2. trilobites are useful as index fossils, because each species existed for a . long period of time over a wide geographic range. long period of time over a small geographic range. short period of time over a wide geographic range. short period of time over a small geographic range. 3. how would you date a sample of rock that you suspect as being one of the earliest on earth? . use a radioactive isotope with a short half-life. use a radioactive isotope with a long half-life. use an index fossil. use a microfossil.