Are the oldest fossils in top layer, middle layer, or bottom layer? Explain why that is?

Bottom layer.

Over time, layers are added. This is by the earth naturally getting older from animals, weather, rock, etc.

So, if you have an animal that died and it was covered by dirt from its time, more dirt is going to pile on top later on

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The oldest layers of rock will be at the bottom because as years go on things die and create new settlement that forces the older rock to move further down in to the ground