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).
Syracosphaera pulchra Lohmann 1902 HOL pirus type
Description: Coccolith structure - dome-shaped, formed of layer of crystallites in hexagonal array with the crystallographic c-axis perpendicular to the surface. In LM the rim is bright and central part dark.
Coccospheres weakly dimorphic, piriform.
Coccolith form - tube about 10 crystallites high, tiered wall fabric; basal flange;
distal cover recessed into tube, domal with ring of 8-10 pores around
contact with tube; internal ridges surround pores in at least some
specimens. Circum-flagellar coccoliths more elevated and with pyramidal central boss.
Life-cycle association: The association of these holococcoliths with Syracosphaera pulchra has been demonstrated by observation of numerous combination coccospheres, Cros et al. (2000), Geisen et al. (2002). Saugestad & Heimdal (2002). oblonga-type holococcoliths also occur in assocaition with S. pulchra and this is interpreted as a result of pseudo-cryptic specieation (Geisen et al. 2002).
Syracosphaera pulchra Lohmann 1902 HOL oblonga type
Description: Coccolith structure - dome-shaped, formed of layer of crystallites in hexagonal array with the crystalographic c-axis perpendicular to the surface. In LM the rim is bright and cenral part dark.
Coccospheres weakly dimorphic, piriform.
Coccolith form - the coccoliths have simple dome-shaped, with wall showing heaxoganla mesh fabric. Circum-flagellar coccoliths more elevated and with pyramidal central boss.
Life-cycle association: The association of these holococcoliths with Syracosphaera pulchra has been demonstrated by observation of numerous combination coccospheres, Cros et al. (2000), Geisen et al. (2002). Pirus-type holococcoliths also occur in assocaition with S. pulchra and this has been interpreted as result of pseudo-cryptic speciation (Geisen et al. 2002).
Fossil record: Can be common in well-preserved Quaternary sediments (Crudeli et al. 2006).
Synonym: Calyptrosphaera oblonga Lohmann 1902
Zygrhablithus bijugatus (Deflandre in Deflandre and Fert, 1954) Deflandre, 1959 [Zygolithus]
Description: Large rod-shaped holococcolith with small basal disk.
Remarks: In well-preserved material (E.g. Bown 2005) it can be seen that these are cavate holococcoliths, i.e. they have an internal cavity. Usually howevr with overgrowth this cavitiy is filled and the holococcolith develops nirregular form.
Synonyms (list from Aubry 1988):
Of these I. claviformis, S. enigmatica and R. semiformis were all based on isolated half or quarter specimens of Z. bijugatus.
Nannoliths - informal grouping. Used here in the sense of Young & Bown (1987)
Description - the following groups of Neogene nannofossils are thought to be related to coccolithophores, but produce structures which are neither heterococcoliths nor holococcoliths.
The term nannolith has been used, especially by palaeontologists, as a convenient term for structures about the same size as coccoliths and occurring with coccoliths, but lacking definite coccolith affinities. In the modern nannoflora, there are fewer groups of cryptic origins, and the term has been less widely used. However, it is useful for calcareous structures that are thought to be formed by haptophytes, but probably by a different biomineralisation process to either heterococcoliths or holococcoliths (Young et al. 1999).
NB The first use of the term in this sense appears to have been by Haq (1978, in Introduction to Marine Micropaleontology), and this was followed by Perch-Nielsen (1985, in Plankton Stratigraphy). By contrast, Aubry (1988 et seq., Handbook of Calcareous Nannofossils) places most of these groups in the Ortholithae.
Syracosphaera pulchra Lohmann 1902
Extant coccolithophore: A common species, with weak preference for oligotrophic areas (Ziveri et al. 2004); coccosphere large, trimorphic with body coccoliths (BCs), spine-bearing circum-flagellar coccoliths (CFCs), and exothecal coccoliths (XCs).
Coccospheres 15-25 µm
|Body coccoliths (BCs) - large (4.5-8 µm); mid-wall flange prominent; inner wall-cycle well developed; central area with three concentric cycles of thin radial laths, occasionally incipient spine in centre|
|Circum flagellar coccoliths (CFCs) - similar to BCs but with robust bifurcate-tipped spine|
|Exothecal coccoliths (XCs) - dome-shaped, rim sub-horizontal, flange-like. Radial cycle formsvertical part of wall, numerous elements with wide slits. Central part formed of several cycles of laths; with a central conical depression.|
Life-cycle: There is a well-estabished alternate holococcolith-bearing phase. Holococcoliths previously refered to as = Calyptrosphaera oblonga and Calyptrosphaera pirus, see Cros et al. (2000), Geisen et al. (2002), Saugestad & Heimdal (2002). Since the oblonga-type and pirus-type holococcoliths are well differentiated we suspect that S. pulchra consists of two species which are only differentiable in the holococcolithophore life-cycle stage (Geisen et al. 2002, 2004).
Fossil record: BCs commonly recorded from Pliocene and Pleistocene sediments, reasonably reliable default identification for large (>5µm) Syracosphaera coccoliths.
Description: Holococcoliths are formed of numerous, minute rhombohedral calcite crystallites. They are produced during the haploid phase of the life-cycle of a wide range of coccolithophores that bear heterococcoliths in their diploid life-cycle phase.
|Coccosphere of Syacrosphaera pulchra holococcoliths||Coccosphere of Helicosphaera carteri holococcoliths||Combination coccosphere of Calcidiscus quadriperforatus||Fossil holococcoliths - Syracolithus schilleri|
Remarks: Although diverse holococcolith assemblages have been described from Cretaceous and Palaeogene assemblages, they are virtually absent from Neogene nannofloras. This is rather surprising, given that modern nannofloras include >60 holococcolithophores (see, e.g., Kleijne, 1991; Jordan & Kleijne, 1994; Young et al. 2003), but most species produce very small holococcoliths (often <2μm) with low preservation potential.
Only the relatively few holococcoliths with a reasonable Neogene fossil record are included here. Where the heterococcolith phase is known the taxonomy has been adjusted, as recommended by Thomsen et al. (1991), Cros et al. (2000) and Young et al. (2003).
Range: Mesozoic to Recent
Synonym: Family CALYPTROSPHAERACEAE Boudreaux & Hay, 1969. It used to be conventional to place all holococcoliths in the Famly Calyptrosphaeraceae, however this is manifestly artificial, since it is now known that holococcoliths are formed during the haploid life-cycle stage of species belonging to numerous different families. Moreover, the type species of the Family Calyptrosphaeraceae, Calyptrosphaera oblonga , is now known to be a life-cycle phase of Syracosphaera pulchra, hence the name Calyptrosphaeraceae is a junior synonym of Syracosphaeraceae.
It is convenient to group the holococcoliths together for identification purposes, but giving this artificial grouping a formal taxonomic name (e.g. Calyptrosphaeraceae) is not defensible.