Suppose you find a rock with a goniatite fossil and another rock with an ammonite fossil? Which rock is older? How do you know?

Goniatite: 390-400million years old

Ammonite: 65-66 million years old

Since ammonites are usually food to the tylosaurus, (existed 66 million years ago) you could estimate ammonites are 65-66 million years old.

And for goniatites, all I know is that it's from the Devonian period