Prototaxites: the Devonian giant that was probably not a fungus

Prototaxites formed massive, columnar structures in the landscapes of the Silurian and Devonian. Some fossils are several metres long and about a metre across, while the land plants of the time were far shorter. Since the nineteenth century palaeontologists have tried to place the organism in a known group: a tree trunk, an alga, a lichen, a rolled mat of liverworts and a giant fungus have all been proposed.

The fungal interpretation was strengthened by carbon isotope analyses. The variable isotopic composition of samples looked like the signature of a heterotrophic organism taking carbon from various components of its environment, rather than a photosynthesising plant. The internal structure, composed of numerous tubes, also brought hyphae to mind. On that basis Prototaxites often appeared in reconstructions as an enormous fungus towering over low vegetation.

More recent structural and chemical studies have weakened that confidence. The expected signals of chitin and chitosan were not found in the material, and the arrangement of the tubes proved hard to reconcile with the anatomy of known fungi. The authors of a 2025 analysis proposed that Prototaxites may represent an entirely extinct lineage of complex land organisms that cannot be forced into a modern kingdom.

The title of this entry contains the word "may" deliberately. Science does not choose the most spectacular reconstruction once and for all; every hypothesis has to fit new data. Prototaxites still teaches something important about the early land surface, even if it ceases to be a fungus: ancient ecosystems contained forms of life without exact modern counterparts, and a fossil shape is not enough to establish relationships. Resolving it requires anatomy, chemistry and geological context together. Each of those lines of evidence is preserved differently in fossils and has its own limitations. The absence of chitin may also result from fossilisation, so one analysis does not close the discussion, though it substantially shifts the burden of proof.

What the evidence actually shows

The headline must remain conditional—and the newest evidence weakens the fungal hypothesis further. Prototaxites formed leafless columns, reaching roughly eight metres in the largest forms, from the late Silurian into the Devonian. Loron and colleagues examined an exceptionally preserved, 407-million-year-old specimen of P. taiti from the Rhynie chert. Three-dimensional anatomy revealed tube arrangements and branching patterns without a good counterpart in known fungi, while spectroscopy distinguished its organic signature from fossil fungi preserved in the same rock. The authors conclude that this species is better placed in an extinct eukaryotic lineage than treated as a confirmed fungus. That result does not automatically establish the biology of every species assigned to Prototaxites, but the popular phrase “giant Devonian fungi” can no longer be presented as settled fact.