Fossils explained 47

that surrounds the Pseudopodia are to and for anchor-age. The ectoplasm and endoplasm remain in contact through the aperture, which is often, but not always, present in the youngest chamber. This article gives an overview of general aspects of foraminiferal biology and palaeontology. As the group is very diverse, especially in morphological and bio-logical aspects, it must be taken into account that there are many exceptions to the general comments that we make herein.

The foraminifera are a group of unicellular organisms (Kingdom Protista, Phylum Sarcodina, Class Foraminiferida) that range in age from the Cambrian to Recent, and that adapt to either a planktonic or benthic existence. The soft tissues of the cell enclose and are enclosed by a hard, preservable test ( Fig. 1) in most foraminifera. Because the test is an internal organ (tests are internal, shells external), it can be likened to a human skeleton. However, whereas our skeleton is composed of numerous separate bones, the foraminiferan test is a single unit without 'moving parts'. This may be composed of either secreted organic matter (tectin), secreted mineral matter (calcium carbonate or opaline silica) or agglutinated particles.
Foraminiferan tests are chambered, either one or many chambers being present. The chambers are divided by walls called septa (singular, septum) and are linked by single or multiple holes in each septum called foramina (singular, foramen). In life, the foraminifer produces pseudopodia from the ectoplasm FOSSILS FOSSILS gastropod shells' the size of coarse-grained sand, which he had found in the stomach of a shrimp. In later translations of Leeuwenhoek's work these 'shells' have been incorrectly referred to as cockles, but the old Dutch word used ('slakhoorntje') in fact means small snail shell. This can be interpreted as representing foraminifera. Apart from the second mention of foraminifera, this is also one of the few records of predation by other animals -in this case a shrimp -on these protists.
Later attempts to classify the foraminifera placed the group in the Mollusca, namely in the genus Nautilus, because of the chambered appearance of the tests. In 1781 Lorentz Spengler was among the first to note that the septa of foraminifera had no siphuncles, and that their septa were curved in the opposite direction to those of Nautilus. In 1826 Alcide d'Orbigny made the same observation, and was the first to name the group Foraminifera. This name refers to foramenbearing, rather than siphon-bearing, molluscs and only in 1835 was it discovered the protoplasm of foraminifera consisted of only one cell, and that therefore they were unrelated to the Mollusca.
The suprageneric taxonomy used by d'Orbigny was based on morphology, especially chamber arrangement. Contemporary scholars like Parker, Jones and Brady argued to the contrary, and although morphology was still prominently present in Brady's 1884 classic monograph, most of his higher taxa were based on wall composition.
In the most recent classification of the foraminifera, test composition and microstructure is very important. The four main groups are based on this, and comprise: organic walled tests (one order); agglutinated tests (four orders); calcium carbonate tests (ten orders); and tests formed of opaline silica (one order). In palaeontological studies, the most often encountered orders have calcium carbonate (CaCO 3 ) tests. For those foraminfera with carbonate tests, this can take the form of low magnesium calcite, high magnesium calcite, or aragonite.

Alternation of generations
The life cycle of the foraminifers is complicated by an alternation of generations between sexual and asexual generations. The typical foraminiferan lifecycle is dimorphic, that is, the two generations in any given species are dissimilar in appearance. The asexually reproducing generation is called schizont and the sexually reproducing generation is gamont. The gamont generation is smaller, but has a larger first chamber (the proloculus), and is called megalospheric. The agamont generation is larger, but has a smaller proloculus and is called microspheric. The agamont generation reproduces asexually by division into numerous smaller daughter cells, with half the number of chromosomes of the parent (reduction division, meiosis). These grow to form the gamont generation. The gamont 'parent' reproduces by dividing without further reduction in chromosome number (mitosis). Pairs of daughters fuse (sexual reproduction) to double the overall chromosome number and grow to be the agamont generation. In certain specialised groups of foraminifera the sexual generation is repressed, resulting in a life cycle with successive macrospheres.
It is now recognized that many species show variations to this simple life cycle, often concerning the inclusion of another asexual generation following the agamont. This generation, the schizont, can be recognized by a smaller proloculus than that of the gamont, but considerably larger than that of the agamont.

Shell morphology
Tests may consist of one (unilocular) or more (multilocular) chambers. In unilocular forms both the chamber and the protoplasm grow at the same time. In multilocular forms protoplasm growth is gradual and continuous, but chamber growth is episodic. Each time a new chamber is required, a growth cyst is formed. The pseudopodia occupy the space of this new chamber and build a thin, organic inner wall upon which the calcareous or agglutinated wall is constructed.
Contrary to organisms with continuous growth, it is not possible to develop a generally applicable growth model (using a limited set of parameters) for all forms. The test morphology (Fig. 1) of simple taxa can be described using a limited set of parameters like expansion rate, rate of overlap over the previous chamber, chamber shape and place of the aperture in the chamber. Complications to this pattern are the development of multiple apertures in some taxa, complex test sculpture and the development of internal structures. All scale bars represent 0.1 mm unless otherwise stated. Specimens come from the Pliocene of Cabaruyan Island (Luzon, Philippines) unless otherwise stated. All specimens deposited in the Nationaal Natuurhistorsich Museum, Leiden.

Palaeoecology of benthic foraminifera
Although their main distribution is undoubtedly marine, foraminifera also occur in some freshwater environments. The two most important life-styles are planktonic and benthic. Benthic foraminifera do not exclusively live at the sediment-water interface, but may inhabit the sediment down to 10-15 cm depth. Few species position themselves above the sedimentwater interface and some build large, branching tests for filter feeding.
The main determining factors of position in the sediment are food availability and bottom-water oxygenation, which are partly coupled. At the sedimentwater interface the seawater may be rich in oxygen and high-quality organic matter is often available. Decay of organic material consumes oxygen, so that the oxygen concentration decreases with depth and the remaining organic carbon will mainly be refractory. The steepness of these gradients depends on the organic carbon and oxygen flux, which often varies seasonally. The distribution of foraminifera in this type of sediment is stratified and epifaunal, and shallow and deep infaunal species can be recognized. Epifaunal species live at the sediment-water interface and feed on organic carbon 'raining' from the overlying watermass. They include taxa like Planulina wuellerstorfi (Schwager) and Hoeglundia elegans (d'Orbigny). These species often reproduce opportunistically in order to respond optimally to food availability. Within the sediment, but above the oxygen-depletion level, the shallow infaunal group is found, including typical species such as Uvigerina spp., Bolivina spp. (Fig. 1O) and Melonis spp. Even deeper and in very low oxygen concentrations are the deep infaunal species, which are characterized by very thin, polyporous tests, for example, Chilostomella spp. and Globobulimina spp. (Fig. 1N). That this is a generalization of an otherwise much more variable patterns is seen by the occurrence of high concentrations of Globobulimina pacifica at the sediment-water interface in oxygen deficient environments, and the occurrence of otherwise epifaunal species in crustacean burrows at 10 cm depth within the sediment. The above generalizations are more applicable to deep marine (oceanic) environments. In shallow environments foraminifera occur at greater depth in the sediment, but the occurrence of species is much more homogeneous.
Within the photic zone, symbiont-housing foraminifera occur, which are restricted by parameters other than those that control the distribution of non-symbiont-bearing species. Symbiont-housing foraminifera are commonly called larger foraminifera because of their size. The largest reported foraminifera have a diameter of 150 mm. In soft bottom conditions large foraminifera occur next to non-symbiont housing foraminifera, but in hard substrate environments, such as coral reefs, symbiont-bearing foraminifera dominate the fauna. Endosymbiosis is only profitable in warm and oligotrophic conditions. In other areas within the photic zone chloroplast husbandry occurs, in which the algae are digested, but the chloroplasts are left intact and functioning.

Geological utility of foraminifera
Benthic foraminifera are very common in many types of marine sediment. This makes them useful for palaeoceanographic, palaeoenvironmental and stratigraphical applications. In oceanic and shelf edge sediments planktonic foraminifera have a better stratigraphical resolution, but, particularly, in shallow tropical conditions, larger foraminifera are the most often used stratigraphical markers, as can be seen in the SE Asian letter classification and in Caribbean biostratigraphy. Distribution patterns of benthic foraminifera have been used to find palaeoceanographic current systems.
The test of calcareous foraminifera is a good source of stable oxygen isotopes for use in palaeotemperature analysis. For such analyses on benthic foraminifera, it is necessary to know the microenvironment as pore-water characteristics leave an imprint on the stable-isotope composition of the test. ∆ 18 O, or the over-representation of the more heavy oxygen molecule to a standard, has been mainly dependant on ice-volume. This was discovered by comparing the oxygen-isotope signal of an epifaunal species of foraminifera with that of planktonic foraminifera from the same sample. Deep ocean water is more stable in temperature and salinity, and by observing the difference of the ∆ 18 O the ice volume effect could be excluded and sea-surface temperature could be calculated.