Geology Reference
In-Depth Information
Major
foraminiferal wall structures
seen in thin sec-
tions and studied in SEM are agglutinated, micro-
granular calcareous, porcelaneous calcareous and hya-
line calcareous (Fig. 10.22).
Wall composition and microstructures are the most
important features for classifying foraminifers on a
suprageneric level, and are the subjects of current criti-
cal evaluations (Piller 1990). Note that the groups men-
tioned below are those of more conventional classifi-
cations. Loeblich and Tappan (1988) recognized 12
suborders acknowledging specific skeleton mineralogy
(e.g. Involutinina originally with aragonitic walls, Fig.
10.27, see Pl. 111/15) or specific lamellation pattern
(e.g. Globigerinina, Pl. 73/9).
The wall of the
agglutinated textulariid foramin-
ifera
consists of particles held together by organic ma-
terial and cement of different composition. The par-
ticles can be coarse (see
Ammobaculites
, Pl. 69/7;
Haddonia
, Pl. 75/9;
Reophax
;
Tolypammina
, Pl. 51/2,
Pl. 111/18) or fine (e.g.
Alpinophragmium,
Pl. 111/1;
Chrysalidina,
Pl. 72/3). In more complex Mesozoic taxa
an outer microgranular wall covers a complex inner
layer that may be folded inward into the chambers, pro-
ducing labyrinthic or alveolar structures (see
Alveo-
septa
, Pl. 69/8;
Orbitolina
, Pl. 70,
Pseudocyclammina
,
Pl. 69/1). Textulariid foraminifera occur in various ma-
rine environments. They are often dominant in transi-
tional environments with brackish and marsh condi-
tions as well as at bathyal and abyssal depths.
Fig. 10.21.
Major wall structures of foraminiferal groups.
Agglutinated walls are characterized by individual grains from
the sea floor bound together in a cement, which may be cal-
careous, ferruginous or organic. Fusulinid foraminifera have
a microcrystalline calcitic wall, that is often differentiated
into several units. The porcelaneous walls of miliolinid fora-
minifera are submicroscopical crystals of magnesian calcite,
usually in random orientation. Rotaliinid foraminifera have
glassy-looking (hyaline) walls, which may be differentiated
into layers. AG - agglutinated grains, ALV - alveoles, BL -
basal layer, C - cement, IL - inner layers, OL - outer layer,
ORL - organic lining. Arrows indicate the optical orientation
of calcite crystals. Modified from Brasier (1980) and Bignot
(1985).
Fusulinid foraminifera
with
microgranular calcar-
eous
wall structures are represented by smaller and
larger foraminifera. The wall of the smaller foramini-
fer consists of microgranular calcite cemented by mi-
crocrystalline calcareous cement (see
Earlandia
, Pl. 67/
2;
Endothyranopsis
, Pl. 67/1;
Cribrogenerina
, Pl. 67/
10;
Tuberitina
, Pl. 67/2). Larger fusulinid foraminifera,
(Fusulinacea; Viséan to Late Permian; Pl. 68) are mor-
phological highly differentiated.
Miliolinid foraminifera
exhibit a
porcelaneous cal-
careous
wall structure without pores, characterized by
a shiny, smooth appearance of the test. Examples are
Austrocolomia
, Pl. 111/23;
Alveolina
, Pl. 75/2;
Galea-
nella,
Pl. 111/4-5;
Hemigordius
, Pl. 67/7).
and septal addition, (3) apertural characters and modi-
fications, and (4) chamber form and ornamentation.
These criteria are by far the most important character-
istics used in the classification and taxonomic differ-
entiation. Further properties particularly important for
discriminating modern foraminifera are life habits and
habitats, protoplasma features, ontogenetic changes, and
genetics. The last criterion has become increasingly
more meaningful for higher-level classification of an-
cient foraminifera as well.
The
rotaliinid foraminifera
are characterized by
hya-
line calcareous
wall structures. In cross sections or-
ganic layers or calcareous laminations may be visible.
Lamellar tests can be recognized by thicker early cham-
bers, thinner later chambers and a single-thickness fi-
nal chamber. Lamellar walls are characteristic for taxa
