400-million-year-old organism grew 8 metres tall, confusing scientists for 165 years. It may belong to a lost branch of life

Many years before the land on Earth was filled with trees, an enormous, branchless organism rose up to eight metres above the ground. This organism used to live around 400 million years ago, when early plants had just started to spread across land, but its unusual structure has made it one of the most difficult fossils for scientists to classify in any category.

The organism which has been named Prototaxites has made scientists confused about its category for almost 165 years. Was it a giant fungus, an unusual plant or something else entirely? A recent study suggests that none of these explanations fits the available evidence. Instead, the organism might have been part of a completely extinct branch of life with no other surviving descendent of it. These findings that have been published in Science.

A fossil that puzzled scientists for generations


The story of trying to understand these organisms began in the 1850s, when geologist John William Dawson examined fossils discovered near Gaspé Bay in Quebec, Canada. He initially though that they were remains of decaying trees and gave the organism a name meaning “first conifer”.

But Prototaxites did not look like a tree because of its massive, upright body that did not have branches, while its internal structure did not fit neatly into any known categories of plant life. As researchers investigated the fossils, its classification became a long-running puzzle.

In 2001, Francis Hueber, a paleontologist at the National Museum of Natural History, argued that the organism was most likely an enormous fungus. That explanation gained further support in a 2017 study, which reported structures resembling those found in certain modern fungi in a fossil fragment of a smaller species, Prototaxites taiti.

However, the fungal theory was not universally accepted and questions remained about the unusual anatomy.

Why the giant fungus theory is questioned

To investigate the mystery, researchers co-led by University of Edinburgh paleobotanist Alexander Hetherington examined three fragments of P. taiti. They looked at its microscopic anatomy and its chemical composition to see if it fell into any known category or group of organisms.

The results did not provide any convincing evidence for a fungal identity. The way its anatomical structures connected differed from that of known fungi, while its chemical composition made a plant or algal origin very unlikely.

Other possibilities also failed to explain the evidence. Its cell walls argued against an animal origin, while its anatomy did not fit a lichen, which involves a partnership between a fungus and another organism.

Hetherington underlined the difficulty in comments to Science Magazine: “In the books and books of anatomy written about living fungi, we never find structures like that.”

After examining these characteristics, the team concluded that the organism could not be placed within any group of life that exists today.

An extinct branch with no living equivalent

The researchers suggest that P. taiti belonged to a previously undescribed group of eukaryotes, organisms whose cells contain a nucleus. If this interpretation is correct, the group represents an entirely extinct branch of life with no known modern equivalent.

Exactly what happened to this group of live remains unknown. The findings don’t answer all questions about Prototaxites, and more research may change its classification as scientists study more evidence.