Final answer:
An index fossil is used to correlate and recognize a particular stratum by providing a way to date and align the sedimentary layers it is found in. It is an organism that lived for a short geological time, making it useful for pinpointing specific time periods.
Step-by-step explanation:
A fossil that may be used to correlate and recognize one particular stratum is known as an index fossil. These fossils are incredibly useful for both dating and correlating the strata in which they are found because they come from organisms that lived for a short geologic time period and are therefore indicative of a narrow span of geologic time.
Fossil correlation uses the presence of fossils in sedimentary rocks to determine how outcrops of rock layers relate to one another in age. This correlation process often involves examining a fossil assemblage, which is simply the group of fossils discovered within a rock layer.
Notably, the use of index fossils for correlation is a form of biostratigraphy, which is the branch of stratigraphy focused on using fossils to determine relative ages and correlate successions of sedimentary rocks. Index fossils help indicate the relative age of the rocks because these organisms are easily recognizable, were widely distributed geographically, but existed for a comparably short period of geologic time.