Ann Arbor Times

A recent study led by researchers at the University of Michigan suggests that the extinction of dinosaurs triggered significant changes in continental landscapes, particularly affecting river systems across what is now western North America. The findings, published in Communications Earth & Environment and supported by the National Science Foundation, provide new insights into how large animals can shape their environments.

Paleontologist Luke Weaver and his colleagues examined rock formations from regions including the Williston Basin and Bighorn Basin. They observed that geologic layers deposited after the Cretaceous-Paleogene (K-Pg) mass extinction were markedly different from those laid down before. Traditionally, scientists attributed these changes to factors like sea level rise or other non-biological causes. However, Weaver’s team found evidence suggesting that the absence of dinosaurs allowed forests to grow denser, which stabilized riverbanks and changed river patterns.

“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, assistant professor in the U-M Department of Earth and Environmental Sciences. “It’s rarely thought that life itself could actually alter the climate and the landscape. The arrow doesn’t just go in one direction.”

Dinosaurs are believed to have acted as “ecosystem engineers” by clearing vegetation and keeping areas between trees open. Their disappearance allowed forests to expand unchecked. This led to rivers becoming more stable with broader meanders due to increased sediment stabilization from tree roots.

The research team analyzed rock layers such as the Fort Union Formation—deposited after dinosaur extinction—which feature distinctive bands previously thought to be pond deposits resulting from rising sea levels. Upon closer examination during fieldwork as graduate students, Weaver and his co-authors determined these were actually point bar deposits formed inside large river bends.

“What we realized was that the pajama stripes actually weren’t pond deposits at all. They’re point bar deposits, or deposits that form the inside of a big meander in a river,” said Weaver, also assistant curator of fossil mammals at the U-M Museum of Paleontology. “So instead of looking at a stillwater, quiet setting, what we’re actually looking at is a very active inside of a meander.”

They also noted an increase in lignite—a type of coal derived from plant matter—in post-extinction layers. According to Weaver: “By stabilizing rivers, you cut off the supply of clay, silt and sand to the far reaches of the floodplain, so you’re mostly accumulating organic debris.”

To confirm whether these geological shifts coincided with dinosaur extinction events elsewhere in North America’s interior basins—not just isolated locations—the team searched for evidence such as an iridium-rich layer marking asteroid impact fallout at K-Pg boundary sites.

“Lo and behold, the iridium anomaly was right at the contact between those two formations, right where the geology changes,” he said. “That discovery convinced us that this isn’t just a phenomenon in the Williston Basin. It’s probably true everywhere throughout the Western Interior of North America.”

Weaver credited discussions about modern ecosystem engineers like elephants for helping him draw parallels with dinosaurs’ ecological roles: “That was the light bulb moment when all of this came together,” he said. “Dinosaurs are huge. They must have had some sort of impact on this vegetation.”

Collaborating with Mónica Carvalho from U-M Museum of Paleontology—who studies plant evolution across mass extinctions—and co-author Courtney Sprain from University of Florida helped reinforce their hypothesis about ecosystem restructuring after dinosaur loss.

“To me, the most exciting part of our work is evidence that dinosaurs may have had a direct impact on their ecosystems,” said U-F’s Courtney Sprain. “Specifically, the impact of their extinction may not just be observable by the disappearance of their fossils in the rock record but also by changes in sediments themselves.”

Weaver highlighted implications for understanding rapid environmental change today: “The K-Pg boundary was essentially a geologically instantaneous change to life on Earth;...the changes we’re making...are going to appear just as geologically instantaneous.... What’s happening in our lifetimes is [in]...the blink of an eye in geologic terms...so [it] is our best analog to our very abrupt restructuring...”