Daughter taxa (time control age-window is: 0-800Ma) | Granddaughter taxa | ||||
Species with trifurcate ray-tips | |||||
Discoaster surculus 6-rayed with trifurcate ray-tips, central branch directed proximally | |||||
Discoaster pseudovariabilis Central branch of trifurcate tip notched | |||||
Discoaster subsurculus Like D. exilis but with knob or tongue between the bifurcations | |||||
D. variabilis and variants | |||||
Discoaster variabilis 6-rayed discoaster with well-developed bifurcate tips; slightly concavo-convex and proximal and distal sides well-differentiated. | |||||
Discoaster loeblichii Like D. variabilis but bifurcations asymmetric | |||||
Discoaster mendomobensis Like D. variabilis but bifurcations narrow and tapering, central area broad |
Taxonomy:
Distinguishing features:
Parent taxon (Discoaster): Radiate nannoliths with each ray formed of a discrete crystal-unit, with the c-axes perpendicular to the nannolith surface.
This taxon: 6-rayed discoasters with bifurcations
Farinacci & Howe catalog pages:
Morphology:
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.
Lith size: 3->22µm; |
Geological Range:
Last occurrence (top): at top of NN16 zone (100% up, 2.5Ma, in Gelasian stage). Data source: Total of ranges of the species in this database
First occurrence (base): within NN5 zone (13.53-14.91Ma, base in Langhian stage). Data source: Total of ranges of species in this database
Plot of occurrence data:
D. variabilis group compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 11-9-2024
Short stable page link: https://mikrotax.org/Nannotax3/index.php?id=425 Go to Archive.is to create a permanent copy of this page - citation notes |
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