Fossil mushrooms from Miocene and Cretaceous ambers and the evolution of Homobasidiomycetes

Two species of fossil mushrooms that are similar to extant Tricholomataceae are described from Cretaceous and Miocene ambers. Archaeomarasmius leggetti gen. et sp. nov., from mid-Cretaceous amber of New Jersey, resembles the extant genera Marasmius and Marasmiellus. Two fruiting bodies of Archaeomarasmius were found. One consists of a complete pileus with stipe, and the other consists of a fragment of a pileus. The latter was accidentally exposed, and zxsubsequently was used for molecular systematics studies (attempts to amplify ribosomal DNA were unsuccessful) and electron microscopy. The spores are smooth and broadly elliptic with a distinct hilar appendage. Protomycena electra gen. et sp. nov., which is represented by a single complete fruiting body from Miocene amber of the Dominican Republic, is similar to the extant genus Mycena. Based on comparison to extant Marasmieae and Myceneae, Archaeomarasmius and Protomycena were probably saprophytes of leaf litter or wood debris. The poor phylogenetic resolution for extant homobasidiomycetes limits the inferences about divergence times of homobasidiomycete clades that can be drawn from Archaeomarasmius and Protomycena. The ages of these fossils lend support to hypotheses that the cosmopolitan distributions of certain mushroom taxa could be due to fragmentation of ancestral ranges via continental drift. Anatomical and molecular studies have suggested that there has been extensive convergence and parallelism in the evolution of homobasidiomycete fruiting body form. Nevertheless, the striking similarity of these fossils to extant forms suggests that in certain lineages homobasidiomycete macroevolution has also involved long periods during which there has been little morphological change.     

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.