The meteorite that killed the dinosaurs ended up creating the Amazon rainforest, study says

Dinosaur and fossil buffs are intimately familiar with a meteorite impact that drove Tyrannosaurus rex and all non-avian dinosaurs to extinction some 66 million years ago. But it is often overlooked that the impact also wiped out entire ecosystems. A new study shows how those victims, in turn, led to another particularly profound evolutionary outcome: the rise of the Amazon rainforest of South America, the most spectacularly diverse environment on the planet. However, the abundance of tropical species and habitats in the Amazon now faces its own existential threat due to the unprecedented destruction of human activity, including the clearing of land for agriculture.

The new study, published Thursday in Science, analyzed tens of thousands of plant fossils and represents “a fundamental advance in knowledge,” says Peter Wilf, a geoscientist at Pennsylvania State University, who was not involved in the research. “The authors demonstrate that the extinction of the dinosaurs was also a massive reboot event for neotropical ecosystems, setting their evolution on a whole new path that leads directly to the extraordinary, diverse, spectacular, and seriously threatened rainforests of the region in the present”.

These ideas, Wilf adds, “provide a new impetus for the conservation of the living evolutionary heritage in the tropics that sustains human life, along with millions of living species.”

Carlos Jaramillo, a paleobiologist at the Panama-based Smithsonian Tropical Research Institute and co-lead author of the study, agrees that the evolutionary and ecological effects of the meteorite have implications for the rapid destruction of the Amazon rainforest and other key habitats today. in day all over the planet. “We can relate this to today,” he says, “because we are also transforming landscapes, and that lasts forever, or at least a long time.

Analysis of some 50,000 pollen grains and 6,000 fossil leaves reveals that the meteorite that wiped out non-avian dinosaurs also gave rise to the Amazon rainforest.

Read | The first dinosaur preserved in a nest with fossilized embryos found in China

Today’s rainforests are an integral part of life on Earth. The Amazon, in particular, plays a crucial role in regulating the freshwater cycle and the planet’s climate. However, paleontologists in western Europe and North America have paid little attention to tropical forests and have focused instead on temperate latitudes. Many academic and hobbyist fossil hunters have also tended to dismiss hot and humid places as a lost cause for the finds because they have assumed that the conditions there would prevent organic materials from being preserved long enough to fossilize. “It is this combination of factors that has led us to this absence of a lot of data in the tropics,” says Bonnie Jacobs, a paleobiologist at Southern Methodist University, co-author of a contextualizing essay that was published with the new study in Science.

Scientists already knew that the effects of the meteorite collision and its aftermath, at least in temperate zones, varied according to local conditions and the distance from the Chicxulub impact crater on Mexico’s Yucatan Peninsula. New Zealand’s forests, for example, escaped relatively unscathed. But the researchers had no idea how the event changed the rainforests of Africa or, until now, those of South America.

Along with most of his co-authors, Jaramillo is from Colombia and specifically wanted to investigate the origins of the tropical forests of his home country. The new study, which you conceptualized as an undergraduate student, represents nearly 12 years of effort. “It took us a long time,” he says, “because we had to start from scratch.”

Whole trees are almost never preserved in the fossil record, so Jaramillo and his colleagues turned to fossilized pollen and leaves for information. Pollen is well preserved over time and is widespread in the fossil record. Like leaves, it differs morphologically between species, helping researchers determine what types of plants lived in ancient habitat.

Jaramillo and his colleagues searched 53 sites in Colombia for rocks that formed during the Upper Cretaceous period, just before the meteorite impact, and others that formed over 10 million years later, in the Paleogene period. From these rocks, the team amassed and analyzed about 50,000 fossil pollen grains and 6,000 fossil leaves to characterize the types of plants that made them. Recent separate findings indicate that the leaves of plants that receive more light have a higher density of veins, as well as a higher proportion of a natural isotope called carbon 13. The researchers studied these characteristics among the fossils collected to reconstruct the structure of the past of the region. forests.

Read also | Comet at the edge of the solar system killed the dinosaurs: study

Their findings paint a picture of a sudden and cataclysmic annihilation of life after impact, but also of a phoenix-like rebirth in the millions of years after. Before the meteorite, the authors determined, the forests of South America featured many conifers and a brightly lit open canopy that supported a lush undergrowth of ferns. Dinosaurs likely played a key role in maintaining these Cretaceous forests by cutting down trees and clearing vegetation, among other things. However, moments after the Chicxulub meteorite impact, this ecosystem was irrevocably altered. The fires, which probably burned for several years, engulfed the southern forests of South America. Along with many of the animals they supported, a total of 45 percent of the continent’s tropical plant species disappeared, according to the authors’ calculations.

It took six million years for forests to return to the level of diversity they had before the meteorite, and species that slowly regrowed were completely different from those that came before. Legumes, plants that form symbiotic relationships with bacteria that allow them to fix nitrogen from the air, were the first to appear and enriched previously nutrient-poor soil. This influx of nitrogen, along with phosphorous from the meteorite ash, allowed other flowering plants to thrive alongside legumes and displace conifers. As the flowering species competed for light, they formed dense canopies of leaves and created the layered Amazon rainforest we know today, characterized by a blanket of productivity at the top and a dark understory below.

Regan Dunn, a paleoecologist at La Brea Tar Pits and Museum in Los Angeles, who was not involved in the new study, agrees that their findings are not only key to revealing the past, but also to putting current anthropogenic threats into perspective. . In particular, it notes the authors’ calculation that 45 percent of plant species became extinct after the meteorite collision, because “current estimates suggest that at least this number of plant species will be globally threatened in the world. Amazon basin in the next 30 years only by human activities. “

“The question remains: How will human impact change the composition and function of Amazonian forests forever?” Dunn says.

The new findings show how extensive mass extinction events can alter “the course of everything,” says Jacobs. We’re in the middle of another such event today, he adds, but this one is powered by a single species, and there’s no place far from the metaphorical impact crater “because humans are ubiquitous.”

However, unlike past mass extinction events, Jacobs says, this time “we are not powerless to stop it.”


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