Ferns from the Arroyo Chacay flora (Huitrera Formation, Eocene) Río Negro Province, Argentina

Abstract

During the Eocene, a mixed paleoflora containing austral-gondwanan and neotropical elements was established in northern Patagonia. Fossils preserving these assemblages have been recovered from the Huitrera Formation, at well-known localities as Laguna del Hunco, Pichileufu, Confluencia, Pampa de Jones and Arroyo Chacay, and show a diverse assemblage of angiosperms and conifers. The pteridophytic element, while present, is less known for these fossil floras. In this contribution, seven fern taxa based on fertile and sterile leaf compressions are described from the Huitrera Formation in the Arroyo Chacay locality. Several fern families are identified, as Osmundaceae (cf. Osmunda and Todea amissa M. Carvalho), a putative Thyrsopteridaceae (“Dicksonia” patagonica Berry), Blechnaceae (cf. Parablechnum), and Aspleniaceae (cf. Asplenium). Additionally, a taxon of unknown affinities, referred to the fossil genus Cladophlebis, is also recorded. These elements add to the total known diversity of ferns in the locality, previously represented by the Eupolypod Tapelrayen helgae M.A.Machado, E.I.Vera, Passalia et Ponce and Eupolypod incertae sedis, and the thelypteridaceous Cyclosorus sp. cf. C. interruptus. It is worth mentioning that two of the recorded taxa, T. amissa and “Dicksonia” patagonica, have been previously identified in the Huitrera Formation at other localities (Laguna del Hunco and Pichileufu, respectively), while the remaining identified taxa are new for the unit. Using the nearest living relative approach to infer ecological requirements of the recovered fossil ferns and evaluate climatic conditions supports a temperate to warm and humid regime in northern Patagonia during the Eocene, as previously postulated.