Dinosaur and fossil enthusiasts are well acquainted with the meteorite attack that threatened the Tyrannosaurus rex and all non-avian dinosaurs to extinction about 66 million years ago. But it is often overlooked that the impact has wiped out entire ecosystems as well. A new study shows how those victims in turn led to another particularly profound evolutionary outcome: the emergence of South America’s Amazon rainforest, the most spectacularly diverse environment on Earth. Yet the abundance of tropical species and habitats in the Amazon is now facing their own existential threat due to unprecedented destruction from 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 advance in knowledge,” said Peter Wilf, a geoscientist at Pennsylvania State University who was not involved in the study. “The authors show that the extinction of dinosaurs was also a massive reset for neotropical ecosystems, setting their evolution on a whole new path that leads straight into the extraordinary, diverse, spectacular and critically endangered rainforests in the current region.”
These insights, adds Wilf, “give new impetus to the conservation of the living evolutionary heritage in the tropics that supports 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 meteorite’s evolutionary and ecological effects have implications for the rapid human-caused destruction of the Amazon rainforest and other important habitats. all over the planet. “We can relate this these days,” he says, “because we also transform landscapes, and that takes forever – or at least a very 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 created the Amazon rainforest.
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Modern rainforests are an integral part of life on Earth. The Amazon in particular plays a crucial role in regulating the freshwater cycle and the Earth’s climate. Still, Western European and North American paleontologists have paid little attention to tropical forests, but instead in temperate latitudes. Many academic and amateur fossil hunters also tend to write off warm, wet locations as a lost cause for finds, assuming that conditions there would prevent organic materials from being preserved long enough to fossilize. “It is this combination of factors that has led us to the lack of much data in the tropics,” said Bonnie Jacobs, a paleobiologist at Southern Methodist University who co-authored a contextualizing essay that, along with the new study in Science was published.
Scientists already knew that the effects of the meteorite collision and its aftermath – at least in temperate regions – varied with local conditions and distance from the Chicxulub impact crater on Mexico’s Yucatán peninsula. For example, New Zealand forests have escaped relatively unscathed. But researchers had no idea how the event changed the tropical rainforests of Africa or, until now, that of South America.
Together with most of his co-authors, Jaramillo is from Colombia and wanted to specifically investigate the origins of the tropical forests of his homeland. The new study, which he conceived 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.”
Entire trees have almost never survived in the fossil record, so Jaramillo and his colleagues turned to fossilized pollen and leaves for insights. Pollen holds well over time and is widespread in the fossil record. Like leaves, it differs morphologically from species to species, helping researchers determine which types of plants lived in an ancient habitat.
Jaramillo and his colleagues searched 53 sites in Colombia for rocks formed during the Late Cretaceous, just before the meteorite impact, and others that formed over 10 million subsequent years, in 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 created them. Recent separate findings indicate that leaves of plants that receive more light have a higher vein density, as well as a higher ratio of a naturally occurring isotope called carbon 13. The researchers studied those characteristics of the collected fossils to structure the region’s past. to put together. forests.
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Their findings paint a picture of a sudden, cataclysmic destruction of life after the impact – as well as a phoenix-like rebirth in the millions of years since. Prior to the meteorite, the authors noted, the forests of South America contained many conifers and a brightly lit open canopy that supported a lush undergrowth of ferns. Dinosaurs likely played a key role in preserving these Cretaceous forests by, among other things, toppling trees and clearing vegetation. Within moments of the impact of the Chicxulub meteorite, however, this ecosystem had changed irrevocably. Fires, likely burning 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 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 slowly regrowed were completely different than before. Legumes – plants that form a symbiotic relationship with bacteria that allow them to extract nitrogen from the air – were the first to appear, enriching the previously nutrient-poor soil. This influx of nitrogen, along with phosphorus from the meteorite’s ash, enabled other flowering plants to thrive alongside the legumes and crowd out conifers. As flowering species competed for light, they formed dense blades of leaves and created the stratified Amazon rainforest we know today, which is characterized by a blanket of productivity at the top and a dark undercoat 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 the findings are key not only to revealing the past, but also to putting current anthropogenic threats into perspective. In particular, she notes the authors’ calculation that 45 percent of plant species became extinct after the meteorite collision, because “current estimates suggest that in the Amazon basin, at least for the next 30 years, human activities alone will face global threats. “
“The question remains, how will human impact forever change the composition and function of Amazonian forests?” Dunn says.
The new findings show how massive mass extinction events could “change the course of everything,” Jacobs said. Today we are in the midst of another such event, she adds, but it is driven by a single species – and no place far from the metaphorical impact crater “because humans are ubiquitous.”
But unlike previous mass extinction events, Jacobs says, this time “we’re not powerless to stop it.”