In Earth’s ancient prehistory, there is a chapter waiting to be told known as Romer’s gap. The researchers identified a gap in the fossil record of tetrapods between 360 million and 345 million years ago, after fish began adapting to land and more than 80 million years before the first dinosaurs.
While mysteries remain about evolution’s experiments with living things during that 15-million-year gap, a fossilized tree described in a new paper offers more insight into what was happening during this time in nature’s laboratory.
Called Sanfordiacaulis densifolia, the tree was six inches in diameter with a trunk nearly 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 spiraling branches that radiated 2 1/2 feet outward. Robert Gastaldo, professor of geology at Colby College in Maine and author of the study published Friday in the journal Current Biology, compared it to “an upside-down toilet brush.” Comically top-heavy, even Seussian, the tree most likely remained upright, intertwining its branches with those of nearby trees.
“This is a totally new and different type of plant” than that 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 usually get plant pieces, or mineralized tree trunks, from Romer’s Gap. We don’t have many whole plants that we can rebuild. “This we can.”
The tree was unearthed near Valley Waters, New Brunswick, in an active private quarry within Canada Stonehammer UNESCO Global Geopark. (A new fossil museum will open in the village later this year.) The area is part of the 350-million-year-old Albert Formation, a geologic layer that has also yielded fossilized fish and trace fossils. Although partial fossils of the same tree species have been found previously, the new discovery represents the only fossil whose trunk and crown have been 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 Museum of New Brunswick 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 rich in 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 dating back to 398 to 327 million years ago and have only been found in coastal wetlands.
Valley Waters Quarry was once a tropical marshy ecosystem surrounding a rift lake, a deep body of water that ran atop a fault zone. Its sediments were similar to those of modern Lake Victoria and Lake Tanganyika in East Africa. The bank containing the tree was knocked down during a catastrophic earthquake, depositing the tree on its side at the bottom of the lake. Subsequent mudslides quickly buried vegetation and snuffed out aquatic life. Sediment filled in around the leaves, three-dimensionally preserving the specimen, which falls somewhere on the evolutionary continuum between a woody tree and an enormous plant.
S. densifolia evolved during a time when the multi-tiered forest structure was still developing and plants were diversifying, King said. It probably lived under the tallest trees, such as those with scaly bark, more than 30 meters tall Lepidodendronbut above all low-growing lycopods and mosses.
“The architecture of this tree suggests that it was growing in this ecological niche of being in the mid-canopy, trying to capture as much sunlight as possible with branches that extended almost as long as the tree was tall,” Ms King said.
“It’s a plant biology experiment that was successful for a certain period of time, 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 then.”