Approximately hundreds of millions of years ago, enigmatic life forms known as Prototaxites reached for the sky. Considered to be the first gigantic organisms to flourish on dry land, certain species of Prototaxites attained heights of up to 26 feet (8 meters) and resembled tree trunks composed of tiny interconnected tubes. The position of these organisms within the broader tree of life has been a subject of debate for over a century and a half. Recent research proposes that this is due to the fact that Prototaxites do not fit into the tree of life as we currently understand it—they belonged to a previously unknown branch.

Researchers from the United Kingdom conducted an analysis of the fossil remains of a Prototaxites species known as 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). The details of their research are outlined in a study posted on the preprint server bioRxiv, which has not yet undergone peer review.

According to the researchers, including Corentin Loron from The University of Edinburgh’s School of Physics and Astronomy, “We conducted an extensive re-examination of P. taiti, leading us to reject the most widely held hypothesis that Prototaxites was a fungus,” as stated in the study. A 2007 paper had previously suggested that Prototaxites was an ancient fungus.

Loron and colleagues reached this conclusion by comparing the anatomy and molecular composition of Prototaxites with contemporary fungi from the 407-million-year-old Rhynie chert, a sedimentary deposit in Scotland that has preserved significant fossil remains from the Early Devonian period (approximately 408 to 360 million years ago). The comparison revealed that Prototaxites taiti was the largest organism in the Rhynie ecosystem, and its anatomy was fundamentally distinct from all known extant or extinct fungi. Furthermore, the researchers 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 identified three defining features of Prototaxites: large structures composed of different tube-like components, compounds similar to lignin (the molecules that contribute to the rigidity of plants), and a heterotrophic lifestyle (consuming other organisms for food).

Based on their investigation, the researchers explained that “we are unable to assign Prototaxites to any extant lineage, reinforcing its uniqueness.” They concluded that the morphology and molecular fingerprint of P. taiti is clearly distinct from that of fungi and other organisms preserved alongside it in the Rhynie chert, suggesting 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). While Bacteria and Archaea are single-celled microorganisms without a nucleus, all multicellular organisms are classified as Eukaryotes. Eukaryotes are generally subdivided into four kingdoms: animals, plants, fungi, and protists (a catch-all category for all eukaryotes that aren’t animals, plants, or fungi). The researchers’ conclusion implies the existence of a new kingdom of life, although they do not explicitly mention this classification.

In essence, long forgotten kingdoms may not just be the stuff of fantasy novels, as the discovery of Prototaxites suggests that there may be more to the tree of life than we currently understand.