Hundreds of millions of years ago, mysterious life forms called Prototaxites towered toward the sky. Believed to be the first giant organisms to thrive on dry land, some species of Prototaxites grew up to 26 feet (8 meters) in height and resembled tree trunks composed of tiny interconnected tubes. Their position in the greater tree of life has been hotly debated for over a century and a half. New research suggests this is because Prototaxites don’t have a place in the tree of life as we know it—they belonged to a previously unknown branch.
Researchers from the United Kingdom analyzed the fossil remains of a Prototaxites species called Prototaxites taiti and concluded that Prototaxites likely belonged to a now-extinct lineage of multicellular terrestrial eukaryotes (organisms, including all animals and plants, whose cells contain a nucleus). Their work is detailed in a study posted to the preprint server bioRxiv, and hasn’t been peer-reviewed yet.
“We conducted an extensive re-examination of P. taiti, leading us to reject the most widely held hypothesis that Prototaxites was a Fungus,” the researchers, including Corentin Loron from The University of Edinburgh’s School of Physics and Astronomy, wrote in the study. A 2007 paper had previously suggested that Prototaxites was ancient fungi.
Loron and colleagues reached this conclusion by “contrasting the anatomy and molecular composition of Prototaxites with contemporary fungi from the 407-million-year-old Rhynie chert,” they added. The Rhynie chert is a sedimentary deposit in Scotland that has preserved significant fossil remains from the Early Devonian period (around 408 to 360 million years ago).
The comparison revealed that the “Prototaxites taiti was the largest organism in the Rhynie ecosystem and its anatomy was fundamentally distinct from all known extant or extinct fungi.” The researchers also did not find evidence of chitin or chitosan molecules in the cell walls of all extinct and current fungi groups known to science.
More broadly, the researchers documented three defining features of Prototaxites: large structures made of different tube-like components, compounds similar to lignin (the molecules that help give plants their rigidity), and a heterotrophic lifestyle (consuming other organisms for food).
“Based on this investigation we are unable to assign Prototaxites to any extant lineage, reinforcing its uniqueness,” they explained. “We conclude that the morphology and molecular fingerprint of P. taiti is clearly distinct from that of the fungi and other organisms preserved alongside it in the Rhynie chert, and we suggest that it is best considered a member of a previously undescribed, entirely extinct group of eukaryotes.”
All living organisms on Earth belong to one of three domains: Bacteria, Archaea, or Eukaryotes (also known as Eukarya). Bacteria and Archaea are single-celled microorganisms without a nucleus, while all multicellular organisms are classified as Eukaryotes. Eukaryotes are generally subdivided into four kingdoms: animals, plants, fungi, and protists (basically a catch-all category for all eukaryotes that aren’t animals, plants, or fungi). The researchers’ conclusion seems to suggest the existence of a new kingdom of life, though they don’t explicitly mention that classification.
In other words, long forgotten kingdoms might not just be the stuff of fantasy novels.
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