There was something that was as high as a pole with eight metres before the trees overthrew it. The fossils of what long has been considered a giant ancient fungus, the Prototaxites, have been re-evaluated in a manner that classifies it neither as a fungus nor as a plant, in what scientists term a lost lineage of complex life entirely.
The puzzle itself has been approximately 165 years old, having been identified and reclassified by curious log-like fossils. Prototaxites can be found in rocks over 400 million years old, at the same time that land ecosystems on Earth were first forming their first alliances between the soil, microbes and small plants. The columnar shape of the organism, a landmark in that sparse landscape, was one of the first on-land giants.
The most recent discovery focuses on one species, Prototaxites taiti, a species that lived in the Rhynie chert of Aberdeenshire, Scotland, 407 million years ago. The Rhynie deposits were known to preserve early terrestrial life in exceptional three-dimensional detail, which means that researchers can not only compare organisms based on their form, but also their molecular fossils. That is important here since Prototaxites has always been an artist of similitude: Sometimes it can resemble plants at a distance, but inside, it is full of tube networks that recall fungi.
Microscopic and 3D-imaging showed an internal structure that was not commensurate with the fungal growth patterns. There are three different types of tubes found in the fossil with some being thick wall tubes and annular banded as well as dense spherical areas known as medullary spots. The structure of those spots consists of complex, intertwined networks of tubes with a branching structure, which researchers believe lacks an analogue in fungi. The team next took an infrared spectroscopy of a chemical fingerprint of the fossil and used machine-learning tools to match that signature with other organisms in the same deposition- plants, bacteria, arthropods and several fossil fungi.
It is there the fungal identity failed. Chemical signals are generally carried by fungi in association with cell walls of chitin, although such signals were absent in Prototaxites. Perylene, a biomarker of some fungal pigments, which had been found in other Rhynie chert fossils, was also sought and was not found in the samples under study. Together with the anatomical findings, the chemical evidence discredited the notion that this organism was a member of any of the fungal families, including the early ancestors of current fungi.
Dr Sandy Hetherington, who co-led the team, said that it was truly thrilling to take a big stride in the Prototaxite debate, which had been in progression since approximately 165 years. They are life, but not the life that we know it, with an anatomical and chemical structure that is neither fungal nor plant life, and thus a completely extinct branch of evolution of life.
Co-first author Laura Cooper put the implications in terms of evolution: Prototaxites, then, is an autonomous experiment by life in the formation of large, complex organisms, which we have the privilege of knowing about due to remarkably preserved fossils.
The fossil is now represented in the Museum collections of National Museums Scotland, and is being added to the Rhynie specimen collections already there, which may be re-examined as understanding advances. What is bigger is that tree of life illustrations are not only filled in by new species; in some cases, a specimen pushes scientists to believe that even entire lineages, entire answers to the living large on land, have existed, thrived, and disappeared without leaving any modern offspring.
