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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
currents) after death. Broadly speaking, calcareous
«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.
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
145 Radiolarians, Upper Eocene, Barbados ( X 70).
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.
SUMMARY
1.
2.
PHYLUM PORIFERA (sponges)
PHYLUM CNIDARIA [or COELENTERATA] (corals, jellyfish, and sea anemones)
Class Anthozoa (corals)
A.
B.
C.
Subclass Rugosa (rugose corals - extinct)
Subclass Tabulata (tabulate corals - extinct)
Subclass Zoantharia
Order Scleractinia (modern reef-building corals)
3.
4.
A.
B.
5.
A.
B.
C.
1.
2.
3.
4.
5.
6.
D.
E.
F.
6.
A.
B.
1.
2.
3.
C.
1.
2.
3.
a.
b.
D.
1.
7.
A.
B.
C.
D.
8.
PHYLUM BRYOZOA (bryozoans)
PHYLUM BRACHIOPODA (brachiopods)
Class Inarticulata
Class Articulata
PHYLUM MOLLUSCA (molluscs)
Class Bivalvia [sometimes called Pelecypoda] (clams and oysters)
Class Gastropoda (snails)
Class Cephalopoda (nautiloids and ammonoids)
Order Nautiloidea
Order Ammonoidea
Order Belemnoida
Order Sepioidea
Order Teuthoidea
Order Octopoda
Class Scaphopoda (tusk shells)
Class Monoplacophora
Class Polyplacophora (or Class Amphineura) (chitons)
PHYLUM ARTHROPODA (insects, spiders, shrimp, crabs, lobsters, barnacles,
ostracodes, trilobites, eurypterids)
Subphylum Trilobita
Subphylum Chelicerata
Class Xiphosura
Class Eurypterida
Class Arachnida
Subphylum Crustacea
Class Ostracoda
Class Cirripedia
Class Malacostraca
Order Decapoda
Order Isopoda
Subphylum Labiata
Class Insecta
PHYLUM ECHINODERMATA (crinoids, blastoids, starfish, sea urchins, sand dollars)
Class Crinoidea
Class Blastoidea
Class Asteroidea
Class Echinoidea
PHYLUM HEMICHORDATA (graptolites)
Class Graptolithina