Researchers have managed to reconstruct the detailed 3-D structure of a fossilized plant more than 400 million years old. The ancient leafy organism, a relative of today’s clubmosses, grew in the Early Devonian era—a crucial time for the development of life on Earth that saw the evolution of complex plants that grew into the first forests. Published in eLife, the new computer visualization of the fossilized plant known as Asteroxylon mackiei reveals details of its rudimentary root system, potentially offering clues as to how modern plants emerged.
“One of the major questions in plant evolution is how you go from very early land plants that don’t have any complex organs such as leaves and roots. How then do roots evolve?” says Alexander Hetherington, a plant scientist at the University of Edinburgh. “This plant develops leaves and has a highly branched rooting system. But it was unclear how closely this rooting system resembles the true roots we see in living plants today.”
The fossil of Asteroxylon mackiei was first dug up in 1964, preserved in flint-like stone called chert at a famous geological site in Scotland. Located near the village of Rhynie, close to Aberdeen in the northeast of the country, the site holds some of the best-preserved remnants of the early colonization of land. Uncovered more than a century ago, the so-called Rhynie chert contains exceptionally well-preserved fossils of a community of plants, fungi, lichen, and animals as they lived some 410 million years ago.
Previous analyses of Asteroxylon mackiei fossils show the plant to be among the most complex of known plants from that time period. It grew to about 30 cm high and featured highly branched shoot and rooting systems. But such fossils offered only a superficial view of how the plant would have looked, developed, and grown in three dimensions.
To get a better look, Hetherington’s team took a drastic step. Using a diamond-tipped blade, they cut the 1964 Asteroxylon mackiei fossil into 31 sections. By photographing each side of each section, they could then use a computer to digitally reconstruct a 3-D image of the complete fossilized plant.
That was a smart move, says Christine Strullu-Derrien, a plant paleobiologist at the Natural History Museum in London, who was not involved with the project. “The work is great because it gives the three-dimensional aspect that you don’t often see with the Rhynie chert,” says Strullu-Derrien. Because the rock formation is mostly silica, Rhynie fossils cannot be reliably analyzed using conventional methods such as synchrotron radiation, she notes. Fossil plants and the background silica have densities that are too similar to distinguish features accurately.
The sliced, scanned, and then reconstructed image of the fossil showed an unusual botanical feature. Asteroxylon Mackiei did not form roots as modern plants do. Rather than roots forming as tissues that branch off from a central stem, the ancient plant showed something called dichotomous branching, in which the central stem forms a fork. One prong then continues growing as before, to maintain the original stem identity, but the second develops as a distinct root.
No roots develop in this way in living plants, suggesting that Asteroxylon Mackiei was an evolutionary dead end and is not a direct ancestor of modern flora. “In their overall structure and the way these things forage for nutrients, they look in some ways like the rooting systems we see today. But we now know their development was fundamentally different,” Hetherington says.
Paper coauthor Liam Dolan, a plant scientist at the Gregor Mendel Institute of Molecular Plant Biology in Vienna, Austria, says the discovery of dichotomous branching will literally see textbooks rewritten. Previous researchers, unable to see inside the solid fossil, had assumed the ancient plant formed roots in a similar way to plants today and had drawn them that way. Indeed, Dolan has coauthored textbooks describing this conventional view. “So, I’m one of the people who’s been promulgating this myth because that was the best there was,” Dolan says. “But now in the second edition of that botanical textbook we can put this model in, and we can stand by it.”
Strullu-Derrien says lots more fossils can be analyzed in this way to address other questions about the evolution and development of plants—such as the symbiotic relationship they often share with fungi. For example, many important samples from the Carboniferous era, about 300 million years ago, are also found preserved in silica. “So, if we have enough blocks,” she says, “we can try the same technique.”
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