On the outskirts of Colorado Springs, researchers have uncovered thousands of fossils showing how life on Earth revived in the aftermath of an asteroid impact 66 million years or so ago that killed most dinosaurs and other life on land and sea.
Taken together, the fossil trove documents an era when evolution, in essence, hit the reset button. While countless species vanished forever, some plants and animals rebounded relatively quickly in the first million years after the devastation, including the mammals ancestral to humankind, the scientists said in research published Thursday in Science.
“This is the age of mammals rising from the ashes of the age of dinosaurs,” said paleobotanist Ian Miller at the Denver Museum of Nature & Science who is a project co-leader. “It really informs how our world came to be today.”
Most mammals were wiped out, but the survivors quickly took hold in the new world. The number of diverse mammal species doubled within 100,000 years of the event, the scientists said. Some of the new species were up to three times as large as those alive right after the blast.
At the same time, ferns reclaimed the wasteland. Over 700,000 years or so, that carpet of ground cover was replaced by groves of palms and then by forests of walnut and other trees, which offered new food sources for rapidly evolving mammals, the scientists said.
“What they have here is pretty remarkable,” said paleontologist Michael Novacek, provost of science at the American Museum of Natural History in New York, who wasn’t involved in the project. “It is supportive of the notion that the evolution of modern mammals has something to do with the recovery from the extinction event.”
The find is the latest in a series of discoveries this year that reveal details of a moment long ago when a city-sized asteroid exploded into the sea near present-day Mexico with the force of a million nuclear warheads. The cosmic hammer blow triggered earthquakes, tidal waves, a global rain of red-hot debris and so much dust that it blocked sunlight world-wide for decades, scientific studies say.
Earlier this week, geoscientists who analyzed the chemistry of fossil plankton from the period said that the oceans rapidly became lethally acidic after the impact, perhaps from storms of acid rain generated by the fallout of pulverized sulfur-rich rocks. That may have eradicated half of all marine life, they said.
Their analysis, published in the Proceedings of the National Academy of Sciences on Monday, suggested it took 40,000 years for sea life to begin its recovery.
on land resuscitated itself in the aftermath.
After decades of searching, Dr. Miller and his colleagues found that evidence where they least expected it: on a stretch of gullies, grassland and stream beds in the southwestern corner of the Denver Basin called Corral Bluffs, where archaeologists and paleontologists had been studying the past off and on since the 1930s. In all that time, only a few fossil fragments had been found there from the era of the asteroid impact.
In 2016, the scientists turned their attention to long-ignored balls of sediment called concretions that littered the area. Such rocks sometimes form around a fossil as a core material, the way a pearl takes shape around a grain of sand.
“I remember seeing one really ugly white amorphous blob,” said vertebrate paleontologist Tyler Lyson at the Denver Museum, who was a project co-leader. “I picked it up, cracked it open with my rock hammer and I could see the cross section of a small skull looking back at me. I almost had tears in my eyes. It was pure elation.”
His colleagues quickly cracked open four more concretions, finding mammal skulls inside each one. “We were finding a skull about every 15 minutes once we figured out what to look for,” said Dr. Miller. “It happened that quickly.”
Across 10 square miles or so (27 square kilometers) of the outcrop, they so far have found more than a thousand animal fossils. The scientists also found thousands of plant specimens from the period, including leaves, stumps, branches and entire saplings. All of them date to the time of recovery after the impact.
Dozens of the fossils will be exhibited at the Denver Museum, the scientists said. The project also is the subject of a new Nova documentary produced for WGBH Boston by the Tangled Bank Studios at the Howard Hughes Medical Institute.
“It is going to set the standard going forward,” said early mammal expert Gregory Wilson at the University of Washington in Seattle. “I’ll be using this as a measuring stick, looking at terrestrial recovery from the mass extinction event.”- WSJ