“What we can say, based on all of those new analyses, is that it’s so different from any modern group we have,” said Corentin Loron, a palaeontologist at Edinburgh University, describing why one of the earliest terrestrial giants still resists a proper name.
Smooth, trunklike pillars, the Prototaxites, had a shocking vertical character long before the forests, before the dinosaur footprints, even before the trees, Devonian scenes. Certain fossils suggest that bodies could grow up to 30 feet (9 meters) tall, and thus were the largest fully terrestrial life-forms known in the time they were, towering above plants that remained less than a meter. Prototaxites was first described in the 19th century, and has been debated ever since, with the various options being that it is rotten wood, a gigantic fungus, a lichen-like alliance, it can explain something, but it fails elsewhere.
The latest redefinition effort is that of extremely well-preserved content in the Rhynie chert of Scotland, which is an unusual window into the primeval land ecosystems that developed in the region of ancient hot-spring activity. It is the power of the deposit that is not spectacle but chemistry, that molecules which might otherwise be lost provide fossilization products that can still be read. The Prototaxites in the Rhynie chert were compared by Loron and others with those of real fossil fungi found in the same rock and in the same burial conditions, minimizing the possibilities of geology being used to explain the disparity.
Prototaxites does not feel any less like a fungus under the microscope: the first impression is of an interweaved tube structure, which resembles the threadlike form of hyphae. However, the similarity becomes weak when examination is made. The tubes in the Rhynie specimen divide and rejoin in structures that are not consistent with established fungal development, and the fossil itself has dense and dark spherical areas with rich internal branching that might have been related to exchange functions either of gases, nutrients or water without being consistent with a known fungal template.
Next there are the molecular hints. The chemical traces of breakdown products of the structural components (chitin and glucan) in fungal walls are evident in fossil fungi that are found in the same chert. Prototaxites does not. These markers are not missing in the analysis of the team in a minor error but rather a basic error: in case it is a fungus that has been preserved together with fungi, the same indicators should have been present. The work maintains that the sample of the Prototaxites does not show any evidence of chitin and glucan and shifts the organism outside the modern day concept of the fungal kingdom. That presents a curious editorial difficulty to biology: what to do with a towering multicellular organism in boxes that fail.
It was done on a species known as Prototaxites taiti, which is found in Rhynie chert and is smaller than the most well-known “tower” types. Critics observe that there is a practical constraint: the genus has approximately 25 named species, and no one specimen can represent much of its variety. The incomplete sampling has been noted by Marc-Andre Selosse of the Natural History Museum in Paris (who was not part of the research) and it has been argued that there is still a possibility of at least some members of the group having a lichen-like mode of life.
There was another twist to the fungal tale which had already been made complicated by previous work: Prototaxites was not acting like a photosynthesizer. Kevin Boyce of the Stanford University, whose earlier work influenced the formulation of the long-running hypothesis of “giant fungus,” has pointed out that Prototaxites must have fed on carbon sources in the environment as opposed to synthesizing its own food by light. but even that ecology does not resolve the problem, since the early terrestrial environments did not provide the rich soils and deep litter beds that modern decomposers are provided with; how such massive animals made a living is also an enigma.
What the Rhynie chert brings is a greater contrast. The linkage of anatomy to molecular signatures within a single, narrow site renders the study more difficult in treating Prototaxites as either an unusual fungus or an uncomfortable plant. In that regard, it is not what it was, but what it can be: a now-uncharacterized extinct eukaryotic lineages, a multicellular experiment in which no surviving descendants and no cozy modern analog existed. At present, Prototaxites is still a landmark, but it lacks a map, a creature, which, once, was the king of the skyline, and still stands just beyond the frontiers of classification.
