How do the remains of long dead animals turn into fossils and why do they become rock-like?
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Fossilisation can occur in a number of ways, but the most common and iconic one (besides limestone which is one giant mass graveyard you build your house out of) is iirc called replacement or substitution or something like that. Basically:
1. Dead animal falls to sea floor.
2. Dead animal gets covered in sediment, like sand.
3. Sediment gets turned via pressure into a loose, water-permeable rock.
4. Water gradually dissolves the dead animal leaving an empty hole the same shape as it, which doesn’t collapse because the sediment has rockified.
5. Minerals dissolved in the water gradually precipitate out of it, sticking to the walls of the cavity (in reality, this is happening at the same time as the dissolving of the original). Because the cast is so finely detailed, the new mineral picks up the same shape and sometimes even patterns as the original. Because the minerals are precipitated so slowly, they form durable crystals that maintain their shape as the rock continues to harden.
6. Lots of geology happens.
7. A nerd digs the fossil out of the rock and goes “lol ancient crystal rock snail”
Most of the time, it’s not actually the organism becoming rock. The organism dies on a soft sediment, makes and impression, and gets covered with more sediment. The organism then slowly decomposes until there is notice left but and imprint of that the animal looks like. That imprint is then filled in with new sediment, and hardens, resulting in the shape of the organism being preserved.
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