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Dinosaur and fossil aficionados are intimately familiar with the meteorite strike that drove Tyrannosaurus rex and all nonavian dinosaurs to extinction around 66 million years ago. But it is often overlooked that the impact also wiped out entire ecosystems. A new study shows how those casualties, in turn, led to another particularly profound evolutionary outcome: the emergence of the Amazon rain forest of South America, the most spectacularly diverse environment on the planet. Yet the Amazon’s bounty of tropical species and habitats now face their own existential threat because of unprecedented destruction from human activity, including land clearing for agriculture.
The new study, 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 dinosaur extinction was also a massive reset event for neotropical ecosystems, putting their evolution on an entirely new path leading directly to the extraordinary, diverse, spectacular and gravely threatened rain forests in the region today.” These insights, Wilf adds, “provide new impetus for 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 hold implications for today’s rapid, human-caused destruction of the Amazon rain forest and other key habitats across the planet. “We can relate this to nowadays,” he says, “because we’re also transforming landscapes, and that lasts 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 nonavian dinosaurs also gave rise of the Amazon rain forest. Credit: Carvalho et al., Science; 2021
Modern-day rain forests are integral to life on Earth. The Amazon, in particular, plays a crucial role in regulating the planet’s freshwater cycle and climate. Yet Western European and North American paleontologists have paid little attention to tropical forests, focusing instead on temperate latitudes. Many academic and amateur fossil hunters have also tended to write off warm, wet locales as a lost cause for finds because they have assumed that conditions there would prevent organic materials from being preserved long enough to fossilize. “It’s 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 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 with local conditions and distance from the Chicxulub impact crater in Mexico’s Yucatán Peninsula. New Zealand forests, for example, escaped relatively unscathed. But researchers have had no idea how the event changed the tropical rain forests 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 his home country’s tropical forests. The new study, which he 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 zero.”
Whole trees are almost never preserved in the fossil record, so Jaramillo and his colleagues turned to fossilized pollen and leaves for insights. Pollen preserves well over time and is widespread in the fossil record. Like leaves, it differs morphologically among species, which helps researchers determine what types of plants lived in an ancient habitat.
Jaramillo and his colleagues searched 53 sites across Colombia for rocks that formed during the Late Cretaceous period, just before the meteorite strike, and others that formed during 10 million subsequent years, in the Paleogene period. From these rocks, the team amassed and analyzed around 50,000 fossil pollen grains and 6,000 fossil leaves to characterize the types of plants that made them. Recent separate findings indicate that plant leaves receiving more light have a higher density of veins, as well as a higher ratio of a naturally occurring isotope called carbon 13. The researchers studied those features among the collected fossils to piece together the structure of the region’s past forests.
Their findings paint a picture of a sudden, cataclysmic annihilation of life after the impact—but also of a phoenix-like rebirth in the millions of years afterward. Prior to the meteorite, the authors determined, South America’s forests featured many conifers and a brightly lit open canopy supporting a lush understory of ferns. Dinosaurs likely played key roles in maintaining these Cretaceous forests by knocking down trees and clearing out vegetation, among other things. Within moments of the Chicxulub meteorite’s impact, however, this ecosystem was irrevocably altered. Fires, which likely burned for several years, engulfed South America’s southerly forests. 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 the forests to return to the level of diversity they had before the meteorite, and the species that slowly grew back were completely different than what 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 they enriched the formerly 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 to displace conifers. As flowering species competed for light, they formed dense canopies of leaves and created the layered Amazon rain forest we know today, which is characterized by a blanket of productivity up top and a dark understory 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 its findings are not only key for revealing the past but also for putting current anthropogenic threats into perspective. She particularly notes the authors’ calculation that 45 percent of plant species went extinct following the meteorite collision, because “current estimates suggest that at least this many plant species will be globally threatened in the Amazon basin in the next 30 years from human activities alone.”
“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,” Jacobs says. Today we are in the midst of another such event, she adds, but this one is driven by a single species—and there is no place far from the metaphorical impact crater “because humans are ubiquitous.”
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