A bizarre ancient life-form, considered to be the first giant organism to live on land, may belong to a totally unknown branch of the tree of life, scientists say.
These organisms, named Prototaxites, lived around 420 million to 375 million years ago during the Devonian period and resembled branchless, cylindrical tree trunks. These organisms would have been massive, with some species growing up to 26 feet (8 meters) tall and 3 feet (1 meter) wide.
Since the first Prototaxites fossil was discovered in 1843, scientists haven’t been sure whether they were a plant, fungus or even a type of algae. However, chemical analyses of Prototaxites fossils in 2007 suggested they were likely a giant ancient fungus.
Now, according to a paper published March 17 on the preprint server bioRxiv, Prototaxites might not have been a humongous fungus after all — rather, it may have been an entirely different and previously unknown life-form. The study has not yet been peer-reviewed.
All life on Earth is classified within three domains — bacteria, archaea and eukarya — with eukarya containing all multicellular organisms within the four kingdoms of fungi, animals, plants and protists. Bacteria and archaea contain only single-celled organisms.
Previous chemical analysis of Prototaxites fossils indicated that they likely fed off decaying organisms, just like many fungi do today, rather than making their food from carbon dioxide in the air like plants.
However, according to this new research, Prototaxites may actually have been part of a totally different kingdom of life, separate from fungi, plants, animals and protists.
The researchers studied the fossilized remains of one Prototaxites species named Prototaxites taiti, found preserved in the Rhynie chert, a sedimentary deposit of exceptionally well-preserved fossils of early land plants and animals in Scotland. This species was much smaller than many other species of Prototaxites, only growing up to a few inches tall, but it is still the largest Prototaxites specimen found in this region.
Upon examining the internal structure of the fossilized Prototaxites, the researchers found that its interior was made up of a series of tubes, similar to those within a fungus. But these tubes branched off and reconnected in ways very unlike those seen in modern fungi.
“We report that Prototaxites taiti was the largest organism in the Rhynie ecosystem and its anatomy was fundamentally distinct from all known extant or extinct fungi,” the researchers wrote in the paper. “We therefore conclude that Prototaxites was not a fungus, and instead propose it is best assigned to a now entirely extinct terrestrial lineage.”
True fungi from the same period have also been preserved in the Rhynie chert, enabling the researchers to chemically compare them to Prototaxites. In addition to their unique structural characteristics, the team found that the Prototaxites fossils left completely different chemical signatures to the fungi fossils, indicating that the Prototaxites did not contain chitin, a major building block of fungal cell walls and a hallmark of the fungal kingdom. The Prototaxites fossils instead appeared to contain chemicals similar to lignin, which is found in the wood and bark of plants.
“We conclude that the morphology and molecular fingerprint of P. taiti is clearly distinct from that of the fungi and other organism preserved alongside it in the Rhynie chert, and we suggest that it is best considered a member of a previously undescribed, entirely extinct group of eukaryotes,” the researchers wrote.
Kevin Boyce, a professor at Stanford University, led the 2007 study that posited Prototaxites is a giant fungus and was not involved in this new research. However, he told the New Scientist that he agreed with the study’s findings.
“Given the phylogenetic information we have now, there is no good place to put Prototaxites in the fungal phylogeny,” Boyce said. “So 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.”
More research into Prototaxites fossils needs to be done to determine if they were fungi or a completely different type of life, and what caused them to go extinct millions of years ago.
“The conclusion that it is a completely unknown eukaryote certainly creates an air of mystery and intrigue around it — probably not likely to be solved until more fossils are discovered or new analytical techniques developed,” Brett Summerell, a plant pathologist and fungi expert at the Botanic Gardens of Sydney, Australia, who not involved in this new study, told the New Scientist.
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