Because of the conditions necessary for their preservation, not all types of organisms that existed in the past have left fossils that can be retrieved.
- In order for a fossil to form, the organism's remains must not be significantly disturbed by a scavenger/decomposer or destroyed by erosion and other natural forces. Therefore, organisms or parts of organisms that make up fossils are most likely buried quickly and deeply.
- example: woolly mammoth found in ice, insects found in amber, animals found in peat bogs, mass burials from flash floods or volcanic ash falls.
- Soft body parts, such as skin, muscle, fat, and internal organs, deteriorate rapidly and leave no trace. Casts of such tissues are rarely found. Similarly, organisms that are soft-bodied creatures, like jellyfish, are very uncommon fossils while hard body parts (such as teeth and shells) fossilize easier.
- Molds can be made of organisms. However, the organism must be buried in sediment after which its tissues dissolve and are replaced by dissolved minerals which make it a solid. Without the correct minerals this process cannot take place.
- The fact that extremely few living things are preserved long enough after death to become fossils makes the large collection s of fossils in the museums of the world quite remarkable.