A reconstruction of an ancient tree from one of Earth’s first forests reveals that the plants were topped with fronds(蕨类植物的叶子) and not leaves. The crown (树冠) of an ancient tree found in a place from which stone is dug out in New York has shed light on the look of the world’s earliest forests believed to have thrived about 390 million years ago.
The discovery of this 390-million year-old, six-foot upper portion of an ancient tree trunk allowed scientists to create a composite picture of the entire plant when they put it together with fragments of a trunk found in the same site a year before and with tree stumps recovered more than 130 years ago. The remains have been widely reported as "evidence of the Earth’s oldest forest," according to a paper published in this week’s Nature.
"The basic point of this paper is two things," says lead author William Stein. "We now have clear evidence what these stumps really were, "and we also have "real strong evidence of the structure of these forms."
From the fossil reconstruction, the team of scientists determined that a tree comprising all these parts could grow about 30 feet tall. According to Stein, the base would have been massive—on the order of 2.5 feet in diameter—and a large, single trunk with vertical ridges, topped by a leafless crown of a material resembling fronds on ferns(蕨类) and palms.
By piecing together the fragments, the team was able to get an idea of what forest might have looked like 390 million years ago. Stein estimates these trees would have been "fairly closely spaced," about 3 to 16 feet apart, and that they would have dropped a load of fronds from their branches onto the forest floor. Amongst these trees were likely smaller plants and shrubs. These trees likely let more sunlight through than modern-day counterparts, for their branch structures did not fan out as far, forming an umbrella-like shape.
Stein notes that the rise of forests with trees like these caused the removal of CO₂ from the air and temperatures to drop, creating climates like those experienced today. The drop CO₂ in levels, he says, likely led to the evolution of flat leaves on trees to attract and retain more of the gas, which plants need. Up next, he says: research will focus on "the internal structure of the plants to work out how they grew" as well as" how they functioned physiologically, particularly the relationship with CO₂. " What possibly resulted in the evolution from leafless tree to flat leaves