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Coccolithales
nanno-taxonomy (8)
Coccolithales (2)
Calcidiscaceae (5)
Coccolithaceae (6)
Description. Discoidal nannoliths of 3-40 elements radiating from a common centre. C-axes vertical, so nannoliths appear dark in plane-polarised light. Some early forms also include a cycle of birefringent units.
Taxa included: Discoaster, Catinaster.
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6-rayed discoasters with bifurcations. |
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Discoaster brouweri group 6-rayed discoasters without bifurcations, and forms closely related to D. brouweri with 3 to 5 rays |
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Discoaster pentaradiatus group symmetric 5-rayed discoasters |
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6-rayed discosters in which the birfurcations meet to form a rim, which extends proximally |
Sizes of Discoasters
I have been looking at published size ranges for Discoasters and in general they appear to be somewhat optimistic. For example in Aubry’s Handbook of Cenozoic Calcareous Nannoplankton she lists a size range of Discoaster tamalis as 10-12 microns. In general the members of this taxon I have observed have been smaller than the lower end of this range, usually in the 4-8 micron range. Similarly, D. prepentaradiatus is listed as 9-12 microns. If I saw one of these that was 9 microns I would think it was unusually large a 12 micron specimen would send me running down the hall to show somebody. These are just examples, most of the other Discoasters (also in other publications) are reported as being similarly large. Are the Discoasters of the Gulf of Mexico a dwarf population, or are authors describing giants?
some thoughts
Good point Mike, I think there are a few factors at work. First most discoasters range in size siginifcantly. Second, if you are looking at pelagic sediments with lots of discoasters then you naturally concentrate on just the largest ones, but if you are doing biostrat in shelf sedients where discoasters are rare a hen's teeath you look at very last specimen. Third with declining preservation the ends get knocked off rays and so discoaters get smaller. Fourth determining size ranges accurately is hard work and sometimes authors have probably just measured published photos, which of course are the biggest specimens going.
Jeremy
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