midterm1 question Layers A, B & C

Q: I was wondering if you could explain question 29 on midterm 1 to me.

#29. You're on a dig in Montana and find a fossil in a layer that's on top of a 125 Ma volcanic layer. In Mongolia (very far away) you find three sedimentary layers (A(bottom) ,B, and C(top)), each containing a different type of fossil. The fossil in layer B is the same species as the one you found on the dig in Montana. Which of the below statements is true? It might help you to draw a picture.

I. The fossil in layer A is older than that in B
II. All three fossils are younger than 125 Ma
III. Fossil A is older than 125 Ma
IV. Fossils B and C are younger than 125 Ma

a. I. and III.
b. I and IV
c. III. and IV.
d. I. and II.
e. noodly appendage!

The answer is (b) but if you could explain why "III. Fossil A is older than 125 Ma" is wrong.  Since B and C are younger 125 Ma and layer A is underneath the volcanic layer, then doesn't that make it older?


A:  We know that the layer that has the B-type fossil is less than 125 million years old from the Montana site.  But we don't know how much less... it could be 5 million years old from what we're told.  Additionally, we know that layer A is older than layer B, but we don't know how much older.  So all we can say is that fossil A is older than fossil B, but that could still be much younger than 125 million years.