FOSSIL MUSHROOMS FROM MIOCENE AND CRETACEOUS AMBERS AND THE EVOLUTIONOF HOMOBASIDIOMYCETES

Two species of fossil mushrooms that are similar to extant Tricholomat aceae are described from Cretaceous and Miocene ambers. Archaeomarasmi us 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 pile us with stipe, and the other consists of a fragment of a pileus. The l atter was accidentally exposed, and zxsubsequently was used for molecu lar systematics studies (attempts to amplify ribosomal DNA were unsucc essful) and electron microscopy. The spores are smooth and broadly ell iptic with a distinct hilar appendage. Protomycena electra gen. et sp. nov., which is represented by a single complete fruiting body from Mi ocene amber of the Dominican Republic, is similar to the extant genus Mycena. Based on comparison to extant Marasmieae and Myceneae, Archaeo marasmius and Protomycena were probably saprophytes of leaf litter or wood debris. The poor phylogenetic resolution for extant homobasidiomy cetes limits the inferences about divergence times of homobasidiomycet e 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 st udies have suggested that there has been extensive convergence and par allelism in the evolution of homobasidiomycete fruiting body form. Nev ertheless, the striking similarity of these fossils to extant forms su ggests that in certain lineages homobasidiomycete macroevolution has a lso involved long periods during which there has been little morpholog ical change.