Septulum

Definition

Fig. 1. The structure of Orbitolina.A-D: Oblique and vertical views of the cone base. These plasticine models sculptured in the years around 1955 by M. Richel (* 1896 - † 1984) were never published. E-F: Details of a septulum in the radial zone of the discoidal chamber. In model E the septulum is cut below the roof and above the bottom of the chambers. The apparent folding of the section results from the adjustment of the endoskeletal structure to the crosswise-oblique arrangement of the stolon axes (arrows) that produce so-called ramps. In model F the septulum is cut in the middle of the chamber and shows a part of the chamber bottom with the face of the previous chamber. In the middle of the chamber, the section of the septulum appears unfolded. G-H: Random thinsections of Orbitolina sp. from Southwestern France, Albian. Transmitted light micrographs. In Fig. G the approximate positions of sections H-K are indicated. Note the inverse orientation: the sections face downward. G-K: Random sections of Orbitolina sp. from Southwestern France, Albian. Transmitted light micrographs. The approximate position of sections H-K are indicated in section G that is very close to the axial plane. Section H demonstrates details of the reticular zone, section I details of the marginal zone and section J details of the radial zone. The transverse section K shows the ramps produced by the crosswise-oblique stolon system. a: aperture; af: apertural face; b: beam; cl: chamberlet; e: epiderm; f: foramen; gr: coarse grains in the septum that obscure the structural pattern; hor: horizontal section in the plasticine model; marg: marginal zone; r: rafter; ra: ramp; rad: radial zone; ret: reticular zone; s: septum; sf: septal face; sl: septulum; sut: suture of the chambers. Double arrows in E and F: crosswise oblique foraminal axes; (Hottinger, 2006; fig. 71)[1] CC/BY-NC-SA)

• according to Hottinger (2006):

SEPTULUM - (French: cloisonette); endoskeletal, wall-like partition extending from the lateral wall into the chamber lumen, dividing it into compartments (chamberlets). In imperforate forms, these partitions are produced by local thickening of the inner part of the shell wall. Their disposition has a close relation to the arrangement of the foramenal axes. In lamellar-perforate species, the septula are produced by folded inner lamellas.

Remarks: In describing alveolinid structures, there has never been any difficulty in distinguishing between a septum (closing off a chamber) and a septulum (partitioning the chamber into chamberlets) ever since Carpenter (1862), Douvillé (1906) and Reichel (1936-1937). In orbitolinids however, there is considerable confusion. Carpenter (1862) described "Patellina" (= Orbitolina) lenticularis with a "large chamber layer". This term corresponds in modern terminology to a single, discoidal (or annular) chamber separated from the next "layer" by a septum. R. Schroeder kept up the idea of the chamber layer to at least 1973 but abandoned it in his 1975 paper. The concept of a chamber layer in orbitolinids leads to the interpretation of the orbitolinid radial partition as "septum" and of secondary, exoskeletal partitions as a "septulum" which is inconsistent with the structural interpretation of all other imperforate foraminifera. In French papers, where the orbitolinid mode of growth was correctly recognized as a uniserial stack of chambers, we find the simultaneous use of septum (for the true septum), cloison for endoskeletal radial main partitions and cloisonette (for beams in the exoskeleton).

See also

• endoskeleton
• foramenal axis

References

Carpenter, Parker & Jones (1862), Introduction to the study of Foraminifera, Ray Society, London, 319 p. + 22 pls.

Douvillé (1906), Évolution et enchaînement des Foraminifères, Bulletin de la Société géologique de France, Paris, (4ème série), tome 6ème, p. 588-602 + pl. XVIII.

Hottinger (2006), Illustrated glossary of terms used in foraminiferal research. Carnets de Géologie, Memoir 2, ISSN 1634-0744 Reichel (1936-1937), Étude sur les Alvéolines, Mémoires de la Société paléontologique Suisse, Bâle, vols. LVII & LIX, 147 p. + 11 pls.

Schroeder (1973), El corte de Aulet (Prov. de Huesca) Evolucion de las Orbitolinas en el limite del Cretaceo inferior superior. In: XIII Coloquio Europeo de Micropaleontologia España 1973, Enadimsa, Madrid, p. 141-149.

Schroeder (1975), General evolutionary trends in Orbitolinas, Revista Española de Micropaleontologia, Madrid, Numero Especial (1975), p. 117-128.

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