Final answer:
The fossil used to correlate rock layers and has a wide geographical distribution is known as an index fossil. These are significant for geologic dating and correlation since they are easily recognized and lived for a short period geologically.
Step-by-step explanation:
The kind of fossil that is used to correlate rock layers and must be easily recognized, abundant, and widespread geographically is known as an index fossil. Index fossils are organisms that lived for a short geologic time period, which makes them particularly useful for both dating and correlating the strata in which they are found. When an index fossil is observed in a rock layer, it can assist geologists in determining its relative age and correlating it with other rock layers that have the same index fossil present.
For instance, Belemnites, which only existed during the Mesozoic era, serve as index fossils for that time period. Finding a Belemnite in a rock layer indicates that the rock is from the Mesozoic, which spanned from 252 to 66 million years ago. The presence of index fossils across different geographic regions can also help geologists correlate rock layers that were deposited simultaneously but may consist of different sediment types like sandstone or shale.