Figure 19.1
-Figure 19.1 shows an outcrop of sedimentary rock whose strata are labeled A-D. If x indicates the location of fossils of two closely related species, then fossils of their most-recent common ancestor are most likely to occur in which stratum?
A) A
B) B
C) C
D) D

Question

Figure 19.1
-Figure 19.1 shows an outcrop of sedimentary rock whose strata are labeled A-D. If x indicates the location of fossils of two closely related species, then fossils of their most-recent common ancestor are most likely to occur in which stratum?
A) A 
B) B 
C) C 
D) D

Quizplus · Accepted Answer

The principle of superposition 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, the most-recent common ancestor of two closely related species would have lived prior to the divergence of those species and would be found in a layer older than the layers where the species are found. Since x is located between the layers of species fossils, it can be inferred that this layer (labeled C) is older than the species fossils and therefore the most likely location for the fossils of their most-recent common ancestor.

Figure 19.1 -Figure 19.1 Shows an Outcrop of Sedimentary Rock Whose Strata