The Giant Rhizomorphs Creeping Across the Silurian Landscape — Prototaxites

Previous reconstruction of Prototaxites（感謝Mary Parrish, National Museum of Natural History提供）

The colonization of land by plants can be traced back to the Ordovician Period. By the Late Silurian, terrestrial vegetation had begun to develop more complex forms and a greater diversity of species. Even so, these early plants remained relatively small in stature, with the tallest individuals reaching only about one to two meters in height. At the same time, however, an extraordinary organism appeared on land: a gigantic fungus that could rival modern trees in size. This organism, known as Prototaxites, could reach lengths of up to eight meters and diameters approaching one meter, making it the largest terrestrial organism of its time.

In an era when terrestrial ecosystems were still sparse and biological productivity was limited, Prototaxites may have played a crucial role as a major carbon source for other organisms. Evidence supporting this ecological role was discovered in 2001, when Francis Hueber reported traces of arthropod borings within the “trunk-like” structures of Prototaxites, indicating that animals interacted directly with these massive fungal bodies.

Since its discovery, the biological identity of Prototaxites has been the subject of extensive debate. Early interpretations proposed that it represented a vascular plant. Others suggested that it might have been a giant alga or a large liverwort. Later, carbon isotope analyses indicated that Prototaxites was heterotrophic, shifting the prevailing view toward a fungal identity or possibly a symbiotic association between fungi and algae. Within fungi, its affinities have most often been suggested to lie either within Basidiomycota or Glomeromycota.

In 2023, a study led by Vivi Vajda offered a new interpretation of Prototaxites. The researchers compared fossil material with rhizomorphs from several living basidiomycete fungi, including Armillaria mellea, Postia rennyi, and Fomitopsis pinicola. Rhizomorphs are root-like structures composed of thousands of fungal hyphae aligned in parallel, forming elongated, hollow strands. These structures typically grow beneath tree bark or underground and function primarily in the transport of nutrients.

Through detailed morphological comparisons, combined with analytical techniques such as Fourier-transform infrared spectroscopy, Raman spectroscopy, and X-ray analyses, the researchers examined structural similarities between these modern rhizomorphs and Prototaxites. The results revealed striking morphological correspondence, leading the authors to propose that Prototaxites was not a fruiting body but rather an enormous fungal rhizomorph.

Rhizomorphs of fungi（圖片來源：Distant Hill，採用 CC BY-NC-SA 2.0 授權）

Based on this interpretation, the authors argued that structures specialized for nutrient transport would be unlikely to grow vertically like a fruiting body. Instead, Prototaxites likely spread horizontally across the ground surface or through the subsurface, extending in multiple directions to absorb nutrients from widely scattered sources. Earlier assumptions that Prototaxites grew upright may have arisen simply because the fossils were initially mistaken for trunks of conifer-like plants, an interpretation that persisted for many years despite limited supporting evidence.

Appearance of Prototaxites according to Vivi Vajda et al.（圖片來源：Vivi Vajda et al，採用 CC BY 4.0 授權）

Fossilized tissues of Prototaxites also contain abundant quartz near the inner hollow region of the structure. This mineral composition suggests that these organisms lived in environments influenced by volcanic activity. During growth, silica-rich waters likely permeated the rhizomorph tissues. The presence of dissolved silica allowed rapid mineralization of internal cellular structures, helping preserve exceptionally detailed cellular anatomy within the fossils.

The appearance of such massive rhizomorph structures during the Silurian and Devonian likely reflects ecological conditions in early terrestrial ecosystems. Vegetation was sparse, and nutrients in soils were widely dispersed. Under such circumstances, organisms capable of developing extensive networks for nutrient acquisition would have gained a significant advantage. Modern fungal rhizomorphs possess central cavities that facilitate oxygen transport, and the formation of structures as large as those attributed to Prototaxites would have required substantial oxygen supply. The hollow central architecture therefore may have functioned not only in nutrient movement but also in oxygen transport.

As terrestrial ecosystems continued to evolve, vascular plants and land animals became increasingly abundant. With the expansion of vegetation and the growing complexity of ecological interactions, nutrient resources became more readily available—but competition intensified. These changing conditions may have removed the ecological advantage that once favored the enormous rhizomorph systems of Prototaxites. As a result, fungi may no longer have needed to produce such gigantic structures. Over time, organisms that once relied on fungi as a primary carbon source or as substrates for burrowing may have shifted their ecological focus toward plants instead.

Author: Shui-Ye You

Reference:

Vajda, V. et al. (2023). Prototaxites reinterpreted as mega-rhizomorphs, facilitating nutrient transport in early terrestrial ecosystems. Can J Microbiol.