Fans of dinosaurs and fossils are intimately familiar with the meteorite attack that led to the extinction of Tyrannosaurus rex and all non-avian dinosaurs about 66 million years ago. But it is often overlooked that the impact also ended up with entire ecosystems. A new study shows how these victims, in turn, led to another particularly profound evolutionary outcome: the emergence of the Amazon rainforest of South America, the most spectacular environment on the planet. Still, the generosity of the Amazon species and habitats now faces its own existential threat due to the unprecedented destruction of human activity, including clearing land for agriculture.
The new study, published Thursday in Science, analyzed tens of thousands of plant fossils and represents “a fundamental breakthrough in knowledge,” says Peter Wilf, a geoscientist at Pennsylvania State University who did not participate in the research. “The authors show that the extinction of the dinosaurs was also a mass restart event for neotropical ecosystems, putting their evolution on a whole new path leading directly to the extraordinary, diverse, spectacular and severely threatened rainforests of the region. current “.
These ideas, Wilf adds, “provide a new impetus for the conservation of living evolutionary heritage in the tropics that supports human life, along with millions of living species.”
Carlos Jaramillo, a paleobiologist at the Smithsonian Tropical Research Institute based in Panama and co-author of the study, agrees that the evolutionary and ecological effects of the meteorite have implications for the rapid human-caused destruction of the Amazon rainforest. and other key habitats. all over the planet. “We can relate it to our days,” he says, “because we’re 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 the non-avian dinosaurs also gave rise to the Amazon rainforest.
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Today’s rainforests are an integral part of life on Earth. The Amazon, in particular, plays a crucial role in regulating the planet’s freshwater cycle and climate. However, paleontologists in Western Europe and North America have paid little attention to tropical forests, focusing on temperate latitudes. Many academic and amateur fossil hunters have also tended to remove hot, humid sites as a lost cause of finds, as they have assumed that conditions there would prevent the conservation of organic materials long enough to fossilize. “It is this combination of factors that has led us to this absence of much data in the tropics,” says Bonnie Jacobs, a paleobiologist at Southern Methodist University, who co-authored a contextualizing essay published with the new study in Science. .
Scientists already knew that the effects of meteorite collisions and their consequences — at least in temperate zones — varied according to local conditions and the distance from the Chicxulub impact crater on the Yucatan Peninsula, Mexico. The forests of New Zealand, for example, escaped relatively unscathed. But researchers have had no idea how the event changed the rainforests of Africa or, so far, those of South America.
Along with most of its co-authors, Jaramillo is from Colombia and wanted to specifically investigate the origins of the tropical forests of his home country. The new study, which he conceptualized as an undergraduate student, represents nearly 12 years of effort. “It cost us a lot,” 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 pollen and fossilized leaves for information. Pollen is highly conserved over time and is widespread in the fossil record. Like the leaves, it differs morphologically between species, which helps researchers determine what types of plants lived in an ancient habitat.
Jaramillo and his colleagues searched 53 sites throughout Colombia for rocks that formed during the Late Cretaceous period, just before the meteorite attack, and others that formed during 10 million years. later years, during the Paleogene period. From these rocks, the team collected 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 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 compose the structure. of the region’s past. forests.
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His discoveries draw a picture of a sudden and cataclysmic annihilation of life after impact, but also of a phoenix-like renaissance in the millions of years that followed. Before the meteorite, the authors determined that South American forests had many conifers and an open canopy with lots of lighting that supported a lush undergrowth of ferns. It is likely that dinosaurs played a key role in maintaining these Cretaceous forests, felling trees and clearing vegetation, among other things. However, within minutes of the impact of the Chicxulub meteorite, this ecosystem was irrevocably altered. The fires, which probably burned for several years, engulfed South American forests. According to the authors’ calculations, a total of 45% of the continent’s tropical plant species disappeared along with many of the animals they supported.
Forests took six million years to return to the level of diversity they had before the meteorite, and the species that grew slowly were completely different from those before. Legumes, plants that form symbiotic relationships with bacteria that allow them to fix nitrogen in the air, were the first to appear and enriched the soil that previously had no nutrients. This flow of nitrogen, along with the phosphorus from the meteorite ash, allowed other flowering plants to thrive alongside the legumes and displace the conifers. As flowering species competed for light, they formed dense canopies of leaves and created the layered Amazon rainforest we know today, which is characterized by a blanket of productivity at the top and a dark undergrowth at the bottom. .
Regan Dunn, a paleoecologist at La Brea Tar Pits and Museum in Los Angeles, who did not participate in the new study, agrees that his findings are not only key to revealing the past, but also to putting current anthropogenic threats into perspective. She particularly points out 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 Amazon Basin in the next 30 years just for human activities. ”
“The question remains: How will the human impact change the composition and function of the Amazon forests forever?” Diu Dunn.
The new findings show how widespread mass extinction events can alter “the course of everything,” Jacobs says. Today we are in the middle of another event of this type, he adds, but this is driven by a single species, and there is no place far from the metaphorical impact crater “because humans are ubiquitous.”
But unlike past mass extinction events, Jacobs says, this time “we’re not powerless to stop it.”