Fossil leaf from the earliest Eocene
(~55.5 million years) of the
Bighorn Basin, Wyoming.
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Natural History Highlight
Mississippi to Montana: Plants Danced to Climate’s Quick Tune
In the cover story of the November 11 issue of Science a team of scientists led by Scott Wing, of the Department of Paleobiology at the Smithsonian Institution, announced their finding that a period of rapid global warming 55 million years ago caused major changes in where plants grew. This sudden global warming, called the Paleocene-Eocene Thermal Maximum, or PETM, occurred in about ten thousand years, lasted for 80 to 100 thousand years, and lifted global temperatures 4-8 °C. The PETM was caused by a massive release of carbon (about 5,000 gigatons) to the atmosphere and ocean, making it an analog for the global warming that is expected as humans add carbon dioxide to the atmosphere by burning fossil fuels. Previous work has linked the PETM with mass extinctions of bottom-dwelling marine organisms and the migration of mammals across Arctic land bridges, but the paper by Wing and colleagues last fall was the first to document similar effects on land plants. The research team included Smithsonian postdoctoral fellow Francesca Smith and former Smithsonian postdoctoral fellow Guy Harrington (now at University of Birmingham, England), as well Jonathan Bloch (University of Florida), Douglas Boyer (Stony Brook University), and Katherine Freeman (Pennsylvania State University).
The work was based on fossil leaves and pollen found in the Bighorn Basin of northwestern Wyoming. The Bighorn Basin is a well-known treasure trove for fossils because enormous amounts of sediment were deposited there as the Rocky Mountains rose; those sediments and the fossils they contain are now exposed in spectacularly eroded badlands. Fossil mammals collected and identified by Bloch and Boyer were key to finding the PETM interval, and measurements of the stable isotope composition of organic carbon in the rocks performed by Smith and Freeman demonstrated that the leaf and pollen fossils studied by Wing and Harrington were from the same time as the PETM carbon release and climate change. The leaf fossils on which the report is based are the first ever discovered from the PETM interval.
Image below: Fossil bean leaf (Machaerium) 55 million years old, from rocks deposited during the PETM. Leaves like this are not known from the Paleocene in Wyoming, nor from Eocene deposits just above the PETM. (Click on image for full resolution.)
Plant fossils from before and after the PETM document the typical northern forests of the time, including relatives of dawn redwood, alder, sycamore, walnut and sassafras. Fossils from the PETM itself show that for a short time the area supported an entirely different type of vegetation dominated by members of the bean family, and including warmth-loving relatives of poinsettia, sumac and paw-paw. Some of the plant fossils found in the PETM in Wyoming have long been known from rocks of similar age in Mississippi, Louisiana and Texas. These southern plants spread from the Gulf Coast to Wyoming, a distance of some 1,000 miles, in ten thousand years or less. The southern plants occurred throughout the PETM in Wyoming, then disappeared, presumably unable to survive there as global climate cooled at the end of event. The return to cooler climate brought a different set of plant invaders to Wyoming. Relatives of linden and wingnut emigrated from Europe to North America across Arctic land bridges during the peak warmth, then established themselves in Wyoming forests after the global heat wave passed. A similar pattern of intra- and intercontinental migration is seen in animals that lived during the PETM, suggesting they, too, moved in concert with rapid climate change.
Wing says that during the PETM plant ranges changed about as quickly as they did when glaciers retreated from the northern continents in the last 20,000 years. Similarly rapid changes in flora might be expected in the future as a result of human-induced global warming – if habitat destruction doesn’t block the paths by which plants disperse to their preferred climate zone. “No two events are ever exactly the same,” said Wing, “but the history of life on this planet has a lot to tell us about our possible future.” Or as Mark Twain put it: “History may not repeat itself, but it sure does rhyme.”
Date posted: 1 March 2006