How can scientists understand what the relevant age of a fossil is?

There are layers of earth that are deposited over time and based on where a fossil is in the layers a scientist can understand what the age of the fossil is relevant to another fossil in a different layer of earth. The law of superposition says that new sediment is deposited on top of old sediment so, the sediment in the bottom layer is the oldest and the sediment at the top is the youngest.

Because of this the fossils in the bottom layer are the oldest and the fossils near the top are the youngest. Say you have 10 layers of rock, and the top layer is number 1 (the youngest) and the bottom layer is number 10 (the oldest). Even if a scientist can't tell exactly how old a fossil is if they know it is in a layer deeper than another fossil they know it is older than that fossil so, a fossil from layer 9 is older than a fossil from layer 5 which is also older than a fossil from layer 3. This is relative age.

If we take it a step further... If a scientist knows that the fossil in layer 4 is 65 million years old than we not only know the fossils in layer 5-10 are older than the one in layer 4, but that they are all older than 65 million years old. So, the fossils in layer 1-3 would be less than 65 million years old. Scientists can know how old a specific fossil is sometimes based on whether or not it is an index fossil (a fossil that only occurred during a very small, specific interval of time), or if it is possible to carbon date it or something in the same layer.