What characterizes an index fossil?

An index fossil is characterized by being widespread and existing for a relatively short geological time frame. This combination makes index fossils particularly valuable for dating and correlating the age of rock layers in different geographic locations. Because they were only present for a limited duration, finding an index fossil in a rock layer indicates that the layer can be dated to a specific time period, helping geologists and paleontologists establish a timeline of Earth’s history.

In contrast, a fossil that exists over a long period or is limited to a specific geographic location does not provide the same clarity for determining the age of rock layers, as it could be present in many different geological eras or confined regions. The distinctive nature of index fossils, therefore, lies in their brief temporal existence coupled with a broad geographical distribution.