How extinction of dinosaurs reshaped earth’s entire landscape

By Stephen Beech

Dinosaurs' extinction "re-engineered" Earth's surface, according to new research.

The reptiles had such an "immense" impact on the planet that their sudden exit led to wide-scale changes in landscapes - including the shape of rivers, say American scientists.

Geologists have long recognized the stark difference in rock formations from just before dinosaurs went extinct to just after, but put it down to sea level rise, coincidence, or other factors.

However, University of Michigan paleontologist Dr. Luke Weaver has now shown that once dinosaurs died off, forests were allowed to flourish, which in turn had a significant impact on rivers.

He says the newly dense forests stabilized sediments and corralled water into rivers with broad meanders.

Dr. Weaver and his colleagues examined locations throughout the western United States that depicted sudden geologic changes that happened at the boundary between the age of dinosaurs and the age of mammals.

He suggests that dinosaurs were likely enormous "ecosystem engineers" - knocking down much of the available vegetation and keeping land between trees open and weedy.

The result was rivers spilled openly, without wide meanders, across landscapes.

But once the dinosaurs perished, forests were allowed to flourish.

The team's findings, published in the journal Communications Earth and Environment, show how rapidly the Earth can change in response to catastrophic events.

Dr. Weaver said: "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.

"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 became extinct after a large asteroid slammed into the Yucatan Peninsula.

Scientists looking for evidence of the asteroid saw that the rocks overlying the fallout debris were starkly different from the rocks below.

Dr. Weaver, Assistant Professor in Michigan's Department of Earth and Environmental Sciences, and his colleagues began investigating the sudden geologic change in the Williston Basin, an area that spans eastern Montana and western North and South Dakota, as well as north-central Wyoming's Bighorn Basin.

The researchers began to suspect changes in geology was somehow related to the mass extinction of dinosaurs - called the Cretaceous-Paleogene, or K-Pg, mass extinction.

They started to examine what types of environments were represented by these different rock formations.

Dr. Weaver said: "What we realized was that the pyjama 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.

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"So instead of looking at a still-water, quiet setting, what we're actually looking at is a very active inside of a meander."

The large river deposits were bracketed by layers largely composed of lignite, a low-grade form of coal formed by carbonized plant matter.

Dr. Weaver, who is also assistant curator of fossil mammals at the University of Michigan Museum of Palaeontology, and his colleagues believed they formed because with the stabilising effect of dense forests, rivers flooded less frequently.

He said: "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."

The evidence that would clinch whether the change occurred right after the K-Pg mass extinction was a fine layer of sediment loaded with iridium, an element typically only delivered to Earth by cosmic rays.

However, when the asteroid slammed into Earth, it carried with it a payload of the element, which settled over much of the planet in a fine layer.

The iridium-rich layer of sediment, which defines the K-Pg boundary, carries around three orders of magnitude more iridium than typical sediments, and is called the iridium anomaly.

The research team then focused on an area the Bighorn Basin where the K-Pg boundary hadn't been located.

Looking at places of geologic change between the dinosaur-bearing formation and Paleocene-mammal-bearing formations, Dr. Weaver took samples of a fine line of red clay about a centimetre in width.

He said: "Lo and behold, the iridium anomaly was right at the contact between those two formations, right where the geology changes.

"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."

Still, the mystery of why the geology of landscapes should have changed so much before and after dinosaurs' extinction remained.

But then Dr. Weaver encountered a series of talks about how present-day animals - such as elephants - influence the ecosystem in which they live.

He said, "That was the light bulb moment when all of this came together.

"Dinosaurs are huge. They must have had some sort of impact on this vegetation."

Study co-author Dr. Courtney Sprain, of the University of Florida, said: "To me, the most exciting part of our work is evidence that dinosaurs may have had a direct impact on their ecosystems.

"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 the sediments themselves."

Dr. Weaver says the K-Pg extinction event is also a lesson in how the record of Earth might change as a result of human-caused climate change and loss of biodiversity.

He said: "The K-Pg boundary was essentially a geologically instantaneous change to life on Earth, and the changes we're making to our biota and to our environments more broadly are going to appear just as geologically instantaneous."

Dr. Weaver added: "What's happening in our lifetimes is the blink of an eye in geologic terms, and so the K-Pg boundary is our best analogue to our very abrupt restructuring of biodiversity, landscapes and climate."