LEAFLESS CROWN: A reconstruction of an ancient tree from one of Earth's first forests reveals that the plants were topped with fronds and not leaves.
The crown of a prehistoric tree found in a sandstone quarry in Gilboa, N.Y., has shed light on the look of the world's earliest forests believed to have thrived during the Devonian period between 360 million and 397 million years ago.
The 2004 discovery of this 380-million year-old, six-foot uppermost portion of an ancient tree trunk allowed paleobotanists to create a composite picture of the entire plant when they put it together with fragments of a trunk found a the same site a year later and with tree stumps recovered more than 130 years ago in another rock quarry 10 miles away. The remains have been widely touted as "evidence of the Earth's oldest forest," according to a report published in this week's Nature.
"The basic point of this paper is, well, two things," says lead author William Stein, a biologist at the Binghamton University in New York State. "We now have clear evidence what these stumps really were," part of the class Cladoxylopsida believed to be related to modern-day ferns, and we also have "real strong evidence of the morphology 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—with a large, single trunk and longitudinal ridges (probably part of the tree's vascular system), topped by a leafless crown of a material resembling fronds on ferns and palms. These fronds apparently had a structure somewhat similar to fingers protruding from the palm of a hand, with multiple branches that split into thinner branchlets. These wispy appendages would have done the work of photosynthesis for the tree and also have borne the spores with which the plant reproduced.
The researchers, including Christopher Berry, a geoscientist from Cardiff University in Wales, and paleontologists from the New York State Museum were able to classify the tree crown into the genus Wattieza, Berry says, "because the very small leaflike appendages have distinctive characteristic recurved tips," meaning they flop back toward the trunk of the tree. Berry has studied other ancient specimens from this genus in Belgium and Venezuela. Stein says the team is "sticking with just the genus," as far as classification goes, because "we can't distinguish species from genera with the fragments we have."
By piecing together the fragments, the team was able to get an idea of what a forest ecosystem might have looked like 360 million years ago. Stein estimates these Wattieza trees would have been "fairly closely spaced," about three to 16 feet apart, and that they would have dropped a load of litter from their branches onto the forest floor. Amongst these trees were likely smaller plants and shrubs and, by the late Devonian, precursors of modern-day conifers called Archaeopteris, as well. Arthropods, which live on detritus, such as millipedes, centipedes and now-extinct spiderlike organisms, may have lived below these trees, which likely let more sunlight through than modern-day counterparts, because their branch structures did not fan out as far and were ascendant, forming a gobletlike shape.
In an editorial that accompanied the paper, Brigitte Meyer-Berthaud and Anne-Laure Decombeix, paleobotanists at the French Agricultural Research Center for International Development and the University of Montpellier in France, respectively, note that the Gilboa tree seemed to be constructed to optimize "mechanical stability and reproduction." In contrast to modern-day trees, which require more complex vascular systems to grow to more hulking sizes, they write: "The Gilboa tree represents an economical alternative where, beyond the necessary investment in spores to ensure reproduction, the products of photosynthesis were mainly devoted to vertical growth of the trunk."
Berry notes that the rise of forests with trees like the Gilboa caused the removal of carbon dioxide from the air and temperatures to drop, creating climates like those experienced today. The drop in carbon dioxide levels, he surmises, likely led to the evolution of flat leaves on trees to attract and retain more of the gas, which plants need for photosynthesis. 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 atmospheric carbon dioxide."
