There are three major conditions that allow organisms to become a fossil. The organism itself must have hard parts. The environment where it comes to rest must be covered with fine-grained sediment, like mud, rapidly. The fossils only represent a small fraction of the biological life that the fossils come from.

As the fossil is buried and becomes a rock, the conditions that a rock goes through will on whether the fossil will be recognized. For example, if many shells are preserved in a sedimentary rock, they will probably be preserved. But if the area is metamorphosed, then the rock will change and you may never know there were fossils buried in the rock.

Fossilization process