New research suggests dinosaurs were ecosystem engineers that promoted habitat openness in the Late Cretaceous epoch, and their extinction around 66 million years ago likely led to a dramatic reorganization of ecosystem structure: once dinosaurs were extinguished, forests were allowed to flourish; this had a strong impact on rivers: the newly dense forests stabilized sediments and corralled water into rivers with broad meanders.
“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 University of Michigan paleontologist Luke Weaver.
“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 around 66 million years ago.
Scientists looking for evidence of the asteroid saw that the rocks overlying the fallout debris were starkly different from the rocks below.
Dr. Weaver and colleagues began investigating this 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.
“While investigating a previous paper, we examined a rock layer called the Fort Union Formation,” Dr. Weaver said.
“The Fort Union Formation was deposited after the extinction of dinosaurs, and looks like it’s composed of stacks of different colored rocks — ‘pajama-striped looking beds’.”
“The brightly colored rock layers were thought to be pond deposits caused, some researchers thought, by a time of rising sea levels.”
“The rock formation was a stark contrast to the formations lying beneath it, which had waterlogged, poorly developed soils reminiscent of what you might see in the outer edges of a floodplain.”
The researchers began to suspect the change in geology was somehow related to the mass extinction of dinosaurs, called the end-Cretaceous mass extinction.
Moreover, they began to examine what types of environments were represented by these different rock formations.
“What we realized was that the pajama stripes actually weren’t pond deposits at all,” Dr. Weaver said.
“They’re point bar deposits, or deposits that form the inside of a big meander in a river.”
“So instead of looking at a stillwater, 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.
The scientists believed these formed because with the stabilizing effect of dense forests, rivers flooded less frequently.
“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,” Dr. Weaver said.
The evidence that would clinch whether the change occurred right after the end-Cretaceous mass extinction?
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.
This iridium-rich layer of sediment, which defines the Cretaceous-Paleogene boundary, carries about three orders of magnitude more iridium than typical sediments, and is called the iridium anomaly.
The authors then focused on an area the Bighorn Basin where the boundary hadn’t been located.
Looking at places of geologic change between the dinosaur-bearing formation and Paleocene-mammal-bearing formations, they took samples of a fine line of red clay about a centimeter in width.
“The iridium anomaly was right at the contact between those two formations, right where the geology changes,” Dr. Weaver 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.”
Still, the mystery of why the geology of landscapes should have changed so much before and after dinosaurs’ extinction remained.
But then the team encountered a series of talks about how present-day animals such as elephants influence the ecosystem in which they live.
“That was the light bulb moment when all of this came together,” Dr. Weaver said.
“Dinosaurs are huge. They must have had some sort of impact on this vegetation.”
The paleontologists suggested the sudden disappearance of dinosaurs allowed forests to flourish, helping to trap sediment, build point bars and structure rivers.
“To me, the most exciting part of our work is evidence that dinosaurs may have had a direct impact on their ecosystems,” said Dr. Courtney Sprain, a researcher at the University of Florida.
“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.”
The findings were published this week in the journal Communications Earth & Environment.
