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An order of Granuloreticulosia in the class Rhizopodea. Foraminiferans are dominantly marine protozoans, with a secreted or agglutinated shell, or test, enclosing the continually changing ameboid body (see illustration) that characterizes this and other orders of the superclass Sarcodina. Their unique combination of long geologic history, ubiquitous geographic distribution, and exceptional diversity of test composition, form, and structure make the foraminiferans the most useful of all marine fossils for stratigraphic correlation, geologic age dating of sediments, and paleoecologic interpretation. Their tests accumulated in great numbers and are recoverable from small quantities of sediment, rock outcroppings, well cores or cuttings, or ocean dredging and submarine coring.
As is true of most protists with skeletons or tests, systematic differentiation and classification of foraminiferans is based on test composition, microstructure, and gross morphology. Information currently available concerning cytoplasmic characters, life cycles, and so on has shown good agreement with this classification, although the function and origin of many shell characters believed to be of systematic importance (canal systems, pores, septal doubling, and apertural tooth plates) are yet undetermined. There are 11 suborders.
Most foraminiferans are benthic, living upon the sea floor, within the upper few centimeters of ooze, or upon benthic algae or other organisms. They occur from the intertidal zone to oceanic depths, in brackish, normal marine, or hypersaline waters, and from the tropics to the poles. Some modern Lagynacea live in fresh water, but none are known as fossils. Assemblages vary widely in response to local conditions, with the greatest diversity occurring in warm, shallow water. A smaller number, the Globigerinina, are planktonic, living at various depths in the water column from the surface to the bottom, being most numerous between 18 and 90 ft (6 and 30 m). Vertical migration may be diurnal and may occur during ontogenetic development. The preferred depth range of a species may vary geographically in response to temperature differences or to changes in water density.