“Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.” So goes the famous dictum of Sherlock Holmes, echoed by paleobotanist C. Kevin Boyce in summarizing the scientific quest for the truth about Prototaxites a colossal, mysterious creature that once ruled Earth’s earliest land surfaces. For more than 180 years, its actual nature has defied categorization, but a new study by the University of Edinburgh and colleagues has challenged centuries-old assumptions, and Prototaxites does not appear to belong to any recognized kingdom of life.
Prototaxites emerged into scientific history back in 1843, when fossilized material was discovered in Scotland. At first identified as a conifer a mistake, as conifers were yet to evolve the plant’s massive, tube-like structures up to 26 feet defied all the later attempts at categorization. Through the decades, theories varied from vascular plant to giant lichen, algae, and most tenaciously, fungus. A 2007 paper by Boyce presented robust evidence for a fungal identity, based on Prototaxites’ assumed heterotrophic lifestyle and chemical signatures with traces of the modern fungi.
But new research, laid out in a preprint released to bioRxiv, overturns this scenario. Scientists gave a thorough anatomical and chemical examination of Prototaxites taiti fossils from the Rhynie chert a 407-million-year-old sedimentary unit in Aberdeenshire, Scotland, famous for the perfection of early terrestrial fossils. This hot spring-derived, silica-dominated chert has produced a virtual time capsule of the Devonian landscape, conserving not only vegetation but also fungi, arthropods, algae, and bacteria in microscopic preservation.
The comparative strategy of the research team placed Prototaxites in the context of highly characterized Rhynie chert fungi. They used cutting-edge imaging and geochemical methods, such as FTIR microscopy and molecular fingerprinting, to investigate the cell wall structure and composition of the fossils. Specifically, FTIR is used in imagery to provide functional information on molecular composition. They were looking for chitin and chitosan diagnostic biopolymers for all extant fungi. Their findings were confirmed: “We conducted an extensive re-examination of P. taiti, leading us to reject the most widely held hypothesis that Prototaxites was a Fungus,” the authors explained. There was no evidence of chitin or degradation products of chitin, and no anatomy of the fossil matched any of the described fungal groups.
Rather, Prototaxites had a peculiar set of characteristics: large tube-like structures, the occurrence of lignin-type compounds (which in contemporary plants provide rigidity), and a heterotrophic mode of life. The inner tubes branched and recurred in patterns different from those found in fungi, and the chemical fingerprint differed strongly from fungi and plants. The researchers decided, “No extant group was found to exhibit all the defining features of Prototaxites. It is best considered a member of a previously undescribed, entirely extinct group of eukaryotes.”
Reclassification has deep implications for evolutionary biology. All multicellular life on Earth is now categorized into three domains Bacteria, Archaea, and Eukaryotes with the latter subcategorized into four kingdoms: animals, plants, fungi, and protists. Prototaxites, though, seems to be beyond these limits, suggesting the presence of a lost kingdom of eukaryotic life. Kevin Boyce, who once supported the fungal theory, acknowledged the shift in thinking. “Given the phylogenetic information we have now, there is no good place to put Prototaxites in the fungal phylogeny,” he explained. He continued: “So maybe it is a fungus, but whether a fungus or something else entirely, it represents a novel experiment with complex multicellularity that is now extinct and does not share a multicellular common ancestor with anything alive today.”
The Rhynie chert itself is a geologic wonder, with the earliest land ecosystem now represented in unparalleled detail. Silica-rich waters from very ancient hot springs quickly buried organisms before they could break down, even saving sensitive cellular structures for hundreds of millions of years. This remarkable preservation has allowed paleontologists to reconstruct not just the anatomy of early fungi and plants but also their ecological interactions, such as symbioses and disease relationships. The Rhynie chert contribution to calibrating molecular clocks and the tracing of major lineage evolution cannot be more important.
The investigation of Prototaxites also testifies to the advanced state of modern analytical methods. Whereas DNA is not often preserved for more than a few million years, scientists can obtain molecular data from fossilized cell walls and tissues with confocal laser scanning microscopy, Raman spectroscopy, and mass spectrometry. These techniques enable scientists to detect certain biopolymers, including chitin and lignin, and restore ancient pathways of metabolism. In the case of Prototaxites, the absence of chitin and the presence of lignin-like compounds were pivotal in ruling out both fungal and plant affinities.
The implications of this finding lie far outside the field of taxonomy. The presence of a giant, structurally elaborate organism outside of the characterized kingdoms of life provides evidence that the early colonization of land was characterized by evolutionary experimentation on a scale never before conceived. The fossil record in the Rhynie chert, already full of plant-fungal symbioses and early arthropods, now hints at the potential for entire eukaryotic lineages that have left no record.
As the authors point out, their conclusions are drawn from a small number of highly preserved specimens, and the fossil record for this time is still patchy. However, the redefinition of Prototaxites as a distinctive, extinct eukaryotic lineage provides new perspectives on the diversity and complexity of early land ecosystems. The tale of Prototaxites, which was previously considered to be a giant fungus, is now a reminder of the deep secrets still hidden in the ancient rocks of Earth, which await science to dig out and decipher them.
