Page 222 - Introduction to Paleobiology and The Fossil Record

PROTISTS 209
Foraminifera
Foraminifera are shelled, heterotrophic proto-
zoans, 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 classifi ca-
tions 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 micro-
scopes. 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 fi eld data. Moreover
stable isotopes extracted from foraminiferan
tests have provided valuable data on ancient
sea temperatures through the Mesozoic and
Cenozoic.
Morphology and classifi cation
Although many different classifi cations have
been published, shell morphology and miner-
alogy form the prime basis for identifi cation
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 catego-
ries have been documented, organic, aggluti-
nated 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
(a)
non-lamellar monolamellar multilamellar bilamellar
(b)
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 calcare-
ous, recycling fragments of coccoliths or
other forams.
3 Secreted calcareous tests may be subdi-
vided into three categories: porcellaneous,
hyaline and microgranular (Fig. 9.3a).
Porcellaneous tests are formed of small,
randomly oriented crystals of high-
magnesium calcite giving a smooth white
shell. Hyaline tests are formed of larger
crystals of low-magnesium calcite and
have a glassy appearance when well pre-
served. 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 granu-
lar 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