The empty space in rock showing the exact shape of an organism that was buried and decayed there is called a mold.

Here's a breakdown:

* Mold: This is the negative imprint of the organism left behind in the rock. Imagine a footprint in wet sand - that's a mold. The organism decomposes, leaving an empty space that reflects its shape.

* Cast: If the mold later fills with sediment (like sand or minerals), that hardened sediment is called a cast. A cast is a positive replica of the organism.

So, to recap:

* Mold: The empty space, the negative imprint.

* Cast: The filled-in space, the positive replica.

Both molds and casts are important fossils that provide valuable information about ancient life.