Charred Fossils Provide Clues about Early Terrestrialization

Paleobotanist Ian Glasspool investigates wildfires to elucidate Earth’s ancient history

By Kardelen Koldas ’15Photography by Jasper Lowe

November 12, 2021

More than 450 million years ago, life spilled out of the sea onto the land, paving the way for human existence. But what made that transition possible in the first place?

Paleobotanist Ian Glasspool looks for clues by studying rather unexpected phenomena: ancient wildfires.

Wildfires, it turns out, can result in exquisite fossil preservation. “It seems rather counterintuitive,” Glasspool said. But because charring alters the original plant chemistry, it can reduce plants’ susceptibility to decay and crushing. These fossils preserved through wildfires provide key information about Earth’s past, including species evolution and atmospheric oxygen levels. “That’s why a few localities with charred fossils have generated so much knowledge about early land plants.”

But there is a gap in the wildfire record from about 400-385 million years ago when life on land began to flourish. Glasspool is shedding light on that era with two recent findings.

“Essentially, what I’m interested in is trying to strengthen the information on atmospheric oxygen so that we can try and correlate that to the evolution of land animals,” said Glasspool, a research associate in Colby’s Geology Department. “If you’re interested in how we all came to be here, one of the first steps is understanding the baseline conditions for what made terrestrialization possible.”

Ian Glasspool holding a sample — The dark, roughly cubic fragments on this rock are fossil charcoal chunks of an extinct enigmatic organism known as *Prototaxites* that grew more than 12 feet in height and up to three feet in diameter. Scientists most commonly consider *Prototaxites* a basidiomycete fungus. This sample comes from the Welsh borderlands of the UK and is a little less than 420 million years old. Before it was fossilized, it burned at temperatures of at least 700°C, or 1292°F.

As a move in that direction, Glasspool and Robert Gastaldo, the Whipple-Coddington Professor of Geology, Emeritus, have been studying early terrestrial ecosystems to assess the level of wildfire activity. Their research, supported by a National Science Foundation grant, seeks to find evidence of ancient wildfires from about 430-360 million years ago—periods geologists call the Silurian and Devonian—to discern the atmosphere’s evolution.

Glasspool and Gastaldo have been studying fossils from Baxter State Park and Aroostook County in Maine, the Holy Cross Mountains in Poland, and the Rumney borehole in Wales. Glasspool has been interested in seeing if evidence of wildfires existed at these localities and can inform him about atmospheric oxygen concentration.

“If you’re interested in how we all came to be here, one of the first steps is understanding the baseline conditions for what made terrestrialization possible.”
Ian Glasspool, a research associate in Colby’s Geology Department

Oxygen, Glasspool explained, is one of three essential ingredients for fires to thrive. “Wildfires can’t propagate when there’s not enough oxygen in the atmosphere,” he said. Too much of it, however, burns things catastrophically regardless of how wet they are.

And this was key during the sea-to-land transition period when plants were tiny and leafless— much like liverworts and hornworts today—and lived and reproduced in damp conditions. Therefore, for them to burn in a wildfire, oxygen must have been plentiful, he suspected.

To test their hypothesis, Glasspool turned to products of wildfires: fossil charcoal—something he has been examining since graduate school and from his time at Chicago’s Field Museum, where he was a collection manager and is currently an adjunct curator of paleobotany. There, he used fossil charcoal to figure out how atmospheric oxygen fluctuated over the last 400 million years.

Now in Maine, he’s continuing his research with Gastaldo.

Together, the duo has been analyzing samples collected from a variety of sites in Europe and Baxter State Park to identify fossil charcoal. First, they analyze the fossils by using a scanning electron microscope to classify them and look for anatomical features that are characteristics of charcoal.

Having imaged the charred fossils, Glasspool embeds these tiny plant samples—often just a few millimeters across—in resin. Next, he finely polishes them with diamond or aluminum-oxide suspensions. Using a reflected-light microscope, Glasspool bounces light off of these polished specimens and measures the amount of light returned. This informs the pair about the pre- and post-burial conditions of the sample, and whether it’s the product of a wildfire. These measurements can even be used to estimate fire temperatures more than 400 million years ago.

**Top left:** A scanning electron microscope (SEM) image of a Devonian-era plant *Leclerqia* preserved in the Trout Valley Formation at Maine’s Baxter State Park. **Top right:** A composite of images taken with a reflected-light microscope of a *Pachytheca* (an ancient plant) specimen coming from Rumney borehole in Wales. The origin of *Pachytheca* is enigmatic. **Bottom left:** SEM image of a *Cryptophyte* plant sample from the Holy Cross Mountains in Poland. The human hair follicle is used for scale. **Bottom right:** SEM image is of a charred fossil “leaf” belonging to an extinct group of plants informally called the *Trimerophytes*, recovered from rocks of the Trout Valley Formation in Baxter State Park. The same image is provided to scale against a “wheat penny.” (Photos: Courtesy of Ian Glasspool)

This technique led them to two major findings, bringing us closer to understanding how terrestrialization happened.

The first came from materials from the Rumney borehole. There, they found an even earlier record of the first wildfire, extending the known range of fire on the planet by 10 million years, back to 430 million years ago. This is right at the interval from which the very first land plant macrofossils are known. “Given how unfavorable to burning this moisture-dependent, diminutive, less-than-one-inch-high vegetation was, it is remarkable that fire could propagate at all,” said Glasspool. These findings suggest that atmospheric oxygen levels at this time must have been elevated well above present-day levels.

The other important result was from Maine fossils.

Devonian age fossils at Baxter State Park, where Maine’s state fossil Pertica quadrifaria comes from, are a superb example of an early coastal ecosystem, he explained. Glasspool and Gastaldo, with help of Professor of Geology, Emeritus Bob Nelson, uncovered tiny charcoal fragments amongst Baxter fossils—only detectable by dissolving away the rock itself. “Not only were some of these fragments beautifully preserved,” said Glasspool, “but they also provided the first robust evidence of wildfire from a time interval [early-mid Devonian] where a gap in the record had been hypothesized previously.”

This absence of charcoal, he noted, was also used to suggest that atmospheric oxygen dropped below 16 percent, the minimum level needed for wildfires. However, the Baxter charcoal provides evidence that terrestrial arthropods—including arachnids, scorpions, and millipedes—known from this time interval were perhaps not evolving in a low-oxygen world.

But that’s information from one data point in such a large time frame, he stressed.

“We need to look further for more evidence of charcoal in that interval,” said Glasspool, who will carry on with his research for another year. “It looks as though it should be there; it probably just hasn’t been recognized as such yet.”