When mineral grains from shale are dated by isotopic analysis to determine the age of a fossil, the lab date is typically:

• A. Much younger than the fossil
• B. Roughly the same age as the fossil
• C. Much older than the fossil
• D. Unrelated to the fossil's age

Answer: (C)

Isotopic dating of mineral grains in shale gives the age of the minerals themselves, not the fossil. In sedimentary rocks like shale, many grains are detrital, meaning they were weathered from older rocks and only later became part of the shale. Those minerals crystallized long before the sediment was deposited, so their radiometric ages record an older event—and the fossil inside the same rock formed later during or after deposition. That’s why the lab date is typically much older than the fossil. If you can date volcanic ash layers within or above the sediments, you can bracket the fossil’s age more precisely, but dating detrital grains almost always yields an age older than the fossil.