Trilobites are considered index fossils because they are used to date geological strata via their presence. They specifically are used to date Paleozoic rock. Index fossils must be widespread, abundant, distinct and limited to a particular time period to be useful; trilobite fossils satisfy these requirements.
Trilobites were a widespread group of organisms that only existed during the Paleozoic era. Individual species evolved and went extinct during distinct time periods of this era, allowing for more precise geologic dating. In addition, trilobites were fairly large and hard-bodied, leaving abundant and easily identifiable fossils. These advantages make them one of the most useful index fossils available to scientists.