What characterizes index fossils?

Index fossils are characterized by their ability to be widespread in their distribution while existing for a relatively brief geological timeframe. This unique combination makes them highly valuable for dating and correlating the age of rock layers. Because they were present in many different locations, their presence in a geological formation can indicate that the rock is of a similar age to other formations where the same fossils are found. This characteristic allows geologists to create a more accurate timeline of Earth's history and correlate strata from different locations.

The brief existence of index fossils ensures that their presence in a layer strongly suggests a specific time period, as they can quickly evolve and become extinct. Therefore, the brief and widespread nature of these fossils is precisely what allows them to serve as effective indicators for geologic dating and correlation.