The extinction of dinosaurs 66 million years ago didn’t just clear the way for mammals to rise. It also transformed the very shape of Earth’s landscapes.
A new study shows that when dinosaurs disappeared, forests flourished—and that shift reengineered rivers across North America.
For decades, geologists noticed a sharp contrast between rock layers formed before and after the dinosaurs vanished.
The older layers were messy, full of waterlogged soils, while the younger ones showed neat, striped deposits.
Many assumed sea levels or climate shifts explained the change. But University of Michigan paleontologist Luke Weaver and his colleagues suspected something else: the extinction of dinosaurs themselves.
Dinosaurs, the team argues, were “ecosystem engineers.”
By eating plants and knocking down trees, they kept much of the land open and patchy. Without thick forests, rivers spread out loosely across floodplains, with little structure.
When dinosaurs were wiped out by the asteroid strike at the Yucatán Peninsula, forests were finally able to spread. Dense vegetation stabilized soil and funneled water into winding, meandering rivers.
To test this idea, Weaver and his team studied rock layers in Montana, Wyoming, and the Dakotas, especially the Fort Union Formation. At first, these striped rock layers were thought to represent ancient ponds.
But closer analysis revealed they were deposits formed along the insides of big river bends—clear evidence of structured, meandering rivers.
Above and below these river deposits, the team also found lignite, a type of coal made from compressed plants. That suggested dense forests were locking sediments in place, preventing frequent flooding.
The smoking gun was the “iridium anomaly”—a thin clay layer unusually rich in the element iridium, left behind by the asteroid impact.
The researchers found this exact marker right at the boundary where the landscape shifts occurred. That confirmed the changes in rivers happened immediately after the dinosaurs died out.
Weaver was inspired by modern examples of large animals shaping their environments, such as elephants that knock down trees and alter ecosystems. Dinosaurs, being even bigger, likely had an even greater impact.
With them gone, vegetation patterns changed dramatically, reshaping entire landscapes.
This discovery is more than a story about the past. Weaver says it’s also a warning for today.
Just as the sudden loss of dinosaurs rapidly transformed Earth’s rivers, the current wave of human-driven extinctions and climate change could cause equally swift and dramatic shifts in landscapes and ecosystems.
“What’s happening in our lifetimes is the blink of an eye in geologic terms,” Weaver says. “The extinction of the dinosaurs shows us just how quickly the Earth can be reengineered when life changes suddenly.”
Source: University of Michigan.
