Organisms that lived for relatively short time periods are particularly useful for dating rocks, especially if they were distributed over a wide geographic area and so can be used to compare rocks from different regions. These are known as index fossils.

What do you call a fossil useful for dating?

Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation. Index fossils are fossils that are known to only occur within a very specific age range.

Which type of fossil is most useful for dating geologic strata?

A useful index fossil must be distinctive or easily recognizable, abundant, and have a wide geographic distribution and a short range through time. Index fossils are the basis for defining boundaries in the geologic time scale and for the correlation of strata.