Soft portions of the body decay and the sediment around the bones harden to rock. Minerals in the water petrify these bones and, with time, the layers of rock shift. Nature exposes the fossil by using wind and rain to wear away the layers of rock. How Fossils are Formed Dinosaurs might be some of the most exciting fossils around, but lots of other living things were also preserved in stone through the ages. A fossil is the mark or remains of a prehistoric plant or animal thousands of millions of years old.
Some fossils are bones, teeth or shells.
Others are just footprints. Bones turn to stone when mineral-rich water soaks into their pores. When the mineral takes the place of the hard part, that's called replacement . Some are just an outline. These fossils are called prints, imprints or molds.