Giant, Mysterious Spires Ruled the Earth Long Before Trees Did. What Exactly Are These Odd-Looking Fossils?
For more than 150 years, scientists have debated whether Prototaxites—which stood roughly 24 feet tall and 3 feet wide—were an early lichen or fungus, like a “giant mushroom”
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When land plants were still the relatively new kids on the evolutionary block and the world’s tallest trees reached only a few feet in height, giant spires of life poked from the Earth.
Fossils of these mysterious stalks, dating to between 350 million and 420 million years ago, reveal “trunks up to 24 feet … high and as wide as 3 feet,” reported National Geographic’s Anne Minard in 2007.
“Plants at that time were a few feet tall, invertebrate animals were small, and there were no terrestrial vertebrates,” geophysicist Kevin Boyce, now at Stanford University, told New Scientist’s Catherine Brahic in 2007. “This fossil would have been all the more striking in such a diminutive landscape.”
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When scientists first uncovered these fossils in 1843, they didn’t know what to make of them. Initially, they described the organism as a cone-bearing plant in 1859. “It was a very strange and odd thing when people found it,” Kirk Johnson, a paleontologist and director of the Smithsonian’s National Museum of Natural History, told BBC Studios in 2023. “It was shaped like a chunk of wood.”
But the spires, known today as Prototaxites (pro-toh-tax-eye-tees), weren’t made of wood. Researchers offered even more ideas for their identity: kelp-like aquatic algae, fruiting bodies of fungi or even lichens, which are partnerships between fungi and algae. The debate has raged for more than 150 years.
“The problem is that when you look up close at the anatomy, it’s evocative of a lot of different things, but it’s diagnostic of nothing,” Boyce told the University of Chicago Magazine in 2010. Another issue, he added, is its size: “Whenever someone says it’s something, everyone else’s hackles get up: ‘How could you have a lichen 20 feet tall?’”
“A 20-foot-tall fungus doesn’t make any sense,” he said in a 2007 statement. “Neither does a 20-foot-tall algae make any sense, but here’s the fossil.”
Today, Prototaxites are most widely accepted to be fungi. The work of the late Francis Hueber, a paleobotanist with the National Museum of Natural History, had a lot to do with that—he mapped the internal structure of the fossils and found their anatomy to be more fungus-like than plant-like. But he hadn’t found a “smoking gun” that would prove it.
Then, a 2007 study by Hueber, Boyce and others suggested they had found the coveted evidence through chemical analysis. They examined the ratio of two carbon isotopes—called carbon-12 and carbon-13—in fossil Prototaxites samples. If the mystery organisms were plants, that ratio would match up with other plants of the time, since all would have been obtaining their carbon from the same atmosphere through photosynthesis. Instead, the team found the ratio to be significantly different from plants, supporting the fungi hypothesis.
So, in one sense, Prototaxites could have been like giant mushrooms—the fruiting bodies, or spore-producing reproductive organs, of a fungus. Still, they didn’t look anything like the mushrooms of today.
Not everyone was sold on the idea that Prototaxites represented early fungi. Other researchers later proposed that Prototaxites spires were actually gigantic mats of liverwort plants that had somehow rolled up. But in a follow-up letter to the journal editor, Boyce and paleobiologist Carol Hotton doubled down on their fugus conclusion. Science can be messy, and despite more than a century of digging, researchers still debate the precise identity of these huge spires that dominated the early Earth.
If they truly were primitive fungi, Boyce and ecologist Matthew P. Nelsen wrote in 2022, they probably have no living descendants today. But through their fossils, we can imagine a long-lost world where Prototaxites might have towered above everything else.
“It creates, in my mind, one of the most bizarre prehistoric landscapes of all,” Johnson told the BBC.
But even though the spires of yore are long gone, don’t feel too bad for fungus-kind. The largest organism on Earth is still a huge fungus, located in eastern Oregon. While it doesn’t protrude dramatically toward the sky, the fungal mat—a single organism—spreads across nearly four square miles of forest.
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