EOCENE RAINFORESTS OF PATAGONIA AND THEIR HIGH-LATITUDE CONNECTION TO MODERN MONTANE AUSTRALASIA

The extremely biodiverse caldera-lake deposits at Laguna del Hunco (51.9 Ma) and Río Pichileufú (47.5 Ma) in Patagonia, Argentina present a longstanding biogeographic and paleoclimatic enigma. The sites contain hundreds of plant species along with insects, frogs, and fish, and their biogeographic importance is high due to location at the connection of South America to Australasia via Antarctica during the warm Eocene. However, understanding floral affinities and paleoclimate is impeded by incorrect legacy identifications and leaf-physiognomic proxies calibrated to unrelated extant floras. Our approach relies on systematic floral revisions from large new collections, focusing on the biogeography and climatic tolerances of crown taxa. We find that the strongest signature in the floras is to extremely wet, often montane habitats of subtropical and tropical Australasia. The conifer assemblage includes several genera of that region with demonstrated physiological intolerance to drought: Papuacedrus (Cupressaceae); Dacrycarpus, Acmopyle, and Retrophyllum (Podocarpaceae); and Agathis and Araucaria Sec. Eutacta (Araucariaceae). The fern flora displays a similar pattern (Carvalho et al., this volume), and includes Todea (Osmundaceae), Dicksonia (Dicksoniaceae), and Sticherus (Gleicheniaceae). Among angiosperms, the Australasian connection is represented by Gymnostoma (Casuarinaceae), Akania (Akaniaceae), Eucalyptus (Myrtaceae), and other typical groups such as Atherospermataceae, Monimiaceae, Cunoniaceae, and Proteaceae. Most of the taxa listed have Paleogene fossil records in Australia, and a few have been found in Antarctica. Neotropical affinities of the floras are few and mostly montane, and affinities to Andean temperate rainforest are tenuous. Vein density of fossil leaves is generally low and also supports greatest similarity to lower-productivity subtropical to tropical montane rainforests, rather than to highly productive, megathermal lowland rainforests. It has long been known that Eocene warmth at high latitudes facilitated migration of thermophilic biota across Laurasia. It is now clear that high-latitude Eocene interchange, and subsequent regional extinctions with climatic cooling and drying, significantly shaped lineage distributions in Gondwana as well.