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Dinosaurs Disappeared, and Mammals Took Over. Here’s How It Happened.

When an asteroid hit the Earth 66 million years ago, it wiped out not just dinosaurs but 75% of all species. The recent discovery of thousands of fossils in Colorado’s Corral Bluffs is helping scientists understand how plant and animal life recovered from that mass extinction event. The fossils show how mammals evolved, diversified, thrived, and eventually came to dominate the planet.

The findings were detailed in a paper in Science co-authored by Professor Stephen Chester (Brooklyn College and The Graduate Center, CUNY), a paleontologist and mammal specialist. He helped identify, analyze, and classify the fossils to show how mammals fit into the timeline of that first million years after the asteroid hit.

The dramatic story revealed by the fossil find is also told in a terrific PBS NOVA documentary, Rise of the Mammals, and a Denver Museum of Nature & Science exhibition. Tyler Lyson, a vertebrate paleontologist at the Denver museum, led the fossil excavation.

Among the fossils were thousands of plants and about 1,000 animals, including turtles, crocodiles, and 16 mammal species. Most were “three-dimensionally preserved” in egg-shaped rocks called concretions. Entire skulls were found, along with “intact delicate structures” like middle ear elements. Other treasures included turtle shells, microscopic pollen spores, and plant parts — seeds, leaves, roots, branches, saplings, stumps. One significant finding was what the authors called “the oldest known occurrence” of legumes. The emergence of bean pod-bearing plants provided an energy-packed food source that fueled the evolution of large, plant-eating mammals.

Researchers were able to date the fossils and create a timeline that suggests an initial phase of recovery about 100,000 years post-catastrophe, followed by bursts and phases of rich species diversification at 300,000 and 700,000 years. These booms in plant and animal species coincided with warming trends. On the plant side, there was a shift from simple ferns and palms to complex forests. On the animal side, small mammals evolved into larger creatures. “You’re going from a very small dog that you’d see on the streets on New York City to a very large wolf within those hundreds of thousands of years,” Chester told The New York Times.

The findings are especially relevant in the era of climate change. “This study provides a better understanding of how the Earth recovers following a mass extinction event,” Chester told The Graduate Center. “It has significant implications given that we are currently facing what many scientists call the sixth mass extinction.”