marine mammal fossil swamp fern fossil trace fossil index fossil
I gave my students this multiple choice question: Certain fossils can be used to help determine the age of the rock layer in which the fossils are found. Which type of fossil is best to use when making this determination?

Only one answer is correct among these 4 choices:
- marine mammal fossil
- swamp fern fossil
- trace fossil
- index fossil

Could you tell me which one is correct?

index fossil