When dinosaurs vanished, forests flourished and rivers calmed down
Most people know the story: An asteroid hit Earth 66 million years ago and the dinosaurs disappeared. But new research shows their loss also changed the very landscape they used to walk.
Geologists analyzing rock layers in Montana and Wyoming found that the extinction of dinosaurs coincided with an abrupt reorganization of rivers and floodplains across North America. The team says dinosaurs acted as “ecosystem engineers,” and their disappearance allowed forests to grow thicker, rivers to settle into steadier paths and swampy areas rich in coal to take over.
“Before the mass extinction, it would be similar what you see in Africa today with open savannahs maintained by large herbivores,” said study co-author Courtney Sprain, Ph.D., a geologist at the University of Florida. “Back then, you had large herds of triceratops wandering around in this region, flattening vegetation.”
Like modern elephants, the trampling and grazing of giant dinosaurs kept landscapes open. Without stabilizing tree cover, rivers were broad, muddy and easily shifted course.
After the impact, everything quickly changed. In the rock record, unstable streambeds gave way to broad, meandering rivers lined with dense vegetation. Coal seams suddenly appear, evidence of swampy, forested floodplains.
The international collaboration was led by Lucas Weaver, Ph.D., an assistant professor at the University of Michigan. The authors published their findings, which were supported by the National Science Foundation, Sept. 15 in the journal Communications Earth and Environment.
“Very often when we're thinking about how life has changed through time and how environments change through time, it's usually that the climate changes and, therefore, it has a specific effect on life, or this mountain has grown and, therefore, it has a specific effect on life,” said Weaver. “It's rarely thought that life itself could actually alter the climate and the landscape. The arrow doesn't just go in one direction.”
A New Hypothesis in the Rocks
The research documents five new boundary sites marked by iridium — the fingerprint of the giant asteroid impact — right where these sediment changes occur. That timing strengthens the link between dinosaur extinction and landscape transformation.
Previous ideas suggested shifting seas, climate changes or wildfires explained the difference. But those processes don’t match the abruptness or million-year persistence of the new river systems. By carefully analyzing the dates of different sediment layers, the researchers came to realize that the dinosaurs themselves were altering the landscape.
Big herbivores such as Triceratops or duck-billed hadrosaurs, some weighing several tons, flattened plants and prevented thick forests from taking hold. Their herding behavior amplified the effect. Once they vanished, dense forests quickly spread, locking rivers into more permanent channels and cutting off the steady flow of sand and mud that once covered floodplains. The organic matter piling up in these wetter, more stable settings became coal.
The environmental change in the wake of the dinosaur extinction has parallels today. Modern ecologists see similar effects when elephants, bison or other megafauna are removed: Forests encroach and rivers shift, altering the landscape.
The researchers stress their idea is still a hypothesis. They’ve set out predictions, such as signs of more open-canopy forests before the boundary and closed forests after, that other scientists can now test.
“We came up with this based on where we’ve worked, what we’ve seen,” Sprain said. “But we’re telling other scientists: ‘Go out and check if it matches your area.’”