Prototaxites was an extinct lineage of multicellular terrestrial eukaryotes
Prototaxites was the first giant organism to live on the terrestrial surface, reaching sizes of 8 metres in the Early Devonian. However, its taxonomic assignment has been debated for over 165 years. Tentative assignments to groups of multicellular algae or land plants have been repeatedly ruled out based on anatomy and chemistry resulting in two major alternatives: Prototaxites was either a fungus or a now entirely extinct lineage. Recent studies have converged on a fungal affinity. Here we test this by contrasting the anatomy and molecular composition of Prototaxites with contemporary fungi from the 407-million-year-old Rhynie chert. We report that Prototaxites taiti was the largest organism in the Rhynie ecosystem and its anatomy was fundamentally distinct from all known extant or extinct fungi. Furthermore, our molecular composition analysis indicates that cell walls of P. taiti include aliphatic, aromatic, and phenolic components most similar to fossilisation products of lignin, but no fossilisation products characteristic of chitin or chitosan, which are diagnostic of all groups of extant and extinct fungi, including those preserved in the Rhynie chert. We therefore conclude that Prototaxites was not a fungus, and instead propose it is best assigned to a now entirely extinct terrestrial lineage.
(...) Having found no support for the most widely held view that Prototaxites was fungal, we next reviewed possible placement in other higher taxonomic groups. No extant group was found to exhibit all the defining features of Prototaxites, namely: (1) formation of large multicellular structures of varied tube types; (2) a tube composition rich in aromatic-phenolic components; and, (3) a heterotrophic (likely saprotrophic) lifestyle (Table 1). Based on this investigation we are unable to assign Prototaxites to any extant lineage, reinforcing its uniqueness. We conclude that the morphology and molecular fingerprint of P. taiti is clearly distinct from that of the fungi and other organism preserved alongside it in the Rhynie chert, and we suggest that it is best considered a member of a previously undescribed, entirely extinct group of eukaryotes.
