Why is Conchidium an important index fossil for Silurian marine rocks?

Conchidium is a genus of extinct brachiopods, commonly known as lamp shells, that serves as a valuable index fossil for marine rocks dating to the Lower and Middle Silurian Period.1

To be considered a useful index fossil, an organism must be easily recognizable, abundant, widely distributed geographically, and possess a short range through time. Conchidium meets these criteria, allowing geologists to use its presence to define specific boundaries within the geologic time scale and to correlate rock strata across different locations. Its utility is enhanced by its physical characteristics, such as its moderately large, strongly convex shell and prominent linear ridges known as costae, which make it distinct and identifiable in the fossil record.21