A fossil species is found in the same rock layer with Fossil 3 and Fossil 4. When could this species have existed?

If a fossil species is found in the same rock layer as Fossil 3 and Fossil 4 it is reasonable to infer that this species existed during the same time period as Fossils 3 and 4. This inference is based on the principle of stratigraphy specifically the Law of Superposition, which states that in an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest layers are at the top.

Therefore, if all the fossils are at the same layer it indicates that they were deposited during the same geological period. However it is important to note the potential for some exceptions to this rule, such as reworking of sediments or the presence of unconformities, but generally the assumption holds true for a simple stratigraphic setting.

To provide a more precise time frame for when the species may have existed you would need additional information such as the absolute age of the rock layer which can sometimes be determined through radiometric dating techniques or the known age ranges of Fossil 3 and Fossil 4 if they have been previously studied and dated.