FERNS FROM THE LAGUNA DEL HUNCO PALEOFLORA (51.9 MA), PATAGONIA, ARGENTINA, REVEAL NEW BIOGEOGRAPHIC LINKS TO RAINFOREST GONDWANA

The fossil record of Cenozoic ferns in South America is notably scanty, mostly due to the naturally low abundance of ferns in angiosperm-dominated forests, as well as a lack of continuous, systematic sampling on the continent. Likewise, fossil fern identification relies on the uncommon preservation of reproductive structures, leading to a poorly known and uninterpretable biogeography for the South American Cenozoic. We report four fertile fern genera from the Laguna del Hunco paleoflora, a montane rainforest assemblage better known for its extraordinary angiosperm diversity. This flora is preserved in caldera-lake deposits from the early Eocene (51.9 Ma) of Patagonia, Argentina, and has been shown to reflect the floristic connection between South America and Australasia via Antarctica at this time. The genera represent Todea (Osmundaceae), Sticherus (Gleicheniaceae), Dicksonia (Dicksoniaceae) and Pteridaceae (cf. Adiantum spp.), identified based on vegetative characteristics and soral or sporangial morphology. These 4 families share deep time divergences as evidenced by a fossil record that extends back into the Late Paleozoic and Mesozoic. Todea is now restricted to montane, humid subtropical and tropical Australasia and South Africa, and has been reported from late Cretaceous deposits of British Columbia and the Jurassic of Argentina. Todea seems to show a much broader distribution in the past than their extant counterparts, a pattern also seen with most Cenozoic ferns of the northern hemisphere. Dicksonia is present in the wet subtropics and tropics of Australasia and South America, and Sticherus is pantropical and austral, and mostly humid montane. Their fossil records remain sparse; both genera are known from Oligocene deposits of Tasmania, and Dicksonia has been also described from a middle Eocene flora from Argentina. These findings increase the number of tropical elements and more widespread Gondwanan lineages present in Patagonia during the early Eocene, and they support rainforest interpretations. The data contribute to previous biogeographic and rainforest affinities inferred for this paleoflora, which have suggested strongest links to the low latitude West Pacific, with additional connections to the montane neotropics and mid- latitude South America.