The enigmatic giant fossil Prototaxites reveals a new facet of its life in the Early Devonian

Before land plants came to dominate terrestrial habitats, metre-tall, columnar structures of unknown systematic classification – known by the generic name Prototaxites – were the largest land-dwelling organisms during the Early and Middle Devonian. The size and biomass of these organisms suggest that Prototaxites provided a potentially rich food source for arthropods and other terrestrial organisms capable of utilising their robust tissue. Our study of three silicified Prototaxites stem fragments revealed a complex system of tunnels, some of which feature chamber-like expansions and are filled with coprolites of two morphotypes: both are characterised by a prismatic shape with a hexagonal cross-section and differ in size and length-to-width ratio. We introduce a new ichnogenus, Hexacoprus igen. nov., for these coprolites and two ichnospecies, H. zenneri igen. nov. et isp. nov. and H. elongatus igen. nov. et isp. nov. The partial healing of the feeding tunnels, similar to that recognised in modern fungi, proves that the tunnels were formed during the organism’s lifetime. The fact that hexagonal coprolites are now found exclusively in insects, which possess six rectal pads for shaping and compressing faecal matter, suggests that insects adapted to a endophagous lifestyle already existed in the Early Devonian. The simultaneous presence of smaller, elongated coprolites in the same feeding burrows points to coprophagy – the earliest evidence to date of this behaviour in terrestrial organisms. The newly discovered feeding tunnels in most likely Early Devonian Prototaxites specimens from various sites in Germany indicate that insects appeared much earlier in terrestrial habitats, a finding consistent with predictions from molecular phylogenetic studies of the emergence of the Pterygota.