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.