Explanation
Correct answer: D.
Choice D is the best answer because it most effectively uses data from the table to complete the statement about the researchers’ conclusion about fossil shark teeth. The text states that mammalian fossil evidence has placed both MFS and PFBV within Hh4 (6 Ma to 4.75 Ma) but “a more precise determination of the sites’ ages has proved challenging,” prompting Killingsworth and colleagues to attempt to date the sites by comparing average ⁸⁷Sr/⁸⁶Sr ratios for fossil shark teeth from MFS and PFBV to ratios in the global strontium seawater curve. Given that “⁸⁷Sr/⁸⁶Sr ratios in seawater correspond to numerical ages,” a fossil whose ⁸⁷Sr/⁸⁶Sr ratio corresponds to the ratio associated with a particular age in the global strontium seawater curve can be inferred to date to that age. According to the table, the ⁸⁷Sr/⁸⁶Sr ratio of 0.709000 for fossil shark teeth from MFS corresponds to an age of 5.86 Ma, which falls within Hh4. By contrast, the table shows that the ⁸⁷Sr/⁸⁶Sr ratio of 0.709028 for fossils from PFBV would fall between 0.709020 (5.40 Ma) and 0.709040 (4.75 Ma), indicating an age that falls within Hh4 but is more recent than MFS’s age of 5.86 Ma. The data therefore corroborate the conclusion that both sites fall within Hh4 while suggesting that PFBV was likely deposited more recently than MFS was.
Why the other choices are wrong
Choice A
Choice A is incorrect because the table indicates that MFS was deposited closer to the Hh3-Hh4 boundary than PFBV was, not the reverse. The table indicates that PFBV’s ⁸⁷Sr/⁸⁶Sr ratio of 0.709028 corresponds to an age between 4.75 Ma and 5.40 Ma, while MFS’s ⁸⁷Sr/⁸⁶Sr ratio of 0.709000 corresponds to an age of 5.86 Ma. Because the Hh3-Hh4 boundary occurs at 6 Ma, MFS was deposited closer to that boundary than PFBV was. In addition, the text doesn’t establish that mammalian fossil evidence offers less dating precision than ⁸⁷Sr/⁸⁶Sr ratios do; it states only that mammalian fossils have placed both MFS and PFBV in Hh4 and that more precise dating has proved challenging.
Choice B
Choice B is incorrect because it mischaracterizes the level of support for the site age estimates established through mammalian fossil evidence. The table indicates that the average ⁸⁷Sr/⁸⁶Sr ratios from MFS (0.709000) and PFBV (0.709028) both correspond to ages that fall within Hh4 (6 Ma to 4.75 Ma)—5.86 Ma for MFS and an age between 5.40 Ma and 4.75 Ma for PFBV. The ⁸⁷Sr/⁸⁶Sr ratios therefore fully support, rather than only partially support, the previous mammalian fossil evidence placing both sites within Hh4.
Choice C
Choice C is incorrect because the table indicates that the average ⁸⁷Sr/⁸⁶Sr ratios in fossil shark teeth from MFS (0.709000) and PFBV (0.709028) correspond to different ages—5.86 Ma for MFS and an age between 5.40 Ma and 4.75 Ma for PFBV. Because the ratios point to different ages, they don’t support the conclusion that the fossils at the two sites were deposited contemporaneously, or at the same time.