|Daughter taxa (time control age-window is: 0-800Ma)||Granddaughter taxa|
|Species with trifurcate ray-tips|
6-rayed with trifurcate ray-tips, central branch directed proximally
=D. surculus variant with weakly developed bifurcations, central area broad with thin inter-ray areas, giving "webbing" appearance
Central branch of trifurcate tip notched
Like D. exilis but with knob or tongue between the bifurcations
|D. variabilis and variants|
6-rayed discoaster with well-developed bifurcate tips; slightly concavo-convex and proximal and distal sides well-differentiated.
Like D. variabilis but with "flaps" between the bifurcations
Like D. variabilis but with very broad bifurcations and >15 µm in diameter
Like D. variabilis but with very broad bifurcations and <15 µm in diameter
Like D. variabilis but with narrow bifurcations
Like D. variabilis but bifurcations asymmetric
Like D. variabilis but bifurcations narrow and tapering, central area broad
Distinguishing features: 6-rayed discoasters with bifurcations
Farinacci & Howe catalog pages:
Morphology: 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.
The other species in the group intergrade with the dominant Discoaster species but represent distinctive end-member morphologies which are of biostratigraphic value, e.g.: D. petaliformis D. musicus, D. kugleri, D. bollii. 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 are using is just a specific morphotype of D. musicus, but in any case its a distinction that has stratigraphic function. Richard Howe was kind enough to let me use an image of our D. sanmiguelensis concept.
|Metrics||Lith size: 3->22µm;|
Last occurrence (top): near base of Gelasian Stage (13% up, 2.5Ma, in Gelasian stage). Data source: Total of range of species in this database
First occurrence (base): in mid part of Langhian Stage (49% up, 14.9Ma, in Langhian stage). Data source: Total of range of species in this database
Plot of occurrence data:
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