The extinction of dinosaurs had profound consequences for Earth, leading to significant alterations in the planet’s landscapes, including shifts in river systems.

There is a clear distinction between various rock formations in North America before and after the end of the dinosaurs during the Cretaceous-Paleogene (K-PG) extinction event approximately 66 million years ago, triggered by the Chicxulub asteroid impact in the Yucatan Peninsula.

For example, the Green Gray Mudstone, recognized as the Hell Creek Formation from the dinosaur period, transitions into the more vivid pajama-striped layers of the Fort Union Formation, which hosts many lignite-rich charcoals from plant material that surfaced with the rise of mammals.

This transition was initially attributed to the direct impacts of asteroid strikes, such as increased rainfall. However, Luke Weaver from the University of Michigan and his team propose another explanation.

They examined floodplain areas in the western United States, revealing abrupt geological changes around the K-PG boundary, particularly in the Williston Basin, stretching across parts of Wyoming, Montana, and the Dakotas.

The multifaceted colorful layers from the Post-dinosaur period are believed to be deposits formed by rising water levels, creating temporary ponds. However, Weaver and his colleagues did not find supporting literature on water level changes during this era.

“There’s no evidence of extremely high water tables or particularly wet conditions,” he says. While there was an intrusion of seawater inland, the nearest instance occurred at least 300,000 years after the K-PG boundary.

Weaver’s team argues that significant sandstone layers formed post-K-PG boundary are indicative of large, stable rivers, known as Point Bar deposits, instead of temporary pond deposits. These layers can exceed 10 meters in thickness, reflecting the stability of these rivers.

Researchers attribute these findings to the extinction of dinosaurs. They propose that, like today’s large herbivores, dinosaurs were ecological engineers, disrupting vegetation, trampling, and grazing seedlings, inhibiting new plant growth.

“These creatures were colossal compared to modern fauna,” Weaver notes. For instance, while a contemporary elephant weighs around 5,000 kilograms, a Triceratops could weigh at least double that.

As they moved through and destroyed vegetation, the rivers would have flooded periodically instead of winding through forests. This change ultimately led to the expansion of marshy mudstone, according to Weaver. Once the dinosaurs vanished, tree roots stabilized the sediments, allowing water to flow through a meandering riverbed, thus creating point bars.

“This illustrates a landscape where biology plays a crucial role,” Weaver observes. Animals, he argues, significantly modify their environments, much like humans have drastically altered Earth’s landscapes.

Christopher Doughty from Northern Arizona University believes this perspective better explains the observed geological transformations than earlier theories. “In contemporary studies where large animals are removed from ecosystems, tree cover significantly increases,” he mentions. “With the extinction of dinosaurs, there were no longer large animals capable of uprooting trees. This led to a decrease in herbivory and reduced the disturbance of seedlings giving rise to robust tree growth.”

However, Cat Schroder from the University of New Mexico remains skeptical. “While there seems to be a correlation between large dinosaurs and open nutritional landscapes, causality hasn’t been established yet,” she says. “Forests thrived before, during, and after the age of dinosaurs.”

Doughty is using isotopic analysis of fossil leaves to investigate how forest structures have shifted since the dinosaurs went extinct.

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