- If a species existed for a relatively short time then that species is now extinct
- Index fossils are formed from species that existed for a relatively short time
- Therefore, the species that form the index fossils are now extinct

So, the species that form the index fossils are now extinct
Index fossils help scientists estimate the age of a rock because index fossil species only existed for a relatively short time. What happened to the species that are now used as index fossils?
- If a species existed for a relatively short time then that species is now extinct 
- Index fossils are formed from species that existed for a relatively short time