Nannotax3 - ntax_cenozoic - Discoaster calcaris

Citation: Discoaster calcaris Gartner 1967

Variants:

D. neohamatus Bukry and Bramlette, 1969 - with more obviously asymmetric tips, which may not appear bifurcate.
Discoaster brouweri subsp. rutellus Gartner 1967 - with less obviously asymmetric tips and which do not appear bifurcate. Gartner (1967) also states that the blades can be deflected either clockwise or anticlockwise but it is not clear whether this might simply be an effect of varying way-up of the specimens (i.e. some being seen in distal view and others in proximal view).

Taxonomic discussion: D. calcaris occurs in the NN9 to NN11A interval and seems to be associated with the evolution of D. brouweri from D. exilis, it can be hard to differentiate it from either of these species. D. hamatus is a 5-rayed species with the same structure, but it has a shorter and better defined range, hence differentiation of the two is important.

Distinguishing features: 6-rayed with asymmetric ray tips; one bifurcation extended proximally

Morphology: D. calcaris is a 6-rayed species with asymmetric ray tips. There are usually weak bifurcations on the distal side, one of which is underlain by a clockwise deflected (in distal view) proximal extension. The central area is small. A proximal knob is usually present. In many specimens the asymmetry of the ray tips is increased by the blade sloping away from the ray. Diagenetic overgrowth occurs preferentially on the blade, further increasing the asymmetry.

See also: Discoaster hamatus - similar but with 5 rays, and a shorter range;

Geological Range:
Notes: D. calcaris is conventionally regarded as first occurring in NN9 (e.g. Theodoridis 1984, Young 1998). However, similar discoasters with asymmetric ray tips can occur earlier. Notably Lambert (2005) documents occurrences as low as zone NN6 (his drawing A3).
Browning et al. (2017) report D. calcaris as extending higher than D. neohamatus.
Last occurrence (top): in upper part of NN11 zone (60% up, 6.7Ma, in Messinian stage). Data source: Browning et al. (2017)
First occurrence (base): near base of NN9 zone (10% up, 10.4Ma, in Tortonian stage). Data source: Browning et al. (2017)

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 V O

Bukry, D. & Bramlette, M. N. (1969b). Some new and stratigraphically useful calcareous nannofossils of the Cenozoic. Tulane Studies in Geology. 7: 131-142. gs V O

Gartner, S. (1967a). Calcareous nannofossils from Neogene of Trinidad, Jamaica, and Gulf of Mexico. University of Kansas Paleontological Contributions, Papers. 29: 1-7. gs V O

Lambert, B. (2005). Discoaster zonation of the Miocene of the Kutei Basin, East Kalimantan, Indonesia (Mahakam Delta Offshore). Carnets de Géologie - Memoir. 2005(1): 1-63. gs

Müller, C. (1974b). Calcareous nannoplankton, Leg 25 (Western Indian Ocean). Initial Reports of the Deep Sea Drilling Project. 25: 579-633. gs V O

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

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 V O

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

Tags	LITHS: nannolith-radiate, star-shaped, CA: process, CROSS-POLARS: 1ou, V-prominent,
Metrics	Lith size: 10->30µm; Segments: 6->6; Data source notes: size from illustrated specs

Discoaster calcaris

References:

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