Fig. 1. Endoskeletal patterns in discoidal shells. A-D: disposition of apertural axes. A: radial axes alternating in radial position from one stolon layer to the next. This is the most common disposition in imperforate forms with annular stages of growth. B: radial axes superposed in radial position on all stolon planes. C: crosswise-oblique stolon axes alternating in radial position from one stolon plane to the next. D: crosswise-oblique stolon axes superposed on all stolon planes. This pattern characterizes all members of the orbitolitid family. Schematic, not to scale. E-H: all endoskeletal elements are disposed in accordance with the basic patterns of the foraminal axes: E corresponds to pattern A, F to pattern B, G to pattern C and H to pattern D. Stereographs after Hottinger, 1967. Schematic, not to scale. In reality, the patterns are often disturbed by intercalary elements generated as the diameter of the annuli increases during growth. This maintains on the apertural face the mean distances between apertures and their mean diameter constant during ontogeny. Examples: E1-2: Pseudotaberina malabarica, megalospheric generation, from Iran. Middle Miocene. E1: oblique-centered section of spiral-involute stage showing radial disposition of pillars. Laterally, there is a layer of short septula. E2: a transverse section tangential to a septum shows the alternating disposition of the foramina and the pillars. F1-2: New genus (possibly related to Pastrikella) from the Pyrenean Upper Cretaceous in Northern Spain. The endoskeleton consists only of septula. There is but one median annular preseptal passage and it occupies the total radial extension of the annular chamber. There are only two planes of stolons. F1: an oblique section at a low angles with respect to the equatorial plane shows the radial disposition of the apertural axes and of the septula. F2: a transverse section parallel to the shell axis shows that the stolon axes on the two stolon planes are superposed. G1-3: Amphisorus from Rottnest Island near Perth, Australia. Recent. G1: the detail of an equatorial section demonstrates the crosswise-oblique disposition of the pillars on neighboring stolon planes. G2, an equatorial section, demonstrates that pillars are restricted to the equatorial zone of the disc. G3: a transverse section parallel to the shell axis and tangential to an annular septum shows the disposition of the median foramina and pillars altermating in radial position on successive stolon planes. They are flanked by two annular preseptal passages separating them from a lateral layer of septula subdividing the annular chamber. H1-2: Orbitolites spp. from the region of Tremp, Lerida prov., Northern Spain. Pyrenean Lower Eocene (Ilerdian). H1: the comparatively regular disposition of the ramps in sections parallel to the equatorial plane reveals their superposition in consecutive stolon planes. H2: in the transverse section parallel to the axis of the shell this superposition is clearly visible where the section is tangential to an annular septum. Abbreviations: b: beam; f: foramen; pi: pillar; prp: preseptal space; ra: ramp; s: septum; sl: septulum; (Hottinger, 2006; fig. 47) CC/BY-NC-SA)
Fig. 2. Comparison of foraminiferal skeletons. Schematic, not to scale. Lamellation, perforation and canal orifices omitted. A: a planispiral-evolute shell without skeletal structures, composed of simple primary chamber walls with multiple apertures, such as that of Peneroplis. B: a planispiral-evolute shell with an alveolar exoskeleton, such as Pseudocyclammina. C: a planispiral shell with a pillared endoskeleton such as Archaias. Note that in the axial sections of shells with peneropliform, flaring chambers the periphery of the shell and the apertural face are on opposite sides. Consequently, the pillars extending from chamber bottom to chamber roof appear in the axial plane on the side cutting the apertural face as longitudinal and on the other side cutting the periphery as more or less perpendicular sections. D: a spiral shell with a supplemental skeleton restricted to the periphery of the shell, as in nummulitids with a marginal cord. E: a spiral shell with an enveloping canal system and a marginal crest as in Pellatispira. Note the primary lateral chamber walls "emerging" from the supplemental skeleton. These primary chamber walls are covered by secondary lamellae but are perforated in continuation of the primary bilamellar wall. Therefore they are not a part of the supplemental skeleton. The supplemental chamberlets have perforate lateral walls but do not communicate directly with the spiral chambers by retral stolons. They are fed by canal orifices. a: aperture; af: apertural face; alv: alveole; bl: basal layer; ch: chamber; chsut: chamber suture; f: foramen; lh: loophole; mc: marginal cord; mcr: marginal crest; per: periphery; pi: pillar; pr: proloculus; s: septum; schl: supplemental chamberlet; spc: spiral canal; spsut: spiral suture; sulc: sulcus; t: tunnel; up: umbilical plate; (Hottinger, 2006; fig. 63) CC/BY-NC-SA)
- according to Hottinger (2006):
ENDOSKELETON - localized thickenings on the inner surface of the chamber wall that partly or totally subdivide the main chamber lumen in the lee of protoplasmic streams according to a pattern produced by the arrangement of intercameral foramina in successive septa. Plate-like elements (septula), usually perpendicular to the septum, may form more or less complete partitions touching the lateral walls or fusing with elements of the exoskeleton. Discontinuous, columnar partitions are called pillars (or interseptal pillars). A third type of endoskeleton is produced by layers of shell deposited on the chamber floor and coating the previously exposed outer shell surface completely (basal layer) or partially (chomata and parachomata of fusulinids). In different taxa, the three endoskeletal types may occur alone or in varying combinations. Often, endoskeletal elements appear only in the course of ontogeny, usually later than exoskeletal elements. In agglutinated shells endoskeletal elements include the septum and may be recognized by remarkably coarse and irregularly shaped particles that obscure the genetically fixed pattern in contrast to the more ordered exoskeletal elements of the same specimen. Whether the toothplates and their equivalents in lamellar perforate foraminifera and the secondary septa produced by folds of the inner lamella in orthophragminid lamellar architecture are homologous equivalents of the endoskeletal structures of non-lamellar-imperforate foraminifera remains an open question.
Remarks: The term endoskeleton was introduced by H. Douvillé (1906, p. 593 and 602) in a key paper comparing the anatomy of imperforate fusiform shells, i.e. fusulinids, loftusiids and alveolinids. Douvillé had already recognized the close morphological relationship between apertural and endoskeletal patterns. In his monograph on alveolinids, M. Reichel (1936-1937) adopted the term endoskeleton to designate the structural elements subdividing the chamber in contrast to a so-called exoskeleton comprising the lateral and frontal chamber walls including the apertural face. The strict correspondence between the pattern of distribution of the foramina on the septal face and the patterns produced by the endoskeletal elements (in the alveolinids by the septula) were clearly demonstrated by models of the shell cavities and their connections through the septum as if they were a cast of the shell cavities (Reichel, 1936-1937, fig. 27). These patterns are still used today as diagnostic features for the definition of alveolinid genera.
Hottinger (1967) modified and extended (1978) Douvillé's term to include all structures subdividing the chamber lumen and linked to the patterns of intralocular protoplasmic streaming in contrast to exoskeletal partitions that are not affected by such patterns. Thus, the originally descriptive term is expanded to include a functional meaning and extended to all corresponding features in imperforate shells. In some lamellar-perforate foraminifera, comparatively rare structures (such as the hollow pillars in Chapmanina) correspond in shape and position to the definition of endoskeletal features. There is no reason to interpret their function otherwise. So such features may be called endoskeletal without hesitation. They may lead the way to clarify, by comparisons, the significance of true toothplate structures.
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 (1967), Foraminifères imperforés du Mésozoïque marocain, Notes et Mémoires du Service géologique, Rabat, N° 209, p. 5-168
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
Reichel (1936-1937), Étude sur les Alvéolines.- Mémoires de la Société paléontologique Suisse, Bâle, vols. LVII & LIX, 147 p. + 11 pls.
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