"Fossils are formed by being buried in sediment, but it is much more complicated than that because some fossils are made from a skeleton with many peices and others are made from a single object.

What happens usually is that the animal dies - sometimes by being attacked by something else, sometimes dying from something like the attack of parasites - and then all the soft parts disappear very quickly because the bacteria has made them decay, and one is left with a more resistant skeleton, which is buried within the sediment. So there is often a very long complicated history from the moment of death to the fossil being found."

Simon Conway Morris