Final answer:
An index fossil is a widely distributed, short-lived fossil that helps date and correlate rock strata. Belemnites, for example, are index fossils indicating rocks from the Mesozoic era.
Step-by-step explanation:
A index fossil is defined as a fossil from a very short-lived, geographically widespread species that existed during a specific interval of geologic time. These fossils are invaluable to scientists because they can be used to both date and correlate the strata in which they are found. Index fossils are typically organisms that lived in a widely distributed area but for a very short geologic period, making them ideal markers for a specific time.
For instance, Belemnites, a type of cephalopod related to modern squids, serve as index fossils for the Mesozoic era. Since they only existed between 252 and 66 million years ago (Ma), their presence in rock strata directly indicates that the rock is from that era. This becomes crucial when scientists want to understand not only the age of rock layers but also the history of life on Earth and the timing of evolutionary events.