A fossil tree with its roots and leaves still attached has provided a tantalising glimpse of what the Earth’s first forests looked like long before dinosaurs roamed the Earth.

Wattieza trees covered vast swaths 385 million years ago, before even amphibians managed to clamber on to land, and had such an impact that they helped to change the planet’s atmosphere.

They were the monsters of their age and are thought not only to have changed the face of the planet but also to have altered even the chemical composition of the atmosphere.

The plant, which grew to at least 26 feet (8m) in height and probably to more than 40 feet, looked similar to a tree fern with a long, bare trunk that was crowned at the top with branches and leaves.

Millions of the Wattieza trees would have covered the ground in coastal and other lowland regions of the planet 140 million years before the first dinosaurs.

They lived in an era when the carbon dioxide content of the atmosphere was much higher than it is today, but would have absorbed it in huge quantities as they grew.

By extracting the carbon dioxide, they helped to reduce the gas to levels similar to those today. By doing so they signed their own extinction warrants because they had made it possible for broad-leafed plants to evolve 20 million years later and take over.

Each tree of the Wattieza genus would have shed 200 or more branches, each about the length of a man’s arm, as it grew taller, leaving piles of rotting vegetation on the ground for ancient arthropod bugs to eat.

The only creature that has been confirmed to have lived in the forests was a huge millipede, almost half an inch in diameter, but many other creepy-crawlies, such as early forms of spiders, are expected to be found when further research is carried out. The tree that formed the first forests was identified when two fossils, one a trunk with roots and the other a crown of branches and leaves, were found in Schoharie County, New York state.

Analysis of the two fossils revealed that they were from the same plant and of the same type as a forest of tree stumps that was found about ten miles (16 km) away at Gilboa, dating to the mid-Devonian period.

The Gilboa Forest was discovered originally in the 1870s but until now no one has been able to say what the trees looked like because the stumps from the base of trees were all that were left.

The two new fossils, discovered six feet apart in 2004 and 2005, are likely to have come from the same forest and were fossilised after becoming covered with sediment in a river delta.

Christopher Berry, of Cardiff University, was one of the international team of researchers involved in the study, published in the journal Nature, that identified the fossils.

“This is a spectacular find, which has allowed us to re-create these early forest ecosystems,” Dr Berry said.

“Branches from the trees would have fallen to the floor and decayed, providing a new food chain for the bugs living below.

“This was also a significant moment in the history of the planet. The rise of the forests removed a lot of carbon dioxide from the atmosphere. This caused temperatures to drop, and the planet became very similar to its present-day condition.”

Some of the fossilised tree bases at Gilboa are more than three feet wide, compared with the 16 inches of the recently discovered tree.

Dr Berry said this meant that the trees probably grew much bigger than the 26ft specimen. “This is the most dramatic of the mid-Devonian plants — and we’ve probably only found a tiddler,” he said.

Wattieza would have reproduced with spores, like ferns, but their trunks would have had soft centres rather than the solid wood of trees today. It is possible that woody branches were able to photosynthesise, as well as the leaves.

The fossil was dug up by researchers at Binghamton University, New York, and the New York State Museum, which carried out the analysis with Dr Berry.

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