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 pupoidesTritaxia 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 µ m), globular or acicular and are arranged randomly. All hyaline tests are traversed by perforations with variable diameter (0.5 to 15 µ m), 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