What Killed the Dinosaurs?

The first big clue was a thin layer of clay found in rocks all over the world, laid down at exactly the boundary between the age of dinosaurs and what came after. That layer is unusually rich in iridium, a metal that is rare in Earth's crust but common in asteroids.

The second clue was the crater. Buried under sediment near the Yucatan Peninsula lies the Chicxulub crater, about 180 kilometres wide and dated to the same moment as the iridium layer. Drilling into it has recovered shocked quartz and melted rock, which only form under sudden extreme pressure.

Put together, these findings answer what killed the dinosaurs with unusual confidence for an event this old. There is a crater of the right size, in the right place, at exactly the right time, with a global chemical fingerprint to match.

The immediate effects were regional. Anything close by was destroyed by the blast, and enormous waves swept the surrounding coasts. Rock thrown into space fell back through the atmosphere as glowing debris, heating the sky and starting fires across large areas.

The global killer was the dust and sulphur blasted into the upper atmosphere. Sunlight was blocked for months or years. Plants could not photosynthesise, so plant eaters starved, and the animals that ate them followed. Temperatures fell sharply, then rebounded as carbon dioxide released by the impact warmed the recovering world.

This is a genuine scientific debate rather than a fringe view. At almost the same time, enormous volcanic eruptions in India were producing the lava beds known as the Deccan Traps, releasing gases that were already stressing the climate.

Most researchers now think both played a part, with the asteroid delivering the decisive blow to ecosystems that were already under pressure. The precise balance between them is still argued over, and improved dating of both events keeps shifting the picture slightly.

The extinction was not random. Certain traits made survival much more likely:

• •Being small, so less food was needed to stay alive
• •Being able to eat seeds, insects or dead material rather than fresh plants
• •Living in water or in burrows, which buffered the temperature swings
• •Being able to breed quickly and rebuild numbers

One group of dinosaurs came through. Birds are dinosaurs, descended from small feathered theropods, and every bird alive today is a surviving branch of that family tree.

So the honest answer to what killed the dinosaurs is that it killed all of them except the birds. There are roughly eleven thousand bird species alive now, which means dinosaurs are still one of the most successful groups of land vertebrates on the planet.

The Chicxulub crater was not discovered by anyone looking for it. Geophysicists working for a Mexican oil company in the late 1970s noticed an enormous circular magnetic and gravity anomaly beneath the Yucatan and could not explain it.

The connection to the extinction was only made in 1991, more than a decade later, when their data was linked with the iridium layer research. The story is a good illustration of how scientific evidence often sits unrecognised in one field until someone from another notices what it means.

What killed the dinosaurs was an asteroid impact 66 million years ago, whose dust and cold did far more damage than the blast, helped along by ongoing volcanic activity, and the birds are the branch of the dinosaur family that survived it.