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).
Genus Pontosphaera Lohmann, 1902
Description: Coccoliths are muroliths, central area with variable number of perforations. V-units form narrow imbricate outer wall, R-units form inner wall and central-area.
Remarks: Species level taxonomy is poorly worked out in the Neogene. The species included here are all well-established extant taxa, and they are all
frequently cited but some authors also use other taxa (e.g. Varol,
Synonyms: Crassapontosphaera Boudreaux and Hay, 1969; Discolithina Loeblich and Tappan, 1963; Discolithus Huxley, 1868; Koczyia Boudreaux and Hay, 1969.
Pontosphaera versa (Bramlette & Sullivan, 1961) Sherwood, 1974
Description: Simple unadorned plate, typically with two longitudinal slits, and a broad raised margin/rim.
Synonym: Pontosphaera scissura (Perch-Nielsen, 1971) Romein, 1979
Pontosphaera rimosa (Bramlette & Sullivan, 1961) Roth & Thierstein, 1972
Description: Simple unadorned plate with a large central hole; the rim is narrow and not distinct from the plate in XPL.
Pontosphaera punctosa (Bramlette & Sullivan, 1961) Perch-Nielsen, 1984
Description: Pontosphaera with plate that has marginal furrows and small pores, and larger pores towards the centre.
Pontosphaera pulchra (Deflandre in Deflandre & Fert, 1954) Romein, 1979
Description: The plate is scalloped towards its outer edge and pierced by two holes, defining a transverse bar. The scalloping is defined by narrow radial ridges and furrows, which run inwards towards the centre.
Remarks: Like Pontosphaera pectinata but the two central holes are larger.
Pontosphaera pulcheroides (Sullivan, 1964) Romein, 1979
Description: Pontosphaera with a plate that has a row of small pores towards its outer edge and a large central hole spanned by an oblique tranverse bar.
Pontosphaera plana (Bramlette & Sullivan, 1961) Haq, 1971
Description: Simple unadorned plate, typically with two narrow longitudinal slits; margin narrow or inconspicuous.
Remarks: Pontosphaera inconspicua (Sullivan 1964) used for small, early specimens.
Pontosphaera perforomarginata Bown, 2005
Description: Large elliptical Pontosphaera with a plate that is pierced by small pores and two central longitudinal holes that define a conjunct bar. The plate thickens towards the edge but the rim is difficult to distinguish in XPL.
Pontosphaera pectinata (Bramlette & Sullivan, 1961) Sherwood, 1974
Description: The plate is scalloped towards its outer edge and pierced by two longitudinal slits or small holes. The scalloping is defined by narrow radial ridges and furrows, which run inwards towards the centre.