How Plants Used Nitrogen to Survive the Dinosaur Extinction

A fossilized cycad specimen that was sampled for nitrogen isotopes that would indicate the atmosphere it grew in.

Palm ferns, commonly known as cycads or cycadales, adorn many windowsills today. However, few may be aware that the history of these plants dates back almost 300 million years. Recent research suggests that the survival of some species into modern times can likely be attributed to a symbiotic relationship with nitrogen-fixing bacteria. This alliance allowed these ancient plants to thrive in nutrient-poor soils, avoiding competition with flowering plants, as reported in the journal Nature.

Palm ferns, scientifically known as Cycadeen or Cycadales, are popular choices for gardens and indoor plants. While their palm-like trunks and fern-like leaves give them an ancient appearance, the true extent of their history is often overlooked. The first palm ferns emerged in the early Permian period, around 300 million years ago.

These primitive plants later served as a food source for dinosaurs when they were widely distributed, forming the understory of numerous forests. Over time, they declined, and only a few species managed to survive into the modern era. The question arises: How did they achieve this feat?”

A New Insight into the Past of Cycads

All palm ferns currently thriving in tropical and subtropical regions share a commonality: akin to some flowering plants, they engage in a symbiotic relationship with nitrogen-fixing bacteria residing in their roots. These nodular bacteria extract nitrogen from the atmosphere for the palm ferns, rendering them biologically usable. In return, the plants reciprocate with sugars produced during photosynthesis. This mechanism enables palm ferns to flourish even in nutrient-poor soils with low nitrogen content.

However, was this strategy truly the key to the survival of certain palm ferns into the modern era? To ascertain this, researchers led by Michael Kipp from the University of Washington examined the fossilized remains of various species. The analysis incorporated a total of 178 fossils from twelve sites spanning from Antarctica to Greenland, with the oldest palm fern specimens dating back 250 million years and the youngest from 20 million years ago.

By determining the nitrogen isotope ratios in the leaves, Kipp and his colleagues could discern whether the palm ferns historically obtained nitrogen from the soil or through symbiosis with single-celled organisms. These data ultimately revealed whether the method of nitrogen acquisition played a decisive role in the life and extinction of palm ferns.

Symbiosis Ensured Survival

Indeed, according to Kipp and his team, all extinct palm fern species previously depended on obtaining nitrogen directly from the soil. “In the few fossil samples from surviving lineages, not more than 20 to 30 million years old, we observe the same nitrogen signature as today,” notes Kipp. Consequently, these palm ferns may have benefited from a symbiotic relationship with bacteria. Palm ferns without symbiosis with nitrogen-fixing bacteria, on the other hand, faced extinction.

But why did the nitrogen source make such a significant difference for the various palm ferns? The researchers speculate that the reason might lie in the escalating competition with flowering plants, which had become the dominant plant group by the end of the dinosaur era. Even today, gymnosperms, such as conifers and palm ferns, compete with flowering plants for nutrients.

Microbial Allies Against Competition

By learning to acquire nitrogen with the help of bacteria, different lines of palm ferns could evade this competitive pressure and thrive in nutrient-poor soils. Simultaneously, nitrogen acquisition through symbiosis could have assisted these ancient plants in surviving the profound climate changes that Earth faced after the asteroid impact 66 million years ago, suggest Kipp and his colleagues.

Featured Image: A fossilized cycad specimen that was sampled for nitrogen isotopes that would indicate the atmosphere it grew in. Michael Kipp—Duke University