EXTREMELY DIVERSE FOSSIL FLORAS FROM THE PALEOGENE OF PATAGONIA, ARGENTINA: IMPLICATIONS FOR ORIGINS OF HIGH PLANT AND INSECT DIVERSITY IN SOUTH AMERICA

The origins of hyperdiversity in Neotropical biotas are poorly understood because extensive vegetative cover and weathering obscure the early Cenozoic macrofossil record of northern South America. Paleogene fossil floras from middle latitudes of Patagonia are invaluable and mostly overlooked resources for bypassing this problem. These assemblages, which are well-exposed in modern desert outcrops, represent humid subtropical vegetation from the southern reaches of the expanded Neotropical forests that existed during the globally warm early Paleogene.

From recent quantitative, stratigraphically and geochronologically controlled work at Laguna del Hunco, a site with early Eocene (52 Ma) caldera-lake deposits in Chubut Province, Argentina, we have recovered more than 155 leaf species from fewer than 5,000 specimens and more than 200 total morphotypes of leaves, fruits, seeds, and flowers. Fewer than 25% of these entities were previously known. This significantly exceeds the richness of Eocene floras from any other region when adjusted for sample size, yet the potential exists for many more species to be found. Laguna del Hunco features outstanding, detailed preservation equal to the most famous North American Eocene floras such as Florissant, Green River, and Republic. Preliminary work also indicates the highest diversity of well-preserved insect damage of any Cenozoic flora known, including unusually diverse and previously unrecorded examples of specialized endophytic damage such as galls and mines. The Eocene Río Pichileufú flora, from ~160 km NW of Laguna del Hunco, features similar floral diversity as well as diverse insect damage. These results support the hypothesis that Neotropical hyperdiversity, usually attributed to Neogene or Pleistocene events, may have a considerably more ancient history than previously thought.