foraminiferan


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foraminiferan

(fərăm'ənĭf`ərən), common name for members of the class Foraminifera, large, shelled ameboid protozoansprotozoan
, informal term for the unicellular heterotrophs of the kingdom Protista. Protozoans comprise a large, diverse assortment of microscopic or near-microscopic organisms that live as single cells or in simple colonies and that show no differentiation into tissues.
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 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 SarcodinaSarcodina,
the largest phylum (11,500 living species and 33,000 fossil species) of protozoans). It comprises the amebas and related organisms; which are all solitary cells that move and capture food by means of pseudopods, flowing temporary extensions of the cell.
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, class Granuloreticulosa.

References in periodicals archive ?
Our observations demonstrate that coccoliths, and probably also planktonic foraminiferan tests, reach the Challenger Deep intact," said Gooday.
The life forms ranged from coral fragments, to mollusk shells, sea urchin spines, sponge spicules, gastropod shells, and foraminiferan tests.
Benthic foraminiferan species were dominant in 1600 but pelagic species (Cooper & Brush 1993) and relatively few benthic species tolerant of anaerobic conditions (Karlsen et al.
However, benthic foraminiferan ages in the Cenozoic rocks of the Pacific coast of North America have been shown to be unreliable because they are time transgressive, relative to ages derived from planktonic microfossils (Prothero 2001: 389).
By counting the abundance of different types of foraminiferan shells in sediments deposited throughout the northern Atlantic during the Last Ice Age, other researchers have shown that polar Foraminifera had greatly extended their range to the south; the Gulf Stream must have then flowed more-or-less straight across the Atlantic toward Portugal, rather than northward, toward Norway.