Thanks to a recent discovery at Corral Bluffs by the curators of Denver Museum of Nature & Science, we now have a fuller picture of how mammals evolved after the extinction of dinosaurs.

In addition to an October 24 paper published by Science magazine, a NOVA-produced documentary premiering Wednesday, October 30 explains how renowned archaeologist Tyler Lyson and his colleague Ian Miller discovered a trove of mammalian fossils at the Corral Bluffs near Colorado Springs in 2016. This discovery was groundbreaking, as knowledge of how mammals rebounded 66 million years ago after an asteroid killed the dinosaurs—and 75 percent of all species on Earth—was effectively zero before this research. The mammals that did survive the asteroid were so small (many no bigger than a rat) that finding fossils buried in the dirt was almost impossible.

“Fossils that we found before were just fragments, broken turtle shells, the occasional crocodile tooth,” Lyson says. “If you’re super lucky, you might find a bit of a mammal jaw.”

But after looking through the museum’s archives, Lyson changed tact. Instead of painstakingly sifting through the dirt for loose fragments of bone, he started cracking open concretions—egg-shaped rocks that form around fossils. In one day, he and Miller found more completely intact fossils at the bluffs from the era after the Cretaceaus-Paleogene extinction than in his entire career, including four mammal skulls.

Photo courtesy of HHMI Tangled Bank Studios

But the fossils would only truly alter science if the researchers could place them in time. Lyson worked with geologists Will Clyde and Anthony Fuentes of the University of New Hampshire, experts in geological dating, to pinpoint the dates in history when the animals lived.

Now, how they did this gets complicated.

At certain points in history, the Earth’s magnetic field switches (I know, bear with me). Basically, South becomes North and North becomes South.  Rocks can record this switch, and scientists know the exact dates these switches occurred. With some luck, Clyde and Fuentes were able to find a rock at the bottom of the Bluffs and a rock at the top of the Bluffs, where the polarity switched; two exact dates about a million years apart. By dating the top and bottom of the cliffs, Lyson and his team could create a timeline based on where each fossil was found. They realized that Corral Bluffs represents the first million years after the extinction of the dinosaurs, an essential and previously unknown part of history. 

When dinosaurs dominated the landscape, they ate most of the food sources, leaving mammals to fight for scraps. To survive, they were small and ate anything they could find. When the Earth was plunged into a global winter as the sun was blocked by dust from the asteroid, a majority of the plants died and the dinosaurs starved. The dinosaurs were too big and relied heavily on a narrow food source. Only the animals who could subsist off opportunistic scavenging, like the smaller mammals, survived.

But the Earth rebounded quickly. Within 100,000 years, the forests had recovered and the mammals were taking advantage of the dinosaur-free food. “The mammal recovery was intensely intertwined with the plant recovery,” Lyson says.

As the food supply diversified, including the beginning of the protein-packed legume family, animals grew rapidly into 100-pound beasts.

Over the few next hundred thousand years, the mammal fossils found at the Bluffs show an increase in specialization, filling the empty niches left by the dinosaurs. For example, a skull found with only large flat teeth, the oldest herbivore, represents the changing of mammals from opportunistic omnivores to a genus with many different specialized species.

In the documentary, Lyson calls this skull the first major specialization in the mammal fossil record. But there is still much to uncover. “Why mammals diversified after the extinction is still kind of an open question,” Lyson says.

The Corral Bluffs are a paleontological trifecta; it combines animal fossils, historical climate indicators from plants, and precise dating. It represents the closest thing to a complete record of how the Earth recovered after the asteroid—the start of our modern world.

“Very, very rarely do we have the plant, the animal, and the ability to date the rock where the fossils are found,” Lyson says. “That’s what makes it so complete, and it’s all those things together that make this discovery so remarkable.”

Learn more: Watch the documentary Rise of the Mammals on Wednesday, October 30, at 9 p.m. ET on PBS or stream it online at See the collection of fossils in a new exhibit, After the Asteroid: Earth’s Comeback Story, at the Denver Museum of Science and Nature, 2001 Colorado Blvd.