A new study has uncovered how some plants used nitrogen to survive the cataclysmic events that led to the extinction of dinosaurs. Michael Kipp, the lead author of the study and an upcoming assistant professor of Earth and Climate Sciences at Duke University, has provided significant insights into this enduring mystery.
Cycads, once a prevalent source of food for grazing dinosaurs, have mostly vanished, with only a few species existing in tropical and subtropical areas today.
Their decline, which began in the late Mesozoic and continued into the early Cenozoic Era, was accelerated by the asteroid impact and volcanic activity marking the K-Pg boundary 66 million years ago. However, unlike their ancient grazers, some cycads managed to survive until the present day.
The results of the study suggest that the key to their survival was their symbiotic relationship with nitrogen-fixing bacteria in their roots. This relationship allowed the plants to extract nitrogen directly from the atmosphere, similar to modern legumes and other nitrogen-fixing plants.
Through this symbiosis, cycads traded sugars with the bacteria in exchange for atmospheric nitrogen, enabling them to thrive in changing environments.
Kipp’s research was based on the knowledge that the tissues of nitrogen-fixing plants can reveal the atmospheric composition of the past. He set out to analyze old plant fossils to gain insights into ancient atmospheres.
This analysis led to the realization that the surviving cycads were the ones that had developed nitrogen-fixing capabilities, unlike their extinct counterparts.
“Instead of being a story about the atmosphere, we realized this was a story about the ecology of these plants that changed through time,” said Kipp, who spent nearly a decade on this finding, first at UW and then as a postdoctoral researcher at CalTech.
The surviving cycads showed the same nitrogen signatures in their fossils as modern cycads, indicating their reliance on symbiotic bacteria for nitrogen. This contrasted with older, extinct cycad fossils, which lacked this nitrogen signature.
The exact benefits of nitrogen fixation for the survival of these cycads remain an area of ongoing research. It might have aided them in adapting to the dramatic climatic changes post-dinosaur extinction or in competing with rapidly proliferating angiosperms, or perhaps both.
Kipp’s upcoming role at Duke University will further focus on using the fossil record to unravel Earth’s climate history and its potential future.
Much of what we know about ancient atmospheres comes from chemical studies of ancient sea life and sediments, said Kipp. Applying some of those methods to terrestrial plants will add new pieces to this puzzle.
Cycads are a group of ancient, seed-producing plants that have a long and fascinating history. Here are some key aspects about them:
Cycads are among the oldest seed plants on Earth, dating back over 280 million years. They were already ancient when dinosaurs roamed the Earth, thriving during the Mesozoic era.
Cycads resemble palm trees or ferns but are not closely related to either. They typically have a stout and woody trunk with a crown of large, hard and stiff, evergreen leaves. They can vary in size from small, few-centimeter-tall plants to large trees.
Cycads are dioecious, meaning individual plants are either male or female. They reproduce via cones, with pollination often occurring through the wind or insects, notably beetles. Some species also exhibit environmental sex determination, where temperature influences the sex of the plant.
Cycads are primarily found in tropical and subtropical regions of the world. They are native to South and Central America, Australia, Asia, and Africa.
There are around 300 species of cycads, categorized into three families: Cycadaceae, Stangeriaceae, and Zamiaceae. Each family and species have unique characteristics and adaptations.
In their native habitats, cycads play significant roles in the ecosystem. They provide food and habitat for various animal species. However, they are slow-growing and have specific habitat requirements, making them sensitive to environmental changes.
Many cycad species are endangered due to habitat destruction, over-collection, and environmental changes. Conservation efforts include habitat protection, cultivation in botanical gardens, and strict regulation of trade (many species are listed under CITES – the Convention on International Trade in Endangered Species of Wild Fauna and Flora).
Cycads have been used by various cultures for food, medicine, and ornamental purposes. However, care must be taken as many parts of the plant are toxic if not properly processed.
Cycads have a rich fossil record, which helps scientists understand the Earth’s prehistoric environments and the evolution of plant life. Their persistence through various geological periods and mass extinctions makes them an important subject of study in paleobotany and evolutionary biology.
The study is published in the journal Nature Ecology & Evolution.
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