Prototaxites
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Prototaxites Temporal range: 420–370 Ma Late Silurian to Late Devonian |
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| An 1888 illustration of Prototaxites in section. | |
| Scientific classification | |
| Kingdom: | Fungi |
| Phylum: | Glomeromycota |
| Order: | Nematophytales Lang |
| Family: | Prototaxitaceae Hueber |
| Genus: |
Prototaxites Dawson, 1859 |
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Prototaxites /ˌproʊtoʊˈtæksᵻˌtiːz/ is a genus of terrestrial fossil organisms dating from the Late Silurian until the Late Devonian periods, approximately 420 to 370 million years ago. Prototaxites formed large trunk-like structures up to 1 metre (3 ft) wide, reaching 8 metres (26 ft) in height, made up of interwoven tubes around 50 micrometres (0.0020 in) in diameter. Whilst traditionally very difficult to assign to an extant group of organisms, current opinion suggests a fungal placement for the genus. Recent discovery of what are likely algal symbionts makes it a lichen, rather than a fungus in the strict sense.Lichens are classified with the fungal not the algal component, and the main tubular cells of Prototaxites are most like those of the fungal phylum Glomeromycota.
Alternative views suggest that Prototaxites was a vascular plant superficially like yew trees (hence the generic name), a kelp-like alga, or enrolled liverwort mats with associated cyanobacteria and fungal tubular elements.
With a diameter of up to 1 m, and a height reaching 8 m, 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 "trunks" 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 μm across, have thick (2–6 μm) walls and are undivided for their length, and "filaments", which are thinner (5–10 μm diameter) and branch frequently; these mesh together to form the organism's matrix. These thinner filaments are septate – that is to say, they bear internal walls. These septa are perforate - i.e. they contain a pore, a trait only present in the modern red algae and fungi.
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