Paleontologists in Denver have uncovered a treasure trove of fossils that give a thrilling new insight into the rise of mammals after the extinction of the dinosaurs.

The discovery, detailed in a paper published this week in Science, has yielded a colossal amount of data showing how tiny mammal species grew and diversified dramatically after the extinction. The finding throws a spotlight on a previously unknown part of our own history: the very early days of a period that eventually produced all current mammal species on Earth.

The mystery of the first million years

There's still a ton that nobody understands about what happened after the extinction 66 million years ago. Researchers have been piecing together evidence about the event itself, which wiped out around three-quarters of all species on Earth and ended the era of the dinosaurs. "The time afterward has been shrouded in mystery," said paleobotanist Ian Miller, one of the lead researchers in the team that found the fossils, in a Q&A published by the Denver Museum of Nature and Science.

In the period afterward, the predecessors of modern species began to proliferate—including mammals, as well as the group of plants that includes all flowering plants and most trees. The question of how quickly this was achieved, and what allowed the expansion, has remained unanswered. This new fossil discovery comes from that gap, giving paleontologists an unprecedented look at how mammal life forms recovered, adapted, grew, and expanded in the first million years after the extinction.

The cache of bones was discovered when vertebrate paleontologist Tyler Lyson tried out a method of fossil hunting he had learned from colleagues in South Africa: looking for "concretions," which are nodules of rock that form around organic matter, like bone.

Lyson and his colleagues struck bone gold in the Denver Basin, a well-trod fossil hunting ground where the remains that were well-hidden in rocks had been overlooked. "I split open a concretion and saw a mammal skull smiling back at me," said Lyson. "And then I looked around and saw concretions just littering the landscape and was like, 'Oh man, here we go.'"

Within a few minutes, Lyson said, they had found four mammal skulls. Individual fossil mammal skulls are a rare and precious find, so this quantity is extraordinary: "That was one of the most remarkable moments in my life."

Rapid recovery

Over the last three years, Lyson, Miller and their team have extracted more than 7,000 fossils from the site, including 16 species of mammals, 233 species of plant, and over 37,000 pollen grains. So far, they have uncovered at least 40 mammal skulls. This means an absolutely astonishing level of resolution, allowing the researchers to track how mammals evolved over the course of just hundreds of thousands of years rather than the usual millions of years.

The fossils paint a picture of a rapid explosion. Before the impact, the area was richly forested and home to many species of dinosaur, as well as small mammals that were around the size of a porcupine. The extinction wiped pretty much everything out. Afterward, the only mammals left were tiny, rat-sized creatures. Initially, there wasn't a huge amount of nutritious plant life around, either—just ferns.

But within just 100,000 years, much larger species proliferated—up to 6kg, or the size of a raccoon. Plant species had diversified, too; there were now palm-dominated forests. By 300,000 years after the extinction, there were beaver-sized mammals and much more diverse forests.

And within 700,000 years, there were already wolf-sized mammals, 100 times larger than the survivors of the extinction. Alongside them were the first plants in the bean family, which probably offered a helpful source of protein.

Just the beginning

This incredibly detailed picture suggests that the expansion of plant life, which followed periods of planetary warming, may explain why mammal species grew so rapidly in body size. And there was space for them to expand into, filling the empty ecological niches previously filled by dinosaurs.

The hand-in-hand evolution of plants, mammals and climate points to the importance of understanding climate and ecosystem interactions. "It's the first time we can piece together four key things: the animals, plants, temperature, and then the timeline," Lyson told the Denver Museum of Nature and Science. "We can really look at the recovery of a whole ecosystem for the first time."

With data from just one region, it's not yet possible to make global generalizations. It's indisputable that mammals grew and diversified after the extinction, but how universal was this incredibly rapid explosion? "Now it'd be great to see if it's normal or abnormal," said Lyson.

There's still tons of data to wade through just from this one cache, and if there needed to be more exciting news, this discovery will probably lead other scientists to look for more concretions—which means there may be more dinosaur bonanzas to come.

Science, 2018. DOI: 10.1126/science.aay2268 (About DOIs).