Not all fungi, plants or animals may be considered the earliest giants on earth. The tower of column of trunk-like structures, also known as prototaxites, are found as fossils, dating back to rocks over 400 million years old, have not yielded to any biological classification in over 100 years, and a new wave of microscopy and chemistry suggests that they are the point.
During the Devonian, there were no forests and canopies. The living cover of the land was low, sporadic and experimental: initial vegetation near the earth, tiny fungi, tiny arthropods. It is against such a background that the remains of Prototaxites can be read as an invasion by an alien race of pillars, smooth as columns, 8 metres high, no leaves, no flowers, no branches, no recognisable architecture at all. In 1843, the earliest fossils were discovered and the name, which translates to the early yew, was retained, even following the discovery that the organism was not a conifer.
There has never been a dearth of probable possibilities in the long debate over Prototaxites. This has been thrown down the decades as algae, rolled-up mat of plants, lichen-like partnership, and, most steadfastly, a giant fungus. The fungal concept was popularized since most of the specimens are made out of interlaced tubes which can also give the effect of fungal hyphae. The case was further supported by chemical work in the 2000s that indicated a more lifestyle of a decomposer than a photosynthesiser. However, the fit never fitted, since every explanation provided one solution and generated another.
The quality of the evidence of the fossils found in the Rhynie chert of Scotland which forms a particularly good window into early terrestrial environments changes the balance of the debate. It was re-examined by researchers on a Rhynie specimen of Prototaxites taiti-smaller than the renowned metre scale columns, but in sufficient condition to examine its internal structure and chemistry in detail. The resulting image only appears cursory fungal, at a distance.
When examined further on the microscope, the tubes within Prototaxites are not acting like a fungal hyphae. The tubes bifurcate, combine and reconnect in patterns that are not of the sort seen in the construction of known fungi. Others even demonstrate banded walls that make one think of some vascular plant structures, without necessarily transforming the organism into a plant. This is important since Prototaxites has been frequently just treated as fungus until some other evidence contradicts this stance; architecture is among the few places where it can be argued that there is evidence against this default.
The chemistry pushes harder. Fungi in the same fossil deposit still have chemical remnants of major cell-wall constituents. Prototaxites does not: the new analyses did not detect any chitin signal – a characteristic polymer which should be present in fungal cell walls, and which is detectable in fossil fungi found with it. Rather, the Prototaxites chemical profile is unique to the fungal one, and certain measurements suggest a unique chemical fingerprint, as opposed to a degraded form of an old friend.
To confirm that the term “distinct” only reflected a state of poor preservation, the team compared a large number of samples of Rhynie chert on a spectra basis as well as a computationally determined classification. In a machine-learning study, after being trained on the chemistry of several fossil and modern clusters, Prototaxites were consistently separated from fungi and other organisms that were found in the same rocks. It is not known what Prototaxites was, but the result goes along with the idea that it was a branch of a completely extinct eukaryotic lineage- a branch of complex life with no extant representatives.
A single quote is a summation of the discomfort that the fossil has brought over the generations. It is like that, it does not fit into any category comfortably, said Matthew Nelsen, who was not a part of the work, to explain why efforts to shoehorn Prototaxites into modern categories continue to fail.
In case Prototaxites indeed belong to no modern taxa, then the interest is not only taxonomic. This is evidence that body plans of large, complicated structures were experimented with in early life on land, and have since then lost all trace of genes, leaving only an anatomy and chemistry fixed in stone.
