Mammals explode in size within a million years of the asteroid
What happened
A fossil site at Corral Bluffs, Colorado, dug up layer by layer, gave paleontologists a rare thing: an actual timeline of what happened to mammals right after the asteroid. Immediately after the extinction, the largest mammals were about rat-sized, down from raccoon-sized before the impact. Within 100,000 years they were back to raccoon-sized, but a new kind. By 300,000 years they had grown to beaver-sized, and by 700,000 years after the impact, species like Ectoconus ditrigonus weighed over a hundred pounds, a hundredfold increase in body size from the survivors that started it. Paleontologist Tyler Lyson, who led the study, found that new legume plants growing back after the extinction gave these recovering mammals a richer diet than they'd had before, right alongside the disappearance of the dinosaurs that used to eat them.
Why it matters
Mammals had spent the entire age of dinosaurs staying small. Corral Bluffs shows that once dinosaurs and their appetites were gone, that changed astonishingly fast, and Lyson notes mammals would not grow this quickly again for another 30 million years. Every large mammal alive today, from horses to whales to humans, descends from lineages that took this same opening.
How we know
The evidence is a continuous stack of fossils and fossilized pollen preserved in sequence at one site, letting researchers read body size and plant composition together at each dated layer, rather than piecing together scattered fossils from different places and times.
Sources
- Smithsonian Magazine. Fossil Site Reveals How Mammals Thrived After the Death of the Dinosaurs (2019) · Reputable sourcesmithsonianmag.com · The domain "smithsonianmag.com" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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Related timelines
- Age of Dinosaurs → · Picks up immediately after the asteroid that ended the dinosaurs