A fossil is the remains, trace or imprint of a prehistoric living thing preserved on Earth crust.
When a fossil falls on water, normally, new layers start forming under water, that makes that the fossil that was on the top layer be under many other layers.
Earth have four eras (If you don't know what's "era", you can look for it on google): The Precambrian, Paleozoic, Meseozoic and the era in what we live, The Cenozoic Era.The Cenozoic Era has three