Sorry for not going into detail on the fossil dating. I was on my phone at the time I responded. Basically, the geological column is in a certain order (with rare exceptions when one layer gets pushed onto the top of another layer, these instances are easily identifiable though.) It's a well-founded assumption that lower layers of strata are older than higher layers of strata. When it comes to fossils, only certain kinds of fossils are ever found together (you don't ever find mammals and dinosaurs together) sedimentary layers are very difficult to date, but igneous rock is very easy to date. Unfortunately, fossils are almost always found in sedimentary rock.
WIth that in mind, here is the process. Scientists label strata by the fossils that are found in them (since certain fossils are always found together no exceptions) We can then occasionally find igneous rock next to sedimentary rock, and we can use that to date the sedimentary rock, along with those fossils found in it. Then, when a new fossil is found, we look at its position with respect to other fossils in the geological column, and assign it the date based on the known date for other fossils in those layers. (note that many dates for each layer have been measured using multiple different radiometric techniques, and they all line up within a margin of error)
WIth that in mind, here is the process. Scientists label strata by the fossils that are found in them (since certain fossils are always found together no exceptions) We can then occasionally find igneous rock next to sedimentary rock, and we can use that to date the sedimentary rock, along with those fossils found in it. Then, when a new fossil is found, we look at its position with respect to other fossils in the geological column, and assign it the date based on the known date for other fossils in those layers. (note that many dates for each layer have been measured using multiple different radiometric techniques, and they all line up within a margin of error)
