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Temporal range: Cambrian–Recent
|Live Ammonia tepida (Rotaliida)|
Temporal range: Cambrian–Recent
|Live Ammonia tepida (Rotaliida)|
The Foraminifera ("hole bearers", or forams for short) are a phylum or class of amoeboid protists. They are characterized both by their thin pseudopodia that form an external net for catching food, and they usually have an external shell, or test, made of various materials and constructed in diverse forms. Most forams are aquatic, primarily marine, and the majority of species live on or within the seafloor sediment (benthos) with a small number of species known to be floaters in the water column at various depths (plankton). A few are known from freshwater or brackish conditions and some soil species have been identified through molecular analysis of small subunit ribosomal DNA.
Foraminifera typically produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure. These shells are commonly made of calcium carbonate (CaCO
3) or agglutinated sediment particles. About 275,000 species are recognized, both living and fossil. They are usually less than 1 mm in size, but some are much larger, the largest species reaching up to 20 cm.
The taxonomic position of Foraminifera has varied since their recognition as protozoa (protists) by Schultze in 1854, there referred to as an order, Foraminiferida. Leoblich and Tappan (1992) re-ranked Foraminifera as a class as it is now commonly regarded.
Foraminifera have typically been included in the Protozoa, or in the similar Protoctista or Protist kingdom. There is compelling evidence, based primarily on molecular phylogenetics, for their belonging to a major group within the Protozoa known as the Rhizaria. Prior to the recognition of evolutionary relationships among the members of the Rhizaria, Foraminifera were generally grouped with other Amoeboids as phylum Rhizopodea (or Sarcodina) in the class Granuloreticulosa.
Rhizaria is problematic as it is often called a "supergroup", rather than using an established taxonomic rank such as phylum. Cavalier-Smith defines Rhizaria as an infrakingdom within the Kingdom Protozoa.
Some taxonomies put Foraminifera in a phylum of their own, putting them on par with the amoeboid Sarcodina in which they had been placed.
Although as yet unsupported by morphological correlates, molecular data strongly suggest that Foraminifera are closely related to the Cercozoa and Radiolaria, both of which also include amoeboids with complex shells; these three groups make up the Rhizaria. However, the exact relationships of the forams to the other groups and to one another are still not entirely clear.
Modern forams are primarily marine, although some can survive in brackish conditions. They are most commonly benthic, and about 40 morphospecies are planktonic. This count may however represent only a fraction of actual diversity, since many genetically discrepant species may be morphologically indistinguishable.
A number of forams have unicellular algae as endosymbionts, from diverse lineages such as the green algae, red algae, golden algae, diatoms, and dinoflagellates. Some forams are kleptoplastic, retaining chloroplasts from ingested algae to conduct photosynthesis.
The foraminiferal cell is divided into granular endoplasm and transparent ectoplasm from which a pseudopodial net may emerge through a single opening or through many perforations in the test. Individual pseudopods characteristically have small granules streaming in both directions. The pseudopods are used for locomotion, anchoring, and in capturing food, which consists of small organisms such as diatoms or bacteria.
The foraminiferal life-cycle involves an alternation between haploid and diploid generations, although they are mostly similar in form. The haploid or gamont initially has a single nucleus, and divides to produce numerous gametes, which typically have two flagella. The diploid or schizont is multinucleate, and after meiosis fragments to produce new gamonts. Multiple rounds of asexual reproduction between sexual generations is not uncommon in benthic forms.
Abundance of certain Foraminifera is sometimes used by researchers as an indicator of the completeness of vertical mixing in certain seas such as the Celtic Sea.
The form and composition of the test is the primary means by which forams are identified and classified. Most have calcareous tests, composed of calcium carbonate. In other forams the test may be composed of organic material, made from small pieces of sediment cemented together (agglutinated), and in one genus of silica. Openings in the test, including those that allow cytoplasm to flow between chambers, are called apertures. The test contains an organic matrix, which can sometimes be recovered from fossil samples.
Tests are known as fossils as far back as the Cambrian period, and many marine sediments are composed primarily of them. For instance, the limestone that makes up the pyramids of Egypt is composed almost entirely of nummulitic benthic Foraminifera. Production estimates indicate that reef Foraminifera annually generate approximately 43 million tons of calcium carbonate per year and thus play an essential role in the production of reef carbonates.
Genetic studies have identified the naked amoeba "Reticulomyxa" and the peculiar xenophyophores as foraminiferans without tests. A few other amoeboids produce reticulose pseudopods, and were formerly classified with the forams as the Granuloreticulosa, but this is no longer considered a natural group, and most are now placed among the Cercozoa.
Foraminifera are found in the deepest parts of the ocean such as the Mariana Trench, including the Challenger Deep, the deepest part known. At these depths, below the carbonate compensation depth, the calcium carbonate of the tests is soluble in water due to the extreme pressure. The Foraminifera found in the Challenger Deep thus have no carbonate test, but instead have one of organic material.
Four species found in the Challenger Deep are unknown from any other place in the oceans, one of which is representative of an endemic genus unique to the region. They are Resigella laevis and R. bilocularis, Nodellum aculeata, and Conicotheca nigrans (the unique genus). All have tests that are mainly of transparent organic material which have small (~ 100 nm) plates that appears to be clay 
Dying planktonic Foraminifera continuously rain down on the sea floor in vast numbers, their mineralized tests preserved as fossils in the accumulating sediment. Beginning in the 1960s, and largely under the auspices of the Deep Sea Drilling, Ocean Drilling, and International Ocean Drilling Programmes, as well as for the purposes of oil exploration, advanced deep-sea drilling techniques have been bringing up sediment cores bearing Foraminifera fossils by the millions. The effectively unlimited supply of these fossil tests and the relatively high-precision age-control models available for cores has produced an exceptionally high-quality planktonic Foraminifera fossil record dating back to the mid-Jurassic, and presents an unparalleled record for scientists testing and documenting the evolutionary process. The exceptional quality of the fossil record has allowed an impressively detailed picture of species inter-relationships to be developed on the basis of fossils, in many cases subsequently validated independently through molecular genetic studies on extant specimens. Larger benthic Foraminifera with complex shell structure react in a highly specific manner to the different benthic environments and, therefore, the composition of the assemblages and the distribution patterns of particular species reflect simultaneously bottom types and the light gradient. In the course of Earth history, larger Foraminifera are replaced frequently. In particular, associations of Foraminifera characterizing particular shallow water facies types are dying out and are replaced after a certain time interval by new associations with the same structure of shell morphology, emerging from a new evolutionary process of adaptation. These evolutionary processes make the larger Foraminifera prone to be fossil index for the Permian, Jurassic, Cretaceous and Cenozoic (e.g. Lukas Hottinger).
Because of their diversity, abundance, and complex morphology, fossil foraminiferal assemblages are useful for biostratigraphy, and can accurately give relative dates to rocks. The oil industry relies heavily on microfossils such as forams to find potential oil deposits.
Calcareous fossil Foraminifera are formed from elements found in the ancient seas they lived in. Thus they are very useful in paleoclimatology and paleoceanography. They can be used to reconstruct past climate by examining the stable isotope ratios and trace element content of the shells (tests). Global temperature and ice volume can be revealed by the isotopes of oxygen, and the history of the carbon cycle and oceanic productivity by examining the stable isotope ratios of carbon; see δ18O and δ13C. The concentration of trace elements, like magnesium (Mg), lithium (Li) and boron (B), also hold a wealth of information about global temperature cycles, continental weathering and the role ocean in the global carbon cycle. Geographic patterns seen in the fossil records of planktonic forams are also used to reconstruct ancient ocean currents. Because certain types of Foraminifera are found only in certain environments, they can be used to figure out the kind of environment under which ancient marine sediments were deposited.
For the same reasons they make useful biostratigraphic markers, living foraminiferal assemblages have been used as bioindicators in coastal environments, including indicators of coral reef health. Because calcium carbonate is susceptible to dissolution in acidic conditions, Foraminifera may be particularly affected by changing climate and ocean acidification.
Foraminifera have many uses in petroleum exploration and are used routinely to interpret the ages and paleoenvironments of sedimentary strata in oil wells. Agglutinated fossil Foraminifera buried deeply in sedimentary basins can be used to estimate thermal maturity, which is a key factor for petroleum generation. The Foraminiferal Colouration Index  (FCI) is used to quantify colour changes and estimate burial temperature. FCI data is particularly useful in the early stages of petroleum generation (~100°C).
Foraminifera can also be utilised in archaeology in the provenancing of some stone raw material types. Some stone types, such as limestone, are commonly found to contain fossilised Foraminifera. The types and concentrations of these fossils within a sample of stone can be used to match that sample to a source known to contain the same "fossil signature".
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