Order ZYGODISCALES Young & Bown 1997
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 and H. wallichii have been cultured repeatedly (Inouye pers. comm.; Probert & Houdan 2004) and Scyphosphaera apsteinii once (Probert & Houdan 2004). 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), Pontosphaera and Scyphosphaera (Frada et al. 2008). These indicate that the haploid phase forms holococcoliths with distinctive rhomboid-array ultrastructure (formerly included in the genus Syracolithus).