Micropaleontology is the study of fossils that can be identified only by using a microscope. Most microfossils are the remains of one-celled animals or plants, such as foraminifera, radiolarian, and diatoms. Micropaleontology also includes the study of microscopically small multicellular organisms, or body parts or life stages of larger multicellular plants or animals--especially ones that are valuable indicators of the age of rock strata or of the environmental conditions under which these strata formed.
The study of micropaleontology expanded greatly when foraminifera and other microfossils began to be used to identify strata that might yield petroleum. New inventions, such as the scanning electron microscope, now permit scientists to study fossils as small as 0.005 mm (0.0002 in) in diameter; the occurrence of such fossils provides geologists with information useful in mapping the changing positions of continents and oceans.
Another exciting new realm of micropaleontology is the search for the earliest forms of life on Earth. Since 1954, many spherical and filament-shaped structures, which almost certainly represent remains of primitive algae and bacteria, have been discovered in Precambrian rocks.
Jones, Daniel J., Introduction to Microfossils (1969).