Prototaxites is an extinct genus of large macroscopic eukaryote dating from the Late Silurian until the Late Devonian periods. Prototaxites formed large trunk-like structures up to 1 metre (3 ft) wide, reaching 8 metres (26 ft) in height, made up of tiny interwoven tubules around 50 micrometres (0.0020 in) in diameter, making it by far the largest land-dwelling organism of its time. The taxonomy of Prototaxites has long been the subject of debate. It is widely considered a fungus, but the debate is ongoing. Its exact relationship with extant fungus lineages is uncertain. It was almost certainly a perennial organism that grew over multiple years. Several ecologies have been proposed, including that it was saprotrophic like many modern fungi, or that it was a lichenised autotroph. In 2026, research comparing fossils of a prototaxite species with other eukaryotic fossils in the same location found it to be structurally and chemically distinct, suggesting that Prototaxites is not a fungus and instead belongs to an extinct, previously undescribed kingdom of eukaryotic life.
Morphology
With a diameter of up to 1 metre (3 ft 3 in), and a height reaching 8.8 metres (29 ft), Prototaxites fossils are remnants of by far the largest organism discovered from the period of its existence. Viewed from afar, the fossils take the form of tree trunks, spreading slightly near their base in a fashion that suggests a connection to unpreserved root-like structures. Infilled casts which may represent the spaces formerly occupied by "roots" of Prototaxites are common in early Devonian strata. Concentric growth rings, sometimes containing embedded plant material, suggest that the organism grew sporadically by the addition of external layers. It is probable that the preserved trunk-like structures represent the fruiting body, or sporophore, of a fungus, which would have been fuelled by a mycelium, a net of dispersed filaments (hyphae). On a microscopic scale, the fossils consist of narrow tube-like structures, which weave around one another. These come in two types: skeletal tubes, 20–50 micrometres (μm) across, have thick (2–6 μm) walls and are undivided for their length, and generative filaments, which are thinner (5–10 μm in diameter) and branch frequently; these mesh together to form the organism's matrix. These thinner filaments are septate – that is, they bear internal walls. These septa are perforate, containing a germ pore, a trait present only in the modern red algae and fungi. The similarity of these tubes to structures in the early plant Nematothallus has led to suggestions that the latter may represent leaves of Prototaxites. Unfortunately for this hypothesis, the two have never been found in connection, although this may be a consequence of their detachment after the organisms' death.
History of research
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