A joint research project by scientists from China, the U.K. and the U.S. has discovered how the earliest trees on earth grew.
Cladoxylopsida, technically a species of fern, appeared around 390 million years ago.
Researchers from the Nanjing Institute of Geology and Palaeontology, Cardiff University and the State University of New York conducted the research based on well-preserved fossils of tree trunks unearthed in northwest China's Xinjiang Uygur Autonomous Region.
The species became common more than 100 million years before dinosaurs existed. The density of carbon dioxide in the air dropped at that time, allowing tetrapod vertebrates to transition to living on land.
The scientists found that the early tree trunks were composed of numerous individual strands of xylem, the plant's water-conducting cells. The structure allowed the tree to have several sets of annual ring networks, compared to just one set in modern trees.
Xu Honghe, from the Nanjing institute and chief scientist on the project, said each of the xylem strands could expand to add additional annual rings, or divide to produce a new xylem strand system.
"The extinct fern was the earliest tree species that formed forests. Its multi-xylem strand structure was capable of producing trees of large size that cannot be found today," Xu said.
He said the research would help scientists better understand the early evolution of the terrestrial ecosystem, and allow for further comparative study of early and modern trees.
The research was published in the science journal PNAS on Tuesday.