by Ancient cycad plants, which thrived during the Mesozoic Era, played a crucial role in sustaining grazing dinosaurs and other prehistoric animals. However, most cycad species have since gone extinct, leaving only a few surviving in tropical and subtropical regions.
According to a study published in the journal Nature Ecology & Evolution, the surviving cycad species relied on symbiotic bacteria in their roots to obtain nitrogen for growth. Similar to modern legumes and other nitrogen-fixing plants, these cycads traded their sugars with the bacteria in their roots in exchange for nitrogen extracted from the atmosphere.
Lead author Michael Kipp, who specializes in combining geochemistry with the fossil record to understand Earth’s climate history, became interested in the cycads’ ability to fix nitrogen. He conducted his research at the University of Washington and analyzed ancient plant fossils to gain insight into the composition of ancient atmospheres. Surprisingly, most of the fossilized cycads were not nitrogen-fixers, leading Kipp to deduce that these lineages had gone extinct.
Kipp, now an assistant professor at Duke University’s Nicholas School of the Environment, emphasizes that this discovery sheds light on the changing ecology of these plants throughout history. While much of our knowledge about ancient climates comes from studying chemical markers in marine life and sediments, applying similar methods to terrestrial plants is a novel approach.
At the start of the project, there were no published nitrogen isotope data from fossilized plant foliage. It took Kipp some time to develop and refine the method, as well as secure samples of valuable plant fossils. In the few fossil samples from surviving cycad lineages, dating back 20-30 million years, the nitrogen signature matched that of modern cycads, indicating the presence of symbiotic bacteria. However, this nitrogen signature was absent in older and extinct cycad fossils.
The study also raises questions about how nitrogen fixation aided the surviving cycads. It potentially helped them adapt to the drastic climate shifts following the extinction event, or it may have given them an advantage over angiosperm plants, which grew more rapidly after the extinction.
This new technique opens up further possibilities for research, as Kipp believes there is much more to be discovered using this method. Understanding Earth’s climate history allows us to gain insight into its potential future.
