A bizarre ancient organism previously thought to be a giant fungus may actually belong to an undiscovered branch of the tree of life that mysteriously went extinct.
Prototaxites, which lived between 420 million and 375 million years ago, was the first giant terrestrial life form to inhabit Earth. It grew trunk-like structures up to 8 metres tall and 1 metre wide.
Its fossils were first discovered in 1843 and were initially thought to be the trunks of rotten conifers. Their classification has been a matter of intense debate, but in 2007, at Stanford University, California, and his colleagues concluded from the carbon isotopes in the fossils that they were a kind of fungus. This chemical evidence suggested that Prototaxites obtained carbon from other living organisms, as fungi do, rather than taking carbon dioxide from the air like plants.
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Now at the University of Edinburgh, UK, and his colleagues have released a preprint paper in which they say that Prototaxites wasn鈥檛 a fungus at all. Even more confounding, it fits nowhere else in the existing lineages of life.
Their study focuses on one species in the group, Prototaxites taiti, which was found in the Rhynie chert in Aberdeenshire, Scotland. P. taiti was much smaller than some of the giant species of Prototaxites and only grew to a few tens of centimetres in height.
P. taiti鈥榮 structure was made up of tubes, just like a fungus, but its tubes branch and connect in ways that are different to fungi, Loron and his colleagues report.
The Rhynie chert fossil site containing P. taiti has also yielded some bona fide fungi, so Loron and his colleagues did a chemical comparison between these fossil fungi and fossil Prototaxites.
They found that the chemical signature left in the fossils of Prototaxites is completely different to the chemical signature left by fungi subject to the same fossilisation processes. This shows that Prototaxites didn鈥檛 contain chitin, a fundamental structural component in fungal cell walls, the researchers say. Instead, the chemical components they identified were most similar to the fossilisation products of lignin, a polymer found in woody plant tissue.
Loron and his colleagues declined to be interviewed by 快猫短视频 as their research is yet to be peer reviewed. In their paper, they say the Prototaxites group is defined by three main characteristics: the formation of large, multicellular structures of varied tube types, a composition rich in compounds similar to lignin but distinct from plant matter, and they fed on decaying organic material.
The researchers say these three key distinguishing features of Prototaxites are simply not known together in any living lineage. 鈥淲e suggest that it is best considered a member of a previously undescribed, entirely extinct group of eukaryotes,鈥� they write.
No one knows why Prototaxites became extinct, but some scientists have suggested that it was outcompeted by fungi or the rapid explosion of shrubs and trees.
鈥淕iven the phylogenetic information we have now, there is no good place to put Prototaxites in the fungal phylogeny,鈥� says Boyce
He says it was an organism composed of interwoven microscopic filaments that fed on organic matter rather than photosynthesising, as is true of many fungi. 鈥淪o maybe it is a fungus, but whether a fungus or something else entirely, it represents a novel experiment with complex multicellularity that is now extinct and does not share a multicellular common ancestor with anything alive today.鈥�
at the Botanic Gardens of Sydney, Australia, says there are 鈥渢oo many unknowns to say it is a unique lineage at this stage鈥�.
鈥淭he assumption that it was likely to be in the fungi was somewhat nebulous, particularly given the nature of the fossils, and seemed somewhat counterintuitive given the relatively massive size,鈥� he says. 鈥淭he conclusion that it is a completely unknown eukaryote certainly creates an air of mystery and intrigue around it 鈥� probably not likely to be solved until more fossils are discovered or new analytical techniques developed.鈥�
bioRxiv
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