Foraminifers

Foraminifers

The foraminifers or foraminifera (forams for short) are a group of ameoboid single-celled pro- tists belonging to the class Foraminifera, phylum Granuloreticulosa, and kingdom Cercozoa. Typically they are microscopic in size and studied by the paleontological branch called micropaleon­tology. They are also one of the main micropale- ontological groups because they can be used to give accurate relative dates to rocks, among other utilities.

Characteristics

Modern foraminifers are primarily marine, although they can survive in brackish conditions, and a few species survive in fresh waters. They have two modes of life, planktic (marine floaters) or benthic (seafloor dwellers). About 275,000 living and fossil species have been recognized. The foraminifers are usually less than 1 millimeter in size, but some are much larger. The largest recorded specimens reached 19 centimeters. They are closely related to the Radiolaria, which also includes amoeboids with complex shells.

The foraminifers typically produce a shell, or test, some becoming quite elaborate in structure. The test can have either one or multiple chambers, with the multichambered forms being more fre­quent. Adjacent chambers are separated by septa. Most have calcareous tests, composed of calcium carbonate (calcite, aragonite), but others may have organic, agglutinated, or silica tests. The organic tests are composed of protinaceous mucopolysac­charide. The term agglutinated refers to tests formed from foreign particles (coccoliths, diatoms frustules, microfragments of mollusk and echino­derm, sponge spicules, quartz, mica, magnetite, or garnet), glued together with a variety of cements (organic, calcareous, or siliceous). The calcareous and siliceous tests are secreted. Calcareous tests may be again subdivided into three major types: microgranular (composed of equidimensional sub- spherical calcite crystals smaller than 5 microns), porcellaneous (composed of thin inner and outer veneers enclosing a thick middle layer of crystal laths), and hyaline (composed of lamellas of calcite crystals larger than 5 microns, which may be equi- dimensional or radiate).

The name foraminifera is derived from the fora­men, the connecting hole trough the septa between each chamber. The living foraminifers have reticulat­ing pseudopods (reticulopodes) used for locomotion, for anchoring, and in capturing food. The opening by which the pseudopods access the outside is called the aperture. A bidirectional cytoplasmic flow along these reticulopodes carries granules, which may consist of mitochondria, digestive vacuoles, vacuoles containing waste product, and even symbiotic dinoflagellates. For this reason, the pseudopods of foraminifera are also called grano- reticulopodes. Foraminifers normally feed on dia­toms, bacteria, and copepods. Many species have unicellular algae as endosymbionts, such as dino­flagellates, green algae, red algae, golden algae, and diatoms. Some foraminifers are kleptoplastic, retaining chloroplasts from ingested algae to con­duct photosynthesis.

Uses in Science and Technology

The reason why foraminifers are so useful in micropaleontology is that their tests are easily fos­silizable and very abundant in the fossil record. Because of their diversity, abundance, and com­plex morphology, fossil foraminiferal assemblages are useful not only in biostratigraphy and geo­chronology, but also in paleoenvironmental recon­struction, paleoclimatology, and paleoceanography. The oil industry relies heavily on foraminifers in finding potential oil deposits.

Classification

The traditional classification of foraminifera is based on the composition and morphology of their tests. The primary characters used in these classifications are mainly the following: wall composition and structure, chamber arrange­ment, the shape and position of the apertures, and surface ornamentation. At species level, other morphologic features are also used, such as the test shape, chamber shape, number of cham­bers, and so on. The commonly accepted classi­fication of the foraminifera is based on that of the 1987 classification by Loeblich and Tappan, although molecular data and cladistic analysis suggest that this classification may vary substan­tially in the future. Foraminifers were initially considered to be an order in the class Rhizopoda, phylum Protozoa, of which 12 suborders were distinguished. Today, they are considered as class Foraminiferea, of which 14 orders are dis­tinguished: Allogromiida, Astrorhizida, Lituolida, Trochamminida, Textulariida, Fusulinida, Spirillinida, Carterinida, Miliolida, Lagenida, Robertinida, Globigerinida, Buliminida, and Rotaliida.

The foraminifera have a geochronological range from the earliest Cambrian to the present day. Order Allogromiida (before suborder Allogromiina) includes foraminifers with organic tests, such as Allogromia. They are difficult to fossilize and do not have much paleontological interest, but include the ancestor from which evolved all the other groups of foraminifera. They are known from the Cambrian, although probably appeared in the Precambrian. Foraminifers with hard tests were scarce during the Cambrian, Ordovician, and Silurian.

Agglutinated forms were initially included in the suborder Textulariina, but they are now classified in several orders: Astrorhizida, Lituolida, Trochamminida, and Textulariida. They may be composed of randomly accumulated grains or grains selected on the basis of specific gravity, shape, or size. All species are benthic in mode of life. The textularinids first appeared in the Cambrian, 525 million years ago, evolving from organic allo- gromiinids. Earliest agglutinated species were single chambered; the multichambered varieties evolved 20 million years later. They flourished in the Cretaceous, culminating in the large conic or concave-convex tests of the orbitolinids, such as Orbitolina, Palorbitolina, or Dictyoconus. They are significant biostratigraphic markers of the Creta­ceous in prereef sublitoral paleoenvironments.

The order Fusulinida (before suborder Fusulinina) includes microgranular calcitic forms. All species were benthic. They first appeared in the early Silurian, more than 430 million years ago (mya), probably evolving from some ancestral textularinids. The fusulinids began to be very abundant in the Devonian, culminating in the complex test forms of the Late Carboniferous and Permian times, such as Fusulina, Schwagerina, or Triticites. They are very important biostratigraphic markers of the Carboniferous and Permian in sub­litoral paleoenvironments. They were extinguished in the Permian-Triassic boundary event.

The porcellaneous foraminifers are included in the order Miliolida (before suborder Miliolina). Their first appearance occurred in the latest Devonian or Early Carboniferous, 360 mya. They probably evolved from microgranular fusilinids, modifying the wall microstructure. Three major groups may be recognized: milioloids, soritoids, and alveolinoids, all of them benthic dwellers of sublitoral paleoenvi­ronments. The first ones have the known milioline coiling (spiral coiling), rotating in such a manner that between the median planes of consecutive chambers various angles are produced, for example, 72° (Quinqueloculina), 120° (Triloculina), or 180° (Pyrgo). The soritoids, such as Orbitolites and Sorites, have cyclical arrangements (arrangement of cyclical chamberlets in one plane or in concentric layers). Finally, the alveolinoids include milioline or planispiral forms, such as Lacazina and Alveolina. The miliolids flourished in the Late Cretaceous and Eocene, periods for which they are important bio- stratigraphic markers. Although both groups were considered unrelated in the past, Miliolida today includes the suborder Silicoloculinina, foraminifers with siliceous tests and of scarce paleontological interest.

The most diversified group of foraminifers is the old suborder Rotaliina, today subdivided in two orders: Buliminida and Rotaliida. The rotalinids appeared in the Triassic, more than 249 mya, and diversified in the Jurassic. They are benthic, although some exhibit planktonic stages, and display hyaline tests. The morphology of rotalinids tests varies enormously. Some are small foraminifers (micro- foraminifera), with serial arrangement (chambers arranged in one [uniserial], two [biserial], or three [triserial] rows), such as Bolivina, Turrilina, or Bulumina; or with spiral coiling, such as Discorbis, Nonion, or Cibicides. Others are much larger (mac- roforaminifers), with orbitoid arrangement (annu­lar series of equatorial chamberlets covered on both lateral surfaces by lateral chamberlets), such as Orbitoides, Discocyclina, and Lepidocyclina; or with planispiral coiling, such as Nummulites, Assilina, Operculina, and Heterostegina. The rotali- inid macroforaminifers had two radiations in the late Cretaceous and Eocene-Oligocene, periods in which they are very important biostratigraphic markers together with alveolinids.

The earliest foraminifers are all benthic, and the planktonic forms do not appear until the Middle Jurassic. These forms evolved from hyaline rotalinids that display a meroplanktonic mode of life (i.e., ben­thic with planktic stages). All planktic foraminifera are included in order Globigerinida (before suborder Globigerinina). The greenhouse conditions of the Cretaceous caused a major diversification of the glo- bigerinids, appearing in large and complex forms such as Globotruncana, Contusotruncana, or Racemiguembelina. The Cretaceous/Tertiary mass extinction affected more than 90% of the latest Cretaceous planktic foraminifers; surviving scarce species consist mainly of Guembelitria. A rapid radiation of planktic foraminifers occurred during the Paleocene, with the appearance of planktic globi- gerinid and globorotalids. The best Cenozoic plank- tic foraminifera are Globigerina, Orbulina, and Globorotalia, but these genera appeared in the Late Paleogene and the Neogene. The planktic foramin- ifera are the most interesting biostratigraphic mark­ers of the Cretaceous and Cenozoic in the pelagic environment.

Other foraminifer groups are less abundant and interesting, but worth mentioning. All of them are benthic and live to the present day. The order Lagenida included planispiral to uniserial foramini- fers with radiate hyaline tests, such as Nodosaria, Dentalina, or Lenticulina. They appeared in the Late Silurian, evolving from extinct fusulinids or ancestral miliolids, and diversified in the Late Triassic and Jurassic. The order Spirillinida has a test constructed of an optically single crystal of calcite, appeared in the Late Triassic (evolving from extinct fusulinids), and include the old suborders Spirillinina and Involutinina (these latter have a two-chambered test composed of aragonite). The order Robertinida also has a test composed of aragonite and evolved in the Middle Triassic, probably from agglutinated textu- larinids; they are the probable ancestor of benthic Rotaliina and planktic Globigerinina. The order Carterinida includes foraminifers that secrete spic­ules of calcite to form the test; they probably appeared in the Eocene, evolving from texturalinids.

Ignacio Arenillas

See also Chronostratigraphy; Evolution, Organic; Extinction and Evolution; Fossil Record; Fossils, Interpretations of; Geology; Paleontology

Further Readings

Loeblich, A. R., & Tappan, H. (1987). Foraminiferal general and their classification. New York: Van Nostrand Reinhold.

Four Fundamental Forces Gustave Flaubert
Comments are closed.