Well, when a plant or animal dies in a damp environment it will most likely get buried in mud or silt. The tissues over time will decompose and leave bones or shells behind ( depending on what was there originally it can vary) Eventually over time sediment builds up and hardens into rock. I sincerely hope this helps you ~
Mud can prevent oxygen from reaching the dead organism
Explanation:
Fossils are formed in a whole of separate ways, but most are produced when a plant or creature vanishes in a watery atmosphere and is immersed in mud and silt. Soft tissues immediately disintegrate leaving the hard bones or shells behind. Over time debris builds over the top and solidifies into rock.. and mud prevent the oxygen to reach dead organism.