Relative age dating and cross cutting relationships
No bones about it, fossils are important age markers.
The photograph below was captured at Volcano National Park on the Big Island of Hawaii.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
Pretty obvious that the dike came after the rocks it cuts through, right?
After all, a dinosaur wouldn’t be caught dead next to a trilobite.
The narrower a range of time that an animal lived, the better it is as an index of a specific time.