How does a dead animal turn into stone?

A dead animal turns into stone only if almost everything goes right. First it has to be buried by sediment soon after death, because otherwise scavengers and decay destroy the remains. Then mineral-rich groundwater soaks into the empty spaces inside the bone, and minerals precipitate out of that water and fill them. This is called permineralization, and most dinosaur bones were preserved this way.

Sometimes the bone is instead replaced by another mineral, or dissolves completely and leaves an animal-shaped hole in the rock, called an external mold. If sediment later fills the hole, the result is a cast that resembles what the animal looked like. Fossils can also be marks rather than bodies, such as tracks and droppings, which are called trace fossils.

Most of what lived never gets this treatment. A fossil usually preserves only the part that was partly mineral in life, like bones and teeth, and the record is strongly biased toward animals with hard parts. Species known from fossils are thought to be far less than 1% of all the species that have ever lived.

People dug up these stones long before they understood them. In medieval China, fossil bones of ancient mammals were often mistaken for dragon bones and used as medicine. Yet in 1088 the scholar-official Shen Kuo looked at sea fossils in mountains hundreds of miles from the coast and reasoned that a prehistoric seashore had once existed there and shifted over centuries.