‘Amazing’ fossil find reveals how mammals thrived after dinosaurs died

The date he opened the skull is etched into paleontologist Tyler Lyson’s brain: September 10, 2016.

Prior to this discovery, Lyson and his colleagues had struck at Corral Bluffs, Colorado, a site mined by fossil hunters since the 1930s.

As the hours passed at the end of the season in the field, with little to show, Lyson remembered a tip he had learned from South African paleontologists – don’t look for bones but for strange rocks. A whitish mass, a stain like a crushed loaf of bread, caught his eye.

“I picked it up, broke it with my rock hammer and broke it in half. And I could see the cross section of a mammalian skull looking at me,” Lyson said, curator of vertebrate paleontology at the Denver Museum. of Nature & Science, at a press conference on Tuesday.

This mammal, a pig-sized herbivore named carsioptychus, lived shortly after an asteroid carved a 90-mile-wide crater into Earth and ended the age of dinosaurs.

“It was just, you know, I almost had tears in my eyes,” Lyson said.

Lyson had searched for a skull like this for 20 years.

A colleague, Denver Museum paleobotanist Ian Miller, spotted a similar misshapen blob and cut it open. Inside was another mammalian skull. A volunteer opened a third rock: a third skull.

All of these animals lived in ecosystems still recovering from the Cretaceous-Paleogene extinction event, an event that killed 3 out of 4 species 66 million years ago.

Corral Bluffs is a goldmine of post-extinction mammal fossils, as Lyson, Miller and a team of scientists reported in the journal Science Thursday. Rock formations called concretions hid the fossils inside, like chocolate nestled in a candy shell.

After cracking the cracking code from the open fossils, paleontologists found nearly 1,000 vertebrate remains, including mammal bones, turtle shells and crocodilian skulls. They found 6,000 petrified leaves and other plant parts. They also found 37,000 fossilized pollen grains.

Simone Hoffmann, an anatomy professor at the New York Institute of Technology who studies the evolutionary history of mammals and was not a member of the research team, called the new findings ‘superb’ .

“They were able to go into a lot of detail during this crucial period that we were missing before,” Hoffmann said. Rock layers and volcanic ash have provided a timeline for the site, which spans approximately one million years of Earth history.

The many fossils showed changes in biodiversity, and the study authors even deduced the ancient temperatures of plants that once grew there.

Importantly, the site includes layers before and after the mass extinction. Before the asteroid hit, this part of Colorado was about as hot as Miami is today, Miller said. tyrannosaurus rextriceratops, armored ankylosaurs and duck-billed dinosaurs roamed the forests.

That was, until the huge piece of metal moving at 150,000 mph (241,000 km/h) punched a hole in the planet near Mexico.

Firestorms baked Alaska in minutes. Tons of earth thrown into space and falling, warming the atmosphere.

Fossils of mammalian skulls and lower jaws recovered from a Colorado hoard. (HHMI Tangled Bank Studios)

“It was like your oven temperature to bake cookies — what it would take to bake cookies — around the world,” Miller said.

Then came acid rain, a nuclear winter and, finally, rampant global warming.

“The Cretaceous has ended,” Miller said, “and the age of mammals begins.”

Over the past 500 million years, it’s this million-year slice of history that “really informs how our world has become today,” Miller said.

The Corral Bluffs finds complement other sites in New Mexico and Montana that contain less complete post-impact fossils, said Sarah Shelley, a mammalian paleontologist at the Carnegie Museum of Natural History who was not involved in the study. .

Together, the remnants show that the global recovery was not a return but a reset.

Mammals coexisted with dinosaurs for millions of years. The largest mammals weighed around 15 pounds, about the size of a raccoon, but no mammal that large survived the Cretaceous-Paleogene extinction event.

At Corral Bluffs, in the layer of rock representing 100,000 years after the impact, fern spores dominated. The largest mammal that lived to roam this fern world, a rat-like creature, weighed about a pound.

Over the next 200,000 years, ferns gave way to palm trees. The diversity of mammals has doubled. Mammals also gained weight, reverting to the size of raccoons. (Researchers estimated animal mass from tooth and skull size.)

At 300,000 years after the impact, species of nuts appeared; scientists haven’t found walnuts, but they have found walnut pollen. The largest mammal from this period, based on the Corral Bluff fossils, weighed around 50 pounds.

A teenage volunteer working at Corral Bluffs has found the oldest bean pod in the world, dated 700,000 years after the impact. Meanwhile, the mammals continued to grow in size, reaching the size of wolves.

The proximity of so many types of fossils gives the site its richness in detail. “If you have the pollen right next to the fossil, or the plants right next to the fossil,” Hoffman said, “then you know they match at the same time.”

Legumes remain an important food source for mammals and humans alike; soybeans, for example, are among the most widely planted crops on Earth. The study authors hypothesize that the appearance of nutrient-rich plants alongside jumps in mammalian body size is no coincidence.

“We speculate that the introduction of these new plant food sources, particularly higher calorie food sources such as nuts and legumes, may have at least contributed to increased mammalian body mass” , Lyson said.

It’s a logical assumption, but it will be difficult to prove, Shelley said. It is also possible that the opposite is true: that new species of mammals have influenced plant diversity.

Corral Bluff fossils will be the subject of a NOVA special, The rise of mammalsairs on PBS on October 30.

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