Answer: If mud, silt or tar is in the area to preserve the fossil.
Fossils are the remains or traces of dead animals and plants. When the animals die the soft tissues are quickly decayed and consumed by microorganisms but hard bones are left, also in plants leaves, seeds, woody parts remains undecayed by microbial activities. These dead remains are buried under different layers of sediments. These sediments gets aggregated in the form of rocks. The rocks contains these remains are called as fossil bearing rocks. The sediments such as mud, silt or tar exerts pressure on dead remains therefore, allows the preservation of fossil in an area.