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Discoasters

Dicoasters are the most important single group of Cenozoic nannoliths, ranging from the Palaeocene to the Pliocene. The numerous species are of great biostratigraphic value (see also Neogene range chart)

Informal taxon-based terms:
Discoaster nannolith formed by Discoasteraceae.
Eu-discoaster typically Neogene and usually star-shaped discoasters, with planar contact surfaces between elements.
Helio-discoaster typically Palaeogene and usually rosette-shaped discoasters, with curved contact surfaces.
N.B. These terms are useful even if formal taxonomic division into the genera Eu-discoaster and Helio-discoaster is not made. The differences between them are given in Theodoridis (1984) and Aubry (1984).

Orientation:
In virtually all discoasters, there are consistent differences between the two faces (Stradner and Papp 1961; Prins 1971; Romein 1979; Aubry 1984; Theodoridis 1984; Self-Trail and Bybell 1995). Many of the Neogene eu-discoasters consistently have one concave and one convex face and, by analogy to coccoliths, these faces have been termed proximal and distal (e.g. Theodoridis 1984). The two sides are also consistently characterised by various other structures, in particular there are usually sutural ridges on the distal side (Text-fig. 11). These structures can be used to separate the proximal and distal faces of planar eu-discoasters.
In helio-discoasters the rays are usually curved, so laevogyral and dextrogyral faces can be distinguished. Moreover, the curvature is usually stronger on the laevogyral surface. It is not, however, certain which of these faces corresponds to the proximal face in eu-discoasters, and they have varying relationships to discoaster concavity. Hence, the terms proximal and distal have been applied rather inconsistently in this group (compare Stradner and Papp 1961, Prins 1971 and Romein 1979) and are, perhaps, better avoided.
Proximal Concave side of eudiscoaster
Distal Convex side of eudiscoaster
Laevogyral face Side of heliodiscoaster showing left-handed curvature of rays
Dextrogyral face Side of heliodiscoaster showing right-handed curvature of rays


Ray-related terms
Ray disc element.
Free ray part of ray protruding beyond central-area
Ray tip outermost part of ray.
Bifurcate tip ray tip divides into two bifurcations (e.g. D. variabilis).
Simple tip ray tip without bifurcation or proximal extension.
Proximal extension continuation of the ray downward from the tip (e.g. D. brouweri).


Other terms
Central-area portion of discoaster with rays in contact.
Contact-surface surface between adjacent elements (alternative term attachment surface, see appendix).
Disc main part of discoaster, excluding bosses or stems.
Boss low distal or proximal protrusion from centre of disc (alternative term knob, see appendix).
Rosette-shaped discoaster with short free rays (Text-fig. 10).
Segment ray and associated boss or stem elements.
Stem high distal or proximal protrusion from centre of disc (e.g. Discoaster kuepperi).
Star-shaped discoaster with long free rays (Text-fig. 10).
Sutural ridge ridge running along suture.
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