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Fossils reveal the epoch when mammals filled dinosaurs' void
In an undated photo from HHMI Tangled Bank Studios, the site at Corral Bluffs, near Colorado Springs, Colo. The site represents about 300 vertical feet of rock that preserve the extinction of the dinosaurs and the first million years that followed. HHMI Tangled Bank Studios via The New York Times.

by Nicholas St. Fleur

NEW YORK, NY (NYT NEWS SERVICE).- 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. His team’s paper was published Thursday in Science.

Lyson has hunted fossils since he was 10. Although he has found many dinosaurs, uncovering fossils of species that emerged in the immediate aftermath of the dinosaur extinction had proved rather elusive in his field of study.

“You can only find so many triceratops skeletons and partial T. rex skeletons and things like that until you want a larger challenge,” said Lyson. “Finding fossils just after the K-Pg extinction is a huge, huge challenge.”

In spring 2016 he and some colleagues explored a fossil 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. He set out looking for mammal bones sticking out of the ground. But his search proved fruitless.

As he wandered around the bluff, he thought back to 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. He shifted his focus from bones to rocks.

“I found this ugly white-looking rock that looked like it had a little mammal jaw coming out of it,” Lyson 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 to Corral Bluffs that September 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 different 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.

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. With their featherlike leaves, 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.

Then around 300,000 years after the catastrophe, a diverse array of walnuts appeared. That coincided with the jump in diversity and body size of herbivorous mammals, which suggests they were an important food source.

“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 played a role in the development of plants and animals following the extinction event.

One of their most important botanical finds — a fossil bean pod — was made one summer by a high school student while the team was working with Nova for a documentary that will be broadcast Wed., Oct. 30, on the Public Broadcasting Service.

“There she is holding the world’s earliest fossil legume,” Miller said. “She just had this ear-to-ear smile, totally beaming.”

They dated the legume to around 700,000 years after the mass extinction event. That period was tied to another short warming pulse as well as to the appearance of wolf-size mammals. Perhaps the legumes were fueling furry animals, the team suggested.

“We liken them to the protein bars of the ancient world,” Miller said. One remaining question, he added, is whether climate drove the changes in the plants and mammals.

Courtney Sprain, a geoscientist at the University of Florida, said she was impressed by their animal, plant and climate records. “That’s one of the really spectacular things about this paper, just how amazing the preservation is and how good the record is for a variety of different changes following a mass extinction.”

Other paleontologists agreed.

“I looked at this and went, ‘Wow, that’s a lot of skulls!’” said Jaelyn Eberle, a vertebrate paleontologist at the University of Colorado at Boulder, who was not involved in the paper. She said a next step should be to perform micro CT scans on the skulls to determine the brain sizes and compartments, which would provide insight into the animals’ sensory abilities.

The find also helps elucidate how species are able to respond to catastrophic events in a relatively rapid time frame.

“It has obvious ramifications for the current biodiversity and climate crisis as we start to approach similar levels of devastation,” said Anjali Goswami, a vertebrate paleontologist at the Natural History Museum in London. The current crisis, she added, is “ unfortunately caused by our own shortsighted avarice, rather than a chance encounter with an extraterrestrial body.”

© 2019 The New York Times Company

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