About 66 million years ago (the end of the Cretaceous period), a 10-km-wide asteroid crashed into Earth near the site of the small town of Chicxulub in what is now Mexico. The impact unleashed an incredible amount of climate-changing gases into the atmosphere, triggering a chain of events that led to the extinction of non-avian dinosaurs and 75% of life on the planet. In an analysis of thousands of fossil pollen and leaves from before and after the Chicxulub impact, scientists from the Smithsonian Tropical Research Institute and elsewhere found that the cataclysmic impact also caused 45% of plants in what is now Colombia to go extinct, setting the stage for the evolution of the Neotropical rainforests.
Reconstruction of Late Maastrichtian (66 million years ago) paleoenvironment in North America, where a floodplain is roamed by dinosaurs like Tyrannosaurus rex, Edmontosaurus and Triceratops. Image credit: Davide Bonadonna.
In the study, Smithsonian Tropical Research Institute paleontologist Dr. Carlos Jaramillo and colleagues examined fossil pollen (over 50,000 occurrences) and leaves (over 6,000 specimens) from before and after the Chicxulub impact from 39 localities in Colombia.
“We wondered how tropical rainforests changed after a drastic ecological perturbation such as the Chicxulub impact, so we looked for tropical plant fossils,” explained first author Dr. Mónica Carvalho, a postdoctoral researcher at Smithsonian Tropical Research Institute and the Universidad del Rosario.
Pollen and spores obtained from rocks older than the impact show that rainforests were equally dominated by ferns and flowering plants. Conifers were common and cast their shadows over dinosaur trails.
After the impact, conifers disappeared almost completely from the New World tropics, and flowering plants took over. Plant diversity did not recover for over 6 million years after the impact.
The researchers found evidence that pre-impact tropical forest trees were spaced far apart, allowing light to reach the forest floor.
Within 6 million years post-impact, some tropical forests were dense, like those of today, where leaves of trees and vines cast deep shade on the smaller trees, bushes and herbaceous plants below.
The sparser canopies of the pre-impact forests, with fewer flowering plants, would have moved less soil water into the atmosphere than did those that grew up in the millions of years afterward.
“It was just as rainy back in the Cretaceous, but the forests worked differently,” Dr. Carvalho said.
The team found no evidence of legume trees before the extinction event, but afterward there was a great diversity and abundance of legume leaves and pods.
Today, legumes are a dominant family in tropical rainforests, and through associations with bacteria, take nitrogen from the air and turn it into fertilizer for the soil. The rise of legumes would have dramatically affected the nitrogen cycle.
How did the after effects of the impact transform sparse, conifer-rich tropical forests of the dinosaur age into the rainforests of today — towering trees dotted with yellow, purple and pink blossoms, dripping with orchids? Based on evidence from both pollen and leaves, Dr. Jaramillo, Dr. Carvalho and their colleagues propose three explanations for the change, all of which may be correct.
One idea is that dinosaurs kept pre-impact forests open by feeding and moving through the landscape.
A second explanation is that falling ash from the impact enriched soils throughout the tropics, giving an advantage to the faster-growing flowering plants.
The third explanation is that preferential extinction of conifer species created an opportunity for flowering plants to take over the tropics.
“Our study follows a simple question: How do tropical rainforests evolve?,” Dr. Carvalho said.
“The lesson learned here is that under rapid disturbances — geologically speaking — tropical ecosystems do not just bounce back; they are replaced, and the process takes a really long time.”
A paper on the findings was published in the journal Science.
Mónica R. Carvalho et al. 2021. Extinction at the end-Cretaceous and the origin of modern Neotropical rainforests. Science 372 (6537): 63-68; doi: 10.1126/science.abf1969