The oldest generally accepted fossil of mushroom-forming fungi (homoba
sidiomycetes) 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 projec
t was to reassess the identity of Phellinites and refine the minimum-a
ge estimate for the origin of homobasidiomycetes. We examined the holo
type of Phellinites digiustoi and a second collection from the type lo
cality. 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. Fos
sils of Araucariaceae are common in the region where Phellinites was f
ound, and so it is plausible that Phellinites is the bark of an Arauca
ria-like tree. The minimum age of the mushroom-forming fungi, based on
direct fossil evidence, is revised to the mid-Cretaceous.