The correct answer is index fossils. A fossil index is a paleontological or archaeological remain whose presence can be used to date with some precision the stratigraphic unit in which it is found because they are particular or exclusive of a certain period of geological history, or of Prehistory, or indicators of a certain paleoenvironment.
In general, the taxa or cultural modes of the past that gave rise to the guide fossils had a very short duration on a geological -or historical- scale, which means that their presence is limited to chronological -or cultural- coordinates, in the case of prehistoric cultures-, very limited. That is, if a guide fossil appears in a stratum, that stratum can be dated with enough accuracy.