Question

index fossils have usually formed from organisms which had a

wide geographic distribution and existed for only a short time

wide geographic distribution and existed for a long time

narrow geographic distribution and existed for only a short time

narrow geographic distribution and existed for a long time

Explanation:

Index fossils are used to date the rock layers they are found in. A wide geographic distribution means they can be found in many places, helping to correlate rock layers across different regions. Existing for only a short time ensures that the rock layer containing the index fossil is of a specific, narrow time - period. If an organism had a narrow geographic distribution, it would not be useful for correlating rock layers over large areas. If it existed for a long time, it would not help in precisely dating the rock layer as it could be from a wide range of time periods.

Answer:

wide geographic distribution and existed for only a short time