Genera overview - Cenozoic holococcoliths and nannoliths

Comments: Coccolithophores which are only known from a holococcolith-bearing stage are assigned to this family. Holococcolith formation must be a rather precise biomineralisation process so this is probably not a polyphyletic grouping. However, on present evidence it is likely that holo- and heterococcoliths are formed respectively during the haploid and diploid life-cycle phases (Manton & Leedale, 1969; Rowson et al. 1986; Billard, 1994). It is quite likely that many more holococcolith taxa will prove to have heterococcolith equivalents. So for the moment the holo- and heterococcolith classifications should be seen as independent.

Holococcoliths have a very poor fossil record in the Quaternary and Neogene, perhaps largely because most of them are too small (<2µm) to be easily preserved or identified. In the Paleogene, however, there are a number of large, distinctive holococcolith taxa. It is therefore convenient to subdivide the holococcoliths into fossil (predominantly Paleogene) and extant groups. The Paleogene genera are divided into birefringent and non-birefringent groups, whilst the living group is subdivided into monomorphic and dimorphic genera, following Kleijne (1991) and Jordan et al. (1995).

A. Non-birefringent fossil holococcoliths

Holococcoliths which are non-birefringent in plan view (i.e. all crystallites have vertical c-axes), predominantly Paleogene.

Clathrolithus Deflandre, 1954 {large, discoidal with large perforations}
Holodiscolithus Roth, 1970 {discoidal with large perforations; one species H. macroporus occurs in the Neogene}
Corannulus Stradner, 1962 (= Guttilithion Stradner, 1962; Diademopetra Hay, Mohler & Wade, 1966) {discoidal with large central opening and marginal perforations or indentations}
Peritrachelina Deflandre, 1952 {crescent-shaped in plan view}
Orthozygus Bramlette & Wilcoxon, 1967 {basin shaped with a bridge (zygolith)}

B. Birefringent fossil holococcoliths

Holococcoliths showing birefringence in plan view, typically composed of several blocks with a narrow rim showing radial crystallographic orientation.

Daktylethra Gartner in Gartner & Bukry, 1969 {domal with exterior ridges and depressions. N.B.Calyptrosphaera pirus, a living species, is often assigned to Daktylethra , but has a quite different morphology}
Lanternithus Stradner, 1962 {subhexagonal in plan view}
Octolithus Romein, 1979 {discoidal, formed of 4 large and 4 small blocks}
Zygrhablithus Deflandre, 1959 (= Pseudozygrhablithus Haq, 1971; Sujkowskiella Hay, Mohler & Wade, 1966) {discoidal base extended into tall spine}
Quadrilateralis Varol, 1991 {quadrilateral rim of four blocks plus bridge}
[Semihololithus Perch-Nielsen, 1971] {defined as showing combined holococcolith and heterococcolith parts. Included Cenozoic species are assignable to Daktylethra and Zygrhablithus }

C. Extant monomorphic holococcoliths

Genera with monomorphic coccospheres, i.e. only one type of coccolith developed.

Calcicasphaera** Kleijne, 1992 {chalice-shaped coccoliths - calcicaliths}
Calyptrosphaera* Lohmann, 1902 {dome-shaped coccoliths - calyptroliths}
Flosculosphaera** Jordan & Kleijne in Kleijne et al., 1991 {flaring tube-shaped coccoliths with distal cover - flosculoliths}
Gliscolithus** Norris, 1985 {bulb-shaped coccoliths - gliscoliths}
Homozygosphaera* Deflandre, 1952 {basin-shaped coccoliths with bridge - zygoliths}
Periphyllophora** Kamptner, 1937 {basin-shaped coccoliths with bridge extended into leaf-like process - helladoliths}
Syracolithus** Deflandre, 1952 {disk-like coccoliths with variable number of depressions - laminoliths}

D. Extant dimorphic holococcoliths

Genera with dimorphic coccospheres. These have body coccoliths of one type with a second type occurring apically, i.e.around the flagellar opening.

Anthosphaera Kamptner, 1937 emend. Kleijne, 1991 {calyptrolith body coccoliths and apical coccoliths with narrow basal ring and leaf-like process - fragarioliths}
Calyptrolithina Heimdal, 1982 {calyptrolith body coccoliths and apical zygoliths}
Calyptrolithophora Heimdal in Heimdal & Gaarder, 1980 {calyptrolith body coccoliths and apical calyptroliths}
Corisphaera Kamptner, 1937 {zygolith body coccoliths and apical zygoliths}
Helladosphaera Kamptner, 1937 {zygolith body coccoliths and apical helladoliths}
Poricalyptra Kleijne, 1991 {calyptrolith body coccoliths and apical helladoliths}
Poritectolithus Kleijne, 1991 {zygolith body coccoliths and apical helladoliths}
Sphaerocalyptra Deflandre, 1952 {calyptrolith body coccoliths and apical calyptroliths}
Zygosphaera Kamptner, 1936 {laminolith body coccoliths and apical laminoliths}

3. NANNOLITHS

As noted above (Young & Bown, above), the nannolith/heterococcolith divide is subjective. We include here all forms which lack a distinct rim. Since V/R mode calcification has not been identified in any of these taxa we cannot be certain that they are directly related to the coccoliths. However they share with heterococcoliths the characteristics of being formed from a relatively low number of calcite crystals each of which has both its crystallographic orientation and morphology strongly regulated. In addition, for all these, the distribution pattern suggests a planktonic origin.

3a. Nannoliths consisting of several crystal units and showing radial symmetry

Family BRAARUDOSPHAERACEAE Deflandre, 1947

Description: See Bown & Young (above).

Braarudosphaera* Deflandre, 1947 {elements trapezoidal, sutures go to edges of the pentagon. Paleogene species are very diverse and include conical forms}
Micrantholithus Deflandre in Deflandre & Fert, 1954 {elements triangular, sutures go to vertices of the pentagon}
Pemma Klump, 1953 {elements triangular, with a central knob, hole or depression}
Pentaster Bybell & Gartner, 1972 {elements elongated into free rays}
Quinquerhabdus Bukry & Bramlette, 1971 {pentalith greatly elevated}

Family GONIOLITHACEAE Deflandre, 1957

Goniolithus Deflandre, 1957 {pentagonal plate with a distinct rim surrounding a mesh-like array of small crystals. Rare and sporadic stratigraphic distribution}

Family LAPIDEACASSACEAE Bown & Young 1997

Lapideacassis Black, 1971 (? = Scampanella Forchheimer & Stradner, 1973; Pervilithus Crux, 1981) {see Bown & Young, above}

Genera incertae sedis

Biantholithus Bramlette & Martini, 1964 {consist of 6-11 radial elements; LM birefringence is low and with an offset radial extinction cross; the nannoliths are concavo-convex and form spheres (Romein, 1979; Mai et al., 1994)}
Nannoturba Muller, 1979 {mass of radiating rods, uncertain affinities}
?Nannotetrina Achuthan & Stradner, 1969 - see Zygodiscaceae
Pseudotriquetrorhabdulus Wise in Wise & Constans, 1976 {rod-shaped with 6-12 laths; each lath is a separate crystal-unit with c-axis radial, relative to axis of the rod}

Order DISCOASTERALES Hay, 1977

We include in this order nannoliths with a structure of elements radiating from a common centre or axis. They all originate in the Paleocene and evolutionary relationships between them have been suggested by, for example, Romein (1979) and Perch-Nielsen (1985). Nonetheless, it may represent a polyphyletic grouping.

Family DISCOASTERACEAE Tan, 1927

Description: Discoidal nannoliths of 3-40 elements radiating from a common centre. C-axes vertical, so nannoliths appear dark in plane-polarised light. Some early forms also include a cycle of birefringent units.

Catinaster Martini & Bramlette, 1963 {basket shaped, certainly derived from Discoaster, e.g. Peleo-Alampay et al. (in press)}
Discoaster Tan, 1927 (= Agalmatoaster, Clavodiscoaster, Discoasteroides, Eudiscoaster, Gyrodiscoaster, Heliodiscoaster, Hemidiscoaster, Radiodiscoaster, Truncodiscoaster, Turbodiscoaster ) {includes >100 species. The most obvious subdivision is into rosette-shaped species with >8 rays and star-shaped species with <10 rays, and a number of other features parallel this subdivision. Formal classification as proposed by Theodoridis (1984) into the genera Heliodiscoaster and Eudiscoaster has not, however, proven popular}

Family FASCICULITHACEAE Hay & Mohler, 1967

Description: Conical- or top-shaped nannoliths consisting of 10-30 wedge-shaped, radially-arranged elements. Apparently distinct distal cycles are developed in some species but these probably are formed by kinking of the elements rather than being new crystal-units. Suggested to be ancestral to the Heliolithaceae (Romein, 1979).

Fasciculithus Bramlette & Sullivan, 1961 {see family description}

Family HELIOLITHACEAE Hay & Mohler, 1967

Description: Discoidal nannoliths consisting of at least two superposed cycles of crystal units. Suggested to be ancestral to the Discoasteraceae (e.g. Romein, 1979).

Bomolithus Roth, 1973 {in LM in plan view only the central column is bright}
Heliolithus Bramlette & Sullivan, 1961 (= Bomolithus Roth, 1973) {in LM in plan view the entire nannolith is bright}

Family SPHENOLITHACEAE Deflandre, 1952

Description: Conical-shaped nannoliths consisting of several superimposed cycles of elements all radiating from a common point of origin. C-axes of the elements run along their length.

Sphenolithus Deflandre in Grasse, 1952 (= Furcatolithus Martini, 1965; Sphenaster Wilcoxon, 1970) {see family description}

3b. Nannoliths consisting of a single crystal-unit, showing radial symmetry

Family LITHOSTROMATIONACEAE Deflandre, 1959

Description: Relatively large (10-20µm) nannofossils, confined to epicontinental areas. Morphology is plate-like with rays and interconnecting ridges. Strongly reminiscent of the internal spicules in actiniscid dinoflagellates. Show low birefringence in plan view.

Lithostromation Deflandre, 1942 {3-fold symmetry}
Martiniaster Loeblich & Tappan, 1963 (= Coronaster Martini, 1961) {12-rayed platelets}
Trochoaster Klumpp, 1953 {6-fold symmetry}
?Genus Isolithus Lyul'eva, 1989 {3-fold symmetry}
Lacunolithus Lyul'eva, 1989 {platelet with 8 pairs of rays}

Genera incertae sedis

Imperiaster Martini, 1970 {flattened tetrahedron}
Rhomboaster Bramlette & Sullivan, 1961 {+/- rhomboidal}
Tribrachiatus Shamrai, 1963 {initially hexaradiate, formed by two superposed triplets of rays. Triplets rotate through evolution to become parallel giving triradiate nannolith with bifurcate ray tips}
Trochastrites Stradner, 1961 {planar triradiate nannolith with bifurcate ray tips, ?a holococcolith}
[Marthasterites Deflandre, 1959] {this genus is now only used for Cretaceous forms but many species of Tribrachiatus were previously included in it}

3c. Nannoliths consisting of a single crystal unit, and lacking radial symmetry

Family CERATOLITHACEAE Norris, 1965

Description: Horseshoe-shaped nannoliths (ceratoliths) composed of a single crystal unit.

Comments: The extant species, Ceratolithus cristatus , occurs as a single nannolith which is apparently wrapped around the cell. Some cells also bear hoop-shaped coccoliths. Alcober & Jordan (1997) observed C. cristatus hoop-shaped coccoliths inside Neosphaera coccolithomorpha coccospheres which suggests that ceratoliths may, like holococcoliths, be an alternate phase of the life-cycle.

Amaurolithus Gartner & Bukry, 1975 {c-axis vertical, nannolith dark in LM}
Ceratolithus* Kamptner, 1950 {c-axis in plan of ceratolith, perpendicular to long axis, bright in LM}
[Angulolithina Bukry, 1973] {angular V-shaped nannolith with c-axis parallel to length. These are of irregular morphology and distribution, and may well be fragments of a larger non-haptophyte fossil}

Family TRIQUETRORHABDULACEAE Lipps, 1969

Description: Rod-shaped nannoliths formed of three blades (these may bear subsidiary ridges). The entire nannolith behaves as one crystal-unit, crystallographic orientation varies between genera.

Orthorhabdus Bramlette & Wilcoxon, 1967 {one blade wider than the other two, c-axis lies in plane of this blade, and perpendicular to the long-axis of the nannolith}
Triquetrorhabdulus Martini, 1965 {blades arranged at 120° to each other, uncurved, c-axis parallel to length; always shows strong birefringence}
unnamed Genus {T. rugosus and related species have structure distinct from Triquetrorhabdulus sensu Martini, 1965; one blade narrower than the other two, c-axis lies in plane of this blade, and perpendicular to the long-axis of the nannolith; birefringence usually low (depends on how the specimen is lying); often curved. New genus to be proposed (Varol & Young, in prep.)}
[Pseudotriquetrorhabdulus Wise in Wise & Constans, 1976] {formed of a set of laths with radial c-axes and so included in radial nannoliths}

Genera incertae sedis

Florisphaera* Okada & Honjo, 1973 {liths are small tapering plates which form artichoke-like coccospheres. C-axis parallel to the long axis of the plate but birefringence is low due to small size. Very abundant. A peg-like structure on the base of some specimens may indicate a second crystal-unit}
Minylitha Bukry, 1973 emend. Theodoridis, 1984 {kite-shaped plate with raised rims on both sides, c-axis in plane of plate}

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