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The Most Ancient Tree

Meet the primeval

tree Ancestor  The Moroccan Sahar

Image courtesy of Walt Cressler (University  of Pennsylvania).

By: Brahm Rosensweig 

The most ancient tree of them all was quite a success in its time. About 360 million years ago, the proto-tree Archaeopteris was found all over the world, in any sort of habitat at all reasonable for growing. The Earth was blanketed in forests composed entirely of it. Now a group of paleobiologists have awarded Archaeopteris the distinction of being the first true modern tree, thanks to an unprecedented fossilized "stand" of specimens found deep in the deserts of Morocco.

 

What the Archaeoperis might have looked like. Image courtesy of Dennis Murphy.

It started a decade ago when a German paleontologist named Jobst Wendt was studying marine deposits in the Moroccan Sahara. He stumbled across logs that had been buried in ancient marine sediment during the Devonian period, some 350 million years ago. Wendt was shocked that no one had noticed them before, remarking that they were completely exposed to view. The area was once a shallow sea, and some of the logs, which had been 50 or 60 years old, floated out, sank in the water, and became buried in the soft sediment. The area would have lain hundreds of kilometres out off the coastline then, but now was located hundreds of kilometres inland into the desert.

In 1998, Wendt, along with paleobotanist Brigitte Meyer-Berthaud from the Université de Montpelier in France and Virginia Tech professor Stephen Sheckler, an Archaeopteris expert, returned and brought a truckload back from the desert for examination. In an article in the April  issue of the journal Nature, the group describes the importance of the find.

Archaeopteris is a tree with fern-like leaves and a woody, coniferous-like trunk. Though Archaeopteris has been studied for many years, it presented a bit of a conundrum for some time. Until specimens of sufficient quality had been found, its leaves had been classified as ferns, while its wood was considered to come from a completely different plant, a conifer.

Scientists have identified 360 million-year-old fossils of the earliest known modern tree - a plant that paved the way for modern ecosystems.

Fossils of the extinct Archaeopteris have been recognised for some time as intermediate between plants that reproduce by shedding free spores, such as ferns, and plants that produce seeds.

While known to be integral features of the earliest forests, their exact appearance was a mystery because only leaf imprints and bits of wood had been preserved.

But now Brigitte Meyer-Berthaud of the Montpellier University in France and colleagues have described the largest group of anatomically well-preserved Archaeopteris remains ever found.

The team's report is based on 150 fossils found in the Morocco, where tell-tale branches and other woody structures confirm the plant's status as the earliest known modern tree.

The samples contained hundreds of examples of branching trunks and roots, which had never been seen before.

The attachment of branches with swellings at the base to form a strengthening collar and with internal layers of wood dovetailed to resist breaking was the same as modern trees.

"We had always thought this was modern but it turns out that the first woody trees on earth had this exact same design," said co-discoverer, Stephen Sheckler from Virginia Tech.

Detailed studies of trunk slices showed the ancient trees also contained structures that allowed them to sprout new branches if the main axis died - a feature shared by no other plants at that time.

Because Archeopteris had fern-like leaves and reproduced by releasing spores rather than producing seeds, palaeobotanists says they are more like "ancient aunts" than direct ancestors of modern trees, which they suspect came from a sister line of plants.

For the 15 million years they dominated ancient forests, they played a key role in paving the way for modern ecosystems. Their sheer numbers meant they made a significant contribution to the increase in oxygen in the earth's atmosphere that occurred around that time.

And their litter which fell into streams was a major factor in the evolution of freshwater fishes whose numbers and varieties exploded, shaping the evolution of other important marine ecosystems. 

Never before had such excellent examples as Wendt's been seen. It allowed the team to make a number of hither-to impossible observations, concluding that Archaeopteris was indeed the first true tree.

 

One of the things that distinguishes a tree is that it has no finite life span, and can continue to grow for an unlimited amount of time. Archaeopteris was the first long-lived perennial. "Other plants ran out of ability to grow," says Scheckler. "These trees could grow for 100 years or more. They had no apparent life span." This, paleobotanists agree, was a factor that allowed Archaeopteris to dominate the forests.

Longevity is made possible partly by the ring system, which allows the tree to support more height with the advancing years. The new observations also note that Archaeopteris had lateral buds on its trunks and branches. "This was unique to Archaeopteris," says Scheckler. "It was the only plant at that time that could bud and continue growing after the main axis tip died; although seed plants now have that ability." Budding is important in increasing the life span of a plant.

 

A specimen from the Miseryfjellet mountain, Bear Island, Norway.

"It was the first time we had seen trunk branching on Archaeopteris, and we found hundreds of examples," Sheckler explained. "And we found big roots, which had previously been mostly conjecture." Branches on a tree require an intricate dovetailing of the layers that make up the wood to support the weight of the branches, and this was seen for the first time in the new fossils. "The attachment of branches was the same as modern trees, with swelling at the branch base to form a strengthening collar and with internal layers of wood dovetailed to resist breaking," says Scheckler. "We had always thought this was modern but it turns out that the first woody trees on earth had this exact same design."

The impact of this tree was far-reaching. In the late Devonian period the earth's atmosphere was changing from about 10 percent to 1 percent CO2 and from about 5 percent to 20 percent oxygen over a 50-million year period. All plants were responsible for the transformation, but according to Scheckler, Archaeopteris played a major role because it made up 90 percent of the forests during the last 15 million years when these changes were at their peak.

Some time around 345 million years ago, Archaeopteris died out. No one knows why, but the change seems to have happened rapidly. And its contributions to posterity were limited. Archaeopteris, like ferns, reproduced through spores, and not seeds like today's trees. So though Archaeopteris is the first true tree, its method of reproduction rule it out as the actual ancestor of the modern tree as we know it. It seems more likely that honour went to another contemporary of Archaeopteris.

Under the shade of the Archaeopteris tree in Canada

Prior to the Late Devonian, the Earth’s surface was scorched everywhere by relentless sun. Moderating shade arrived with the spread of the first forests composed of the first tree Archaeopteris

CHRONICLES OF CANADIAN PALEONTOLOGY

A 40 cm long frond of Archaeopteris, the first tree.This specimen is on display at the Miguasha Museum and was featured on a Canadian stamp.Photo by BDEC (c).

Fern-like leaves go along with other features that confirm Archaeopteris is the earliest know modern tree.

                                              

Shales and sandstones of the Escuminac Formation (Late Devonian, 380 Ma) are exposed near Miguasha in the Baie des Chaleurs area of Gaspé Peninsula,Canada. These sedimentary rocks were deposited in an estuary in front of a major river draining into a very narrow Proto-Atlantic Ocean. The rocks contain not only extremely important fossil fish, but also critical fossil plants.

Some of the plant debris that swept down this river became water-logged, sank, and was fossilized along with the animals living in the estuary. The plants include spore-bearing fronds that Sir J. William Dawson, the premier paleobotanist in Canada and the Principal of McGill University, described as Archaeopteris. He thought these were fronds of a primitive fern. That assessment held until the 1960s when Charles Beck, paleobotanist at the University of Michigan, demonstrated that conifer-like wood called Callixylon was attached to fern leaflets bearing Archaeopteris foliage. Because the wood and leaflets are part of the same plant, only a single name can apply -- and that name has to be Archaeopteris because Dawson’s name was published first.

Archaeopteris was more than a woody shrub -- it was the earliest known tree, and a sizeable one at that. Stumps up to one metre in diameter and trunks 30 metres tall are known. Archeopteris forests quickly became widespread across Late Devonian lowlands. The canopies of these trees would have provided protective shade and allowed for the accumulation of leaf-litter and humus on the forest floor. Such moist shaded settings had not existed on Earth previously, but these might well have been critical for the colonization of land by tetrapods. To escape the desiccating effects of the sun, the first amphibians would have sought out moist, shaded, equitable environments near water and underneath Archaeopteris trees.