|MadSci Network: Earth Sciences|
Ammonites populated the seas for million of years. Over most of the world the bottom of the oceans are made up of different minerals at different times and areas. Where the fossilized ammonite shell that is now filled with calcite died and buried, later the calium rich soils around it were pushed by extreme preasure to its inside over millions of years to form the mineral.
The other specimen likely died where the ocean was high in silicon or clay type mineral. It just depends on the strata at the time of dying and luck if it ends up in the right situation to be turned into a fossil. Only a small number of animals that have ever been alive have parts that turn into fossils. Most are eaten by bacteria or other animals, or undergo significant decay before burial.
Reference: Prehistoric Life, Shaw and Millar , Penquin.
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