foraminiferan (fərămˌənĭfˈərən) [key], common name for members of the class Foraminifera, large, shelled ameboid protozoans belonging to the phylum Sarcodina. Most foraminiferan shells are calcareous, but some are siliceous, and others are built of sand grains. Initially, the shell contains a single chamber, and new chambers are added in a characteristic linear, spiral, or concentric series as the organism grows. Some shells reach several inches in diameter, but most species are less than a millimeter in diameter. Long, branching extensions of the cell (pseudopodia) reach from openings in the shell and fuse together to form a net in which plankton organisms are trapped. The net may cover an area 10 times the diameter of the shell, and crustaceans of 1 in. (2.5 cm) or more long may be caught by these much smaller protozoans.

A few foraminiferans live in freshwater or brackish water, but the majority are marine. They are found in all seas at all depths and are extremely abundant. Foraminiferans may be red, brown, or white in color. About 30 pelagic species live in the open sea, the most important belonging to the genus Globigerina.

Foraminiferans live near the water surface when young, but gravitate downward with age. When the animals die, the shells drop to the bottom, forming "globigerina ooze." Such ooze constitutes about half the sediments found on the roughly 50 million sq mi (130 million sq km) of ocean bottom that is covered with sediment in warm and tropical seas. Similar deposits in the past have contributed heavily to the formation of sedimentary rock, and the study of fossil foraminiferans has been extremely important in recognizing geological strata and for dating deposits. Layers of limestone or chalk, such as are found in Dover, England, and in Alabama and Mississippi, solidified from similar deposits of ooze in ancient seas. Foraminiferan fossils have been particularly useful in locating domes where petroleum deposits occur. Limestone used in some Egyptian pyramids contains skeletons of foraminiferans, especially of nummulites, which have coin-shaped skeletons.

Foraminiferans are classified in the phylum Sarcodina, class Granuloreticulosa.

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