Heterotrophic protists

Radiolaria

Radiolaria are unicellular, marine microzooplankton characterized by an intricate, siliceous, internal skeleton. They are globally distributed and live at all levels of the water column. They have a long geological history, extending from the Cambrian to the present day, and their rapid evolution makes them useful biostratigraphic indicators.

Morphology

Living radiolarians may be considered as balls of protoplasm with an intricate internal skeleton, or test. Typically the test has a delicate, lattice-like structure formed from siliceous elements, including external spines, loose spicules, and internal bars. In radiolarians, buoyancy is maintained by the presence of fat globules and gas vacuoles that reduce density and by the test structure. Long spines increase drag, and the spherical and conical shapes ofthe tests resist sinking. Two main fossil orders are recognized on the basis of test architecture: the Spumellar and Nassellar (Fig. 13.7). Spumellarians have spherical tests that show radial symmetry (Fig. 13.8), whereas nassellarians have conical, bell-shaped tests.

Ecology/paleoecology

Of all the well-preserved protists, radiolarians have the widest oceanic distribution related, along with other zooplankton, to patterns of oceanic circulation, and species can be linked to water masses with particular characteristics. Analysis of the distribution of fossil assemblages can be used to infer the circulation patterns of ancient currents. Most species show a vertical stratification linked to temperature, and species common in

Radiolarians CharacteristicsSpongy Radiolaria

deep waters in the tropics exist in shallower waters at the poles. Distinct assemblage boundaries have been identified at 50, 200, 400, 1000, and 4000 m, with spumellarians dominating the shallower waters and nassellarians favoring depths below 2000 m. Radiolarians are most abundant in areas of upwell-ing where critical nutrients, particularly silica, are brought up to surface waters. The highest radiolarian diversity occurs at equatorial latitudes where surface waters diverge and cause upwelling. Comparisons ofliving radiolarians with fossil assemblages have helped identify temperature changes in water masses, for example the cooling of the Southern Ocean during the Cenozoic.

Radiolarian oozes

As the robust siliceous test of the radiolarian is less prone to dissolution than most other microfossils, even other siliceous forms, radiolarian material accumulates in marine sediments. Below the calcite compensation depth, or CCD, the depth at which calcite dissolves in the oceans (3000-5000 m), siliceous detritus dominates. In modern oceans radiolarian oozes cover 2.5% of the sea floor, mostly occurring in the deep sea sediments of the equatorial Pacific, below areas of high productivity. Chert horizons are frequently found in the fossil record, particularly in the Mesozoic and Cenozoic, interbedded with chalk. Such nodular cherts are considered to have formed from siliceous organisms.

Radiolarians are useful for Palaeozoic and Mesozoic bio-stratigraphy. They are used in more recent sediments where calcareous microfossils are absent, for example where specimens have been retrieved from rocks deposited below the CCD.

Foraminifere Radiolarije
Fig. 13.8 Ordovician radiolarian from the Southern Uplands.

Foraminifera

Table 13.2 Variation of foraminifera morphology.

Foraminifera are one of the most important fossil groups. Their distribution is global, they inhabit all marine environments, and they have a continuous fossil record from the Cambrian to the present day. Sensitive to differences in water temperature and chemistry, they are useful paleoclimatic and paleoceanographic indictors. They are also important for stra-tigraphic correlation.

Morphology

Foraminifera are unicellular organisms with an internal calcareous shell, or test. In living foraminifera most of the protoplasm is contained within the internal test. Strands of protoplasm, pseudopodia, extrude through pores in the test wall to trap food and aid movement.

Test wall composition, structure, and shape vary and are important features in classification. Three main wall compositions have been documented: organic, agglutinated, and calcareous. Organic walls are thin, flexible membranes and agglutinated tests are constructed from cemented detritus. Practically all fossil foraminifera are calcareous. Three types of calcareous wall exist: microgranular tests are common in Palaeozoic forms and are often recrystallized; porcellaneous tests are translucent; and hyaline tests are transparent.

Test shape is determined by the arrangement and shape of the chambers and is extremely variable (Table 13.2). Most foraminiferan tests are multichambered and progressively larger chambers are secreted in simple rows or in a coil. Linear tests are formed of single (uniserial), double (biserial), or triple rows (triserial). Coiled tests vary in shape from flattened forms (planispiral), where chambers have been added in a single plane, to helical forms (trochospiral), where chambers have been added along a vertical axis. Chamber shape is also very variable. Single-chamber tests may be flask shaped, tubular, or branched. In multichamber forms, chambers may be spherical or club shaped. Within the final chamber there is an opening, or aperture. Apertures may be single or multiple openings of variable shape and construction. The external surface of the test varies from completely smooth to highly ornamented. Test sculpture is considered to assist with buoyancy, anchor the foraminiferan, and deter predators.

Ecology/paleoecology

Foraminifera are very useful paleoenvironmental indicators. Benthic foraminifera morphology is indicative of substrate,

Test shape Chamber shape Aperture type External sculpture

Rounded

Spinose

Single row (uniserial)

Flask-shaped

Rounded

Spinose

Flask-shaped

Spherical

Radiate

Costate

Double row (biserial)

Spherical

Radiate

Costate

Coiled (planispiral)

Coiled (planispiral)

Lunate

Slit-like

Carniate

Lunate

Slit-like

Carniate

Clavate

Sutural

Coiled (trochospiral)

Clavate

Sutural

Raised sutures water depth, and seawater chemistry. Most benthic foraminifera are epifaunal and attach to the substrate using an organic membrane, the pseudopodia, or are cemented. Species associated with silty and muddy substrates tend to be thin-shelled, delicate, elongate forms. Coarse-grained sediments, less rich in nutrients, support sparser populations of thick-shelled, heavily sculptured foraminiferans. Furthermore, in higher energy environments, tests of free-living species are generally stronger (thicker walled). Cementing types, which prefer hard substrates, usually have a flattened or concave basal surface.

Most benthic foraminifera tolerate only normal marine salinities (approximately 35%o), and environments with fluctuating salinities are characterized by low diversity assemblages. Agglutinated foraminifera are common in marshes, whilst perforate calcareous forms favor lagoons. Hypersaline conditions are preferred only by porcellaneous foraminifera. Most foraminifera are found in marine water with normal oxygen levels, but a few thrive in the low oxygen environments of the deep sea. These species are typically small and have smooth, thin-walled, calcareous or agglutinated tests.

Benthic and planktic foraminifera assemblages are important indicators of paleobathymetry. The indicators of paleoenviron-ment and paleodepth are summarized in Fig. 13.9. Planktic foraminifera are amongst the best studied groups of microfossils. They are active predators within the plankton, though some also grow photosynthetic algae within their skeleton. Their modern distribution is in five major zones that reflect modern water temperatures. These zones can be recognized through the last interglacial and glacial periods and help in the reconstruction of the oceans at warmer and cooler times than the present.

The tests of planktic foraminifera are widely used in stable isotope analyses. These stable isotopes vary in relative proportion in the oceans in response to changing climatic features, such as temperature. The isotopic ratios of some elements are recorded by foraminifera as the animals extract material from the water to build their skeletons. Measurements made from the fossil skeletons can be used to estimate features of ancient climates such as ocean temperature, productivity, and amount of polar ice.

Evolutionary history

The earliest known foraminifera are simple, agglutinated tubes from Lower Cambrian rocks. These benthic forms diversified in the Ordovician and calcareous tests first occurred in the Silurian. Multichambered forms originated during the Devonian. Calcareous foraminifera continued to flourish in the Carboniferous and Permian, where a variety of test architecture with a high level of complexity existed. After the end-Palaeozoic mass extinction event benthic foraminifera underwent a major radiation adopting new habitats (lagoons, marshes, and reefs) and expanding into deeper water environments.

Planktic foraminifera first appeared in the Middle Jurassic. During the Cretaceous they underwent a major radiation, their global distribution and rapid evolution making them important zonal fossils for this period. Of the 300 species recognized for the Cretaceous, only five species of planktic foraminifera survived the end-Cretaceous extinction event. A major radiation occurred in early Cenozoic times, although there was a severe extinction of benthic forms at the Palaeocene-Eocene boundary. Foraminifera diversified during the Eocene but suffered a further extinction in tropical taxa associated with cooling at the end of the epoch. Radiation followed in the Miocene, related to a warmer climatic phase.

Open shelf

Lagoon more or less saline than the sea

Lagoons normal

Porcellaneous tests Hyaline tests Agglutinated tests

Porcellaneous tests Hyaline tests Agglutinated tests

Foraminifera Test Wall

3000

Benthic species become less abundant

Agglutinated tests at abyssal depths

Benthic species become less abundant

3000

Agglutinated tests at abyssal depths

Was this article helpful?

0 0
Survival Basics

Survival Basics

This is common knowledge that disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

Get My Free Ebook


Responses

  • Askalu
    What are five important characteristics of heterotrophic protists?
    8 years ago

Post a comment