Flabellum chunii Marenzeller, 1904

Stolarski, Jarosław, 2003, Three-dimensional micro- and nanostructural characteristics of the scleractinian coral skeleton: A biocalcification proxy, Acta Palaeontologica Polonica 48 (4), pp. 497-530 : 503-505

Flabellum chunii Marenzeller, 1904

Septal morphology.— F. chunii (and some other representatives of traditional Flabellidae ) have a scale−like texture on the septal flanks ( Fig. 5A 1 , A 2). Scale−like units here are composed of bundles of fibers arranged quasi−parallel to septal faces (in Stephanocyathus bundles of fibers are arranged quasi−perpendicularly to the septal faces resulting in their smooth surface). Scale−like structures are developed also on the inner side of the wall, and on the axial junction of the septa. Scale−like units continue on distal, growing edges of skeletal elements where they form small tubercles.

Transverse sections.—SEM micrograph of polished and etched sections made approximately in half of the length of S1 ( Fig. 5D 1 , D 2), shows two zones of different etching relief: (1) dRAF zone (mid−septal zone) composed of series of Ą neighboring pits (dCRA), and (2) TD zone composed of layers of fibers orientated quasi−parallel to the septal surface.

The dCRA show twofold structure: inner, deeper hollow is surrounded with circle of short fibers ( Fig. 5D 2). The dRAF zone continue between septa and wall (marginotheca, see Stolarski 1995).

Longitudinal−radial sections.—Longitudinal sections in the RAF plane show narrow “strands” (ca. 15 µm in width) that are arranged fanwise in the plane of the section ( Fig. 6D ). Due to the undulating course of RAF in radial and longitudinal directions, dRAF are visible only in some surfaces of the thin−section. In TLM, each “strand” is composed of regular alternations ( Fig. 6E ) of thicker, brownish, and slightly opaque layers (ca. 1–3 µm width) that are intercalated with thin, rather colorless and more transparent ones (ca. 0.8–1 µm width). Layers within each “strand” have domed shapes, and fade approaching the border of the “strand”. Layers that are brownish in TLM exhibit strong, bright green fluorescence in MFM; thin and transparent layers do not show a chromatic reaction ( Fig. 6F ). Polished and etched sections viewed in SEM ( Fig. 5C 1 , C 2) show ca. 15 µm wide “strands” of generally negative etching relief that are composed of regular alternations of domed hollows (ca. 2–3 µm width), separated by small and narrow ridges (ca. 1.5–2 µm width).

Elemental (C, S, Sr, Mg) X−ray mapping of polished sections using wavelength−dispersive techniques (WDS) showed an overall homogenous distribution of S, Sr and Mg (the latter is a trace element). However, “strands” as viewed in TLM ( Fig. 6A ), or darker regions as viewed in BSE mode ( Fig. 6B ), are enriched in carbon ( Fig. 6C ).

Sectionperpendiculartothewall.—Polished and etched sections perpendicular to the marginothecal wall ( Fig. 5B ) show overall continuity of layers of fibers between dRAF and TD. The main difference between these regions is negative etching relief of broader zones in dRAF region and only narrow zones of negative relief between fiber layers in TD; pattern of layers continuing between dRAF and TD zones is very similar to that observed in S.paliferus in section perpendicular to the septum, however, in Flabellum TD are formed primarily on the inner wall side (layers that continue on outer side of the wall, named tectura by Stolarski (1995), are narrow and fade downwards).

Figures

Fig. 5. Flabellum chunii Marenzeller, 1904. Recent, Great Meteor Seamount, SEAMOUNT2 (1993), DW 152 (January 11, 1993), 30°02.00’N, 28°22.10’W, 470 m. A. Septa and inner side of wall (A1) thickened by fibers arranged in scale−like units (A2, enlargement); ZPALH.23/2/1. B. Marginothecal wall sectioned longitudinally (large white arrow); layers of successive growth increments (dRAF) continue in wall “stereome”, i.e.,TD (small white arrows); ZPAL H.23/2/2. C. Septum longitudinally sectioned in RAF plane. Dissolved or etched components of RAF form narrow “strands” (C2 enlargement); ZPALH.23/2/3. D. Transverse polished and etched section of septum; dRAF zone composed of neighboring dCRA (D1) is from both sides covered with layers of TD fibers (D2) which direction conform to that of scale−like units (i.e., semi−parallel to RAF); ZPALH.23/2/4.All SEM; growth direction within skeletal element i ndicated by black arrow in B.

Fig. 1. Stephanocyathuspaliferus Cairns, 1977. ZPALH.23/1 (originally NMNH 46443 lot). Recent, south of Bonaire, 11°18.8’N, 68°22’W, 384–607 m. Pills sta. P−753. July 26, 1968. A–C. Distal (A), lateral (B), and proximal (C) views of corallum. D, E. SEM of septa and paliform lobes. D. Arrow indicates portion of septum enlarged on E. E. “Patches of microcrystals” (fasciculi of Wise 1972) at the growing septal edge here called the Rapid Acretion Front (RAF).

Fig. 6. Flabellumchunii Marenzeller, 1904. ZPALH.23/2/4. Locality data as in Fig. 5. A–C. Complementary regions of longitudinally sectioned and polished septum (RAF plane): TLM (A), Back−Scattered Electron mode (BSE) (B), and pseudocolor carbon mapping images acquired on the electron microprobe by wavelength−dispersive techniques (C); black/dark blue equals lower concentration (<150 counts per second) whereas yellow−white equals higher concentrations (>150 c/s). Brownish structures exposed at section surface (A), appear darker in BSE mode (B) as it enhances atomic number contrast; elements with lower atomic numbers appear darker, those with higher atomic numbers appear lighter. BSE darker regions (arrows) match exactly to carbon−enriched regions in WDS x−ray mapping image (C, arrows). D–F. TL M (D, E) and MFM (F) micrographs of longitudinally sectioned septum. Organic and mineral phase of dRAF regularly alternate (grayscale enlargement in E); brownish organic components (D) exposed and stained with acridine orangedye, fluoresce (F) with bright−green light. Growth direction within septum indicated by black arrow.