18. Trilobites are useful as index fossils, because each species existed for a a.long period of time over a wide geographic range. b. long period of time over a small geographic range. c.short period of time over a wide geographic range. d. short period of time over a small geographic range.

## Step 1:Identify the criteria for an index fossil. Index fossils help geologists and paleontologists to date rock layers. The key characteristics of an index fossil are that it should be found over a wide geographic area and represent a species that existed for a short period of geologic time.## Step 2:Evaluate the given options against the criteria for an index fossil. An index fossil should not have existed for a long period of time because that would make it difficult to pinpoint a specific geologic time. It should also not be limited to a small geographic range because that would reduce its usefulness in correlating rock layers over different regions.## Step 3:Choose the option that best fits the criteria for an index fossil. The correct answer will be the one that states the species existed for a short period of time and was distributed over a wide geographic range.### **The important formula for an index fossil is:**### ** **