History › Complex life › Age of dinosaurs
K–Pg mass extinction (asteroid impact)
66 million years ago· Event· Life & early humans
Well documentedSolid records, physical evidence or scientific consensus.
An asteroid about 10 km (6 miles) wide struck what is now Mexico's Yucatán 66 million years ago, ending the dinosaurs' reign.
See it on the History map →What happened
The impact at Chicxulub blasted rock and soot into the sky, sparking wildfires and blocking sunlight for months to years. Plants died, food chains collapsed, and roughly three-quarters of species vanished (estimates vary), including all non-bird dinosaurs, flying pterosaurs and shelled ammonites. Huge volcanic eruptions in India, the Deccan Traps, were under way at the same time and may have added stress.
Why it matters
It ended dinosaur rule and cleared the stage for mammals, which diversified rapidly into the roles dinosaurs left empty.
How we know
In 1980 Luis and Walter Alvarez reported a worldwide clay layer rich in iridium, a metal common in asteroids; the buried Chicxulub crater was identified in 1991.
Still debated
How much the Deccan volcanoes contributed alongside the asteroid is still debated.
Inside this (1)
Connections (2)
▸K–Pg mass extinction (asteroid impact) ended Age of dinosaursWell documented
The connection
The asteroid impact at Chicxulub caused wildfires and a global 'impact winter' that shut down plant growth. Large animals that needed lots of food, like dinosaurs, starved. All dinosaurs except early birds died out, ending their reign of about 165 million years.
When and how
Worldwide, 66 million years ago, starting at the impact site off Mexico's Yucatán Peninsula.
How we know
A global iridium-rich layer marks the boundary; no non-bird dinosaur fossils are found above it; the crater's age matches precisely.
Debates and open questions
Whether some dinosaurs survived briefly after the impact, and how much volcanoes contributed, are debated.
▸First mammals spread and diversified after the K–Pg mass extinction (asteroid impact)Well documented
The connection
Mammals had lived in the dinosaurs' shadow for over 150 million years, mostly small and nocturnal. Small, burrowing animals that ate many foods survived the impact's aftermath better. With dinosaurs gone, mammals quickly evolved larger bodies and new forms, filling empty roles within a few million years.
When and how
Worldwide, in the first 10 million years after the impact, about 66–56 million years ago.
How we know
Fossils show mammals growing sharply in size and variety in rocks just above the impact layer, as at Corral Bluffs, Colorado (studied 2019).
Debates and open questions
Some mammal groups began diversifying before the impact, so how much the extinction itself drove their spread is still argued.