Some 400 million years ago, long before dinosaurs or even trees had evolved, an enigmatic organism towered over the landscape like a prehistoric monolith.
Now, new research makes the case that the ancient life form is not a plant, animal or fungi and instead may be a completely unknown form of multicellular life.
“What we can say, based on all of those new analyses, is that it’s so different from any modern group we have,” said Corentin Loron, a palaeontologist at Edinburgh University and a co-lead author of the research, which published in the journal Science Advances last month.
First identified 160 years ago, the fossils — known as Prototaxites — measure up to 30 feet (around 9 meters) tall and have long defied easy classification.
In the 19th century, scientists initially thought Prototaxites was the rotten trunk of a conifer. Subsequent study, however, revealed it was composed of interwoven tubes, rather than the block-like cells that make up plant tissue.
Other scientists argued that it was a lichen-like mass, a symbiotic association between a fungus and algae. In recent years, some researchers thought the organism more closely resembled a fungus, in part because it didn’t appear to produce energy through photosynthesis.
The new research focused on three Prototaxites fossils unearthed in the Rhynie chert, a prehistoric land ecosystem near Aberdeen, Scotland. The Rhynie chert is home to the best-preserved examples of the earliest plants, fungi and fauna that colonized land 400 million years ago, during a period known as the early Devonian. The site was once an ancient hot spring like Yellowstone.
The exceptional preservation of the fossils embedded in the rock at the Rhynie chert allows scientists, with the right tools, to detect the chemical signatures of long-vanished molecules, known as fossilization products.
“We are able to still have signatures that inform us about the original composition of those fossils, meaning it’s not overcooked, it’s not overly transformed by the geology,” Loron explained.
The new analysis by Loron and his colleagues suggests that the biomarkers in Prototaxites fossils were chemically distinct from those of fossilized fungi found at the site and preserved in similar conditions. Fungi fossils preserved in the chert contained compounds from the breakdown of chitin and glucan, key structural molecules in fungi. Prototaxites, however, lacked these biomarkers.
“If Prototaxites was fungi, we would have expected it to follow the same trend as the fungi because they are next to each other in the same burial conditions,” Loron said.
Other structural features — such as a complex branching pattern within dark spherical spots in the fossil that could have carried out a gas, nutrient, water, or served another exchange function — were distinct from all known fungi, whether living or extinct, the researchers noted in the study. Based on these results, it is too early to shoehorn Prototaxites into a specific category, according to the team.
Different species of Prototaxites might have varied in size, but the largest would have really towered over the landscape at a time when plants were less than 1 meter tall, said Kevin Boyce, a professor of Earth and planetary sciences at Stanford University. His work on Prototaxites fossils has shown that the ancient organisms didn’t use photosynthesis to produce energy from light like plants, but likely consumed carbon sources in the environment — just as some living fungi live off decomposing organic matter.
“People have compared it to specific fungi or algae in the past, and they were doing their best with the information they had at the time, but we now have a much better handle on the overall tree of life and Prototaxites is too old for those comparisons to be valid,” Boyce, who wasn’t involved in the study, said in an email.
“You can compare it to mushrooms, but mushrooms just aren’t that old,” he added. “That doesn’t mean Prototaxites is or isn’t a fungus (or anything else), just that its form would have evolved independently from the mushrooms and other complex multicellular examples amongst fungi that we have now.”
Marc-André Selosse, a professor at the Natural History Museum in Paris, said the authors of the new study conducted “wonderful analyses” but noted the research only examined one of 25 known species of Prototaxites. Selosse, who also didn’t take part in the work, said he thought it was still possible that the organism functioned in a lichen-like way.
“The sampling is not encompassing the diversity of Prototaxites species,” Selosse said. “So for me, it doesn’t make it as a finished story.”
Loron said there is still much that is unknown about Prototaxites. For example, it’s not clear how Prototaxites were anchored to the ground or whether the organism, which is thought to have grown slowly, was upright for its whole lifespan. His team is planning follow-up studies on fossilized tubular organisms similar to Prototaxites to further the research.
“Sometimes it’s scary to not know what something is, but it’s also scientifically exciting,” Loron said.