Protozoa

Protozoans are neither animal nor plant, but single-celled eukaryotes that commonly show animal characteristics such as motility and heterotrophy; some groups are able to form cysts. Most are about 50-100 |m in size and are very common in aquatic environments and in the soil. They can occupy various levels in the food chain ranging from primary producers to predators and some groups function as parasites and symbionts.

Foraminifera

Foraminifera are shelled, heterotrophic protozoans, common in a wide variety of Phanero-zoic sedimentary rocks and of considerable biostratigraphic and paleoenvironmental value. The foraminiferans are characterized by a complex network of granular pseudopo-dia. The foraminiferans were traditionally included in the phylum Sarcodina together with the Radiolaria and a range of other non-flagellate protozoans. In modern classifications the foraminiferans are usually regarded as a discrete phylum, the Granuloreticulosa. Cavalier-Smith (2002), for example, regarded the Foraminifera as a member of the infrak-ingdom Rhizaria and placed them within the phylum Retaria together with the Radiolaria.

Foraminifera are easily the most abundant of microfossils and can be studied with simple preparation techniques and low-power microscopes. Consequently, pioneer studies in micropaleontology were based on the forami-niferans and techniques established for the study of this group were extended to many other microfossil taxa. Foraminifera have proved extremely useful in the petroleum industry, where detailed biostratigraphic schemes, particularly for Cenozoic rocks, have helped correlate oil field data. Moreover stable isotopes extracted from foraminiferan tests have provided valuable data on ancient sea temperatures through the Mesozoic and Cenozoic.

Morphology and classification

Although many different classifications have been published, shell morphology and mineralogy form the prime basis for identification of species and higher categories of Foramin-ifera. Most have a shell or test comprising chambers, interconnected through holes or foramina. The test may be composed of a number of materials and three main categories have been documented, organic, agglutinated and secreted calcareous:

1 Organic tests consist of tectin, which is a protinaceous or pseudochitinous substance.

2 Agglutinated ("glued") tests comprise fragments of extraneous material bound together by a variety of cements. The microgranular compound porcellaneous hyaline radial microgranular compound porcellaneous hyaline radial

Testa Hyaline Radial

non-lamellar monolamellar multilamellar bilamellar

Figure 9.3 Main types of foraminiferan test walls: (a) the composition and structure of test walls and (b) lamellar construction.

non-lamellar monolamellar multilamellar bilamellar

Figure 9.3 Main types of foraminiferan test walls: (a) the composition and structure of test walls and (b) lamellar construction.

debris may be siliciclastic, such as quartz, mica grains or sponge spicules, or calcareous, recycling fragments of coccoliths or other forams. 3 Secreted calcareous tests may be subdivided into three categories: porcellaneous, hyaline and microgranular (Fig. 9.3a). Porcellaneous tests are formed of small, randomly oriented crystals of highmagnesium calcite giving a smooth white shell. Hyaline tests are formed of larger crystals of low-magnesium calcite and have a glassy appearance when well preserved. Hyaline tests have two main modes: the radial tests are made up of minute calcite crystals with their c-axes normal to the test surface, whereas granular forms consist of microcrystals of calcite with variable orientations. Both modes usually have a multilayered structure (Fig. 9.3b) and perforations. Hyaline aragonitic tests occur but are much rarer than calcitic tests. Finally, microgranular tests consist of tightly packed, similar-sized grains of crystalline calcite. Most members of this group are known from the Upper Paleozoic.

The gross morphology of a foraminiferan test is governed by the shape and arrangement of the chambers. The group has evolved a wide range of test symmetries (Fig. 9.4) from simple uniserial and biserial forms to more complex planispiral and trochospiral shapes (Fig. 9.5). Chambers also come in a wide

Leishmania Stages

uniserial biserial triserial trochospiral planispiral quinqueloculine

Figure 9.4 Main types of foraminiferan chamber construction.

uniserial biserial triserial trochospiral planispiral quinqueloculine

Figure 9.4 Main types of foraminiferan chamber construction.

Microgranular Foraminifera

Figure 9.5 Some genera of foraminiferans: (a) Textularia, (b) Cribrostomoides, (c) Milionella, (d) Sprirolina, (e) Brizalina, (f) Pyrgo, (g) Elphidium, (h) Nonion, (i) Cibicides, (j) Globigerina, (k) Globorotalia, and (l) Elphidium (another species). Magnification X50-100 for all. (Courtesy of John Murray (b, d, e, g, h, j, k) and Euan Clarkson (a, c, f, i, l).)

Figure 9.5 Some genera of foraminiferans: (a) Textularia, (b) Cribrostomoides, (c) Milionella, (d) Sprirolina, (e) Brizalina, (f) Pyrgo, (g) Elphidium, (h) Nonion, (i) Cibicides, (j) Globigerina, (k) Globorotalia, and (l) Elphidium (another species). Magnification X50-100 for all. (Courtesy of John Murray (b, d, e, g, h, j, k) and Euan Clarkson (a, c, f, i, l).)

spectrum of shapes, from simple spherical compartments through tubular to clavate forms. Moreover, the shape and position of the aperture may vary. Surface ornament may include ribs and spines or be merely punctate or rugose. Foraminifera are classified according to test type and ornamentation (Box 9.2).

Life modes

The foraminiferans have adopted two main life modes, benthic and planktonic. The majority are benthic, epifaunal organisms; they are either attached or cling to the substrate or crawl slowly over the seabed by extending their protoplasmic pseudopodia. Infaunal types live within the top 15 cm of sediment. Most benthic forms have a restricted geographic range. Planktonic foraminiferans are most diverse in tropical, equatorial regions and may be extremely abundant in fertile areas of the oceans, particularly where upwell-ing occurs.

The functional morphology of these groups can now be modeled mathematically (Box 9.3) and potentially can be related to different life modes in the group. Moreover their relationships to different environments, past and present, are well established (Box 9.4).

Evolution and geological history

The earliest foraminiferans are known from the Lower Cambrian, represented by simple agglutinated tubes assigned to Bathysiphon, a living benthic genus (Fig. 9.8). More diverse agglutinated forms appeared during the Ordo-vician while microgranular tests evolved during the Silurian; however, it was not until the Devonian that multichambered tests probably developed. Nevertheless, Carboniferous assemblages have a variety of uniserial, bise-rial, triserial and trochospiral agglutinated tests. Around the Devonian-Carboniferous boundary the first partitioned tests displaying multilocular growth modes (the addition of new chambers in series) appeared. Two families, the Endothyridae and Fusulinidae, dominated Carboniferous assemblages and the porcellaneous Miliolinidae achieved importance in the Permian. The Fusulinidae were generally large, specialized foraminiferans, adapted to carbonate and reef-type facies during the Late Carboniferous and Permian. Despite a high diversity during the Late Permian, they became extinct at the end of the Paleozoic, and the Endothyridae and the Miliolinidae were very much reduced in diversity.

Although Triassic assemblages were generally impoverished, the stage was set for a considerable radiation during the Jurassic. Two hyaline groups, the benthic Nodosariidae and planktonic Globigerinidae, diversified, while the agglutinates, Lituolitidae and Orbitolini-dae, continued. The planktonic foraminifer-ans diversified in the Cretaceous, culminating in the near extinction of the group during the Cretaceous-Tertiary (KT) mass extinction. Two further periods of diversification took place during the Paleocene-Eocene and the Miocene.

Radiolaria_

The radiolarians are marine, unicellular, planktonic protists with delicate skeletons usually composed of a framework of opaline silica (Fig. 9.9). Their name is derived from the radial symmetry, commonly marked by radial skeletal spines, characteristic of many forms. Many others, however, lack radial symmetry. Most radiolarians feed on bacteria and phytoplankton, but also on copepods and crustacean larvae and occupy levels in the water column from the surface to the abyssal depths, although most live in the photic zone commonly associated with symbiotic algae. The radiolarian ectoplasm covers the test and holds symbiotic zooxanthellae, microorganisms enclosed within the cell mass, and perforations, providing some nourishment. The radiolarian endoplasm (surrounded by the capsular membrane) contains the nucleus and other inclusions. The group has two types of pseudopodia: the axopodia are rigid and not ramified, whereas the filipodia are thin, ramified extensions of the ectoplasm.

Morphology and classification

The radiolarian skeleton or test consists of isolated or networked spicules, composed of opaline silica and forming sponge-like structures or trabeculae. Three of the main groups are recognized (Box 9.5) on the basis of skeletal structure and arrangement of

Box 9.2 Classification of Foraminifera Suborder ALLOGROMIINA

• Organic tests, usually unilocular, occurring in fresh, brackish and marine conditions. Not usually fossilized

• Cambrian (Lower) to Recent

Suborder TEXTULARIINA

• Agglutinated tests consist of debris bound together with cement; both septate and non-septate

• Cambrian (Lower) to Recent

Suborder FUSULININA

• Microgranular tests, some with two or more laminae; septate and non-septate forms

• Ordovician (Llandeilo) to Permian (Changhsingian)

Suborder INVOLUTININA

• Aragonitic hyaline tests

• Permian (Rotliegendes) to Cretaceous (Cenomanian) Suborder SPIRILLININA

• Calcitic hyaline tests, planispiral to conical

• Triassic (Rhaetic) to Recent

Suborder CARTERININA

• Tests comprise calcareous spicules in calcareous cement

• Tertiary (Priabonian) to Recent

Suborder MILIOLINA

• Porcellaneous tests, imperforate, both septate and non-septate, which are often large and complex

• Carboniferous (Visean) to Recent Suborder SILICOLOCULININA

• Imperforate tests of opaline silica

• Tertiary (Miocene) to Recent

Suborder LAGENINA

• Calcitic monolamellar tests, hyaline radial

• Silurian (Pridoli) to Recent

Suborder ROBERTININA

• Aragonitic, hyaline radial tests; both septate and finely perforate

• Triassic (Anisian) to Recent

Suborder GLOBIGERININA

• Calcitic, hyaline tests; finely perforate planktonic forms

• Jurassic (Bajocian) to Recent

Suborder ROTALIINA

• Calcitic, hyaline radial; perforate multilocular forms

• Jurassic (Aalenian) to Recent

Box 9.3 Modeling of foram tests

David Raup's theoretical work on the modeling of mollusk morphospace created a paradigm shift in our understanding of shell ontogeny (see p. 332). The skeletons of many groups of organisms can now be generated, mathematically, according to a simple set of equations in each case. The shapes of microfossils can also be modeled in this way, with a set of rules based on the angle of deviation, a translation factor and a growth factor (Tyszka 2006). By varying these, a huge range of possible and impossible tests can be homegrown on the computer (Fig. 9.6). The forms illustrated here are only a subset of the total number of possibilities. Interestingly, these sorts of computer models always generate some bizarre forms. The dysfunctional forms, for example, are geometrically possible but the shapes and volumes of the chambers could simply not function; vacant ranges on the other hand contain fully functional morphologies but these forms have not yet been found in the fossil record. Why not?

Foraminiferans
Figure 9.6 Modeling foraminiferan tests: part of a theoretical three-dimensional morphospace for foraminiferans. GF, growth factor; TF, translation factor; deviation factor. (From Tyszka 2006.)

perforations: the nassellarians (Fig. 9.10) and entactinarians develop a lattice from bar-like spicules, each end having a bundle of spicules. The initial nassellarian spicule is enclosed in the cephalis, and the skeleton develops further by the addition of segments following axial symmetry. By contrast, the initial entactinar-ian spicule is enclosed in a latticed or spongy test with radial symmetry based on a spherical body plan; this is similar to those of the spumellarians (Fig. 9.10), which however have a microsphere (instead of a spicule) internally.

Evolution and geological history

Although some records suggest an origin in the Mid Cambrian or earlier, the radiolarians became common in the Ordovician, and they are often found in deep-sea cherts associated with major subduction zones. The albaillellar-ians together with the entactinarians were the dominant forms, although after the Devonian, spumellarians with sponge-like tests were more prominent (Fig. 9.10).

Spumellarians remained important during the Triassic, with genera such as Capnuchos-

phaera, although the nassellarians had appeared; they continued as the major group through the Jurassic, Cretaceous and Early Tertiary. Late Tertiary forms evolved thinner skeletons, perhaps because of increased competition with the diatoms for mineral resources.

Radiolarian oozes cover about 2.5% of the ocean floors, accumulating at rates of 4-5 mm per 1000 years. Radiolarians are useful in paleo-oceanographic investigations, and they are particularly useful in dating the formation of deep-water sediments accumulating beneath the carbonate compensation depth (CCD), where carbonate-shelled organisms such as foraminiferans cannot survive. Radiolarian cherts and radiolarites commonly occur in oceanic facies preserved in mountain belts and are commonly associated with ophiolites, sections of the ancient ocean crust and upper mantle that have been uplifted (see p. 48), so they are very important in deciphering the origins and destruction of ancient ocean systems such as Tethys.

But the beauty of the radiolarian skeleton has also assured the group's place in the history of art (Box 9.6).

+1 0

Responses

  • pirkko
    How do radiolarian skeletons differ from foram tests?
    8 years ago

Post a comment