Dead and Then Fossilized
Fossilization is the process by which a plant or animal becomes a fossil. This process is extremely rare and only a small fraction of the plants and animals that have lived in the past 600 million years are preserved as fossils. This may be surprising, considering the millions of fossils that have been collected over the years, and the many billions still in the rocks. Those plants and animals that do become fossils generally undergo, with some exceptions, several key steps.
First, the soft tissue that exists during life decays leaving behind only the "hard parts" (bone, shell, teeth). Second, hard parts may be transported and broken. This causes the fossilized remains to be incomplete representations of the living animal. It is much more common to find a fragment of shell or bone than it is to find a complete skeleton. Third and most important, hard tissues become buried and altered. In most cases this involves destroying the original material from which the hard parts were made as minerals are slowly dissolved and replaced by new ones. Often times a hard part is dissolved without being replaced by new material, leaving behind only an impression or mold of the original animal. If this mold is filled with sediment that is later cemented into rock it will make a cast of the original animal.
These fossilization scenarios are a few of many possible processes that turn living organisms into rock-like material. Each process generally follows the three previously mentioned steps: decay, transport, and burial. Think of using the same procedure to bake two cakes and using a different set of ingredients for each cake. In our case, the cakes are fossils and fossilization is the procedure.