A geologist discovers that the number of fossils of a single species decreases significantly in higher rock layers compared to deeper rock layers. Which conclusion can best be drawn from this evidence?

This means that the number of individuals of this species has decreased over time.

Explanation:

When a rock is formed in one place, it goes through a process called weathering, which causes this rock to create layers over time. The deeper the layer, the older it is, on the other hand, the more superficial layers are younger. This is very important when cinetistas want to know how the population of a given species has increased or decreased over the years.

This occurs by visualizing the amount of fossils in each layer of the rock. If scientists can see that the number of fossils of a single species decreases significantly in the higher rock layers compared to the deeper layers, they can conclude that the number of members of that species has decreased over time.

The species became less abundant

Explanation:

If the species decreases it doesn't necessarily means it has become extinct.