Nannotax3 - Coccolithophores - Discoaster hamatus

Citation: Discoaster hamatus Martini and Bramlette, 1963

Notes & discussion: This species seems to have an identical structure to that of the 6-rayed species D. calcaris (q.v.). Typically only large specimens (>c.10 microns) have well developed offset proximal extensions, associated smaller specimens lack these and are better placed in D. bellus for biostratigraphic rigour. Both the D. calcaris and D. bellus morphotypes have longer stratigraphic ranges than true D. hamatus.
Browning et al. (2017) record that the smaller specimens (<15µm) occur above and below NN9 so for stratigraphic rigour the large specimens need to be used.

Morphology remarks: Symmetric 5-rayed discoaster; rays with clockwise deflected (in distal view) proximal extensions. The central area is small with a featureless distal surface and small proximal knob.

Size: Usually only large specimens, (>10 µm) show the characteritic raytip development

References:

Browning, E., Bergen, J. A., Blair, S. A., Boesiger, T. M. & de Kaenel, E. (2017). Late Miocene to Late Pliocene taxonomy and stratigraphy of the genus Discoaster in the circum North Atlantic Basin: Gulf of Mexico and ODP Leg 154. Journal of Nannoplankton Research. 37(2-3): 189-214. gs

Martini, E. & Bramlette, M. N. (1963). Calcareous nannoplankton from the experimental Mohole drilling. Journal of Paleontology. 37: 845-855. gs

Perch-Nielsen, K. (1972). Remarks on Late Cretaceous to Pleistocene coccoliths from the North Atlantic. Initial Reports of the Deep Sea Drilling Project. 12: 1003-1069. gs

Theodoridis, S. (1984). Calcareous nannofossil biostratigraphy of the Miocene and revision of the helicoliths and discoasters. Utrecht Micropaleontological Bulletin. 32: 1-271. gs

Wei, W. (2003). Upper Miocene nannofossil bistratigraphy and taxonomy of Exxon core CH30-43-2 from the Gulf of Mexico. Journal of Nannoplankton Research. 25(1): 17-23. gs

Young, J. R. (1998). Neogene. In, Bown, P. R. (ed.) Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publication Series. 225-265. gs

Tags	LITHS: nannolith-radiate, star-shaped, RIM: free-rays-long, tips extended proximally, tips oblique, PROCESS: boss, boss-proximal, CSPH: not known, V-UNITS: all, R-UNITS: none, DETAILS: 1ou,
Metrics	Lith size: 10->25µm; Segments: 5->5; Sources: size from OD & illustrated specs

Discoaster hamatus

References:

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Daughter taxa: Variants (could also be distinguished informally) (blue => in age window 0-800Ma)
				Discoaster bolivariae 4-rayed D. hamatus
				Discoaster triuncinus 3-rayed D. hamatus