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The Corral Bluffs area near Colorado Springs, where researchers from the Denver Museum of Nature & Science discovered thousands of fossils unveiling the origins of life on Earth as we know it. (HHMI Tangled Bank Studios)

Scientists know how dinosaurs and nearly 75% of all species on Earth were wiped out 66 million years ago: A meteorite larger than Mount Everest slammed into what is now Mexico. 

But what has been less clear was how life recovered — and how long it took — in the critical wake of the devastating Cretaceous–Paleogene extinction event, which gave birth to the rise of mammals and eventually mankind. Less clear, that is, until now. 

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Dr. Tyler Lyson, Curator of Vertebrate Paleontology for the Denver Museum of Nature & Science, photographed on location at the Corral Bluffs fossil site. (Provided by HHMI Tangled Bank Studios)

Scientists from the Denver Museum of Nature & Science spent three years combing through Corral Bluff Open Space, just east of Colorado Springs, and examining thousands of concretions, the term for fossils cocooned inside rocks. What was uncovered is a picture of the origin of modern life and how mammals bounced back that has never truly been understood before.

“Now, we have an amazing fossil record,” said Tyler Lyson, curator of vertebrate paleontology at the museum at a Thursday event unveiling the findings. “We have the vertebrates and we have the plants. And with the plants we can determine temperature. And then probably most importantly we can place all of those in time to look at how quickly did it take for Earth to rebound from a mass-extinction event.”

Some of the biggest discoveries: 

  • It look less than 100,000 years after the mass extinction for mammals to reach the same maximum size as before the extinction — the size of roughly a modern-day raccoon. That’s despite the fact the meteorite reduced mammals to no larger than about a pound.
  • Within 300,000 years of the mass extinction, mammals had reached the size of a small pig.
  • By 700,000 years after the mass extinction, there were 100-pound-plus mammals roaming around.

“That recovery in size is truly phenomenal and truly very, very fast,” Lyson said. “Mammals won’t see a comparable increase in body size for another 30 million years.”

The quick growth can be attributed, researches say, to the discovery of the world’s oldest legumes.

Dr. Tyler Lyson, Curator of Vertebrate Paleontology for the Denver Museum of Nature & Science, holds open a split concretion and reveals the cross section of a vertebrate skull inside. (HHMI Tangled Bank Studios)

“Legumes are rich in protein, so we’ve dubbed this the protein bar moment for the recovery of mammals,” Lyson said. “As the forests recovered and new food sources became available, mammals were taking advantage of those new food sources and becoming larger and larger.”

Finally, the researchers found three distinct warming levels that made the whole thing possible, thus facilitating forest recovery and, in turn, allowing mammals to get larger. 

The museum is calling the project and its discoveries “Earth’s Comeback Story,” which features a display at the Denver attraction and a documentary series, NOVA Rise of the Mammals, which is now streaming online and will air Oct. 30 on PBS.

The findings are already making a splash nationwide and around the world.

For three years — when the current fossil project was launched — the museum has been keeping the whole thing under wraps as it worked to understand what it had found. 

“Our museum has actually been working down there for the better part of 20 years,” said Ian Miller, the museum’s curator of paleobotany. “The rocks down there preserve the end of the time for dinosaurs and the beginning of the time of mammals.”

An overhead shot of the prepared mammal skull fossils and lower jaw retrieved from Corral Bluffs. (HHMI Tangled Bank Studios)

In fact, people have been examining the site for almost 100 years. But it wasn’t until Lyson cracked open a concretion a few years ago and found a mammal skull fossil that the site’s significance was really understood. 

“I was floored, because after 20 years of looking for these very fossils, here I was holding the most complete one I’d ever found in my hand,” Lyson said of the discovery. “It was a moment I’ll never forget. I started yelling.”

Dr. Tyler Lyson, Curator of Vertebrate Paleontology for the Denver Museum of Nature & Science, uses a rock hammer to split open a concretion found at Corral Bluffs. (HHMI Tangled Bank Studios)

He realized there had to be more, and within five minutes, Lyson and his team had found another mammal skull in a concretion. And then many others were discovered.

“We’re going out there and finding more and more fossils,” Lyson said. 

Before Lyons’ discovery, researchers at Corral Bluffs mostly ignored the “unassuming” concretions there.

“Concretions are just a type of rock that forms around a nucleus, like a bone,” Lyons said. “These concretions are not that common in sediments that preserve land-dwelling animals. Because of this, previous work at Corral Bluffs … they overlooked these unassuming rocks and the treasures they contained.”

The time period from which the fossils come is notoriously difficult to track down. “Together the fossil animals and the plants, with an excellent understanding of the time in these rocks, is really almost unheard of,” Miller said. “(We have) started to call this a paleontological trifecta.”

The researchers’ findings were also published Thursday in Science Magazine.

“This could very well be the most important day in the history of the museum,” said museum CEO George Sparks.  “This is probably my proudest moment in my 15 years here as CEO.”

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Jesse Paul is a Denver-based political reporter and editor at The Colorado Sun, covering the state legislature, Congress and local politics. He is the author of The Unaffiliated newsletter and also occasionally fills in on breaking news coverage.

A Colorado College graduate, Jesse worked at The Denver Post from June 2014 until July 2018, when he joined The Sun. He was also an intern at The Gazette in Colorado Springs and The News Journal in Wilmington, Delaware, his hometown.

Jesse has won awards for long form feature writing, public service reporting, sustained coverage and deadline news reporting.

Email: Twitter: @jesseapaul