I have just seen a video of a crab encased in rock, which has become a fossil.
How has that happened in such that solid rock forms around a creature, leaving a permanent imprint in stone?
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Well the creature dies and is covered in soft sediment. That gets covered in more and more soil until the pressure inside is so high that everything gets compressed.
Then the next step is the actual petrification, mineral rich water seeps through the fossil and replaces biological molecules with minerals, creating a rock in exactly the same shape of the dead animal
The conditions for that are very specific, so only a tiny fraction of dead animals will ever become fossils.
Sedimentary rocks are formed when mineral particles are squeezed together and the spaces between the particles get filled in. Soft minerals like limestone actually dissolve in water, so the water carries minerals into the tiny spaces and fills them up. Once all the spaces between the particles are filled in you have a new rock but if you cut it open you can still see the individual rocks that stuck together to form the new rock. Bones and shells are made of minerals so they can be part of that process just like a rock. They become part of the new rock but you can still tell that they used to be bones.
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