Nannotax Enabling nannofossil research

6-rayed discoasters with bifurcations. This is the predominant group of Neogene 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.

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:

	D.loeblichii - asymmetric bifurcations (NN10-11A)
	D. bollii - wide central area and large bosses (NN8-9)
	D. kugleri - wide flat central area, short free rays with notched tips (NN7)
	D. musicus (variant D. sanmiguelensis - wide central area with large distal boss and sutural ridges (NN5-6)
	D. petaliformis - large distal and proximal bosses, triangular bifuractions (NN4-5)
	D. druggii - 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 small er 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.