Marginal zone

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Fig. 1. Henson's "zonation" of discoidal chambers in uniserial-conical shells, namely the marginal, radial and reticulate zones. Compare Fig. 2; from Henson, 1948; (Hottinger, 2006; fig. 20)[1] CC/BY-NC-SA)
Fig. 2. 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. For Henson's zonation of the chamber see Fig. 1. 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)[2] CC/BY-NC-SA)


Definition

  • according to Hottinger (2006):

MARGINAL ZONE – the marginal portion of the discoidal chamber in conical shells or of the lateral parts of an annular chamber in discoidal-evolute shells. It often houses an exoskeleton. In uniserial-conical shells the marginal zone is usually separated from a central area of the chamber by a furrow, the trough. The furrow is produced by a recess of the marginal zone for third or half of the chamber height (in the direction of growth). In discoidal-annular shells, a similar recess produces a pair of circular marginal troughs framing the apertural face, as in Marginopora.


See also

References

Henson (1948), Larger imperforate Foraminifera of south-western Asia. Families Lituolidae, Orbitolinidae and Meandrosipinidae, British Museum (Natural History), London, 127 p. + 16 pls.

Hottinger (2006), Illustrated glossary of terms used in foraminiferal research. Carnets de Géologie, Memoir 2, ISSN 1634-0744

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