The single-celled organisms include animal-like and plant-like protists . The animal-like protists include foraminifera and radiolaria. Most foraminifera have skeletons of calcium carbonate. Their shells accumulate on the sea floor to form limestone. Radiolarians have shells made of silica, and are common contributors to the formation of chert.

The plant-like protists are diatoms, coccolithophores, and dinoflagellates. The shells of diatoms are made of silica, and they accumulate to make up the rock diatomite. The shells of coccolithophores are called coccoliths. They are made of calcium carbonate, and are the main constituent of chalk. Dinoflagellates are organic-walled, and may be extracted from a variety of sedimentary rock types.

Among the microscopic parts of multicellular animals that you may see are ostracodes and conodonts. Ostracodes are a type of shrimp-like animal or crustacean, belonging to Phylum Arthropoda. Ostracodes have 2 valves or shells which are hinged at the top, resembling a tiny kidney bean. The valves are made of calcium carbonate, and are often covered with complex ornamentation consisting of bumps, pits, and ridges. Conodonts are hard parts from an extinct group of elongated, soft-bodied, fish-like or worm-like chordates, which had a well-defined head, a notochord, and a distinct tail with fins. The name "conodont" name means "cone-tooth", in reference to their shape. Conodonts are made of apatite, a phosphate mineral which is similar in composition to our teeth and bones. They were part of a feeding apparatus. The phylum of the conodonts was not determined until the mid-1990's.

Spores and pollen are microscopic plant reproductive structures that are sometimes preserved in fine-grained sedimentary rocks. They are made of organic material, and may be extracted from a variety of types of sedimentary rocks.

Sponge spicules are microscopic needle-like and multi-rayed skeletal elements secreted by sponge cells. Some groups of sponges secrete spicules of calcium carbonate and other groups secrete spicules of silica, or organic fibers.

Microfossils are useful for determining the age of sedimentary rocks, and for interpreting the environment in which the sediments were deposited. Microfossils are often used for biostratigraphic correlation. This means that by studying the microfossils present in different outcrops, it is possible to correlate the sections. Microfossils have been used for biostratigraphic correlation with great success in the petroleum industry. In drill cores hundreds or thousands of feet long, there are many beds with the same rock type, but each bed has a unique assemblage of microfossil species. Thus, it is possible to correlate several cores using the species of microfossils present in each bed.

The amount of time between the evolution (or first appearance) of a species and the extinction (or disappearance) of a species is called its geologic range. Many species of micro-organisms have short geologic ranges, and were widely distributed in the oceans. The microfossils serve as good index fossils or tools for worldwide time correlation. Other organisms, however, have long geologic ranges, and may have inhabited geographically restricted environments, such as reefs or tidal flats. These organisms are not useful for correlation, but they may serve as tools for interpreting ancient sedimentary environments.

1. Short geologic range.
2. Widespread distribution. Many microscopic organisms are part of the floating plankton in the oceans. These species are readily distributed over large areas by ocean currents.
3. Facies independence (not restricted to one particular rock type; present in many rock types). For example, plankton falls into any sediment that is forming on the sea floor.
4. Distinctive and easily recognized form.
5. Preservable, fossilizable hard parts.
6. Abundance. The fossils should be abundant enough to be collected in sufficient quantity for study. Because they are so tiny, hundreds or thousands of microfossils may be present in small sediment samples.