Index Fossils

The encyclopedia Britannica defines index fossils as “any animal or plant preserved in the rock record of the Earth that is characteristic of a particular span of geologic time or environment. 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.” 

That particular span of geological time and the rock strata associated with it define a  geologic period in the geologic column. To fully understand index fossils we need to understand the geologic column. The geologic column was developed by Sir Charles Lyell in 1830. The ten strata systems that compose the “standard geologic column” are the familiar Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Tertiary periods.  This dating of index fossils was done over 100 years before radiometric "dating" techniques were developed. 

adapted from https://www.allaboutcreation.org/index-fossils-faq.htm