Scientists find trilobites during a routine dig. Trilobites were common in the Paleozoic Era (540 to 245 million years ago) and went extinct during the late Permian period (248 million years ago). On another site, the scientists find ammonites. Ammonites lived during the Mesozoic Era (245 to 65 million years ago) and are not found after that period. What can be concluded about the rock layers? Choose all that apply. A The rock layers must have formed at the same time because they both contain biozones.

B The rock layer containing the trilobites must have formed before the one containing the ammonites.

C The rock layer containing the trilobites must have formed after the one containing the ammonites.

D The rock layers must have formed at different times because they contain two different biozones.

A and C

Explanation:

The rock layers must have formed at different times because they contain two different biozones. The rock layer containing the trilobites must have formed before the one containing the ammonites. The key thing to note here is that the Trilobites only lived from 540 to 248 million years ago. And that the Ammonites were from 245 to 65 million years ago. So there's a 3 million year gap between these 2 different creatures where neither of them existed. So the dig where the trilobites were found has to be at least 3 million years older than the dig where the Ammonites were found. So with that in mind, let's look at the options and see what makes sense.

The rock layers must have formed at the same time because they both contain biozones. * This is nonsense no matter what dates you're speaking of. In effect it claims that all rocks that indicate life were created at the same time. So this is definitely a bad choice.

The rock layers must have formed at different times because they contain two different biozones. * This is technically incorrect since it assumes that all biozones represent an unique period of time which is incorrect. However, since the biozones represented by the presence of Trilobites and Ammonites don't overlap, it is true that these these rock layers were formed at different times. I would have preferred if this option said "non overlapping biozones". But on the balance, I'd consider this option to be true, so it should be selected.

The rock layer containing the trilobites must have formed before the one containing the ammonites. * This is definitely true since the last Trilobite died about 3 million years before the first Ammonite lived. So this is a correct choice.

The rock layer containing the trilobites must have formed after the one containing the ammonites.

* This is the opposite of the previous option. And since the previous option was true, this option must be false. So this is a bad choice

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