(Science Trek music) JOAN CARTAN-HANSEN, HOST: How do fossils form?

First, a living thing has to die, perhaps drowning in a lake.

The body sinks to the bottom and the soft parts like tissue and skin decompose.

That leaves behind the hard skeleton.

Over time, the skeleton is covered by mud and sand.

As the skeleton is buried, the minerals in the water enter through the pores of the bone and turn it into stone.

Layers of sediment build up over time.

And eventually, the sediment is compressed into solid rock.

Some time later, the movement of the Earth's crust eventually brings the rock to the surface.

Wind and water wear away the rock layers, sometimes exposing the fossil.

Or paleontologists dig though the rock layers to find them.

It takes just the right circumstances to make a fossil.

But knowing where fossils were found, under what conditions, and how long ago helps create a fossil record and this fossil record helps us better understand the creatures that lived long ago.

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