Nannotax Enabling nannofossil research

Description: Coccoliths are placoliths with rim structure similar to Calcidiscus, i.e. the V-unit forms distal shield and central-area/tube; R-unit forms proximal shield only. As in the Coccolithaceae, growth occurs downward from the proto-coccolith ring which becomes embedded within the structure so that alternating V- and R-units are only visible on specimens where the proximal shield has broken off.

Distal shield sutures usually show laevogyral curvature. The proximal shield is usually formed of a single layer of elements with sub-radial sutures; sometimes a lower layer is developed, with elements showing strong dextral obliquity (in proximal view). The connection between the proximal and distal shields is weak and they frequently separate.

Remarks: Dominant phase of life-cycle non-motile with placolith heterococcoliths. Holococcolith bearing motile phase has been documented in the extant species C. leptoporus and C. quadriperforatus (refs. Gesen et al. 2002, Houdan et al. 2004).

Taxa included: Calcidicus, Umbilicosphaera, Oolithotus, Cryptococcolithus, Hayaster.
All these genera were included within the Coccolithaceae in older classifications but were separated by Young & Bown (1987) into the family Calcidscaceae on the grounds of them sharing a simpler coccolith structure than that of the Coccolithaceae. The grouping has been supported by molecular genetic data (Sáez et al., 2003, 2004).

Selected references: Inouye & Pienaar (1984) - cytology of Umbilicosphaera; Kleijne (1993) - detailed description of extant species; Knappertsbusch (2000) Calcidiscus evolution; Young et al. (2004) - coccolith structure.