Mass extinctions in the fossil record

0:00Dinosaur fossils turn up in layer after layer of rock. Then, suddenly, they stop. Why?

0:08Remember, the lower layers formed first, so they are older. The top layers are the youngest.

0:15The lower layers hold fossils of many kinds of living things: dinosaurs, flying reptiles, sea reptiles, ammonites, ferns and small mammals.

0:27Count the different kinds in each layer. Here, each of these two layers holds eight kinds.

0:34Then comes a thin, dark layer, found in rocks all over the world.

0:39Just above it, most of those kinds are missing. No dinosaurs, flying reptiles, sea reptiles or ammonites are ever found again.

0:49In this column, eight kinds fell to two. Most of the living things in the lower layers died out.

0:56When many kinds of living things die out in a short time, all over the world, it is called a mass extinction.

1:04This one happened 66 million years ago. It ended the age of the dinosaurs.

1:11Scientists estimate that about three in every four kinds of living things died out.

1:17But some kinds lived on, such as ferns and small mammals.

1:22Over millions of years, the survivors spread, and new kinds appeared, such as horses and whales.

1:29So the fossil record shows biodiversity, the number of different kinds of living things, falling sharply and then slowly rising again.

1:38Rocks record five big mass extinctions in Earth's past. The one that ended the dinosaurs is just the most famous.

1:47A mass extinction shows in the fossil record as many kinds vanishing at once, and life slowly recovering above.