J. Nannoplankton Res. 33 (1), 2014, pp23-28 © 2014 International Nannoplankton Association23

A new perspective of the central structure andthe alignment of neogene discoasters
Rui O %P Ga GamaShell International Exploration and Production Inc, USA; Rui.Da-Gama@shell.com

Osman Varol

Varol Research_USA, 5212 Sagesquare Street, Houston, TX77056, USA; osman@varol.com

ABSTRACT: With few exceptions, Neogene star-like Discoasters exhibit diamond shaped distal depressions (concavities) on radial elements. The concavities and the sutures they form appear to be primary morphological features
observed on the distal side of the Discoaster. The concavities, sutures and the inner end of rays, bend and extend
outwards to form processes (distal bosses). On the proximal side, the boss (when present), results from the inward
merging of proximal ridges. Bosses are an integral part of the individual ray element and thus are not often observed
in isolation within the sedimentary record. Herein we discuss the relationship between proximal and distal features
observed on the centre of star-like Discoasters and speculate these may have been joined on 'male/female' structurescomprising two or more Discoasters; on these structures the Discoasters rays are not aligned and seem offset or rotated.
This offsetting suggests a mechanism that facilitates the interlocking and possibly the movement between proximaland distal structures.
Key Words: Nannofossils, Neogene, Discoaster, Structure
Introduction
Discoasters are the most important single group of Cenozoic nannoliths, ranging from the Paleocene to the Pliocene. The first illustrations of this incertae sedis groupwere made by Ehrenberg, 1854. In 1927 Tan introduced
the term Discoaster for stellate or star-shaped nannoliths.
Tan also proved the organic nature of these structures. Theorigin and the structure of Discoasters remains not fully
understood as they are mostly recorded isolated in both
SEM and LM preparations. Various authors have proposed reclassification or splitting of the genus Discoaster,e.g. Ehrenberg, 1854; Tan, 1927; Deflandre, 1954; Loeblich & Tappan, 1963; Prins, 1971; Theodoridis, 1984).
Loeblich & Tappan (1963) validated the genus Discoasterby designating Discoaster pentaradiatus as the type species. Bramlette & Riedel (1954) paved the basis for exist-ing taxonomy. Although there have been many publications discussing taxonomic aspects of the group little has
been added to our understanding of their structure. Theirconcavo-convex curvature suggests disposition around aspherical organism but we cannot be certain whether they
were intracellular or extracellular (Aubry, 1984). Stradner & Papp (1961) is an insightful work that explains
the structure of Discoasters with illustrations but lacks indepth discussions and supportive evidence from real images.
The group includes a variety of morphologies withtwo main characteristics: radial symmetry in plan viewand no birefringence on the polarising light microscope(with very few exceptions e.g. D. pentaradiatus and D.
quintatus). Many Neogene Discoasters have concave andconvex faces designated proximal and distal (Farinacci,Fig. 1 & 2 Discoaster terminology herein. (Proximal view - picture courtesy of Dr. Jeremy Young). Note: In the natural settling position, the C axis ofNeogene Discoasters (with the exception of D. pentaradiatus and D. quintatus) is nearly vertical to the microscope stage and appears non-birefringent.

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3456Fig. 3 Scanning Electron Microscope (SEM) image displaying the absence of concavity; Fig. 4-6 SEM images showing how concavity, sutures andboss are related and control the morphology of the central area in the distal side.

7a7b7c7dFig. 7 (a-d): SEM images displaying the construction of central area.

1989). Their star-like structure is composed of wedgeshaped radially arranged elements or rays (Fig. 1) thatconverge to the centre of the Discoaster (Stradner andPapp, 1961; Aubry, 1984).
The number, shape and relation of these elements determine the overall outline of species. In addition, Neogene star-like Discoasters are classified using other structural elements as shown in Fig.1 & 2, including: a) suturalridges (sutures) on the distal side; b) the form of ray terminations; c) appendages protruding beyond the ray tips
reflecting the extension of proximal ridges (visible fromthe distal side); d) ridges along the extension of the rayand a more or less prominent boss with a knob or star
shape in the centre in both proximal and distal sides.
Discussion
Distal SideWith few exceptions (Fig. 3 & Fig. 7a) the centre of Neogene star-like Discoasters exhibit concavities (diamondshaped distal depressions) in the inner end of radial elements. The concavities are a consistent feature of the distal side and appear to control the overall shape of the cen-

8a8b8ctral area of most Neogene star-like Discoasters. The depthof the concavities determines how pronounced the suturalridges are (Figs. 4-6; Fig. 9a-d). The concavities are a
primary morphological feature for speciation frequently
observed on the distal side. The sutural ridges converge
towards the centre of the Discoaster where they meet andextend outward to form the central boss (Figs. 4-6 andFig. 9a-d). The shape and size of the resultant boss (Fig.
9a-d) is a primary morphological feature for speciation.
Light microscopic images of single ray elements inside view Fig. 8 (a-e) show continuation or extension ofthe ray to form distal and proximal bosses. Although thisidea is not new, no clear evidence from LM and SEM images and discussions were presented to support it. Herein
we show that ray elements and bosses are part of a single
unit rather than separate structures. In addition, SEM images (Fig. 7 (a-d)) show the continuation or extension ofsutures through to the central process and confirm these
are not separate structures but an integral part of the rayelement. Isolated (broken off) bosses are very rarely observed, which provides further evidence for structuralintegrity. The evidence above supports structural observations by Stradner (in Stradner & Papp, 1961) and The-8d8e

Fig. 8 (a-e): LM images showing single ray elements containing both proximal and distal bosses indicating these are integral part of the ray. Thebosses form at the inner end of the ray. On the side view of broken rays of Discoasters the C axis is not vertical to the microscope stage and thus theray element are birefringent. With a gypsum plate, the c-axis is parallel to the gypsum slow ray and appears yellow (tangential c-axis).

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9a9b25

9c9dFig. 9 (a-d): SEM images of the distal side with examples of deep concavities and distinct bosses.

10a10b10c10dFig.10(a-d): SEM images showing examples of proximal bosses and ridges

odoridis (1984). However, due to preservation factors,
the extension of sutural ridges into the central boss is notalways visible. Central boss structures vary considerably
and may assume several shapes: flaring, stellate, slenderor robust Fig. 9 (a-d), Fig.10 (a-e). The distal ridges arepositioned between the outer end of the ray and the concavity and thus are often considerably shorter than theproximal ridges Fig. 9 (a-d).
Proximal side
The proximal side is characterised by the absence of concavities and the presence of more or less prominent ridges
and a central boss. The proximal boss is formed via merging and thickening of proximal ridges (Fig.10 & Fig.11b).
Stack arrangement
On rare occasions, star-like Discoasters appear to bejoined by their bosses to form chains or stacks of two
or more Discoasters (Fig.12 a-c, Plate 1). Within thesestructures a preferential arrangement is inferred with the11a

proximal face of one Discoaster interlocking with the distal face of the adjoining Discoaster. The proximal, distaland sutural ridges together with concavities seem to facilitate interlocking, with a slightly offset or rotated junction necessary in order to lock adjacent structures. The
proposed locking mechanism is supported by the disposition of structures on proximal and distal sides and by theoffset rotation of the individual Discoasters in the chain
structures observed in Fig. 12 (a, b, c) and in Plate 1. Thismechanism possibly allowed some flexibility of movement within the chain arrangement.
The bosses appear to be structurally weak and preferential surfaces for splitting, and thus chains/stacks
are rarely preserved. It is possible that Discoasters werecrystallographically joined by the bosses through a weak‘calcite bridge’ or more likely by an organic ‘adhesive’
possibly made of soft tissue.
Several nannofossil specialists occasionally indicate observing these structures in well preserved nannofossil rich samples, however, they prefer to overlook

11b11cFig.11(a-c) SEM & LM image showing D. brouweri displaying off-centre ridges.

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Plate 1

Discoaster brouweri

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A new perspective of neogene discoasters

12a12b27

12c

Fig.12(a-c) LM image showing chain arrangement of Discoasters.

(non age diagnostic) and consider them a random agglomeration of Discoasters. Further research will allow a better understanding of the mechanisms associated with thealignment of Discoasters.
Conclusions
We have shown that with few exceptions Neogene starlike Discoasters exhibit concavities (or depressions) in theinner ends of their ray elements. The concavities and thesutural ridges in between are primary morphological features for speciation and appear to control the overall shapeof the central area on the distal side of most Neogene starlike Discoasters.
The central structure is formed via merging of individual ray elements and is not a crystallographically separate
structure. This is confirmed by LM images of individualray elements showing vestiges of proximal and distal
bosses to be an integral part of the ray unit, and by SEM
images showing the extension of the sutures through tothe central process.
We demonstrated that star-like Discoasters are occasionally joined by their bosses (proximal surface againstdistal surface) in structures with two or more Discoasters. The Discoasters in these structures are not alignedand seem to show an offset rotation possibly reflecting
the mechanism that facilitates interlocking and a certain
flexibility to the structure. The bosses appear to be structurally weak and preferential surfaces for splitting, andconsequently these structures are rarely preserved.
We aim to raise the awareness of these type of structures and possibly at fomenting further research in orderto better understand the organism that produced the Discoaster.
Acknowledgements
We acknowledge Dr. Dave Rutledge for his proof reading
and suggestions.
Recommendations
Due to rare observations of Discoasters in stack arrangements, we recommend, when possible, to record theiroccurrences and to post into a centralised database (e.g.
Nannotax website).

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