Prototaxites May Be Completely Unknown Branch of Complex Life

For more than 165 years, one of the first giants to rise above Earth’s barren lands has defied classification. Known as PrototaxitesThis imposing, columnar-shaped organism dominated terrestrial landscapes more than 400 million years ago, reaching heights of up to 8 m (26 ft) long before the evolution of trees. A new study by palaeontologists from the University of Edinburgh and National Museums Scotland claims that this enigmatic organism was not a massive fungus, as many scientists thought, but rather belonged to an entirely extinct lineage of complex life.

Prototaxites was the largest terrestrial organism 410 million years ago. Image credit: Matt Humpage.

Prototaxites was the first giant organism to inhabit the Earth’s land surface, appearing during the late Silurian and Devonian periods, approximately 420 to 370 million years ago.

Known thanks to columnar fossils that can reach up to 8 m in length, it dominated the first terrestrial ecosystems well before the appearance of trees.

Widespread in ancient terrestrial environments and obviously consumed by arthropods, Prototaxites played an important ecological role during a pivotal phase of land colonization.

Yet despite more than 165 years of study, its biological identity remains up in the air, with paleontologists debating whether it was a fungus or a member of an entirely extinct lineage of complex eukaryotic life.

In the new study, Dr. Corentin Loron and his colleagues focused on Prototaxites taitia species preserved with exceptional three-dimensional detail in the 407 million year old Rhynie chert in Aberdeenshire, Scotland.

“Rhynie chert is incredible,” said Dr Loron, first author of the study published this week in the journal Scientific advances.

“It is one of the oldest fossilized terrestrial ecosystems in the world and, thanks to the quality of its preservation and the diversity of its organisms, we can pioneer new approaches such as machine learning on fossil molecular data.”

“There are already many other Rhynie chert materials in museum collections for comparative studies, which can add important context to the scientific results.”

Researchers examined a new specimen of Prototaxites taiti – this is the largest known example of the species from the site – allowing detailed anatomical and molecular comparisons with fossil fungi preserved in the same rock.

Microscopic imaging revealed a complex internal organization unlike any known fungus.

The fossil is composed of three distinct types of tubes, including large, thick-walled tubes with annular bands and dense spherical regions called medullary spots.

These spots are made up of complex three-dimensional networks of interconnected tubes, a branching pattern that scientists say has no equivalent in fungal biology.

Using infrared spectroscopy and machine learning-based classification, the authors compared the molecular fingerprint of Prototaxites with that of fossil fungi, arthropods, plants and bacteria from Rhynie chert.

Fungal fossils from the site preserve characteristic chemical signatures associated with chitin-rich cell walls, but these signatures were absent in Prototaxites.

The team also looked for perylene, a biomarker linked to pigment compounds produced by certain fungi and previously detected in other Rhynie chert fossils. The compound was not found in Prototaxites samples.

Taken together, the structural, chemical, and biomarker evidence undermines the notion that Prototaxites belonged to any fungal group, including the earliest relatives or strains of modern fungi.

“It’s really exciting to take a big step forward in the debate on Prototaxiteswhich has been going on for about 165 years,” said Dr. Sandy Hetherington, lead author of the paper.

“They are life, but not as we know it today, exhibiting anatomical and chemical characteristics distinct from fungal or plant life, and therefore belonging to an entirely extinct evolutionary branch of life.”

“Our study, combining analysis of the chemistry and anatomy of this fossil, demonstrates that Prototaxites cannot be placed in the fungal group,” said Laura Cooper, co-author of the paper.

“As previous researchers have ruled out Prototaxites other groups of complex life, we concluded that Prototaxites belonged to a distinct and now entirely extinct lineage of complex life.

“Prototaxites therefore represents an independent experiment that life has made in constructing large, complex organisms, the existence of which we can only know thanks to exceptionally preserved fossils.

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Corentin C. Loron and others. 2026. Prototaxites fossils are structurally and chemically distinct from extinct and extant fungi. Scientific advances 12 (4); doi: 10.1126/sciadv.aec6277