Some 66 million years ago, mammals caught their lucky break.
An asteroid crashed into what is now Chicxulub, Mexico, and set off a catastrophic chain of events that led to the annihilation of nonavian dinosaurs. That day began their furry ascension to the top of a brave new world, the one from which our species would one day emerge.
But little is known about the time period directly after the Cretaceous-Paleogene extinction, or K-Pg event, because the fossil record is lacking.
Now, a team of paleontologists has uncovered a trove of thousands of fossils in Colorado that provides an in-depth look at the first 1 million years following the K-Pg mass extinction event. The finding provides insight into the interactions between animals, plants and climate that occurred in the earliest days of the age of mammals, and that allowed them to grow from the size of large rodents into diverse wildlife we might begin to recognize today.
“We provide the most vivid picture of recovery of an ecosystem on land after any mass extinction,” said Tyler Lyson, a vertebrate paleontologist at the Denver Museum of Nature and Science.
In spring 2016 he and some colleagues explored a site near Colorado Springs called Corral Bluffs. He knew that years earlier, Sharon Milito, an amateur fossil hunter, had found a mammal skull that was confirmed to be from the K-Pg boundary there.
As he wandered around the bluff, he thought of his time as a graduate student working in South Africa. There, he had learned to spot certain rocks called concretions that held fossils captive, like pearls in oysters.
“I found this ugly white-looking rock that looked like it had a little mammal jaw coming out of it,” he said. He cracked it open and found inside part of a fossilized crocodile. “That was the moment when the light bulb went off. If there’s one concretion with fossils inside, there’s got to be more.”
He and his colleagues returned and searched for more of the ugly rocks. “When I cracked open the very first concretion I found a mammal skull,” Lyson said.
It was the most complete mammal from the K-Pg interval that he had ever seen. Within an hour they found four or five more. So far, they have uncovered more than 1,000 vertebrate fossils and 16 mammal species.
“With this discovery, we’re starting to see the entire skull of many of these mammals that we previously only knew from teeth,” said Stephen Chester, a mammalian paleontologist at Brooklyn College and an author on the paper published in Science.
The skulls tell a story of mammalian resilience. Whereas rat-size mammals survived the extinction event, raccoon-size ones perished. About 100,000 years after the K-Pg event, mammals bounced back, with raccoon-size mammals reappearing.
Some 300,000 years after the asteroid struck, more mammals appeared, such as Loxolophus and the small, pig-size Carsioptychus. Within 700,000 years, the capybara-size Taeniolabis and the wolf-size Eoconodons began to thrive.
“You’re going from a very small dog that you’d see on the streets on New York City to a very large wolf within those hundreds of thousands of years,” Chester said.
The team also collected more than 6,000 fossilized leaves and analyzed more than 37,000 pollen grains. Together the items describe the reemergence of plant life, which may have been be a crucial factor in the evolution of mammals.
First came the ferns. They proliferated across the wasteland for many hundreds of years to a couple of thousand years, paving the way for forests to rebound. Next, the palms paraded in, dominating the green scene for hundreds of thousands of years.
About 300,000 years after the catastrophe, an array of walnuts appeared. That coincided with the jump in diversity and body size of herbivorous mammals.
“We call that world the ‘Pecan Pie World,’ ” said Ian Miller, a paleobotanist at the Denver Museum of Nature and Science. He added that this epoch also coincided with a warming period in the fossil record, which could indicate that a shifting climate also played a role in the development of plants and animals.
One of their most important botanical finds — a fossil bean pod — was made one summer by a high school student. “There she is holding the world’s earliest fossil legume,” Miller said. “We liken them to the protein bars of the ancient world.”