I can tell you all about fossils.

A fossil is the remains of a  prehistoric plant or animal embedded in a rock and preserved in petrified (organic matter) form.Fossils are the remaining traces of once living animals. Most fossils are the remains of extinct animals.

When animals die they usually get completely destroyed- either by another animal eating it, or because of it decaying (to rot). Sometimes when the animal is buried before it is destroyed and when that happens the remains if the animal are preserved as fossils. Most fossils are from invertebrates, animals without a back bones.