interchange of tropical taxa occurred with the American as well as the Australasian tropics, although the latter is far better understood because of a general lack of reliable megafloral data from the Paleogene Neotropics.