Fossils are the remains or traces left of prehistoric life.
The chances of being fossilized are improved if:
- there is rapid burial.
- the organism has hard parts.
There are many types of fossils:
- Mold - when a buried shell or bone is dissolved away by underground water. Just shows surface details.
- Cast - when a mold is filled with minerals.
- Petrified - internal voids filled with minerals.
- Replacement - the organism's material is replaced with minerals. Can show microscopic detail !
- Amber - hardened resins from trees trap organisms.
- Carbonization - fine sediments encases organism. Pressure over time leaves only a thin layer of carbon.
- Impression - when carbon of carbonization is lost, leaving just details in sediment.
- No change - more recent organisms may not be altered. Example: mastodon teeth on the island.
Traces - indirect evidence of life:
- Burrows - the Devil's Corkscrew.
- Tracks -
- Coprolites - droppings.
- Gastroliths - stomach stones.
- Fossils can be used to correlate (show relationships among) sediments.
- William Smith (1769-1839). English canal builder who noticed each rock strata contained unique fossils.
- Smith used fossils to correlate strata over wide areas.
- Principle of biotic succession: fossil organisms succeed one another in a definite and determinable order. Time periods can be recognized by its fossil content.
- Certain organisms are widely distributed but lived for a short time. These fossils are called index fossils.