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Fossil soils reveal emergence of modern forest ecology earlier than previously thought

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IMAGE: This video shows the rooting system of the ancient tree Archaeopteris at the Cairo fossil Forest site. view more 

Credit: Charles Ver Straeten

While sifting through fossil soils in the Catskill region near Cairo, New York, researchers uncovered the extensive root system of 386-million-year old primitive trees. The fossils, located about 25 miles from the site previously believed to have the world's oldest forests, is evidence that the transition toward forests as we know them today began earlier in the Devonian Period than typically believed.

"The Devonian Period represents a time in which the first forest appeared on planet Earth," says first author William Stein, an emeritus professor of biological science at Binghamton University, New York. "The effects were of first order magnitude, in terms of changes in ecosystems, what happens on the Earth's surface and oceans, in global atmosphere, CO2 concentration in the atmosphere, and global climate. So many dramatic changes occurred at that time as a result of those original forests that basically, the world has never been the same since."

Stein, along with collaborators, including Christopher Berry and Jennifer Morris of Cardiff University and Jonathan Leake of the University of Sheffield,have been working in the Catskill region in New York, where in 2012 they uncovered "footprint evidence" of a different fossil forest at Gilboa, which, for many years has been termed the Earth's oldest forest. The discovery at Cairo, about a 40-minute drive from the original site, now reveals an even older forest with dramatically different composition.

The Cairo site presents three unique root systems, leading Stein and his team to hypothesize that much like today, the forests of the Devonian Period were composed of different trees occupying different places depending on local conditions.

First, Stein and his team identified a rooting system that they believe belonged to a palm tree-like plant called Eospermatopteris. This tree, which was first identified at the Gilboa site, had relatively rudimentary roots. Like a weed, Eospermatopteris likely occupied many environments, explaining its presence at both sites. But its roots had relatively limited range and probably lived only a year or two before dying and being replaced by other roots that would occupy the same space. The researchers also found evidence of a tree called Archaeopteris, which shares a number of characteristics with modern seed plants.

"Archaeopteris seems to reveal the beginning of the future of what forests will ultimately become," says Stein. "Based on what we know from the body fossil evidence of Archaeopteris prior to this, and now from the rooting evidence that we've added at Cairo, these plants are very modern compared to other Devonian plants. Although still dramatically different than modern trees, yet Archaeopteris nevertheless seems to point the way toward the future of forests elements."

Stein and his team were also surprised to find a third root system in the fossilized soil at Cairo belonging to a tree thought to only exist during the Carboniferous Period and beyond: "scale trees" belonging to the class Lycopsida.

"What we have at Cairo is a rooting structure that appears identical to great trees of the Carboniferous coal swamps with fascinating elongate roots. But no one has yet found body fossil evidence of this group this early in the Devonian." Stein says. "Our findings are perhaps suggestive that these plants were already in the forest, but perhaps in a different environment, earlier than generally believed. Yet we only have a footprint, and we await additional fossil evidence for confirmation."

Moving forward, Stein and his team hope to continue investigating the Catskill region and compare their findings with fossil forests around the world.

"It seems to me, worldwide, many of these kinds of environments are preserved in fossil soils. And I'd like to know what happened historically, not just in the Catskills, but everywhere," Says Stein. "Understanding evolutionary and ecological history--that's what I find most satisfying."

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This work was supported by Natural Environment Research Council of the UK.

Current Biology, Stein, Berry, and Morris et al.: "Mid Devonian Archaeopteris Roots signal Revolutionary Change in Earliest Fossil Forest" https://www.cell.com/current-biology/fulltext/S0960-9822(19)31569-6

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