Prototaxites reinterpreted as mega-rhizomorphs, facilitating nutrient transport in early terrestrial ecosystems

The enigmatic fossil found in successions ranging from the Middle Ordovician to the Upper Devonian was originally described as having conifer affinity. The current debate, however, suggests that they probably represent gigantic algal-fungal symbioses. Our re-investigation of permineralized specimens from two localities, the Heider quarry in Germany and the Bordeaux quarry in Canada, reveals striking anatomical similarities with modern fungal rhizomorphs . We analysed extant fungal rhizomorphs and fossil through light microscopy of their anatomy, Fourier transform infrared spectroscopy, X-ray microscopy, and Raman spectroscopy. Based on these comparisons, we interpret the as fungi. The detailed preservation of cell walls and possible organelles seen in transverse sections of reveal that fossilization initiated while the organism was alive, inhibiting the collapse of delicate cellular structures. has been interpreted to grow vertically by many previous workers. Here we propose an alternative view that represents a complex hyphal aggregation (rhizomorph) that may have grown horizontally similar to modern complex aggregated mycelial growth forms, such as cords and rhizomorphs. Their main function was possibly to redistribute water and nutrition from nutrient-rich to nutrient-poor areas facilitating the expansion for early land plant communities.