Lecture 4

Test morphology and

wall composition

There are three basic kinds of test wall.

• In the first, the test is formed by an organic membrane composed of tectin.

• The test (Allogromiids) is unilocular, thin and flexible as in Shepheardella, Allogromina, Neogullmia, Myxotheca and Kibisidytes.

• In detail, the structure of the memberane (Allogromiids) to be quite complex, consisting of a spongy, fiberous, network in Shepheardella and laminated in Myxotheca with the fibers of one layer laid down at right angles to those of adjacent layers.

Composition and structure of the test wall

Agglutinated Wall Structure

In this group, this memberane becomes the foundation for an agglutinated wall as in Rhizammina, Eggerellina, Astrohiza, Reophax, Bathysiphon, Rhabdammina, Gaudryina, Trchammina, Miliammina, Lagenammina, Ammobaculites, Saccammina, and Haplophragmoides.

In the majority of agglutinating forms, the cement is mixed with organically bound iron which on oxidation hardnes the test and gives it a red-brown colour.

Ammodiscus incertus Cyclammina

Dorothia pupoides Tritaxia tricarinata

Reophax cylindracea

Arenobulimina macfadyeni

Calcareous Wall Structure

• Very early in foraminiferal studies the major groups of

calcareous genera were distinguished as porcelaneous or glassy

according to their appearance in reflected light.

Porcelaneous

• In the white, porcelain-like group of calcareous genera, the

wall is composed of three layers: a thick median layer of laths

in random array with thin inner and outer veneers.

• In smooth, shining species the laths in the surface veneer are

arranged parallel to the surface, in a 'tile-roof' or parquet

floor' pattern. In rough-walled species as Quinqueloculina

berthelotiana the laths of the external veneer are arranged

perpendicularly to the surface to give a cobble pattern.

The main types of secreted, calcareous walls.

Microgranular

An important group of calcareous Foraminifera occur in the Upper Paleozoic and as they have a dark wall which sometimes includes agglutinated grains they were considered by the early workers to belong to the agglutinated group as in Paleotextularia.

Hyaline or glassy

Two major types of hyaline wall were distinguished on the basis of optical characters observed in thin sections or fragments under crossed nicols of the polarizing microscope.

These are first, radial structure and the second structure showed no extinction pattern.

In both cases the test is built of units composed of numerous platelike or rhomboidal crystals each about 1µm in diameter.

These units are enclosed in an organic memberane and irregularly sutured together.

The crystals forming the wall of porcelaneous (imperforate) tests are

very small (0.1 to 2 µm), globular or acicular and are arranged

randomly.

All hyaline tests are traversed by perforations with variable diameter

(0.5 to 15 µm), density and location.

Calcareous tests are by far the most abundant and fall into three

suborders, each with a different wall structure: the Miliolina, Fusulina

and Rotaliina.

Porcelaneous tests of the Miliolina appear a distinctive milky white in

reflected light and an amber colour in transimitted light.

They are constructed of tiny needles of high magnesian calcite randomly

arranged for the most part, but the outer surfaces are built with

horizontally or vertically arranged needles.

Examples of wall structures in the five foraminiferid suborders

Test morphology

•The foraminiferal test is either unilocular (non septate) or multilocular, being composed of more than one chamber and divided by septa.

•Unilocular tests may simply possess an open end which serves as an aperture.

•In the multilocular group the aperture is usually restricted and when a new chamber is added it becomes an internal foramen (plural: foramina).

•The foramen is often modified and different from the aperture.

•A tooth or teeth may be present in the aperture and in some genera the tooth is developed as a plate or tube which extends back to the previous foramen.

Annular complex: With annular discoidal "equatorial" layer and layers of lateral chamberlets on each side, giving a generally flattened spheroidal shape.

Biserial: Chambers arranged in two alternating rows. The initial part may be trochospiral or planispiral.

Trochospiral: The chambers are coiled in a helicoids spiral as in the gastropod Trochus.

High trochospiral: Trochoid genera with a very high spire exist and triserial arrangement with three chambers to the turn is common.

Uniserial: The chambers are arranged in a single series, straight (rectilinear) or curved (curvilinear).

Uniserial arrangement

Siphonodosaria

Stilostomella subspinosa

Stilostomella paleocenica