Exoskeleton

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Fig. 1. Alveolar exoskeleton and polygonal network. A-C: simple alveolar layer in Everticyclammina virguliana (Koechlin), Mechra Klila, Northeastern Morocco, Uppermost Jurassic. A: stereograph, schematic, not to scale. B: tangential section. Note the large size of the alveoles in a postseptal position. C: para-equatorial, non-centered section showing septa and the basal coat at the bottom of the chamber, resembling a basal layer. E-G: polygonal network in Spirocyclinidae. E: Choffatella tingitana Hottinger, megalospheric generation, in tangential section near to the equatorial plane. Note the clear differentiation of beams and rafters. D: stereograph of spirocyclinid polygonal network. Note the curved pigeon holes in preseptal position which in axial section might be mistaken for foramina. F-G: extension of beams into a corrugated sheet that replaces endoskeletal pillars in Hottingertidae. F: stereograph representing a part of an axial section. Not to scale. G: Alveosepta powersi (Redmond), Northeastern Morocco, Upper Jurassic. Equatorial section of megalospheric specimen. alv: alveoles; b: beam; bl: basal layer; corr: corrugated median extension of beams; f: foramen; ph: pigeon holes; s: septum; sf: supplementary foramina; sph: sphaeroconch; ssut: septal suture. Arrows: direction of growth. After Hottinger, 1967; (Hottinger, 2006; fig. 45 [1] CC/BY-NC-SA)

Definition

  • according to Hottinger (2006):

EXOSKELETON - localized thickenings on the inner surface of the chamber's outer walls that subdivide the chamber lumen into blindly ending compartments. They form geometric patterns which are independent in number and direction from those determined by protoplasmic circulation through foraminal systems, i.e. endoskeletal patterns. Exoskeletal elements may consist simply of partitions (beams) perpendicular to the septum and to the lateral chamber wall, that produce simple alcoves, or of a tapestry of alveoles of various kinds coating the internal surface of the outer chamber walls. Two main alveolar types may be distinguished: 1) branched or unbranched alveoles with a blind ending of rounded outline below the external wall surface: the partitions lack a differentiation into beams and rafters and 2) pigeon-holes with a blind ending polygonal in outline below an abaxial, that is produced by partitions differentiated into beams and rafters. Exoskeletal elements may be very deep, subdividing the chamber almost completely as in Dictyopsella.


Remarks: M. Reichel (1936-1937) introduced the term "expressis verbis" as a contrary of Douvillé's "endoskeleton" for alveolinids. He thus designated the free, outer chamber walls including the frontal wall with its apertures as exoskeletal, all internal partitions (septula, basal layer and chamber floor) as endoskeletal. This concept was supported by an obvious differentiation of the wall texture in the external cover of the chamber walls considered at that time to be mainly the expression of a particular "behaviour" in the diagenetic process. Nowadays, we know that this particular kind of differentiation in the porcelaneous wall is textural and corresponds to the general differentiation of all lamellar-perforate and non-lamellar walls into inner and outer layers. In agglutinated foraminifera, there are often true structural elements to be classified in separate categories in much the same geometric way as Reichel conceived it, for they carpet the internal surfaces of the free lateral chamber walls (exoskeleton) or follow the foraminal pattern in the septa (endoskeleton). In discoidal agglutinated shells, exoskeleton and endoskeleton are separated by open spaces running parallel to the septum, the annular passages. In uniserial-conical constructions, the main radial partitions of a chamber may be continuous, extending from the shell center to the periphery of the chamber and comprise an inner, endoskeletal and an outer, exoskeletal part (as in Orbitolina). Thus, Reichel's (1936-1937) term exoskeleton was conserved but its definition restricted to true structures as opposed to the also exclusively structural elements of the endoskeleton (Hottinger, 1967, 1978). Therefore, the endoskeleton as conceived here comprises septal structures only when such elements consist of blind-ending recesses carpeting also the lateral chamber walls (Hottingerita) or when more complex structures derive from such a feature (Alveosepta). Banner's "hypodermis" (1966, according to the English manuscript) is not synonymous with "exoskeleton" but a general term for all kinds of alveolar layers, including the pigeon-holes of the polygonal subepidermal network, but excluding simpler "pseudalveolar" structures as in Orbignya or Voloskinovella. As the distinction of polygonal networks and various alveolar structures is of great taxonomic and stratigraphic importance, we do not recommend the use of the term hypodermis.

See also

References

Banner F.T. (1966), Morphology, classification and stratigraphic significance of the Spirocyclinidae. Voprosy Mikropaleontologii, Moskva, N° 10, p. 201-224 + 20 pls. (in Russian)

Hottinger L. (1967), Foraminifères imperforés du Mésozoïque marocain. Notes et Mémoires du Service géologique, Rabat, N° 209, p. 5-168

Hottinger L. (1978), Comparative anatomy of elementary shell structures in selected larger Foraminifera. In: Hedley R.H. & Adams C.G. (eds.), Foraminifera. Volume 3.- Academic Press, London, p. 203-266.

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

Reichel M. (1936-1937), Étude sur les Alvéolines. Mémoires de la Société paléontologique Suisse, Bâle, vols. LVII & LIX, 147 p. + 11 pls.

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