65 million years ago

Extinction of the dinosaurs

Extinction of the dinosaurs

About eight months ago on our timeline (really 65 million years ago), the dinosaurs became extinct. Quite why they disappeared is a mystery. Nobody is certain, although there are clues.

In many parts of the world there is a thin layer of clay that contains a lot of iridium — a kind of metal. This metal is usually very rare on the Earth, but it is common in meteorites. Some people think that the iridium proves that a meteorite or asteroid about 10 km across crashed into the Earth. It exploded and threw up a huge amount of dust and rock, which blocked out the sunlight for several months. This killed the plants (although their seeds were safely buried in the ground, waiting for the return of the sun); the herbivores died through lack of food; and the carnivores that ate the herbivores also died. Only the scavengers, eating rotting plants and dead animals, survived.

However, other scientists say that a period of huge volcanic eruptions altered the climate and this caused the extinction. They point to the thousands of metres of lava flows in India that built up at the same time as the meteorite is said to have struck.

But it is not as simple as this. A great many species had become extinct during the million years before the asteroid strike or volcanic eruption took place and yet others went into extinction a short time afterwards. It was not just the dinosaurs that became extinct: a number of groups of animals disappeared at the same time, including the flying reptiles, ichthyosaurs and plesiosaurs, ammonites, belemnites and most of the marine microscopic plant life (phytoplankton). Yet many other groups of marine organisms, together with fish, mammals, birds, tortoises and turtles, crocodiles, lizards and snakes and most of the plants were not affected. What could have happened that killed some things, but not others?

This mass extinction is a puzzle because it took place over several million years. It was not just one big extinction event. Perhaps for those species heading towards extinction anyway, the meteorite strike or the volcanic activity was just 'the straw that broke the camel’s back'.

Whatever happened, about 60 per cent of all species disappeared 65 million years ago.

The Earth is now 46 years oldBack to 100-0 million years ago

The Earth is now 46 years old.