An absolute treasure trove of rare fossils has been uncovered in central Colorado. The collection reveals the stunning speed at which mammals emerged and diversified once the dinosaurs were gone.
New research published today in Science describes thousands of new mammal, reptile, and plant fossils recovered from the Corral Bluffs site in Colorado’s Denver Basin. The fossils span the 1 million years that followed the Cretaceous-Paleogene mass extinction (KPgE)—an event that resulted in the extinction of non-avian dinosaurs. These fossils show how our planet and biosphere recovered after the mass extinction and how quickly mammals were able to dominate terrestrial environments.
Key to this discovery was the search for concretions—a special type of rock that forms around bones and other bits of organic matter. Concretions tend to be ignored in favour of other paleontological clues, but Tyler Lyson, the lead author of the new study and a paleontologist from the Denver Museum of Nature and Science, decided to switch things up when sifting through rocks in Corral Bluffs back in 2016.
“I saw this ugly, white-coloured amorphous blob that looked like a loaf of bread,” said Lyson at a press conference this week. “I used my rock hammer to crack it open, and I could suddenly see the cross-section of a mammal skull staring back at me.”
Lyson yelled for his team to come and take a look. Following a brief moment of celebration, Ian Miller, a paleobotanist from the Denver Museum and a co-author of the study, picked up another rock nearby and cracked it open, revealing yet another mammal skull. This was repeated again and again, and the scientists realised they’d made a monumental discovery.
“It was pure elation,” said Lyson. “It all happened so quickly—you don’t have discovery moments like this in paleontology very often.”
In total, the researchers uncovered well over 1,000 vertebrate fossils, 233 distinct fossilised plants and plant materials (like fossilised spores), and an assortment of other fossilised organisms. Incredibly, the researchers documented 16 new species of mammals—so many that they have yet to fully describe them all, saying future research will be devoted to fully detailing the new finds.
To say fossils from this time period are rare would be an understatement. And that’s a shame, because much of the current era can be traced back to this critical time, when an assortment of animals and plants emerged and diversified in the wake of an event that wiped out 75 per cent of all species on Earth. Some 66 million years ago, a gigantic comet or asteroid plunged into the Earth, triggering a mass extinction that wiped out all non-avian dinosaurs. For mammals, however, this proved to be a fortuitous event, allowing them to emerge from a dinosaurian shadow.
The dearth of fossil evidence has made it difficult for scientists to study the post-KPgE recovery period. In particular, the fragmentary nature of the few fossils that do exist, such as broken chunks of jaws and bits of teeth, aren’t connected to any coherent narrative about this recovery period. That’s what makes this latest find so special, as the fossils are providing an uninterrupted record spanning the first million years following the mass extinction.
“Fossil sites often give us ‘snapshots’ of life on Earth by preserving species from a single point in time,” David Grossnickle, a biologist from the University of Washington who wasn’t involved with the new research, wrote in an email to Gizmodo. “What really stands out to me about this study is that it presents a near-continuous fossil record for a period of 1 million years during a critical time in history, capturing the beginning of the Age of Mammals. It’s analogous to finding the entire first chapter of a book rather than a single page.”
By matching these fossils to a specific timeframe, the researchers were able to piece together three critical stages of post-extinction recovery and document the kinds of animals and plants that emerged during this million-year timeframe. A major takeaway was the speed at which mammals diversified, grew in size, and took over, which they did by occupying empty niches and taking full advantage of new plant species.
As the authors note in the new paper, the diversity of mammalian species doubled beginning around 100,000 years after the impact event, a time when their maximum body size reached the same levels seen during the Late Cretaceous. These animals were no larger than rats or raccoons—weighing around 8 kilograms (18 pounds)—and they subsisted primarily on the carbon-rich detritus left over from the mass extinction. Non-avian dinosaurs struggled, resulting in their disappearance.
Things really began to change around 300,000 years post-KPgE. By this stage, ferns began to take over terrestrial landscapes, enabling larger body sizes among herbivorous mammals. The largest of these animals were now three times bigger than before, weighing over 25 kilograms (55 pounds) and attaining the size of pigs. Examples found at Corral Bluffs include Carsioptychus coarctatus and Oxolophus, species of small, four-legged herbivores.
“Plants are the basis of terrestrial ecosystems,” said Miller during the press conference. “These mammals were responding to new kinds of plants on the landscape.”
By 700,000 years after the impact event, mammals got even bigger—the largest were the size of modern wolves. The heaviest mammals during this period weighed between 35 to 50 kilograms (77 to 110 pounds). These developments coincided with the emergence of legumes and possibly walnuts—calorie-rich food sources that powered these larger animals.
“One major storyline is the rapid increase in mammalian size after the mass extinction event, likely related to the evolution of more herbivorous diets and filling of open ecological niches,” Grossnickle told Gizmodo. “The quality of fossil preservation is great, and the fossils are accompanied by a thorough record of geological and environmental information, which makes for a more complete evolutionary history of the time.”
Paleontologist David Polly from Indiana University, who wasn’t involved with the new research, said the fossils are important because they’re showing us the ways in which the biosphere returned to “normal” (i.e. similar to Late Cretaceous conditions) and the ways in which it irrevocably changed.
“To some extent, the asteroid caused a blip from which there was a rapid recovery. But at the same time, they show that there were substantial differences in the species that made up post-asteroid ecosystems and that new specialties evolved in the wake of the extinction that had not been seen before,” said Polly in an email to Gizmodo. “One important thing these fossils show is that within 100,000 years, plant and mammal species were about as diverse as they had been before the asteroid. A hundred thousand years may seem like a long time, but it is very short in a geological time frame, and much quicker than recoveries from other extinctions.”
Polly said the quick recovery bolsters the “blip” hypothesis—the notion that Earth’s environment and ecosystem largely remained on course after the asteroid. At the same time, however, the new fossils demonstrate that “within 300,000 years, mammals and plants became more diverse than they had before the impact” and that, by 700,000 years, entirely new plants and animals had emerged. These changes, he said, “show that ecosystems were really functioning differently and that new opportunities existed that were not available in the Cretaceous.”
Looking ahead to future research, Lyson said he’d like to get a better sense of the ecosystems in which each animal lived, and how these animals fit together in larger evolutionary tree of life. As for Miller, he’d like to uncover more fossil evidence of legumes and walnuts to bolster the association between the emergence of these plants and the rise of mammals.