• Ascomycota;
  • Basidiomycota;
  • Dikarya;
  • fungal evolution;
  • higher fungi;
  • land colonization;
  • mycelial cords

The fossil record paints a thin picture of early terrestrial life. Useful diagnostic features are rare in the organic-walled fossils of the first land colonizers, and at first glance the Silurian–Devonian Tortotubus protuberans seems no exception. Now, new material from New York, Gotland and Scotland reveals the ontogenesis and affinity of this problematic organism. Its filamentous early stages (previously referred to Ornatifilum lornensis) demonstrate simple septal perforations and a bilayered cell wall; threads of entwined filaments, bounded by an elaborately sculptured surface, arose via the retrograde growth and subsequent proliferation of secondary branches. This morphology and pattern of growth together indicate an affinity with the ‘higher’ fungi (Dikarya) and document the formation of differentiated mycelium. The presence of complex mycelial fossils in the earliest Silurian corroborates the likely contribution of fungi to the colonization of land and the establishment of modern sedimentological systems; their rise seemingly accompanied the diversification of early embryophytes and the vegetation of the terrestrial biosphere.