Getting to the Root of How Earth’s Massive Coal Seams Formed

Close-up of a lycopsid tree. Did fungus break down these plants? Image from Wikipedia.
Close-up of a lycopsid tree. Did fungus break down these plants? Image from Wikipedia.

Writing about science is a tightrope walk. You can practice as much as you want, and during preparation you have lifelines in the form of editors and experts you can phone for answers, but in the end it’s just you out there, trying to toe the line suspended between attention and accuracy. Eyes are on you for any misstep, and even a perfect performance comes with an often-unrecognized risk. That’s because science is a process, not a static collection of facts, and in an instant a new discovery or study can make the rope vanish beneath your feet.

A few months back my Phenomena neighbor Robert Krulwich wrote a post titled “The Fantastically Strange Origin of Most Coal on Earth.” It’s a lovely little story, all about how a delay in microbial evolution allowed the vast forests of over 300-million-years-ago to become compressed into the fossil fuels we rely on. “[W]hen those trees died,” Krulwich writes, “the bacteria, fungi, and other microbes that today would have chewed the dead wood into smaller and smaller bits were missing.”

Paleontologists call this the “lag hypothesis.” And it turns out to be wrong.

Back in March, about two months after Krulwich’s post went up, Stanford University geoscientist Matthew Nelsen and colleagues published a paper in PNAS that set the record straight in the very title: “Delayed fungal evolution did not cause the Paleozoic peak in coal production.” What seemed like a neatly-solved question once again turned into a conundrum.

The key to the puzzle is lignin. This is the sturdy stuff that often gives bark, wood, and even the cell walls of many plants their resilience. And in the thick forests of the Carboniferous, over 300 million years ago, lignin was supposed to be the stuff that microoganisms and fungi just couldn’t chew up. With no decomposers up to the task, the enormous trees of the time and other plant material piled up for burial rather than breaking down.

A 19th century rendition of a Carboniferous forest. Source.
A 19th century rendition of a Carboniferous forest. Source.

Yet new discoveries have totally reversed what paleontologists expected of those primordial forests. The bulk of the great Carboniferous swamp biomass consisted of trees called lycopsids, Nelsen and colleagues write, but up to 80% of these plants was made of a kind of bark that has no modern equivalent. In other words, these trees did not rely on lignin to support themselves.

And it gets better. “Carboniferous fossils provide direct evidence that fungi were taxonomically and ecologically diverse,” Nelsen and coauthors point out, and paleontologists have already uncovered Carboniferous wood “infiltrated with fungi and possessing damage consistent with white rot decay or other forms of fungal degradation of lignified tissue.”

The lag was in our understanding, not fungus evolution. Lignin wasn’t as critical as had been thought, and, even then, fungus and other decomposers were still capable of busting up the material. And this makes the vast coal seams created by these forests even stranger. If not a reprieve from becoming compost, what could have made such a glut of fossil fuels? The answer, Nelsen and colleagues suggest, probably has to do more with how those forests became buried.

Carboniferous forests were incredibly productive, throwing up plant life faster than the dead plants could decay, creating a literal logjam of organic material in the hot, humid habitats. This happened in a glacial world, but as those stores of ice melted the thick tangles of slowly-decaying plants were buried and eventually compacted down. The Earth’s crust had its own role to play, too. The sweltering forests grew in areas of the planet that were shunted beneath the surface as Pangaea coalesced, the movement of the Earth providing the geological forces necessary to create the fuel for the Industrial Revolution and the climate change we’ve brought upon ourselves.


Nelsen, M., DiMichele, W., Peters, S., Boyce, C. 2016. Delayed fungal evolution did not cause the Paleozoic peak in coal production. PNAS. doi: 10.1073/pnas.1517943113

2 thoughts on “Getting to the Root of How Earth’s Massive Coal Seams Formed

  1. Of course part of the cause was the sea level changes caused by the ice sheets. This lead to the whole coal cycle, as areas went from open ocean thru beaches to swamps and back again. This is also seen in the Paradox basin of Ut in terms of changes in salt deposition. The sea level rise would bury the decaying swamp plants with sand as the beaches encroached etc.

  2. Seems to me that there is an awful lot of Cretaceous coal around too. Shouldn’t explanations of coal formation account for that too?

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