Which statement about index fossils is true?

• A. They are fossils used to determine the color of rocks.
• B. They are fossils created by meteor impacts.
• C. They are fossils used to define and identify geologic periods.
• D. They are fossils found only in igneous rocks.

Answer: (C)

Index fossils are fossils that serve as time markers for Earth's history. They come from organisms that lived for a relatively short period, were widespread across large areas, and are easy to recognize. Because of these traits, they let scientists define the boundaries of geologic periods and identify the ages of rock layers in different places. For instance, finding a distinctive ammonite in a rock layer can indicate a specific slice of time, and the same fossil across different regions helps correlate those layers. This is why the statement that index fossils are used to define and identify geologic periods is true. They aren’t used to determine rock color, they aren’t fossils formed by meteor impacts, and they’re typically found in sedimentary rocks, not only in igneous rocks.