A fossil found in a surface rock layer is of the same species as a fossil found in a deeper rock layer. However, the two fossils differ slightly in structure. What do differences in the structures of the fossils indicate?

The slight differences in the structures of the fossils indicate that the species has been evolving and adapting to its environment.

Explanation:

When we think of fossil remains of species we often tend to think that it is a species that has gone extinct or has changed enough so that its descendents are nowadays classified as different species. While this is true in big portion of the cases, it is not true entirely, as there are numerous extant species from which there are fossils from hunderds of thousands of years ago or even millions of years ago, and the changes are so little that they are still the same species.

When such fossils are examined and compared to the living members of the species it is almost always noticed that even though they are both of the same species there are some slight differences in their structures. This is due to evolution, as the organisms are adapting to the new environmental conditions, so in order to be able to survive they experience some changes that will help them in that. An excellent example are multiple species of opposums that exist nowadays and have experienced only very slight and even unnoticeable structural changes for tens of millions of years.