Cut into the trunk of a pine tree, and you will see a familiar series of concentric rings, each corresponding to a season of growth. But not all stumps tell the same story. A study published in November in the Proceedings of the National Academy of Sciences USA reveals that the world's oldest trees had a very different structure.

Some 370 million years ago cladoxylopsid trees stood at least eight meters tall, capped by branches with twiggy appendages instead of leaves. They looked a bit like spindly palm trees. Today their scant remains reveal little about their insides; in most cases their innards had rotted before the trees fossilized, and storms had filled them with sand. But the recent find of two well-preserved fossils in China has exposed the trees' inner workings—which are like no other species studied before.

Credit: From “Unique Growth Strategy in the Earth's Frist Trees Revealed in Silicified Fossil Trunks from China,” by Hong-He Xu, Christopher M. Berry, William E. Stein, Yi Wang, Peng Tang and Qiang Fu, in Proceedings of the National Academy of Sciences USA, Vol. 114, No. 45; November 7, 2017

At its heart, the mature cladoxylopsid tree was hollow. Around the edges ran thick, vertical strands containing xylem, the tubelike structures that carry water through many plants. Modern trees add new layers of multiple xylem as they grow, creating a woody trunk with a single set of concentric rings. But in cladoxylopsids, “each strand of xylem had its own growth rings,” says paleobotanist Christopher M. Berry of Cardiff University in Wales, who co-authored the study with colleagues at the Chinese Academy of Sciences in Nanjing and Binghamton University, S.U.N.Y.

Peering into a single cladoxylopsid tree stump would be like looking at dozens of smaller “trees,” the woody strands held together by the plant's soft tissue. As the cladoxylopsids grew, these columns of xylem split themselves apart—most likely to supply water to the expanding plant. Rings of wood then formed around the newly created strands.

Over a tree's lifetime these strands would weave and cross, forming an intricate latticework around a hollow core. “It's just incredibly complex,” Berry says. He likens these networks of flexible tissues and structures to the Eiffel Tower—if said tower could grow, stretch and rip itself apart over time.

Although the cladoxylopsid tree has no living descendants today, it does have an important legacy. Brigitte Meyer-Berthaud, a paleobotanist at the French National Center for Scientific Research, who did not participate in this work, explains that these trees were among “the major carbon reservoirs of the Paleozoic,” a time period from 542 million to 251 million years ago. Cladoxylopsids made up our planet's first forests, capturing carbon from the atmosphere and playing a part in modulating Earth's climate. Given this fact, maybe we should study these trees for the forests.