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 the non-avian 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 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 on Thursday in Science.

Dr. 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 proven rather elusive in his field of study.