Lecture 9 PHYLUM PROTOZOA

Transcription

1 Lecture 9 PHYLUM PROTOZOA The Protozoa include a diversity of one-celled aquatic organisms, most of which are less than one millimetre in diameter. A shell is secreted by members of two groups, the Foraminifera and the Radiolaria, which are common as fossils. Foraminifera The foraminiferan body consists of a blob of protoplasm which carries out all the bodily functions of respiration, feeding, excretion and reproduction which in the many-celled animals are performed by separate organs. The protoplasm lies mainly within the shell, or TEST, but some extends through one or more openings, APERTURES, to enswathe it, and form thread-like extensions, the PSEUDOPODIA, which are used for movement, and to capture other micro-organisms like diatoms for food. The test is calcareous, horny or agglutinated, i.e. made up of foreign particles like sand or shell fragments cemented by an organic matrix. It may consist of a single chamber (fig. 143e) or more usually, of several communicating chambers which are arranged spirally (fig. 143g), or in row or zigzag line. The size range is from a few hundredths of a millimetre to a few centimetres. Foraminifera are mainly marine but a few forms live in brackish water and fresh water. They are enormously abundant in the sea, occurring at all depths from the shore-line to the abyssal zone, and in all latitudes from the poles to the equator. In parts of the ocean where little detritus accumulates, the remains of calcareous Foraminifera may build up extensive deposits, or oozes. Fossil forms, too, may constitute much of the bulk of certain limestones, as in, for example, the nummulitic limestones from which the Egyptian Pyramids were in part built. Mode of life Most Foraminifera are bemhonic, but a number, e.g. Globigerina are planktonic. The benthonic forms may be sessile or vagrant, the latter crawling sluggishly by means of their pseudopodia. The majority occur in the neritic zone, and here the test is often a flattened disc shape or is coiled in a flat top-like spiral. Planktonic forms are most abundant at shallow depths, between about 6 and 30 m; they are dispersed by currents, and they often have a globular calcareous test with fragile spines (fig. 143k). Many species are restricted in their occurrence to a narrow range of physical conditions, being especially sensitive to temperature and salinity, and in the case of benthonic forms to the nature of the sea-bed (e.g. whether sandy or muddy) and to turbulence. Fossil forms, too, show an environmental preference, though this may be obscured by accidental transport (by

2 currents) after death. Broadly speaking, calcareous

3 «3 (n) Etiestheria x10 (m) Beyrichia couodont 143 Foraminifera and oilier microfossils. a-j and g-k, foraminifera. e and f, radiolarians. 1, a conodont. m, an ostracod. n, brancbiopod. Foraminifera are commonest in warm regions, and since temperature falls with depth of water, in surface waters. Larger forms, too, are mainly restricted to warm areas. Agglutinated forms are more characteristic of colder waters. Some common examples are: Saccammina (fig. 143d). The test is agglutinated and consists of a globular chamber with a short neck. Benthonic. Silurian to Recent. Lagena (fig. 143e). The test is calcareous, and consists of a single flask-shaped chamber. Benthonic. Jurassic to Recent. Nummulites (figs. i43a,b and 144). The test is calcareous. It is a flattened disc shape made up of many whorls coiled in a plane spiral, each whorl enveloping the preceding one. The whorls are divided by septa into chambers, each connected with its neighbour by a slit-like opening. Benthonic. Lower Tertiary. Rotalia (fig.i43g,h). The test is calcareous. The chambers are coiled in a low spiral with all the whorls showing on one side, but only the last whorl on the other side. Benthonic. Upper Cretaceous to Recent. Globigerina (fig. 143k). The test is made of calcite and is perforated by many fine pores. It consists of several spherical chambers, coiled in an irregular spiral. Planktonic. Ternary to Recent. Related forms occur in the Upper Cretaceous.

4 144 Internal structures in foraminifera. Left, section of a Permian fusulinid (X 7'5). Right, section of a nummulite showing the septa which partition the spirally coiled whorls (x 4'2); Nummulites, Bracklesham Beds, Eocene, Isle of Wight. Geological distribution Foraminifera range from the Ordovician to the present day. Most of the Palaeozoic forms had an agglutinated test. Calcareous forms are common in limestones of the Upper Palaeozoic, for example in the Lower Carboniferous as in Britain, and also in marine limestones of the Permian which occur in areas like Central Russia and parts of the United States where fusulinids, forms like wheat grains (fig. 143), are an important constituent of the rocks in which they occur. They are readily identified in thin sections of rocks, and are of great value stratigraphically (fig. 144, left). Planktonic Foraminifera did not appear until the Mesozoic, and they are relatively common in the Chalk where globigerinids occur. The nummulites were 'giant' (benthonic) forms (5-20 mm) which characterised, and often contributed massively to, the Lower Ternary deposits of the Tethys region, ranging eastwards from the Mediterranean area to the East Indies. Several species of Nummulites occur in the Eocene rocks of the Hampshire Basin (fig. 144, right). Economic use In the search for oil many hundreds of thousands of feet of sedimentary rock are penetrated by drilling each year. It is important that such

5 145 Radiolarians, Upper Eocene, Barbados ( X 70).

6 drilling should be stratigraphically controlled, that is to say, at any time the geologist must be able to identify the stratigraphical age of the rock being encountered in the borehole. The value of fossils for this purpose is obvious. However, the technique of drilling involves tearing the rock at the bottom of the hole into small chips which are then flushed to the surface. Any macrofossils are, therefore, macerated in this process and their value is correspondingly reduced. The small size of Foraminifera, however, means that many specimens may be obtained from one small rock chip perhaps no more than one centimetre across. It so happens that the majority of oil deposits are in the younger sedimentary rocks and in marine facies. Foraminifera are often abundant in these rocks and since many species have a short geological range they are of very considerable value in oil exploration and also in oil-field development. Radiolaria The Radiolaria (Cambrian to Recent) are marine planktonic organisms which secrete a symmetrical skeleton with an elaborate latticework composed typically of silica (figs. 143e and 145). At the present day they have a wide distribution but their remains are conspicuous mainly in the deeper parts of the ocean where calcareous shells such as Foraminifera dissolve. Fossil forms are found in cherts and siliceous limestones of various ages, e.g. in the Ordovician of Girvan (Southern Uplands). Such cherts are not necessarily very deep-sea deposits, though a radiolarian earth of Miocene age in Barbados is possibly an authentic example.

7 SUMMARY 1. PHYLUM PORIFERA (sponges) 2. PHYLUM CNIDARIA [or COELENTERATA] (corals, jellyfish, and sea anemones) Class Anthozoa (corals) A. Subclass Rugosa (rugose corals - extinct) B. Subclass Tabulata (tabulate corals - extinct) C. Subclass Zoantharia Order Scleractinia (modern reef-building corals) 3. PHYLUM BRYOZOA (bryozoans) 4. PHYLUM BRACHIOPODA (brachiopods) A. Class Inarticulata B. Class Articulata 5. PHYLUM MOLLUSCA (molluscs) A. Class Bivalvia [sometimes called Pelecypoda] (clams and oysters) B. Class Gastropoda (snails) C. Class Cephalopoda (nautiloids and ammonoids) 1. Order Nautiloidea 2. Order Ammonoidea 3. Order Belemnoida 4. Order Sepioidea 5. Order Teuthoidea 6. Order Octopoda D. Class Scaphopoda (tusk shells) E. Class Monoplacophora F. Class Polyplacophora (or Class Amphineura) (chitons) 6. PHYLUM ARTHROPODA (insects, spiders, shrimp, crabs, lobsters, barnacles, ostracodes, trilobites, eurypterids) A. Subphylum Trilobita B. Subphylum Chelicerata 1. Class Xiphosura 2. Class Eurypterida 3. Class Arachnida C. Subphylum Crustacea 1. Class Ostracoda 2. Class Cirripedia 3. Class Malacostraca a. Order Decapoda b. Order Isopoda D. Subphylum Labiata 1. Class Insecta 7. PHYLUM ECHINODERMATA (crinoids, blastoids, starfish, sea urchins, sand dollars) A. Class Crinoidea B. Class Blastoidea C. Class Asteroidea D. Class Echinoidea 8. PHYLUM HEMICHORDATA (graptolites) Class Graptolithina