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In Earth’s ancient prehistory, there is a chapter waiting to be told known as the Romer Gap. Researchers have identified a pause in the fossil record of tetrapods between 360 and 345 million years ago, after fish began to adapt to land and more than 80 million years before the first dinosaurs.
While mysteries remain about evolutionary experiments with living things during that 15-million-year span, a fossilized tree described in a new paper offers greater insight into some of what was happening during this period in nature’s laboratory.
Called Sanfordiacaulis densifolia, The tree had a diameter of six inches and a trunk almost 10 feet tall composed not of wood, but of vascular plant material, such as ferns. Its crown had more than 200 finely striated compound leaves emanating from spiral-shaped branches that radiated 2½ feet outward. Robert Gastaldo, professor of geology at Colby College in Maine, author of the study, which was published Friday in the journal Current Biology, compared it to “a toilet brush turned upside down.” Comically heavy, even Seussian, the tree probably stood upright by intertwining its branches with those of neighboring trees.
“This is a totally new and different type of plant” than what had been found in the Late Paleozoic, said Patricia Gensel, a biology professor at the University of North Carolina at Chapel Hill and another author of the paper. She added: “We normally get pieces of plants, or mineralized tree trunks, from Romer’s Gap. We don’t have many entire plants that we can rebuild. We can do this one.”
The tree was unearthed near Valley Waters, New Brunswick, in an active private quarry within Canada. Stonehammer UNESCO World Geopark. (A new fossil museum will open in the town later this year.) The area is part of the 350-million-year-old Albert Formation, a geological layer that has also produced fossilized fish and trace fossils. Although partial fossils of the same tree species had previously been found, the new discovery represents the only fossil whose trunk and crown were preserved together.
“It’s very rare to find something so well preserved and unique,” said Matt Stimson, an author of the study who works at the New Brunswick Museum and who first excavated S. densifolia with another study author, Olivia King of Saint Mary’s University. “It’s like finding a cactus in the middle of a Canadian boreal forest.”
Trees with spongy trunks and vascular tissue first appeared between 393 and 383 million years ago. Their woody counterparts entered the fossil record about 10 million years later. Trunks and stumps make up the majority of tree fossils from 398 million years ago to 327 million years ago, and have been found only in coastal wetland areas.
The quarry at Valley Waters was once a tropical swamp ecosystem surrounding a rift lake, a deep body of water that runs over a fault zone. Its sediments were similar to those of present-day Lakes Victoria and Tanganyika in East Africa. The bench containing the tree dislodged during a catastrophic earthquake, depositing the tree on its side at the bottom of the lake. The ensuing landslides quickly buried vegetation and wiped out aquatic life. Sediments were filled in around the leaves, three-dimensionally preserving the specimen, which lies somewhere on the evolutionary continuum between a woody tree and a huge plant.
S. densifolia evolved during a time when the tiered structure of the forest canopy was still developing and plants were diversifying, Ms King said. It probably lived under the tallest trees, such as those with scaly bark taller than 30 meters. lepidodendronbut above the low growing ones lycopods and mosses.
“The architecture of this tree suggests that it was growing in this ecological niche of being in the center of the canopy, trying to capture as much sunlight as possible with branches that extended almost as long as the height of the tree,” said Ms. King.
“It’s a plant biology experiment that was successful for a while and then it wasn’t,” Dr. Gastaldo said. “We don’t see anything like this in any of the forests we’ve been able to evaluate since.”