Is Phellinites digiustoi the oldest homobasidiomycete?

The oldest generally accepted fossil of mushroom-forming fungi (homobasidiomycetes) is Phellinites digiustoi, from the Jurassic of Patagonia. The next-oldest homobasidiomycete fossil does not occur until about 70 million years later, in the mid-Cretaceous. The goal of this project was to reassess the identity of Phellinites and refine the minimum-age estimate for the origin of homobasidiomycetes. We examined the holotype of Phellinites digiustoi and a second collection from the type locality. Thin sections were prepared from both collections. Phellinites has a cellular structure, confirming that it is biological in origin, but there are no hyphae, spores, or other fungal structures. Rather, Phellinites appears to be the outer bark (rhytidome) of a conifer. Fossils of Araucariaceae are common in the region where Phellinites was found, and so it is plausible that Phellinites is the bark of an Araucaria-like tree. The minimum age of the mushroom-forming fungi, based on direct fossil evidence, is revised to the mid-Cretaceous.