Big ideas
What killed the dinosaurs
6 min read
The short version
Sixty-six million years ago an asteroid about ten kilometres across hit what is now the Yucatán coast of Mexico. The impact threw enough dust and sulphur into the air to darken the sky for years, plants stopped growing, and every dinosaur except the birds died out — along with about three quarters of all species.
The evidence for an impact
In 1980 the Alvarez team found a worldwide clay layer at the Cretaceous–Paleogene boundary enriched in iridium, an element rare in Earth's crust but common in asteroids.
The crater was identified a decade later: Chicxulub, roughly 180 km wide, buried under the Yucatán Peninsula, dated to the boundary within tens of thousands of years. Shocked quartz and impact glass spherules ring the Caribbean and North America.
How it killed
The immediate blast and tsunami were regional. The global killer was the atmosphere: vaporised rock and sulphur aerosols cut sunlight sharply for years, collapsing photosynthesis on land and in the oceans.
Food chains built on living plants failed first. Small omnivores and scavengers living on seeds, insects and detritus fared best — which is a large part of why birds, mammals and crocodilians came through.
What survived
Birds, mammals, lizards and snakes, turtles, crocodilians, amphibians and most freshwater life came through. On land, body size mattered: nothing much above 25 kg survived.
In DinoAtlas the boundary is stark — occurrences drop off a cliff at 66 Ma and only the avian branch continues.
Common questions
- How big was the asteroid that killed the dinosaurs?
- Roughly 10–15 km across, leaving the ~180 km Chicxulub crater beneath the Yucatán Peninsula.
- Did all dinosaurs die out?
- No. Birds are dinosaurs and survived; all non-avian dinosaur lineages died out at the boundary.
- Was it volcanoes or an asteroid?
- The impact is the accepted primary cause. The Deccan Traps eruptions were a contributing stressor, and their exact weight is still debated.