Dating a fossil
One example of this is ironstone nodules in the Mazon Creek fossil beds that contain amazingly details fossil plants and insects.
Unaltered Preservation - The original material of the organism has not been changed to another substance.
Some fossils, called index fossils, are particularly useful in correlating rocks.
For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.
Very rare, and typically found in very recent fossils.
Some examples include freezing (woolly mammoths in the permafrost), mummification, fossilization in amber.