Fossils serve as time markers. The life-time of a species typically lasts from one hundred thousand to millions of years. A species, once extinct, can never arise again. In the last 700 millions years, and especially in the last 550 million years, various fossil groups have been identified as especially useful index fossils, that replace each other through the geologic record. Fossils are also good indicators of the environmental conditions in which the rock formed, especially the place and the position where the organism was fossilized.
Fossils allow to reconstruct past water depth, temperature, salinity, and many other factors.
Naturally these inferred environmental conditions are more reliable when there are still living descendants in the fossil groups available for observation and study. These inferences are less reliable if a group is completely extinct. Finally, the silicate, carbonaceous and carbonate fossils can compose a major part of a rock, or at least a significant component of the rock. Algae and corals are two of the most efficient rock-builders, although many other fossil groups can also be considered important components. Sedimentary rocks can be divided into different groups.