About 400 million years ago, long before dinosaurs or even trees evolved, a mysterious organism towered over the landscape like a prehistoric monolith.
Now, new research suggests that the ancient life form was not a plant, animal, or fungus, but may have been a completely unknown multicellular life form.
“Based on all these new analyses, we can say that it was very different from any modern group we have,” said Corentin Loron, a paleontologist at the University of Edinburgh and co-lead author of the study, which was published last month in the journal Science Advances.
First discovered 160 years ago, the 30-foot-tall fossils, known as protosugi fossils, have long defied easy classification.
In the 19th century, scientists initially believed that the original cedar stone was the decayed trunk of a coniferous tree. However, subsequent studies showed that it was composed of interwoven tubes rather than the lumps of cells that make up plant tissue.
Other scientists believe it is a lichen-like substance that is a symbiotic relationship between fungi and algae. In recent years, some researchers have argued that the organism is more similar to a fungus, in part because it does not appear to produce energy through photosynthesis.
The new study focuses on three protozoan fossils unearthed from the Rainey Chert, a prehistoric terrestrial ecosystem near Aberdeen, Scotland. Lenny Chert is home to some of the best-preserved examples of the earliest plants, fungi and animals that colonized the land 400 million years ago, in the early Devonian period. The site was once a hot spring as old as Yellowstone.
The fossils embedded in the Rainey chert rocks are so well preserved that scientists can use the right tools to detect the chemical signatures of long-lost molecules known as fossil products.
“We can still tell by the signature what the original composition of these fossils was, meaning it wasn’t overcooked or overly altered by geological conditions,” Loren explained.
A 410-million-year-old Protaxities fossil found in Rennie, Aberdeenshire, Scotland. ——Neil Hanna
unanswered questions
New analysis by Loren and his colleagues shows that biomarkers in the fossilized protandora are chemically different from biomarkers in fossil fungi found at the site and preserved in similar conditions. Fossils of fungi preserved in flint contain compounds produced by the breakdown of chitin and glucan, key structural molecules of fungi. However, Prototaxus lacks these biomarkers.
“If Protozoa was a fungus, we would expect it to follow the same trends as fungi because they were found next to each other under the same burial conditions,” Loren said.
Other structural features – such as the complex branching patterns within the fossil’s black spherical blobs, which may have served gas, nutrient, water or other exchange functions – are unlike any known fungus, living or extinct, the researchers noted in the study. Based on these results, it’s premature to shoehorn Protozoa into a specific category, the team said.
From right, researchers Sandy Hetherington, Corentin Loron and Laura Cooper, who led the new study, display some of the protocedar fossils at the National Museums Collection in Scotland. ——Neil Hanna
Kevin Boyce, a professor of earth and planetary sciences at Stanford University, said that different species of Prototaxus can vary in size, but the largest ones can literally tower above the ground when the plants are less than 1 meter tall. His study of fossilized cedar trees suggests that ancient organisms did not use photosynthesis to generate energy from light, as plants do, but instead may have consumed carbon sources in the environment—much like some living fungi survive by breaking down organic matter.
“People have compared it to specific fungi or algae in the past, and they did their best with the information they had at the time, but we now have a better handle on the entire tree of life, and Protozoa is so old that those comparisons are invalid,” Boyce, who was not involved in the study, said in an email.
“You can compare it to mushrooms, but mushrooms are not that ancient,” he added. “This doesn’t mean that Protozoa is or isn’t a fungus (or anything else), just that its form evolved independently of mushrooms and other complex multicellular examples in fungi that we have now.”
Marc-André Selosse, a professor at the Museum of Natural History in Paris, said the authors of the new study performed a “wonderful analysis” but noted that the study examined only one of the 25 known species of Protozoa. Selosse, who also was not involved in the work, said he thinks it’s still possible that the organism functions in a lichen-like manner.
“Sampling did not cover the diversity of Protozoa species,” Selosse said. “So to me, it’s not a complete story.”
Loren says there’s still a lot that’s unknown about protocedars. For example, it’s not clear how protoplasts were anchored to the ground, or whether the organism, thought to be slow-growing, remained upright throughout its life cycle. His team is planning follow-up studies of fossil tube-like organisms similar to prototaxites to further advance the research.
“Sometimes it’s scary not knowing what something is, but it’s also exciting from a scientific perspective,” Loren said.
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