Questions: What is an important characteristic of an index fossil? A. It must have been very large. B. It must have lived for a short geologic period of time. C. It must have lived in a very specific geographic region. D. It must not be extinct.

The answer is B: It must have lived for a short geologic period of time.

Explanation for each option:

A. It must have been very large.
This is incorrect. The size of the organism is not a determining factor for an index fossil. Index fossils are identified based on their temporal and geographic distribution, not their size.

B. It must have lived for a short geologic period of time.
This is correct. An important characteristic of an index fossil is that it represents a species that existed for a relatively short period of geologic time. This allows geologists to use them to accurately date the rock layers in which they are found.

C. It must have lived in a very specific geographic region.
This is incorrect. While some index fossils may have a limited geographic range, the key characteristic is their short-lived presence in the geologic record, not their geographic specificity.

D. It must not be extinct.
This is incorrect. Many index fossils are of species that are extinct. The extinction of a species does not disqualify it from being an index fossil; in fact, many index fossils are from extinct species.

In summary, the defining feature of an index fossil is its presence during a short, specific time period, which makes option B the correct choice.