What is "prototaxites"

Prototaxites 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 wide, reaching in height, made up of interwoven tubes just 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.