A towering, trunk-like organism that sprouted from the ground 400 million years ago has turned into quite the prehistoric mystery. These organisms — known by the genus name Prototaxites — look somewhat like living stalagmites, or cacti without any arms. They’ve long been thought of as fungi, making them an ancient relative of modern mushrooms. However, it turns out that Prototaxites are something else entirely.
A new study published in Science Advances has determined that Prototaxites are a unique form of life that doesn’t fit the description of either fungi or plants. After examining a fossil from the 407-million-year-old Rhynie chert (a sedimentary deposit in Scotland), researchers found that the organisms were chemically and anatomically distinct from fungi.
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First Fossil Evidence of Prototaxites
Back when Prototaxites lived from the late Silurian to the Late Devonian (about 420 to 370 million years ago), they were the largest land-dwelling organisms before being overtaken by taller trees — some Prototaxites grew to be 8 meters (26 feet) tall.
The first fossil of Prototaxites was collected in 1843, but since then, scientists have been uncertain about its position in the tree of life. While the organisms don’t fit in the same classification as algal groups or land plants, they do contain some features reminiscent of fungi.
For example, they may have subsisted on dead or decaying organic matter (making them saprotrophs, just like most fungi). Prototaxites fossils also contain interwoven masses of tubes, another trait that led some scientists to believe that they were fungi.
Prototaxites: Unfit to Be a Fungus
The new study, however, has debunked the idea that Prototaxites were fungi after inspecting a fossil preserved in the Rhynie chert, which shed light on the structural and chemical properties of the organisms.
“It’s really exciting to make a major step forward in the debate over Prototaxites, which has been going on for around 165 years,” said lead co-author Sandy Hetherington, an evolutionary paleobiologist at the University of Edinburgh, in a statement. "They are life, but not as we now know it, displaying anatomical and chemical characteristics distinct from fungal or plant life, and therefore belonging to an entirely extinct evolutionary branch of life.”
The team of researchers behind the study examined the fossil — from a species known as Prototaxites taiti — with 3D imaging techniques involving lasers, allowing them to see the structural differences between Prototaxites and fungi. With this closer look, they found a network of three types of tubes, deemed more complex than the inner structure of fungi.
In addition, the fossil had medullary spots (areas with a mix of dense tiny tubes and branching larger tubes) that appeared to be distinct from any known fungi.
An Unknown Branch of Life
The researchers also created a machine learning model to identify the chemical fingerprints of Prototaxites, which were compared with those of fungi and five other groups of organisms preserved in the Rhynie chert.
This model reliably recognized Prototaxites as a separate group, and further analysis revealed a chemical contrast between these organisms and fungi; fossilized fungi from the Rhynie chert had compounds derived from chitin and glucan (key molecules in fungi), but these were entirely absent in the Prototaxites taiti fossil.
With this updated knowledge of Prototaxites' structure and molecular composition, the researchers concluded that the extinct organisms are associated with an unknown lineage of multicellular life.
“As previous researchers have excluded Prototaxites from other groups of large complex life, we concluded that Prototaxites belonged to a separate and now entirely extinct lineage of complex life,” said co-first author Laura Cooper, a PhD student at the University of Edinburgh. “Prototaxites, therefore, represents an independent experiment that life made in building large, complex organisms, which we can only know about through exceptionally preserved fossils.”
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