A fungal plant pathogen discovered in the Devonian Rhynie Chert.

are integral to well-functioning ecosystems, and their broader impact on Earth systems is widely acknowledged. Fossil evidence from the Rhynie Chert (Scotland, UK) shows that were already diverse in terrestrial ecosystems over 407-million-years-ago, yet evidence for the occurrence of the subkingdom of that includes the phyla and ) in this site is scant. Here we describe a particularly well-preserved asexual fungus from the Rhynie Chert which we examined using brightfield and confocal microscopy. We document gen. et sp. nov. that we attribute to . The fungus forms a stroma-like structure with conidiophores arising in tufts outside the cuticle on aerial axes and leaf-like appendages of the lycopsid plant . It causes a reaction in the plant that gives rise to dome-shaped surface projections. This suite of features in the fungus together with the plant reaction tissues provides evidence of it being a plant pathogenic fungus. The fungus evidently belongs to an extinct lineage of ascomycetes that could serve as a minimum node age calibration point for the as a whole, or even the crown group, along with some other previously documented in the Rhynie Chert.