Polygonal subepidermal network
- according to Hottinger (2006):
POLYGONAL SUBEPIDERMAL NETWORK - exoskeletal structure formed by a layer of always undivided, deep, tubular recesses in chamber walls, ending blindly below a thin, often transparent epiderm or similar structure. They generate a polygonal pattern at their distal end and open proximally into the chamber lumen with rounded mouths that are slightly restricted between lateral partitions of the chamber differentiated in beams and rafters.
Remarks: In foraminifera with greatly inflated chambers like ‘’Bradyina’’ or ‘’Gyroconulina’’, the beams may produce a polygonal pattern of their own rather than form a row of partitions perpendicular to the septum. Introduced by H. Douvillé in 1906 under the name "réseau sous-épidermique", the term was used mainly in the description of orbitolinids where already in 1930 Davies distinguished major and minor partitions. In the later Anglosaxon literature, these details were not considered as important: Cox (1937) described the Loftusia exoskeleton as "alveolar layer" and Henson (1947) preferred to use the general term "subepidermal partition" for all elements subdividing the chamber lumen into "pidgeon-holes" or "subepidermal cells". The general term "subepidermal partition" was extended by Henson himself and by later authors to any kind of lateral chamber partition, thus depriving the term of any significance for studies of comparative or functional anatomy. For this reason, we recommend dropping the use of the term "subepidermal partition". Henson's term "pigeon-holes" rather than "subepidermal cells" is appropriate to distinguish these from ordinary alveoles or alcoves.
A polygonal network has not been found in any living foraminifer. Therefore, that the recesses were coated by the organic lining can not be directly confirmed. The interpretation of the polygonal network as an exoskeletal structure rather than as a particular kind of wall texture (as Henson, 1947, had already pointed out) is supported by their common combination: lamellar Fabiania and its relatives exhibit a perforate epiderm covering their subepidermal network, advanced Cuneolina and Dicyclina have a paraporous epiderm and many fusulinids bearing a parakeriotheca combine this wall texture with an exoskeleton. The extremely thin, often transparent epiderm in agglutinated foraminifera suggests, that the polygonal network is a device to keep symbionts exposed to light and in the immediate vicinity of the location where gas exchange through the shell should be enhanced by particular, porous textures. It may be of taxonomical importance at a supra-generic level to distinguish at least two kinds of polygonal networks. The first one to appear in the Mesozoic (Haurania and Amijiella, Middle Lias) is comparatively coarse and deep. There is probably no clear differentiation of an epiderm, just a thin outer wall covering the polygonal ends of the blind recesses. The differentiation of beams and rafters does not seem to be very orderly, even in evolute, fan-shaped to discoidal shells such as Timidonella or Alzonella. The second group that appeared not much later with Pseudocyclammina liasica, has finer meshes and a clearly differentiated, extremely thin epiderm. This type characterizes most late Mesozoic conical foraminifera as well as the peneropliform spirocyclinids. The question arises: is there an evolutionary series of exoskeletonal development from simple to complex, such as Praekurnubia (Late Middle Jurassic) with simple beams as exoskeleton, through Kurnubia (Early Upper Jurassic) with a comparatively simple polygonal network, to Rectokurnubia (Late Upper Jurassic) with a deep and more complex polygonal network. Similar evolutionary trends have been proposed for reticulinellids and conical agglutinated foraminifera. In the ontogeny of specimens belonging to one of the numerous genera involved no such series of exoskeletal structural complication has been observed. In my view, the question remains unresolved (for discussion see Hottinger, 1978, table 1, p. 255).
Cox (1937), The genus Loftusia in South Western Iran, Eclogae geologicae Helvetiae, Basel, vol. 30, N° 2, p. 431-450.
Douvillé (1906), Évolution et enchaînement des Foraminifères, Bulletin de la Société géologique de France, Paris, (4ème série), tome 6ème, p. 588-602 + pl. XVIII.
Davies (1930), The genus Dictyoconus and its allies: a review of the group, together with a description of three new species from the lower Eocene beds of northern Baluchistan, Transactions of the Royal Society of Edinburgh, vol. 56, p. 485-505.
Henson (1947), New Trochamminidæ and Verneuilinidæ from the Middle East, The Annals and Magazine of natural History, London, (Ser. 11), vol. XIV, N° 117, p. 605-630 + 5 pls.
Hottinger (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