Anyone ever taught to count rings in order to tell how long a tree lived for will know that tree rings are time made visible. But the orderly layers of growth that modern trees use are only one strategy for becoming large and tall. And according to a new paper being published in the journal Proceedings of the National Academy of Sciences, the very earliest trees grew in a very different way: as a ring that expanded outward.
“It’s kind of a design of a plant that is cheap,” Gregory Retallack, who studies fossilized plants and soils at the University of Oregon and was not affiliated with the new study told Newsweek. “People had found them years ago in New York and we didn’t really understand them all that well,” he says of truly ancient trees belonging to a group called the cladoxylopsids, which lived more than 375 million years ago.
More than five years ago, Xu Honghe, who studies fossilized plants at the Nanjing Institute of Geology and Paleontology in China, and his colleagues stumbled on an incredibly well preserved tree trunk in northwest China. The trunk had been mineralized—that’s not quite the same process as petrification, it actually leaves the plant cells in better condition, so scientists can see each individual cell hundreds of millions of years later.
That tree trunk allowed Xu and his colleagues to finally solve the mystery of how the cladoxylopsids grew. Scientists have found their remains around the world since that first New York discovery, but never in as good a condition. Xu and his colleagues were able to slice hundreds of pieces off the fossil to examine, including looking along both the side-on and top-down angles—the equivalent of studying both the grain and rings of modern wood.
That dual appoach let the team identify the cladopsid’s network of tunnels that carried water and nutrients up the tree’s structure. Where modern trees are solid, with orderly rings of tubes growing along the outer edge each year, the prehistoric tree’s tubes grew in a ring surrounding tissue that Retallack compares to lettuce. This tissue then likely died as the tree aged and left it hollow. Today, he adds, the closest trees come to this ancient structure is in palm trees.
“There is no other tree that I know of in the history of the Earth that has ever done anything as complicated as this,” Chris Berry, a co-author of the study and a specialist in fossil plants at Cardiff University in the United Kingdom, said in a press release. “The tree simultaneously ripped its skeleton apart and collapsed under its own weight while staying alive and growing upwards and outwards to become the dominant plant of its day.”
Retallack says the unusual structure may not have done the cladoxylopsids any favors. He says the donut-like construction would have let them grow tall and weedy and that they may have been destroyed by a catastrophe anyway, but that the cheap structure left them vulnerable to being overtaken by more organized trees. “There’s being outcompeted and there’s bad luck,” he says. “A lot of bad joojoo happened too.”