Oldest petrified forest unearthed: Gilboa

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Reconstruction of Gilboa Forest. Credit: Frank Mannolini, New York State Museum

The oldest known fossil forest, dating back 385 million years, has been investigated after being unearthed for the first time since it was discovered and buried by dam workers back in the 1920s.

The Gilboa fossil forest, in the Catskill Mountains in upstate New York, has for several years been recognized as the oldest known fossil forest. And yet it has long been hard for investigators to access.

Wattieza, the oldest known tree, stood up to 10 m tall and resembled modern tree ferns. Image: Frank Mannolini.

Fossils of hundreds of large tree stumps (the Gilboa or Wattieza tree) preserved in the rocks were discovered in the 1920's during excavation of a quarry to extract rock to build the nearby Gilboa Dam. Only sketchy information was recorded about the geological context in which the fossils were found, and following completion of the dam the quarry was backfilled. Until now, the only way the Gilboa fossil forest could be investigated was from museum specimens and from small exposures of other levels in nearby streams.

In May 2010, the quarry was partially emptied as part of a dam maintenance project. Researchers were monitoring the site with contractors, Thalle Construction Company and the New York City Department of Environmental Protection. Professor Bill Stein, Binghamton University and Frank Mannolini, New York State Museum saw that the original quarry floor was exposed, and that the roots and positions of the trunk bases had been preserved.

As Chris Berry of the Cardiff School of Earth and Ocean Sciences explains: "For the first time we were able to arrange for about 1,300 square meters to be cleaned off for investigation. A map of the position of all the plant fossils preserved on that surface was made."

The researcher's findings were just published in the journal Nature. They describe the bases of these Gilboa trees as spectacular bowl-shaped depressions up to nearly two meters in diameter, surrounded by thousands of roots. These trees stood as high as 10 meters in height. One of the biggest surprises was that the researchers found many woody horizontally-lying stems, up to about 15 cm thick, which they have demonstrated to be the ground-running trunks of another type of plant [aneurophytalean progymnosperm], only previously known from its upright branches. They also found one large example of a tree-shaped club moss, a type of tree that commonly forms coal seams in other, younger rocks across Europe and North America.

Berry said: "All this demonstrates that the oldest forest at Gilboa was a lot more ecologically complex than we had suspected, and probably contained a lot more carbon locked up as wood than we previously knew about. This will enable more refined speculation about the way in which the evolution of forests changed the Earth.

"Personally, the chance to walk on that ancient forest floor, and to imagine the plants that I have been studying as fossils for more than 20 years, standing alive in the positions marked by their bases, was a career highlight. Seven years ago colleagues Linda and Frank found us a fossil of a complete Gilboa tree. That was amazing. But this time we've got the whole forest!"

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