In my Biology class at my high school we are learning about evolution and I got confused about relative dating and when I asked my teacher she didn't know the answer so I though you might. My textbook is a 7th edition Prentice Hall Biology book. On page 575 it says "If these sedimentary layers contain fossils, scientists use the layers to determine when certain organisms existed in relation to others." It looks to me like they are saying that the layers date the fossils. But, on page 578 it talks about the trilobite being a good index fossil and it says, "Some species of these shelled marine animals only lived between 500-600 million years ago. Scientists know, therefore, that sedimentary rock layers containing trilobite fossils must be between 500- 600 years old." On this page it looks like the fossils are used to date the layers. So I guess my question is, how can fossils date the layers when layers date the fossils? Thanks for your time. Stephanie

You have asked an excellent question that deals with the self-referential nature of almost anything we do. We simply don't have time to check absolutely everything, so we infer that (for example) the sun will rise tomorrow because it always has in the past, or a package we sent to Aunt Millie will get there before her birthday because UPS has always delivered things on time before.

A small part of your problem may be a need to understand upper and lower bounds; for that, see this previous answer.

But most of your problem is that you are not following the chain of reference to its beginning.

Essentially all fossils are found in sedimentary rocks, that is, rocks that are formed by compression of dirt or mud or sand. We can't tell the age of a layer of sedimentary rock directly because it is made up of tiny bits of older rocks, of all different ages. If you use radiometric dating, you will get different ages from different samples of a sedimentary rock layer because all the bits were formed at different times.

But volcanic ("igneous") rocks are ancient lava flows, and all of a layer of igneous rock will have been "fresh" when it was laid down. So we can use radiometric dating to establish its age.

Suppose that a layer of sedimentary rock appears between two igneous layers. We naturally assume that the layers were deposited one after the other. This means that the age of the sedimentary rock must be in between the ages of the two layers of igneous rock, and any fossils in that sedimentary rock layer are expected to have the same age as the rock itself. That's how we date fossils.

After studying a large number of such instances, paleontologists may notice that fossils of a certain animal (such as a particular species of trilobite) only occur between 500 and 600 million years ago. We can therefore infer that any rock layer with that sort of trilobite fossil is between 500 and 600 million years old, even if there is no layer of igneous rock around to provide confirmation.

Since trilobite fossils are quite common--and easy to identify because of the different sorts of bumps and spines seen in different species--we can use trilobite fossils to date any sedimentary rocks in which they appear. But remember this... the use of fossils as a way to date rocks is always preceded by finding a sufficiently large number of the same sort of fossils in rock layers that can be dated by radiometric methods.