Please choose the correct answer to the question: 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?

Answer1: They became extinct. Answer2: They changed their diets. Answer3: They hid in marine sediments. Answer4: They migrated to new environments.

Answer format: answer1/answer2/answer3/answer4
Answer: answer1