Why are Index fossils a good tool for determining the age of a rock layer? Because index fossils are of organisms that existed for a long period of time. Finding them in a rock layer, will determine the life span of similar animals found today.

Because index fossils are of organisms that existed for a long period of time. When finding them in a rock layer, you can be certain that the rock layer was formed in that long-time frame.

Because index fossils are of organisms that existed for a long period of time. Finding them in a rock layer, will bring you good luck.

Because index fossils are of organisms that existed for a short period of time. When finding them in a rock layer, you can be certain that the rock layer was formed in that small-time frame.