Nannotax3 - ntax_main - Calcidiscus bicircus Nannotax3 - ntax_main - Calcidiscus bicircus

Calcidiscus bicircus


Classification: ntax_main -> Coccolithales -> Calcidiscaceae -> Calcidiscus -> C. pacificanus group -> Calcidiscus bicircus
Sister taxa: C. bicircus, C. gerrardii, C. pacificanus, C. parvicrucis, C. parvus, C. scullyae, C. kamikurii


Taxonomy:
Citation: Calcidiscus bicircus Bown, 2005
Taxonomic rank: species

Distinguishing features:
Parent taxon (C. pacificanus group): Medium-sized Eocene calcidiscid coccoliths with narrow or closed central-area.
This taxon: Medium to large (5-9 µm) circular-subcircular placoliths with a non-birefringent distal shield and narrow to closed central area surrounded by a narrow, bright tube cycle.

Farinacci & Howe catalog pages:
C. bicircus *

Search data:
LITHS: placolith, circular, elliptical, CA: closed,
CSPH: equant, monomorphic, CROSS-POLARS: rim-bicyclic, V-prominent, R-prominent,
Lith size: 5->9µm;
The morphological data given here can be used on the advanced search page. See also these notes

Geological Range:
Last occurrence (top): within NP17 zone (37.32-40.40Ma, top in Priabonian stage). Data source: Bown et al., 2007
First occurrence (base): within NP12 zone (50.50-53.70Ma, base in Ypresian stage). Data source: Shamrock & Watkins 2012

Plot of occurrence data:

References:

Bown, P. R. & Newsam, C. (2017). Calcareous nannofossils from the Eocene North Atlantic Ocean (IODP Expedition 342 Sites U1403–1411). Journal of Nannoplankton Research. 37(1): 25-60. gs O

Bown, P. R. (2005d). Palaeogene calcareous nannofossils from the Kilwa and Lindi areas of coastal Tanzania (Tanzania Drilling Project 2003-4). Journal of Nannoplankton Research. 27(1): 21-95. gs O

Bown, P. R., Dunkley-Jones, T. & Young, J. R. (2007). Umbilicosphaera jordanii Bown 2005 from the Paleogene of Tanzania: confirmation of generic assignment and a Paleocene origination for the family Calcidiscaceae. Journal of Nannoplankton Research. 29(1): 25-30. gs

Jones, A. P. & Dunkley Jones, T. (2020). Middle Eocene to Early Oligocene calcareous nannofossils from the Nanggulan Formation, Java, Indonesia. Journal of Nannoplankton Research. 38(1): 57-79. gs

Self-Trail, J. (2011). Paleogene Calcareous Nannofossils of the South Dover Bridge core, Southern Maryland (USA). Journal of Nannoplankton Research. 32(1): 1-28. gs

Shamrock, J. L. & Watkins, D. K. (2012). Eocene calcareous nannofossil biostratigraphy and community structure from Exmouth Plateau, Eastern Indian Ocean (ODP Site 762). Stratigraphy. 9(1): 1-11. gs

Shepherd, C. L. & Kulhanek, D. K. (2016). Eocene nannofossil biostratigraphy of the mid-Waipara River section, Canterbury Basin, New Zealand. Journal of Nannoplankton Research. 36(1): 33-59. gs O

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Calcidiscus bicircus compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 11-4-2026

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Comments (2)

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Its images seem to Biscutum blackii from Late Cretaceous species. If being so and found within NP14-NP17 zone assemblage, it may have been reworked from old sediments.
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C. bicircus certainly does resemble Biscutum but it occurs in Tanzania in sites with negligible reworking and has a consistent range there so it probably is an example of homoeomorphy not reworking.