Nannotax Enabling nannofossil research

Taxa included: The extant families Helicosphaeraceae and Pontosphaeraceae and extinct family Zygodiscaceae. These show highly variable shape but similar structure and there is strong palaeontological evidence for their evolutionary connections (Romein 1979; Aubry 1989).

Coccolith structure: V-units form outer rim, in Pontosphaeraceae this is a narrow cycle of anticlockwise imbricating elements, in the Helicosphaeraceae a helical flange. The R-units form a proximal plate of rather regular inward-growing elements and a distal blanket, which typically appears as a mass of minute tangentially elongated crystallites. Growth does not occur downwards from the proto-coccolith ring and so the alternating belt of V-R nuclei remains clearly visible on the proximal surface.

Life-cycles and culture studies: Helicosphaera carteri has been cultured repeatedly (Inouye pers. comm., Probert unpubl. data) and Scyphosphaera apsteinii once (Probert unpubl. data). No life-cycle transitions have been observed in these cultures, but combination coccospheres have been observed for Helicosphaera (Cros et al. 2000, Geisen et al. 2002). These indicate that the haploid phase forms holococcoliths with distinctive rhomboid-array ultrastructure (formerly included in the genus Syracolithus).