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D. variabilis group
Edited by Lyndsey on Tue, 01/03/2011 - 15:41
Discoaster variabilis group
6-rayed discoasters with bifurcations. This is the predominant group of Early and Middle Miocene discoasters. N.B. Asymmetric 5-rayed specimens of all these species occasionally occur, and, especially in the Early Miocene, 7-rayed forms.
The dominant species are succesively D. deflandrei (NN1-5), D. exilis (NN6-10), and D. variabilis + D. surculus (NN10-16) but they intergrade and are of limited proven biostratigraphic value, except for the LOs of D. variabilis and D. surculus.
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| D. deflandrei |
D. exilis | D. variabilis |
D. surculus |
The following forms mostly intergrade with the dominant Discoaster species but represent distinctive end-member morphologies which are of biostratigraphic value:
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- asymmetric bifurcations (NN10-11A) |
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- wide central area and large bosses (NN8-9) |
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- wide flat central area, short free rays with notched tips (NN7) |
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D. musicus (variant D. sanmiguelensis - wide central area with large distal boss and sutural ridges (NN5-6) |
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- large distal and proximal bosses, triangular bifuractions (NN4-5) |
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- large variant of D. deflandrei (NN2-3) |
NB Discussion of species concepts in this group is very welcome.
Mike Styzen - In the GOM D. sanmiguelensis is used as a marker and is distinguished as being somewhat smaller than D. musicus with shorter rays. One might argue that what we're using is just a specific morphotype of D. musicus, but in any case it's a distinction that has stratigraphic function. Richard Howe was kind enough to let me use an image of our D. sanmiguelensis concept.
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Neogene: D. variabilis group; Discoaster variabilis
Comments
Discoaster "9" (Shell US)
On the GOM charts I published I have a marker slightly older than the LAD of D. brouweri that I base on the LAD of a species designated as Discoaster 9 (P 1.8).
I have always regarded this taxon to be the last gasp of D. surculus, and thus the last gasp of the D. variabilis group. It is a robust six rayed form, usually between 5 and 10 microns with paralell sided rays which have either squared off (overgrown?) or slightly notched tips. If the central area has a knob I call it Discoaster 9a. As I recall both have the same top.
The equivalent GOM benthic foram marker is Christellaria S (LAD Lenticulina cf. wallacei)
Other workers Have another equivalent Discoaster marker Discoaster "brouweri A". This is not the same bug as Discoaster 9. I'll leave it to someone who actually uses that species to describe it.
Jeremy Young - I have
Jeremy Young - I have amended this page by adding the tables so it can act as a sort of key to the taxa
Zonation schemes
I have recently joined this site and have just begun to browse around. The first I have noticed though is the use of the NN/NP zonation by Martini. It is my understanding that this is the favored system in Europe, but wondered if you might consider including the CN/CP of Okada and Bukry as well?