What do scientists know about fossils found in deeper layers of rock compared to

those found in higher layers?

They know that layering, or bedding, is the most obvious feature of sedimentary rocks. This Law of Superposition is fundamental to the interpretation of Earth history, because at any one location it indicates the relative ages of rock layers and the fossils in them. Layered rocks form when particles settle from water or air. That is kinda easy : D