The new study, published on 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 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.”
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.
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.”
Yet unlike past mass extinction events, Jacobs says, this time “we are not powerless to stop it.”
