CENOZOIC CALCAREOUS NANNOPLANKTON CLASSIFICATION

Part 2, Holococcoliths & nannoliths

Jeremy R. Young, Palaeontology Dept., NHM Cromwell Road, London, SW7 5BD, UK &

Paul R. Bown, Dept. of Geological Sciences, UCL, Gower Street, London, WC1E 6BT, UK

The original version of this ms was printed in the Journal of Nannoplankton Research, issue 19/1


NANNOFOSSIL HIGHER CLASSIFICATION (Young & Bown 1997)
INTRODUCTION
FAMILY LEVEL OVERVIEW
ACTIVE MAP OVERVIEW
REFERENCES
MESOZOIC (Bown & Young 1997)
1. HETEROCOCCOLITHS
2. HOLOCOCCOLITHS
3. NANNOLITHS
CENOZOIC (Young & Bown 1997)
1. HETEROCOCCOLITHS
2. HOLOCOCCOLITHS
3. NANNOLITHS


2. HOLOCOCCOLITHS

Family CALYPTROSPHAERACEAE Boudreaux & Hay, 1969

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 (<2Ám) 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.

B. Birefringent fossil holococcoliths

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

C. Extant monomorphic holococcoliths

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

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.

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).

Family GONIOLITHACEAE Deflandre, 1957
Family LAPIDEACASSACEAE Bown & Young 1997
Genera incertae sedis

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.

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).

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).

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.

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

Family LITHOSTROMATIONACEAE Deflandre, 1959

Description: Relatively large (10-20Ám) 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.

Genera incertae sedis

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.

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.

Genera incertae sedis

Return to: top; Introduction, family-level overview, Mesozoic heterococcoliths, holococcoliths & nannoliths; Cenozoic heterococcoliths, holococcoliths & nannoliths

This page was produced by Jeremy Young, feedback and corrections welcome.