Tracing the Evolution of Dicksoniaceae and Thyrsopteridaceae (Cyatheales) in South America: New Records from the Cretaceous.

Background and aims: Two new species of cyathealean ferns are described based on sterile and fertile frond fragments from the Cretaceous of Patagonia, Argentina. The new species are compared with other extant and extinct cyathealean ferns in Dicksoniaceae and Thyrsopteridaceae.

Methods: The fern foliage macrofossils came from the Lower Cretaceous (Albian) Kachaike Formation, Santa Cruz Province, and the Upper Cretaceous (Maastrichtian) La Colonia Formation, Chubut Province. They were studied and photographed using standard techniques. Spores were also macerated from four fertile Kachaike Formation specimens and examined using light and scanning electron microscopy. Spores from a fertile La Colonia Formation specimen were described and figured in previous studies.

Key results: The frond material from the Kachaike Formation is placed in the new species Dicksonia hallei; it is morphologically consistent with the modern genus Dicksonia, although its spores are distinct and conform to the dispersed spore species Cyathidites minor. The material from the La Colonia Formation cannot be placed in an existing genus and is assigned to the new genus and species Rhabdopteris chubutensis; its spores are also similar to the dispersed spore species Cyathidites minor. Cyathidites minor-type spores are known from several other Mesozoic cyathealean foliage taxa.

Conclusions: Dicksonia hallei is currently the oldest credible macrofossil record of Dicksonia and Dicksoniaceae in South America, indicating that Dicksonia has been present on the continent at least since the Early Cretaceous. Because it has fertile pinnae and pinnules with highly reduced laminar tissue (skeletonized pinnae and pinnules) bearing Cyathidites minor-type spores, Rhabdopteris likely has affinities to the family Thyrsopteridaceae. The Thyrsopteridaceae have an extensive fossil record worldwide encompassing several fossil foliage genera.