Sunday, 19 May, 2013

New light on ancient trees

It’s a basic question about the evolution of life: When was the first forest, and what lived there? For almost a century, the Riverside quarry in Gilboa, near Albany, New York, has been considered the grand-daddy of fossil forests, with hundreds of tree stumps dating from about 390 million years ago.

These strange Eospermatopteris trees contained no wood, but some stood more than 10 meters tall, says William Stein, an associate professor of biology at the nearby University of Binghamton.

Although Eospermatopteris did not have leaves, it was topped by a crown of branches.

The development of trees is a milestone in the development of life on land — as trees offer habitat for animals, alter the soil and landscape, and affect the atmosphere by using up carbon dioxide.

The Riverside quarry was excavated to supply stone for a dam in the 1920s, and it was at that site that paleontologist Winifred Goldring studied fossils of big, ancient trees. Ever since, her work has been considered essential evidence for arboreal evolution.

In 2010, Stein and Frank Mannolini of the New York State Museum obtained access to the same site for 10 days after contractors exposed the old rock in a search for stone to rebuild the dam. Although many of the Eospermatopteris stumps had been removed in the 1920s, the researchers found fossils of their roots “beautifully preserved in the ancient soil,” says Stein.

Palming it

The tree is comparable to a modern palm, Stein says, with “branches that act like fronds; they are very large structures that allow photosynthesis and reproduction.”

Those branches are studded with branchlets — but no leaves — that pick up energy from the sun, Stein says. “All photosynthesis takes place on the branchlets that surround the frond; there is a hand-like structure with four fingers and hundreds of little branchlets surrounding it.”

Leaves are not the only tree feature that’s missing, Stein says. “They are without the standard woody tissue you would expect in a tree of this size, and we don’t really understand how it works. Our best guess is that they are hollow, like an overgrown bamboo, with a very extensive outer structure that is thicker in the larger trees.”

Heard it through the grapevine?

Eospermatopteris was known from the 1920s, but the real surprise was fossils of a large, woody rhizome plant about as big around as an anaconda. (Botanical blip: A rhizome is a vine-like plant that runs along the ground.)

This was no average rhizome — but rather a monster up to 15 centimeters in diameter. At the site — a coastal location that repeatedly flooded — the rhizome apparently cohabited with Eospermatopteris, Stein says. “We can see it growing around the root mounds, which indicates that they were well aware of the tree’s presence.”

Based on microscopic samples of mineralized plant material, “to my surprise, we found that this was an Aneurphytalian, a very early group of woody plants.”

“It’s amazing to walk around these trees, to see where they were placed, the Aneurophytales looking like snakes, to see three major tree types when we thought there was only one,” says Stein. Based on evidence from nearby sites, the site probably also featured insects and fish, although it was too early for land-dwelling animals.

Radical rhizome

The Aneurophytalean rhizome may have grown like a vine on the Eospermatopteris trees, Stein says. “There is a pretty good indication that it climbed. That makes a lot of ecological sense, but the evidence is circumstantial.”

Rhizomes are typically thought of as “prostrate or semi-prostrate, producing leaves that go upward,” says Stein. “This is in our heads, based on botanical terminology, as opposed to the actual plants, which will do whatever they want” based on the abilities evolved by their ancestors.

Whether it was a rhizome or a vine, Stein says the Aneurophytalean is also a very early ancestor of all woody, seed-bearing plants. “Ultimately, wood was good invention,” says Stein. “Once it had the capability to grow, there is nothing beside orientation and some structural adaptations standing in the way of these rhizomes becoming a tree. They inherited the earth.”

Tags: biodiversity, evolution, forest, fossil, William Stein