This particular index fossil is called Venericardia planicosta.

It is found all over the world, BUT it is only found in rocks formed 54 to 35 million years ago during a period of time called the Eocene.

Therefore, it can be considered an index fossil.

If you found this fossil in a rock layer somewhere, what could you say about the age of those rocks?

a) The rocks are from the Eocene
b) The rocks are from the Cretaceous

Since Venericardia planicosta is known to have lived only during the Eocene, then rocks containing this index fossil must have been formed during the Eocene.

Remember that Venericardia planicosta is only found during the Eocene. Try again...