How the Chicxulub impactor gave birth to modern rainforests –

How the Chicxulub impactor gave birth to modern rainforests

About 66 million years ago, a huge asteroid crashed into what is now Yucatán, plunging Earth into darkness. The impact transformed the rainforests, giving rise to the kingdom of flowers.

Today’s rainforests are biodiversity hotspots and play an important role in the world’s climate systems. A new study published today in Science sheds light on the origins of modern rainforests and can help scientists understand how rainforests will respond to a rapidly changing climate in the future.

The study led by researchers from the Smithsonian Tropical Research Institute (STRI) shows that the impact of an asteroid that ended the reign of the dinosaurs 66 million years ago also caused the extinction of 45% of the plants in what is now Colombia , and ushered in the reign of flowering plants in modern rainforests.

“We wonder how the rainforests changed after a drastic ecological disturbance like the Chicxulub impact, so we looked for tropical plant fossils,” said Mónica Carvalho, first author and joint postdoctoral fellow at STRI and the Universidad del Rosario in Colombia. . “Our team examined more than 50,000 fossil pollen records and more than 6,000 leaf fossils before and after the impact.”

In Central and South America, geologists scramble to find fossils exposed by roadblocks and mines before heavy rains wash them away and the jungle hides them again. Before this study, little was known about the effect of this extinction on the evolution of the flowering plants that now dominate the American tropics.

Carlos Jaramillo, STRI staff paleontologist and his team, mostly STRI fellows, many of them from Colombia, studied pollen grains from 39 sites including rock outcrops and cores drilled for oil exploration in Colombia, to paint a picture. large regional image of forests before and after impact. Pollen and spores obtained from rocks older than the impact show that the rainforests were equally dominated by ferns and flowering plants. Conifers, such as the relatives of the Kauri pine and the Norfolk Island pine, sold in supermarkets at Christmas (Araucariaceae), were common and cast their shadows on dinosaur trails. After the impact, conifers almost completely disappeared from the New World tropics and flowering plants took over. Plant diversity did not recover for about 10 million years after the impact.

The leaf fossils told the team a lot about the past climate and the local environment. Carvalho and Fabiany Herrera, a postdoctoral research associate at the Negaunee Institute for Conservation Science and Action at the Chicago Botanical Garden, led the study of more than 6,000 specimens. Working with Scott Wing at the Smithsonian’s National Museum of Natural History and others, the team found evidence that rainforest trees before impact were widely spaced, allowing light to reach the forest floor. Within 10 million years of impact, some tropical forests were dense, such as today, where tree leaves and vines cast a deep shadow over smaller trees, shrubs, and herbaceous plants found under. The more scattered crowns of the pre-impact forests, with fewer flowering plants, would have moved less water from the ground into the atmosphere than those that grew in the millions of years after.

“It was just as rainy in the Cretaceous, but the forests worked differently.” Carvalho said.

The team found no evidence of legume trees before the extinction event, but there was great diversity and abundance of legume leaves and pods afterward. Today, legumes are a dominant family in rainforests and, through associations with bacteria, they take nitrogen from the air and turn it into fertilizer for the soil. The increase in legumes would have drastically affected the nitrogen cycle.

Carvalho also worked with Conrad Labandeira at the Smithsonian’s National Museum of Natural History to study insect damage to leaf fossils.

“Insect damage to plants can reveal in the microcosm of a single leaf or the extension of a plant community, the basis of the trophic structure in a tropical forest,” Labandeira said. “The energy that resides in the mass of plant tissue that is transmitted along the food chain, ultimately to boas, eagles and jaguars, begins with insects that skeletonize, chew, pierce and suck, extract, crack and they pierce plant tissues … The evidence for this food chain of consumption begins with all the diverse, intensive and fascinating ways insects consume plants. “

“Before the impact, we see that different types of plants have different damages: the feeding was specific to the host,” said Carvalho. “After the impact, we found the same type of damage in almost all the plants, which means that the feeding was much more generalist.”

How did the aftermath of the impact transform the sparse and coniferous-rich tropical forests of the dinosaur era into the tropical forests of today: towering trees dotted with yellow, purple, and pink flowers, dotted with orchids? Based on the evidence from both pollen and leaves, the team comes up with 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 across the landscape. A second explanation is that the ashfall from the impact enriched the soils of the tropics, giving faster-growing flowering plants an advantage. The third explanation is that the preferential extinction of coniferous species created an opportunity for flowering plants to take over the tropics.

“Our study follows a simple question: How do rainforests evolve?” Carvalho said. “The lesson learned here is that under rapid disturbances, geologically speaking, tropical ecosystems do not simply recover, they are replaced and the process takes a long time.”

The Smithsonian Tropical Research Institute, based in Panama City, Panama, is a unit of the Smithsonian Institution. The institute promotes understanding of tropical biodiversity and its importance to human well-being, trains students to conduct research in the tropics, and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems.


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