Basal layer

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Fig. 1. Basal layer and flosculinisation. A: Unidentified, large miliolid, Fabularia roselli Caus and Dendritina sp. Randomly oblique sections. Uppermost Middle Eocene, Spanish Pyrenees. B: Idalina antiqua Munier-Chalmas et Schlumberger. More or less centered, subaxial sections. Santonian, Spanish Pyrenees. Note the proloculus wall illustrating the reduced thickness of the outermost layer of the primary chamber wall. The difference in the opacities of the basal layer and the primary outer chamber wall demonstrates how dissimilarly the discrete submicroscopic, porcelaneous wall textures react to diagenesis. C: Fabularia verseyi Cole, oblique section, from Jamaica, Middle Eocene, showing irregular tubiform, anastomosing passages in the basal layer. D-F: random sections of Pseudochubbina globularis (Smout), subspherical, and P. kassabi De Castro, flaring, showing parallel, rarely anastomosing tubiform passages in the basal layer, and an outer layer of more regular parallel chamberlets similar to attics, as also in Fabularia. Note the thin spirotheca extending into a lateral cover of the flaring portion of the test. Campanian, Iran. G: Alveolina daniensis Drobne, oblique sections near the equatorial plane. Compaction of the sediment has mechanically separated outer from inner whorls, revealing the thin spirotheca. Note lines of accretion parallel to the septum in the basal layer of flosculinized whorls. Lowermost Eocene, Slovenia. H: Alveolina tenuis Hottinger, axial section, showing columella produced by polar thickening of the basal layer. Note tubular passages in the columella, continuous in subsequent chambers, without interruption by preseptal spaces. Middle Eocene, Aquitaine, Southwestern France. Abbreviations: bl: basal layer; prp: preseptal space or passage; s: septum; spth; spirotheka (outer wall of sucessive spiral chambers); tph: trematophore. Scale bars: 1mm; (Hottinger, 2006; fig. 18 [1] CC/BY-NC-SA)

Definition

  • according to Hottinger (2006):

BASAL LAYER - in imperforate foraminifera: the parts of a chamber wall coating the previous coil and/or the septal face of the previous chamber. As they are not in contact with the ambient environment, these portions of the wall lack the differentiations in the texture of the outer wall. Basal layers represent or are part of the endoskeleton, and may be sculptured by ribs in the chamber lumen or form endoskeletal elements reaching the ceiling of the chamber, i.e. pillars or septula.


Remarks: The basal layer is a very important element of the architecture of imperforate foraminifera. In spiral-involute and miliolid shells it may become much thicker than the external chamber wall (flosculinisation); in fusiform shells it may form a double columella reaching both poles of coiling. Thickened basal layers often have numerous tubular passages of irregular shape connecting successive chamber lumina, with (edomiids) or without (elongate alveolinids) intermediate preseptal spaces. In our view the basal layer is homologous with the "basal skeleton" of fusulinids (chomata and derived structural elements). Where tubular chamberlets are vertically superposed, the basal layer may form regular floors parallel to the chamber roof, as in elongate Praealveolina.


See also


References

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

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