Foraminifera (forams for short) are single-celled organisms (protists) with shells or tests (a technical term for internal shells). They are abundant as fossils for the last 540 million years. The shells are commonly divided into chambers that are added during growth, though the simplest forms are open tubes or hollow spheres. Depending on the species, the shell may be made of organic compounds, sand grains or other particles cemented together, or crystalline CaCO3 (calcite or aragonite).

Fully grown individuals range in size from about 100 micrometers to almost 20 centimeters long. Some have a symbiotic relationship with algae, which they "farm" inside their shells. Other species eat foods ranging from dissolved organic molecules, bacteria, diatoms and other single-celled algae, to small animals such as copepods. They catch their food with a network of thin pseudopodia (called reticulopodia) that extend from one or more apertures in the shell. Benthic (bottom-dwelling) foraminifera also use their pseudopodia for locomotion.

There are an estimated 4,000 species living in the world's oceans today. Of these, 40 species are planktonic, that is they float in the water. The remainder live on or in the sand, mud, rocks and plants at the bottom of the ocean. Foraminifera are found in all marine environments, from the intertidal to the deepest ocean trenches, and from the tropics to the poles, but species of foraminifera can be very particular about the environmentin which they live. Some are abundant only in the deep ocean, others are found only on coral reefs, and still other species live only in brackish estuaries or intertidal salt marshes.

Foraminifera are among the most abundant shelled organisms in many marine environments. A cubic centimeter of sediment may hold hundreds of living individuals, and many more dead shells. In some environments their shells are an important component of the sediment. For example, the pink sands of some Bermuda beaches get much of their color from the pink to red-colored shells of a foraminiferan. In regions of the deep ocean far from land the bottom is often made up almost entirely of the shells of planktonic species.

The study of fossil foraminifera has many applications beyond expanding our knowledge of the diversity of life. Fossil foraminifera are useful in biostratigraphy, paleoecology, paleobiogeography, and oil exploration.
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Foraminifera provide evidence of the relative ages of marine rocks
There are several resons that fossil foraminifera are especially valuable for determining the relative ages of marine rock layers. They have been around since the Cambrian, over 500 million years ago. They show fairly continuous evolutionary development, so different species are found at different times. Forams are abundant and widespread, being found in all marine environments. Finally, they are small and easy to collect, even from deep oil wells.


Foraminifera provide evidence about past environments
Because different species of foraminifera are found in different environments, paleontologists can use the fossils to determine environments in the past. Foraminifera have been used to map past distributions of the tropics, locate ancient shorelines, and track global ocean temperature changes during the ice ages. If a sample of fossil foraminifera contains many extant species, the present-day distribution of those species can be used to infer the environment at that site when the fossils were alive. If samples contain all or mostly extinct species, there are still numerous clues that can be used to infer past environments. These include species diversity, the relative numbers of planktonic and benthic species, the ratios of different shell types, and shell chemistry.

The chemistry of the shell is useful because it reflects the chemistry of the water in which it grew. For example, the ratio of stable oxygen isotopes depends on the water temperature, because warmer water tends to evaporate off more of the lighter isotopes. Measurement of stable oxygen isotopes in planktonic and benthic foram shells from hundreds of deep-sea cores worldwide have been used to map past surface and bottom water temperatures. This data helps us understand how climate and ocean currents have changed in the past and may change in the future.


Foraminifera are used to find petroleum
Some species are geologically short-lived and some forms are only found in specific environments. Therefore, a paleontologist can examine the specimens in a small rock sample like those recovered during the drilling of oil wells and determine the geologic age and environment when the rock formed. As a result, since the 1920's the oil industry has been an important employer of paleontologists who specialize in these microscopic fossils. Stratigraphic control using foraminifera is so precise that these fossils are even used to direct sideways drilling within an oil-bearing horizon to increase well productivity.


Very little is known about how most species of foraminifera live. The few species that have been studied show a wide range of behaviors, diet, and life cycles. Individuals of some species live only a few weeks, while other species live many years. Some benthic species burrow actively, though slowly, through sediment at speeds up to 1cm per hour, while others attach themselves to the surface of rocks or marine plants. Foraminifera are abundant enough to be an important part of the marine food chain, and their predators include marine snails, sand dollars and small fish.
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Traditionally, classification of foraminifera has been based primarily on characters of the shell or test. Wall composition and structure, chamber shape and arrangement, the shape and position of any apertures, surface ornamentation, and other morphologic features of the shell are all used to define taxonomic groups of foraminifera. New research is adding molecular data on relationships among species that may greatly affect how these organisms are classified.

Chamber arrangements commonly found in living species are shown in figures 1-6. The following terms are used: Unilocular refers to a shell made of a single chamber Uniserial refers to chambers added in a single linear series Biserial refers to chambers added in a double linear series Triserial refers to chambers added in a triple linear series Planispiral refers to chambers added in a coil within a single plane like the chambered nautilus Trochospiral refers to chambers added in a coil that forms a spire like a snail shell Milioline refers to an arrangement where each chamber stretches the full length of the shell and each successive chamber is placed at an angle of up to 180 degrees from the previous, relative to the central axis of the shell Arborescent refers to an erect, branching series of tubes. Terms such as planispiral-to-biserial and biserial-to-uniserial are used when the mode of chamber addition changes during growth.

Of the various kinds of wall composition and microstructure found in foraminifera, three basic types are common among living species. Agglutinated shells may be composed of very small particles cemented together and have a very smooth surface, or may be made of larger particles and have a rough surface. Hyaline shells are made of interlocking microcrystals of CaCO3, and typically have a glassy appearance and pores that penetrate the wall. Porcelaneous shell walls are composed of microscopic rod-shaped crystals of CaCO3. These have a milky, translucent to opaque look and generally lack pores beyond the initial chambers. In some porcelaneous species, small depressions in the surface ornamentation give the appearance of pores. Another type of wall structure, called microgranular, is made of tightly packed equidimensional rounded grains of calcite. This wall type is found in many Paleozoic foraminifera including the fusulinids.

Figures 1-6. These images were captured using the Environmental Scanning Electron Microscope at the UC Museum of Paleontology, Berkeley, CA.
Figure 1
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Figure 6