How the Extinction of Dinosaurs Reshaped Earth’s Rivers and Forests
By David Freeman - September 16, 2025
The extinction of the non-avian dinosaurs has been told countless times as the story of a sudden catastrophe, a world-shaking asteroid strike that erased some of the most remarkable creatures to ever walk the Earth. The Chicxulub impact, around 66 million years ago, has rightly earned its place as one of the defining events in the history of life. Yet a new wave of research suggests that the end of the dinosaurs was not just about animals disappearing. It also transformed the very landscapes they once shaped, in ways that can still be read in the rocks today.
A team of geologists and paleontologists led by Lucas Weaver at the University of Michigan and colleagues from the United States, Australia, Belgium, and beyond has pieced together a striking case. Their findings, published in Communications Earth & Environment in 2025, argue that the extinction of large-bodied dinosaurs fundamentally altered river systems, vegetation patterns, and continental sedimentation across North America. In doing so, they position dinosaurs not just as participants in ancient ecosystems but as engineers of the environments themselves, whose sudden disappearance created a wholesale reorganization of the land.
The research begins with a puzzle long noted by geologists working on the Cretaceous–Paleogene boundary, the thin interval of rock that separates the age of dinosaurs from the age of mammals. In the marine realm, the extinction event is marked by a sharp turnover in microfossils, a collapse in food webs, and the presence of a globally distributed iridium-rich layer left behind by the impact. On land, the same horizon is equally unmistakable but less often appreciated for its sedimentary signatures. Across wide swaths of North America, from the Powder River Basin of Wyoming to the Williston Basin of Montana and the Alberta plains, the rocks tell of a dramatic change. Cretaceous strata, the last rocks to form under the dinosaurs, show unstable meandering rivers, shallow floodplains clogged with clastic sediments, and soils that were often waterlogged. Immediately above, in the Paleogene, the picture changes. Rivers become broad and stable, coals accumulate across wide areas, and the floodplains are starved of sediment. This transformation has been recognized for decades, but until recently it was attributed to gradual environmental changes unrelated to the extinction.
Weaver and his collaborators challenge that interpretation. Through the identification of five new boundary sections in the Bighorn and Williston basins, confirmed by the presence of iridium anomalies associated with Chicxulub ejecta, they show that the facies shift is synchronous with the extinction itself. The coincidence is too precise, too geographically widespread, and too persistent to be explained by slow climatic or tectonic processes alone. Instead, they argue that the extinction of dinosaur megafauna provides the missing piece.
To understand this idea, it helps to think about the role of large herbivores in ecosystems today. In African savannas, elephants are known as landscape architects. By uprooting trees, trampling vegetation, and grazing extensively, they prevent the spread of dense forest and maintain open grasslands. Remove elephants from such systems and forests advance rapidly, changing the character of the landscape and influencing everything from fire regimes to river courses. Similar patterns have been documented following the extinction of mammoths and other Ice Age giants. Large herbivores exert a disproportionate influence on vegetation structure, and by extension on the geomorphology of the land.
The argument made by Weaver’s team is that dinosaurs played a comparable role in the Cretaceous, but on an even grander scale. Herbivorous dinosaurs such as Triceratops, weighing up to fifteen tonnes, and the vast herds of hadrosaurs that roamed the floodplains, would have constantly disturbed vegetation cover. Their size and numbers meant that dense forests could not easily establish. The result was a patchy, open landscape prone to erosion, where rivers shifted course frequently, splays broke through levees, and clastic sediments were spread widely across the floodplains. In this world, unstable rivers and waterlogged soils were the norm, and the geology of the Late Cretaceous reflects that.
With the impact of Chicxulub and the sudden disappearance of these giants, the stabilizing pressures on vegetation changed overnight. Forests, no longer checked by constant disturbance, spread across the floodplains. In a warm Paleocene climate, dense stands could establish within decades, and in a few centuries closed-canopy forests may have dominated vast tracts of North America. That vegetational shift would have stabilized river courses, reducing the frequency of avulsions and starving distal floodplains of sediment. In turn, organic matter accumulated in thick deposits of coal and lignite, marking the base of the Fort Union Formation across many basins. These were not isolated local changes but continent-wide transformations linked directly to the absence of dinosaurs.
The study carefully examines alternative explanations that have been proposed over the years. Some researchers pointed to increased rainfall in the Paleocene, others to a rising water table, still others to the transgression of the Cannonball Sea in North Dakota and surrounding areas. Yet each of these explanations fails to account for the full picture. Paleosol studies show that the Hell Creek Formation of the latest Cretaceous already had high water tables. Magnetostratigraphy places the transgression of the Cannonball Sea several hundred thousand years after the boundary. And detailed sedimentological work shows that the “variegated beds” of the Fort Union Formation are not pond or marsh deposits but point-bar deposits formed by large, stable meandering rivers. The evidence aligns more convincingly with a biological cause.
The concept of “dinosaurs as ecosystem engineers” emerges as the unifying framework. By acting as living agents of disturbance, dinosaurs prevented the establishment of dense forest cover and sustained open habitats across millions of years. Their extinction represents not just the loss of charismatic fauna but the loss of a critical ecological function. Once removed, landscapes reorganized in fundamental ways, leaving a clear signature in the geological record.
The implications extend far beyond sedimentology. The rise of closed-canopy forests in the Paleocene would have shaped plant evolution. Seed size among flowering plants increased, a pattern consistent with adaptation to shadier understories where larger seeds confer an advantage for seedlings. Animals too were influenced. Many Cretaceous mammals were small, ground-dwelling, or semi-fossorial, suited to open habitats. In the Paleocene, with the spread of forests, arboreal and fruit-eating lineages began to appear, setting the stage for primates and other modern groups. The cascading effects of dinosaur extinction therefore reached into evolutionary trajectories that continue to resonate.
Regional differences add nuance to the story. In mountain-distal basins such as Williston, the facies shift is sharp and consistent, with the development of multistoried channels and extensive coal deposits. In mountain-proximal areas such as the Raton Basin of Colorado and New Mexico, the change is expressed differently, reflecting local sediment supply and tectonic setting. Yet even with variation, the underlying pattern remains the same: the K–Pg boundary coincides with an abrupt and lasting reorganization of fluvial systems.
Testing the hypothesis further will require detailed work. Weaver and colleagues outline several ways forward. Continental boundary sections should consistently show a link between impact signals and facies change. Proxies for canopy structure, such as light-sensitive leaf traits or isotopic indicators, should reveal a shift from open to closed vegetation across the boundary. Fluvial deposits above the boundary should record longer-lived, more stable meanders compared to those below. Each line of evidence offers a way to probe the role of dinosaurs as agents of landscape engineering.
The broader message is that life and Earth are entwined in feedback loops more intricate than often appreciated. The disappearance of dinosaurs was not only a biological event but also a geological one, with rivers, soils, and sediments bearing witness to the change. Life shapes land, just as land shapes life, and in the case of the end-Cretaceous extinction, the sudden removal of giants reset that relationship.
It is a perspective that encourages us to think differently about ecosystems today. The loss of elephants, rhinos, or large grazers in modern landscapes has well-documented effects on vegetation and geomorphology. By looking back to the end of the dinosaurs, we see that such processes are not minor but capable of reshaping continents. The past serves as a reminder of the power of biology to alter the face of the Earth itself.
The rocks of Montana, Wyoming, and Alberta preserve the record of this transformation with extraordinary clarity. At sites such as the Hell Creek and Fort Union formations, one can trace with the hand a line where dinosaurs disappear, where iridium from the impact still lingers, and where rivers take on a different form. It is a line that marks not just extinction but also the beginning of a new chapter in Earth’s landscapes.
By reframing dinosaurs as ecosystem engineers, the research gives us a richer, more textured view of their role in deep time. These animals were not only evolutionary marvels but also dynamic shapers of their environments, their bulk and behavior echoing in the sediments that survive them. When they fell, the land itself shifted, and the Paleocene world that followed was forever different.
on September 16, 2025, 4:40 pm
