Fossil forest in Canada’s Arctic: Three new extinct walnut species discovered
Less than 1,000 kilometers from the North Pole, 45 million years ago there was a species-rich forest whose remains were discovered almost 40 years ago. But only now has a research team discovered that three previously unknown walnut species were also growing there.
Where only a few centimeters of polar willow and dwarf birch grow today, enormous tree stumps and roots protrude from the ground, surrounded by branches, leaves, cones and seeds, almost as if they had just fallen. Back in 1985, geologist Brian David Ricketts literally stumbled across the fossilized remains of a forest that grew on the Arctic island of Axel Heiberg, now part of Canada, during the mid-Eocene period, 45 million years ago.
Only one year later, James Basinger, now Professor Emeritus of Earth Sciences at the University of Saskatchewan, was one of the first scientists to explore the fossil forest with his team. He still remembers his impressions during his first expedition to the site and describes them in an email to Polar Journal AG:
“Most impactful is the jarring contrast between past and present. Stumps more than a meter in diameter, still rooted in the soils in which they grew. These stumps stand in stark contrast to the arctic tundra of today. These are vastly different environments, yet both are found in the Far North, separated not in space but in time.
As we looked more closely, the remarkable preservation of the fossils becomes apparent. Leafy litters look like your backyard compost. Wood that looks and feels almost modern. And of course cones and nuts, including the walnuts, lying on the surface.”
“There aren’t really that many places around where you can go to see fossils that are preserved that well,” said Professor Steven Manchester, curator of paleobotany at the Florida Museum of Natural History, in a museum news release.
Rare form of conservation
The reason for the exceptionally good state of preservation of the fossil plant material is the special way in which it has been preserved, which has only occurred very rarely in the Earth’s history: Mummification. This form of fossilization only takes place under very special conditions, which apparently existed at the foot of the Geodetic Hills on Axel Heiberg 45 million years ago.
“The walnuts were growing on the floodplain of a meandering river. Sediments were periodically deposited by periodic flooding of the river flats, and buried the walnuts,” Professor Basinger continues. “Fossilization depends upon many factors, including rapid burial, exclusion of air/oxygen in a waterlogged sediment to prevent decay. And for the extraordinary quality of preservation on Axel Heiberg Island, mummification of fossils, it is necessary that the fossils not be deeply buried, where they would be turned to coal. And of course for the last 10 million years they have been cold, even frozen, as a result of global climate deterioration that led to the Ice Age.”
After the first expedition to the fossil forest, many more took him to the island over the following two decades to collect more of the well-preserved fossil material. “Repeated collection of fossil nuts emerging from sediments through erosion was necessary to accumulate the extraordinary collection we now have.” A collection that includes more than a thousand nuts and seeds – enough material to identify the species.
Previously unknown walnut species
However, the characteristics of the fossil walnuts differ from those of previously known species. Professor Manchester analyzed the nuts using computer tomography, among other things, and found that the walnut trees that grew on Axel Heiberg 45 million years ago belong to three new but already extinct species: Juglans eoarctica, J. nathorstii and J. cordata.
In a study recently published in the International Journal of Plant Sciences, Professor Manchester as lead author and Professor Basinger together with two colleagues describe the new species. The egg- to pear-shaped nuts are between 10 and 29 millimeters long, making them somewhat smaller than the walnuts that grow today.
Professor Basinger described in a museum press release that he found some nuts in one spot, suggesting that animals must have collected and deposited them there. Some of the nuts even had small holes in them, most likely from mouse-sized rodents.
The discovery of the new species also contributes to a better understanding of the evolutionary history of the walnut family, which was long thought to have originated in Asia. But fossil finds indicate that they first appeared in warm, humid areas in North America or Europe.
A green island in the middle Eocene
Back then, Axel Heiberg was also located at roughly the same latitude as today and was therefore exposed to the polar day and night just like today. Nevertheless, thanks to a much warmer climate than today, a magnificent forest was able to develop that resembled today’s redwood forests on the west coast of North America.
In addition to walnuts and redwoods, cedars, hickories, hemlocks, larches, birches and gingko trees also grew here. The annual rings of well-preserved stumps show how the trees had adapted to the eternal light in summer and the months of darkness in winter: In summer, the trees grew quickly and formed broad light-colored lines in their wood and in winter they stopped growing, leaving only narrow dark lines.
Only a small part of the original forest can still be seen on the island in the form of fossil remains. Professor Basinger explains that the fossil forest covers an area of about half a square kilometer. However, the area on which the walnuts were found is only measured in square meters.
Julia Hager, Polar Journal AG
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