They have excavated a site south of Denver that looks very much like a present-day Amazonian rainforest, full of trees and other plants, the team at the Denver Museum of Nature and Sciences said.
The fossils of more than 100 kinds of towering conifer trees, huge ferns and blooming flowers challenge scientists' long-held assumption that a desolate Earth took about 10 million years to recover from an asteroid collision that wiped out the dinosaurs 65 million years ago.
The rainforest would have been lush only a million years after the catastrophe -- which suggests either that life recovered more quickly than anybody thought, or that pockets of territory were somehow sheltered from the effects of the asteroid, the researchers said.
"It not only recovered, it went crazy," said Kirk Johnson, paleontology curator at the Denver Museum of Nature and Science. He reported the findings in the latest issue of the journal Science.
In fact, scientists said it might be the earliest example on record of a true tropical rainforest.
"This site certainly raises more questions than it provides answers," Johnson said.
Most scientists believe that an asteroid slammed into the Earth 65 million years ago near what is now the Yucatan Peninsula of Mexico, creating a huge crater and throwing billions of tons of dust and rock into the air. It would have dimmed the sun's light for centuries and most species of plants and animals died.
For the next 10 million years, the fossil record is "very boring", with just a few species of plants, animals and insects, Johnson said.
"A million years after the extinction of the dinosaurs was a really peaceful time," he said. The big carnivores were gone and the ancestors of modern mammals were small, scrambling creatures.
And on the east slope of the Rockies, it seems, a pocket of rain forest grew.
Nowadays, tropical rainforests are found near the equator and all have high rainfall and a year-round constant temperature. They harbor many different species of plants and animals and many of the trees have large leaves with smooth edges shaped to help water drip off easily.
The site, in Castle Rock, Colorado, about 25 miles south of Denver and in the foothills of the Rockies, is rich in fossils that look like they came from such an environment.
It has the remains of large leaves, fronds and roots and fossil casts of entire tree trunks, Johnson and colleague Beth Ellis report in Friday's issue of Science.
It looks just like the leaf litter found at a present-day rainforest in Peru, with large, smooth-edged leaves.
"When you split the rocks sometimes the leaves actually peel out and blow in the wind," Johnson said. "You can actually see the cell structure of the leaf."
He said in the rainforest leaves are often chewed by insects, as are the fossil leaves. "We found one about five minutes ago -- we found a place where a great big hole had been bitten out of a leaf," said Johnson, who took a break from "swinging a pickax" to explain his work.
The site, just off an interstate highway, contains "tons of new species", he said. "The site itself produced about 104 different kinds of things and about 80 undescribed extinct species," he said.
Now Colorado is semi-arid, but 60 million years ago the remnants of a huge inland sea that spread from the Arctic to the Gulf of Mexico remained. It could have provided moisture that would have swept around and been dumped on the east slope of the mountain range.
The site was discovered in 1994 by a state highway worker. It is scheduled to be demolished later this year in a road-widening project.