Taphonomy is the study of what happens to a living thing between its death and its discovery. After an animal dies, scavengers feed on it, bacteria break down the soft tissue, and weather cracks the bones. Left on the surface, a skeleton can fall apart within years or decades.

Whether a fossil forms is a race between destruction and burial. If sediment (mud, sand or ash) covers the body fast, the bones are shut away from scavengers, sunlight and most of the oxygen. Settings where that happens are:

• a river delta or flooded plain, where a flood drops a blanket of mud
• a volcanic ash fall
• a sandstorm or a collapsing sand dune
• a landslide, or the bottom of a lake or sea