For approximately the last two hundred years, this succession of fossils in sedimentary rock has been used to argue that the earth has undergone successive events.
For instance, trilobite-bearing beds are supposed to reflect a time when trilobites were the dominant life form on earth, and dinosaur-bearing beds are supposed to reflect a time when dinosaurs were dominant on the earth.
Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you.
The essence of fossil succession, however, remains the same whether individual fossils, of groups of them, are used.The constant extension of ranges simultaneously reduces the credibility of the geologic column and organic evolution, and makes it easier for the Genesis Flood to explain an increasingly-random fossil record. Instead, they tend to be found at specific horizons, and these horizons can be located in rocks all over the world.