foraminifera and other unicellular microfossils The biological kingdom Protista contains all those relatively simple organisms which consist of a single cell containing a nucleus and other internal structures. Their dimensions range from a micron to several centimetres. Most protists occur as single cells but some occur as loose aggregates of several cells. Many forms are capable of photosynthesis and hence resemble plants in the way that they obtain energy. Others ingest other living organisms from their surroundings and thus are animal-like. A third group is capable of obtaining energy by both methods. This diverse group of organisms is subdivided into a number of groups but, since most lack any form of skeleton, only a few have a fossil record and, of these, only the coccolithophorids, dinoflagellates, diatoms, foraminifera, and radiolarians are important.
Coccolithophorids (Coccolithophorida) are an important group of planktonic marine protists which are capable of photosynthesis. They are spherical or oval and generally less than 20 micrometres (μm) in diameter. The living cell possesses a gelatinous sheath in which are embedded calcareous plates termed coccoliths. As an individual cell grows older, the gelatinous sheath becomes calcified and the plates become rigidly fused. The older coccoliths are gradually shed from the sheath and sink to the ocean floor. Hence, fossil coccolithophorids are generally seen as isolated plates, although entire skeletons are sometimes found. The plates are minute (2 to 30 μm) and variously shaped (Fig. 1). The coccolithophorids range in age from Triassic to Recent, and form a major constituent of Mesozoic and Tertiary chalks. Their extremely small size, great abundance in many deep-water deposits, and widespread geographic distribution make them valuable in biostratigraphic correlation. They can also be used to infer oceanic palaeotemperatures and salinities.
Diatoms (Bacillariophyceae) are a group of photosynthesizing protists characterized by a siliceous cell wall consisting of two elements (valves) with one valve overlapping the other like the lid on a box (Fig. 1). The cell size varies from 5 to 2000 μm. The members of one group of mainly marine diatoms are circular with a radial symmetry (centric diatoms). Those of a second group, mostly confined to freshwater environments, are elliptical with bilateral symmetry (pennate diatoms). Cells usually occur singly but some forms are colonial or filamentous. The cell wall is usually delicately ornamented and perforated by minute holes. The oldest known diatoms are found in the Jurassic. From the Cretaceous onwards, diatom ooze containing more than 30 per cent diatom skeletons has been deposited in high-latitude areas of the deep ocean. Diatoms are valuable in biostratigraphic correlation and, because many fossil diatoms have living relatives, they are also valuable in palaeoenvironmental work.
Dinoflagellates are globular protists with two flagella (thread-like structures) of unequal length. Their size range is from about 5 to 2000 μm. Dinoflagellates exist in two forms. In the motile stage, the organism propels itself through the water by active beating of the flagella. In this form, the organism may either have a flexible cell wall or it may have a rigid armour of cellulose plates. Some forms bear simple processes. Whenever conditions are unsuitable, the organism becomes dormant and enters the cyst stage. Only the cysts are preserved in the fossil record. They comprise a cellulose skeleton with a variety of spines or processes (Fig. 1). Dinoflagellates first appear in the Cambrian. Modern forms occur not only in marine environments but also in quasi-marine situations, such as estuaries, and in some large lakes. The dinoflagellates are useful in both biostratigraphy and palaeoenvironmental studies.
Radiolarians (radiolaria) are a group of marine protists characterized by a perforated central chitinous or membranous capsule which contains the bulk of the protoplasm and the nucleus. They range in size from 100 to 2000 μm. The central capsule is rarely found fossilized but they possess a delicate skeleton of opaline silica or strontium sulphate which may be preserved. This skeleton is a lattice of variable morphology made up of needles (spicules), bars, and spines (Fig. 1).
Radiolaria are relatively uncommon as fossils, but occur sporadically from the middle Cambrian onwards. All modern forms are marine, most being free-floating forms occurring either at the surface or close to the sea bed. Radiolarians are used in biostratigraphic correlation of oceanic sediments and are particularly useful in cases where calcareous microfossils have been subject to dissolution. Fossil Radiolaria occur in a wide range of sediments, but of particular interest is radiolarian ooze and its presumed fossil equivalent, radiolarian chert. Radiolarian ooze is a relatively rare deep ocean sediment which consists of more than 30 per cent radio-larian skeletons. It is formed in equatorial deep-sea areas where the water is rich in dissolved carbon dioxide and is readily able to dissolve any calcium carbonate present. This occurs at water depths of about 4500 m (the carbonate compensation depth) and more. A few fossil examples of deposits of this type, termed radiolarian cherts, are known.
Foraminifera (technically Foraminiferida, but variously termed foraminiferids, foraminiferans, or forams) are the most important protists from a palaeontological point of view. They are amoeboid organisms whose cell is protected by a test (or protective shell) comprising one to many chambers. The test may have one or more large openings or apertures. The protoplasm is extruded as pseudopods via the aperture and through any perforations that may be present in the test. The pseudopods form a complex network which is used to capture food (bacteria, other protists). On the basis of test structure and composition, five subdivisions of the group can be recognized. The most primitive forms, the Allogromiina, secrete a thin non-rigid test composed of a chitin-like, organic material termed tectin. Similar material occurs as a thin lining within the chambers of most other foraminifera. Some Allogromiina tests may be covered in loosely attached grains of sediment. The second foraminiferan group, the Textulariina, includes all those forms whose test is composed of grains of organic and mineral matter (e.g. sand grains, diatoms, other foraminifera, sponge spicules) bound together by an organic, calcareous, or ferric oxide cement. These are called agglutinated tests. While some species are very selective in terms of the size, texture, or composition of the grains they use, others are fairly catholic.
Foraminifera with calcareous tests are by far the most abundant forms and fall into three suborders, each possessing a different wall structure. Fusulinina have tests which consist of minute crystals of calcite. These crystals are all approximately the same size and are closely packed together. In thin sections under the microscope it can be seen that the calcite crystals are arranged either randomly, producing a granular appearance, or aligned at right angles to the surface of the test, giving the wall a fibrous appearance. In most fusulinine foraminifera, granular and pseudo-fibrous layers of microcrystalline calcite are combined to produce a multi-layered test wall.
The Miliolina have tests consisting of tiny needles of high-magnesium calcite. The calcite crystals are randomly oriented except adjacent to the outer and inner surface of the test, where a layer constructed of horizontally or vertically arranged needles occurs. This arrangement gives milioline tests a porcellaneous appearance. They are distinctively milky white when viewed in reflected light and amber in transmitted light.
The final group, the Rotaliina, possess tests which appear hyaline (glassy) when viewed in reflected light and grey to clear in transmitted light. However, the clarity is not always apparent, being obscured by a thick test wall, by a large number of fine perforations, or by the surface of the test being ornamented by spines. In thin section, the calcite crystals in rotaliine walls have a variety of arrangements including monocrystalline, granular, and radial crystalline.
The walls of many foraminifera are perforated by narrow straight or branched pores. In life, these pores allow the transfer of food particles from the external to the internal cytoplasm. Radial pores are characteristic of the Rotaliina but they also occur in some of the more complex Textulariina and Fusulinina. Where they are present, these pores give the test wall a pseudo-radial or pseudo-fibrous appearance in thin section.
Forms range from a single, non-chambered flask- or sac-shape to very complex, chambered discs (Fig. 2). In multi-chambered forms, the chambers may be arranged in a great variety of modes including linear, planispiral, and trochospiral patterns. The chambers are connected together via pores or foramina in the dividing walls. It is from this characteristic that the group name is derived. Most foraminifera are small, generally less than 1 mm across. However, some forms are considerably larger; a few measure as much as 100 mm in diameter.
The earliest foraminifera occur in Cambrian rocks, where a few simple tubular membranous forms are recorded. Agglutinated tests first occur in Ordovician rocks, and textulariines dominate foraminiferal assemblages in the early Palaeozoic. Coiled tests appeared in the Silurian but chambering did not arise until the Devonian. Although forms with calcareous tests first evolved in the Ordovician, they remained relatively uncommon until the Devonian. There followed a general trend in many lineages towards increasing complexity both of the wall structure and chamber arrangements. The first planktonic species evolved in the Jurassic, and all belong to the Rotaliina.
Foraminifera occupy a wide range of marine habitats, from estuaries and tidal flats to the deep ocean. The great majority are benthic, some being free-living while others live attached to objects. Characteristic suites of species occur in each environment, although an individual species may be found in more than one habitat. Foraminifera are therefore extensively used as environmental indicators, providing information on substrates, palaeosalinities, water depths, palaeotemperatures, etc. Their small size and relative abundance makes them particularly useful in this respect. A few forms (about fifty modern species) have adapted to a planktonic existence by increasing the horizontal dimensions of the test, reducing the thickness of the test, increasing pore size, developing supplementary apertures, or by enlargement of the aperture. Although there are only a few planktonic species, they are extremely abundant in modern oceans, and much of the deep ocean floor is covered with an ooze largely composed of their tests. Planktonic foraminifera can be used to obtain data relating to surface water temperatures and oceanic circulation patterns.
The small size and abundance of foraminifera in many marine sediments make them useful in biostratigraphy. Planktonic forms have been intensively studied and some species can be used for stratigraphical correlation on a near worldwide scale. Benthic species are less useful, although they can be used for local correlation.
Allan N. Insole
Bibliography
Bignot, G. (1985) Elements of micropalaeontology. Graham and Trotman, London.
Brasier, M. D. (1980) Microfossils. Allen and Unwin, London.
Jenkins, G. and and Murray, J. W. (1981) Stratigraphical atlas of fossil Foraminifera. Ellis Horwood Ltd, Chichester.
Lord, A. R. (1982) A stratigraphic index of calcareous nannofossils. Ellis Horwood, Chichester.
