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.

An impact origin was suggested in 1981, but the link to the extinction was only confirmed in 1991, 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.

The iridium clue was found almost by accident. Around 1980 the geologist Walter Alvarez and his father, the physicist Luis Alvarez, were studying the boundary clay in limestone near Gubbio in Italy. They hoped to use iridium as a kind of clock. Tiny amounts of meteorite dust fall on Earth at a fairly steady rate, so measuring iridium in the thin clay might show how long it had taken to settle. Instead they found far more iridium than any slow rain of dust could explain.

The same spike soon turned up at sites in Denmark and New Zealand, and the impact idea was born, a decade before the crater was firmly identified. Critics pointed out that the last dinosaur fossils often lie some way below the boundary, as if they died out earlier. Palaeontologists answered with a principle called the Signor-Lipps effect. Because fossilisation is rare, the last fossil you find of any species will usually come before its true last appearance, so a sudden extinction can look gradual in the rocks.

The extinction was just as severe in the sea. The ocean food web rests on microscopic plankton that photosynthesise near the surface, and when sunlight failed, they suffered badly. Rocks just above the boundary show that most species of tiny shelled plankton vanished, replaced for a time by a few hardy survivors. With the base of the chain cut away, the animals higher up starved in turn. The great marine reptiles, including mosasaurs and long-necked plesiosaurs, disappeared completely. Animals on the deep sea floor that fed on dead material drifting down from above generally fared better, because their food did not depend on fresh growth.

One of the most telling contrasts involves ammonites and nautiluses, two groups of shelled relatives of squid. Ammonites had flourished for hundreds of millions of years, yet they died out at the boundary, while nautiluses survived and still live today. A leading explanation lies in their young. Ammonites seem to have produced huge numbers of tiny hatchlings that drifted near the surface, exactly where the food supply collapsed. Nautiluses lay a few large eggs that develop slowly, over many months, in deeper water, and the young hatch at a relatively large size. A different breeding strategy may have decided which branch lived.

The rocks just above the impact layer also record how life came back. In North America, samples from immediately above the boundary contain far fewer pollen grains than usual and a sudden abundance of fern spores, a pattern called the fern spike. Ferns are good pioneers. Their spores are tiny, numerous and carried by the wind, and many can regrow from underground stems that survived the disaster. For a time they seem to have dominated devastated landscapes, much as they can colonise fresh volcanic ground today, before flowering plants and trees gradually returned.

Mammals had lived alongside dinosaurs for well over a hundred million years, but most stayed fairly small. With the large dinosaurs gone, mammals diversified rapidly into the empty roles. Within about the first fifteen to twenty million years after the impact, the fossil record includes early bats, primates, horse ancestors no bigger than a dog and the first relatives of whales, which began as four-legged land animals. The extinction did not guarantee that mammals would flourish, but it removed the competitors that had held them back. Your own ancestors were among the small survivors whose descendants filled that new world.

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.