Fans of dinosaurs and fossils are closely acquainted with the meteorite strike that led to the extinction of Tyrannosaurus rex and all nonavian dinosaurs about 66 million years ago. But it is often overlooked that the impact has also wiped out entire ecosystems. A new study shows how those victims, in turn, led to another particularly profound evolutionary outcome: the emergence of the Amazon rainforest in South America, the most spectacular diverse environment on the planet. However, the Amazon’s abundance of tropical species and habitats now faces their own existential threat due to unprecedented destruction of human activity, including deforestation of agricultural land.
The new study, published Thursday in Science, looked at tens of thousands of plant fossils and represents “a fundamental advance in knowledge,” says Peter Wilf, a geo-scientist at Pennsylvania State University who was not involved in the research. “The authors demonstrate that the extinction of dinosaurs was also a massive reset event for neotropical ecosystems, putting their evolution on a whole new path, leading directly to the extraordinary, diverse, spectacular and critically endangered tropical forests of the region today.”
These ideas, Wilf adds, “provide a new impetus for the conservation of the living evolutionary heritage of the tropics that supports human life, along with millions of living species.”
Carlos Jaramillo, a paleobiologist at the Smithsonian Tropical Research Institute in Panama and co-author of the study, agrees that the evolutionary and ecological effects of the meteorite have implications for the rapid man-made destruction of the Amazon rainforest and other key habitats. along the planet. “We can relate this to our days,” he says, “because we also transform landscapes and that lasts forever – or at least for a very long time.”
Analysis of about 50,000 pollen grains and 6,000 fossil leaves reveals that the meteorite that wiped out the Nonavian 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 freshwater cycle and the planet’s climate. However, paleontologists in Western Europe and North America have paid little attention to tropical forests, focusing instead on temperate latitudes. Many academic and amateur fossil hunters also tended to eliminate hot and humid locations as a lost cause of the discoveries, as they assumed that conditions there would prevent the preservation of organic materials long enough to fossilize. “This combination of factors 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 that was published with the new study in Science.
Scientists already knew that the effects of the meteorite’s collision and its consequences – at least in temperate areas – vary depending on local conditions and the distance from the Chicxulub impact crater on Mexico’s Yucatán Peninsula. New Zealand forests, for example, have escaped relatively unscathed. But researchers had no idea how the event changed tropical rainforests in Africa or, so far, those in South America.
Along with most of his co-authors, Jaramillo is from Colombia and specifically wanted to investigate the origins of tropical forests in his home country. The new study, which he conceptualized as a student, represents almost 12 years of effort. “It took us a long time,” he says, “because we had to start from scratch.”
Whole trees are almost never kept in the fossil record, so Jaramillo and his colleagues turned to pollen and fossilized leaves for information. The pollen is well preserved over time and is spread in the fossil record. Like the leaves, they differ morphologically between species, which helps researchers determine what types of plants lived in an ancient habitat.
Jaramillo and colleagues searched 53 sites in Colombia to find rocks that formed during the Late Cretaceous, just before the meteorite struck, and others that formed over the next 10 million years during the Paleogene. 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 produced them. Recent findings indicate that the leaves of plants that receive more light have a higher density of veins, as well as a higher ratio of a natural isotope called carbon 13. Researchers have studied these features among fossils collected to divide the region’s past structure. forests.
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Their discoveries paint the image of a sudden, cataclysmic annihilation of life after impact – but also of a phoenix-like rebirth millions of years later. Before the meteorite, the authors established, the forests of South America had many conifers and a brightly lit open canopy, which supported a lush underground of ferns. Dinosaurs probably played key roles in maintaining these Cretaceous forests by felling trees and clearing vegetation, among other things. However, in a few moments of impact of the Chicxulub meteorite, this ecosystem was irrevocably modified. The fires, which probably burned for several years, covered the southern forests of South America. Along with many of the animals they supported, a total of 45% of the continent’s tropical plant species have disappeared, according to the authors’ calculations.
It took six million years for the forests to return to the level of diversity they had before the meteorite, and the species that grew slowly were completely different from what came before. Legumes – plants that form symbiotic relationships with bacteria that allow them to fix nitrogen in the air – were the first to appear and enrich the previously nutrient-poor soil. This influx of nitrogen, along with phosphorus from the meteorite ash, allowed other flowering plants to thrive along with legumes and move conifers. As flowering species competed for light, they formed dense leaf canopies and created the stratified Amazon rainforest we know today, which is characterized by a productivity blanket at the top and a dark sub-forest at the bottom. .
Regan Dunn, a paleoecologist at the La Brea Tar Pits and Museum in Los Angeles who was not involved 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. It notes in particular the authors’ calculation that 45 percent of plant species became extinct following the meteorite collision, because “current estimates suggest that at least as many plant species will be threatened globally in the Amazon Basin in the next 30 years. years only from human activities. ”
“The question remains: How will the human impact change the composition and function of the Amazon forests forever?” Dunn says.
The new findings show how widespread mass extinction events can change “everyone’s course,” says Jacobs. Today we are in the middle of another such event, she adds, but it is run by a single species – and there is no place far from the metaphorical impact crater “because humans are ubiquitous.”
However, unlike past mass extinction events, Jacobs says, this time “we are powerless to stop it.”