Evolutionary theory assumes that a particular creature evolved from its ancestors, lived successfully for a period, then became extinct as its descendants evolved better ways of surviving.
In other words, that creature had a defined ‘evolutionary life-span’.
Other major index fossils are small or microscopic, part of the floating plankton in the world ocean. They can be found even in small bits of rock, such as wellbore cuttings.
Because their tiny bodies rained down all over the ocean, they can be found in all kinds of rocks.
Thus, marine index fossils older than ~200 million years are normally found in sedimentary strata on land, in areas that were once covered by seas.