MESOZOIC CALCAREOUS NANNOPLANKTON CLASSIFICATION

Part 1 - Heterococcoliths

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

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

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

Introduction

The purpose and philosophy of this contribution are explained in the introductory section (Young & Bown, above). A three-level order-family-genus classification is used, as far as seems reasonable, based on current knowledge. In addition, a set of numbered groupings (1. Heterococcoliths to 3. Nannoliths) are used to provide a logical, but very possibly artificial, organisation, particularly of families and genera incertae sedis. Question marks ("?") preceeding generic names indicate tentative inclusion in the family. Square brackets "[ ]" indicate genera we regard as superfluous.


CONTENTS

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


1. HETEROCOCCOLITHS

1.1. Murolith coccoliths

Description: Wall-like rim, typically higher than it is wide, composed of two crystal-units: the distal/outer cycle (V-unit), which is commonly dominant; and the proximal/inner cycle (R-unit), which is subordinate and sometimes vestigial. The light microscope (LM) cross-polarised light (XPL) image is either bicyclic, with a dark outer cycle and a bright inner cycle, or unicyclic and relatively dark. This image variability is dependent upon the size of the proximal/inner cycle and the orientation of the rim, i.e. whether it is vertical or flaring.

Remarks: The division between the two murolith orders proposed herein, is based upon orientation of the elements in the distal/outer cycle, i.e. imbricating or non-imbricating. This distinction is apparent and stable through most of the Mesozoic but may be less clear amongst some Upper Triassic and Lower Jurassic muroliths, which represent the products of the initial diversification of this group (Figure 1).

1.1a. Imbricating muroliths (loxoliths)

Order EIFFELLITHALES Rood, Hay & Barnard, 1971

Description: Murolith coccoliths with a distal/outer cycle composed of imbricating elements, i.e. in side-view, the sutures are not vertical. This feature is not distinguishable from non-imbrication in the LM. The distal/outer cycle imbrication is clockwise. The term loxolith is applied to this rim structure (Bown, 1987).

Fig. 1 Initial development of the Eiffellithales
Family CHIASTOZYGACEAE Rood, Hay & Barnard, 1973 emend. Varol & Girgis, 1994

Description: Loxoliths with variably-developed proximal/inner-cycles and a central-area spanned by axial, non-axial or diagonal crossbars or a single transverse bar which is, however, usually formed from four, fused bars. LM image includes both unicyclic and bicyclic types.

Comments: This broad taxonomic group includes numerous simple loxolith forms. This simple model of coccolith construction was repeatedly modified through the Mesozoic and these subtle and numerous morphological variations are reflected in a taxonomy which is virtually unworkable. Very few distinctive and well-constrained species exist, and most commonly-used species names are virtually meaningless form-taxa. Even at the generic level there is no consensus on nomenclature, illustrated by the following list of names applied to forms with axial crosses: Bownia Varol & Girgis, 1994, Rothia Varol & Girgis, 1994, Staurolithites Caratini, 1963, Staurorhabdus Noel, 1973, Vekshinella Loeblich & Tappan, 1963, Vagalapilla Bukry, 1969.

A new classification of the group was recently proposed by Varol & Girgis (1994), however, their subdivision is based upon whether the LM rim image is unicyclic or bicyclic, a feature which appears to be of dubious taxonomic significance and is probably homeomorphic within this group. An informal subdivision is applied below.

A. Central-area with axial cross
B. Central-area with transverse bar
C. Central-area with diagonal cross
D. Central-area closed or open with variable bars/grills
E. Central-area with net or vacant
Family EIFFELLITHACEAE Reinhardt, 1965

Description: Loxoliths with a well-developed proximal/inner cycle and typically a wide central-area spanned by axial, non-axial (asymmetric), or diagonal crossbars. The broad, proximal/inner cycle is conspicuous in LM, creating a strongly bicyclic image.

Family RHAGODISCACEAE

Description: Loxoliths with a dominant distal/outer-cycle and a central-area typically filled by a plate of granular calcite. The central structure may be spine-bearing, perforate or massive. The LM image is generally unicyclic.

1.1b. Non-imbricating muroliths (protoliths)

Order STEPHANOLITHIALES Bown & Young 1997

Description: Muroliths with a distal/outer-cycle composed of non-imbricating elements, i.e. in side-view, the sutures are vertical or near-vertical. This feature is not distinguishable from imbrication in the LM. The term protolith is applied to this rim structure (Bown, 1987). Figure 2 provides a schematic overview for this group.

Figure 2 Phylogeny of the Mesozoic Stephanolithiales
?Family CALCIOSOLENIACEAE Kamptner, 1927

Description: Coccoliths are rhombic muroliths, usually termed scapholiths. The central-area is spanned by numerous transverse bars. See Young & Bown (below) for further discussion.

Family PARHABDOLITHACEAE Bown, 1987

Description: Protoliths with high rims and a central-area spanned by an axial cross or transverse bar but commonly filled by broad, often tall spines. The LM image is usually bicyclic, with the proximal/inner cycle well-developed. Commonly observed in side-view.

Family STEPHANOLITHIACEAE Black, 1968

Description: Protoliths with low rims, weakly-developed or vestigial proximal/inner cycles, and a central-area spanned by one to numerous bars. Coccolith outline may be polygonal. LM image is usually unicyclic and inconspicuous, although a number of genera do exhibit bicyclicity, e.g. Rotelapillus, Stephanolithion and Stoverius (Figure 2).

1.2. Placolith coccoliths

Description: Broad and thin rim, usually constructed from two superimposed, appressed shields joined by a tube-cycle. Precise understanding of the relationship between rim-cycles is often lacking, but where known, the shields are constructed from two crystal-units which may be complexly intergrown and superimposed, leading to a multicyclic, surficial appearance. LM image is wholly dependent upon the relative development of the two crystal-units (V and R), which is extremely variable, but usually consistent within families. When V- and R-units are relatively equally developed, i.e. each forming an entire shield, the LM image is predominantly dark but often bicyclic, the inner cycle being narrow and bright, e.g. Biscutaceae, Prediscosphaeraceae. When the V-unit is weakly developed, and it is often reduced to peg cycles or is vestigial, then both shields are almost wholly constructed from R-unit crystals and the LM image is bright, e.g. Watznaueriaceae . Mesozoic placoliths generally have simple, monocyclic proximal shields, which vary little from family to family. Cenozoic placoliths have more-complexly constructed proximal shields which are often bicyclic.

1.2a. Non-imbricating (or radial) placoliths and related taxa

Order PODORHABDALES Rood et al., 1971 emend. Bown, 1987

Remarks: This order includes the Biscutaceae and other closely related forms, which have placolith (or modified placolith) coccoliths with shields formed from elements which display little or no imbrication and, typically, equal development of V and R crystal-units, i.e. one shield (distal) formed from V-units, the other from R-units (proximal). The V/R development is reflected in consistent LM images which are of low birefringence but high relief in phase contrast. Shield elements are typically joined along radial sutures, but these may often curve or kink. Element curvature is consistently dextrogyre (veeing anticlockwise) and obliquity is broadly dextral (clockwise) in the distal shield, and the same in the proximal shield when viewed proximally.

The rim constructions of the Cretarhabdaceae, Tubodiscaceae and Mazaganellaceae are not well understood, and these families are only tentatively assigned to this order.

Family AXOPODORHABDACEAE Bown & Young 1997
Type genus: Axopodorhabdus Wind & Wise in Wise & Wind, 1977.

Description: Placoliths with two narrow shields and a wide central-area, spanned by axial crossbars or granular plates with variable numbers of perforations; the central structure generally supports tall, hollow spines. The distal shield is formed from V-unit elements joined along radial or near-radial sutures which show little or no imbrication. The proximal shield and inner cycle (if present) are formed from R-units. LM image is generally dark, but bright inner-cycles may be developed. The image is characterised by clearly visible shield elements, often giving a 'beaded' appearance around the inner edge of the shields.

Comments: We have abandoned the name Podorhabdaceae in favour of Axopodorhabdaceae due to problems associated with the type genus of the former family (Podorhabdus, type species P. grassei Noel, 1965). Many authors believe that the holotype illustrations of P. grassei represent a species of Discorhabdus, which should then be classified in the Family Biscutaceae. It would be extremely undesirable to use Podorhabdus for those coccoliths presently within Discorhabdus, or to use Podorhabdaceae to replace Biscutaceae; the informal term podorhabdid is useful, and widely used, to described the coccoliths classified together in the Axopodorhabdaceae.

The Axopodorhabdaceae has been dramatically overdivided, with numerous, monospecific genera distinguished only by the number of central-area perforations (Figure 3). The family requires a species-level review, and revision is not attempted here.

Family BISCUTACEAE Black, 1971

Description: Placoliths with two broad shields, which may or may not include a distal innercycle (tube-cycle), and a small central-area which may be vacant, filled with granular calcite or spanned by variably-oriented bars or axial crossbars. The distal shield is formed from V-units and the proximal shield and tube-cycle are formed from R-units; the distal shield elements are joined along radial or near-radial sutures (+/- kinks) and show little or no imbrication. LM image is generally dark, but bright inner-cycles are common. Coccospheres are well known and usually spherical or cylindrical. A diagramatic overview of the family is shown in Figure 4.

Remarks: The taxonomic significance of conspicuous tube-cycles has led to a variety of classification schemes within this family. In a study of the oldest (Early Jurassic) biscutaceans, de Kaenel & Bergen (1993) proposed considerable revision to this family. These authors distinguish Palaeopontosphaera (widely regarded as a junior synonym of Biscutum) from Biscutum by the presence of a birefringent, distal inner cycle. However, the type species holotype (EM) of Biscutum was a proximal view and thus the presence or absence of a tube-cycle cannot be determined. Moreover, they argue that the aforementioned holotype is very nearly circular and emend the genus in such a way as to render it equivalent to Bidiscus Bukry, 1969 (= Discorhabdus Noel, 1965) (i.e. circular to subcircular, unicyclic placoliths), a drastic change compared to its normal and widespread usage.

The holotype in question is badly damaged (around 40% is missing) and most probably not lying flat, and it is therefore very difficult to prove that this specimen is circular. In fact, Black (in Black & Barnes, 1959) clearly states that the holotype is composed of "two unequal elliptical discs" (p.325), and discusses at some length the geometrical changes in the shape of the rim elements around the ellipse (p.326). We therefore propose that the traditional usage of Biscutumis retained, i.e. subcircular to elliptical biscutacean coccoliths which typically have a distal, inner tube-cycle. This causes least disruption in the relatively stable Biscutaceae taxonomy and is as justifiable as the case presented by de Kaenel & Bergen (1993), given the available evidence. The inclusion of the earliest biscutaceans in the genus Similiscutum is, however, followed here.

The genus Discorhabdus (= Bidiscus) is distinguished on its circular outline. In many coccolith groups, outline is not a stable, taxonomically significant feature, however Discorhabdus represents a coherent, long-lived group in which outline is consistently circular. There are also additional morphological features which distinguish them from the rest of the family, e.g. most species lack an inner distal tube-cycle and many Jurassic representatives have large spines.

Family CALYCULACEAE Noel, 1973

Description: Modified placoliths constructed from a high, broad, flaring distal-shield composed of radial, non-imbricating elements, and a proximal-shield which is often reduced to a simple, narrow cycle of elements; central-area structures are highly variable, including numerous bars, concentric structures, nets and grills. The distal shield is formed from V-units and the proximal shield is formed from R-units. LM image is generally dark, but the high distal shield often shows some birefringence, reflecting the thickness of the crystals; commonly observed in side-view. There is some evidence of coccosphere dimorphism (Goy, 1981).

Family PREDISCOSPHAERACEAE Rood, Hay & Barnard, 1971

Description: Elliptical to circular placoliths with two shields and a central-area spanned by crossbars which support tall, complexly-constructed spines. The distal shield is typically bicyclic, with a broad outer cycle, usually constructed from 16 non-imbricating elements (V-units) and a narrow inner cycle (tube-cycle) (R-units). The proximal shield is formed from R-units. The LM image is bicyclic, with the outer cycle dark, and inner cycle bright.

Rim structure uncertain but tentatively placed within the Podorhabdales
Family CRETARHABDACEAE Thierstein, 1973

Description: Placoliths with two shields and a central-area spanned by a variety of structures, most commonly fibrous axial crossbars with subsidiary lateral bars and a solid central spine or process (Figure 5). The distal shield is generally bicyclic, with a narrow outer cycle and a dominant, broad inner cycle; the elements are usually radial or near radial and do not appear to imbricate. The relationship between these cycles and the coccolith crystallography is presently uncertain. LM image is moderately birefringent.

Family MAZAGANELLACEAE Bown, 1987

Description: Three-shielded placoliths with a wide central-area spanned by a variety of bars, grills, nets or plates. The distal shield may be high and flaring. The distal shield is formed from one cycle of non-imbricating elements, joined along radial sutures; the exact structural relationship between the different shields is unknown at present. LM image is generally dark, but birefringence increases when the distal shield is high.

Family TUBODISCACEAE Bown & Rutledge 1997 (in Bown & Young 1997)
Type genus: Tubodiscus Thierstein, 1973.

Description: Elliptical placoliths composed of two narrow shields and a third, narrow, proximally-situated collar-cycle which is variable in height. The central-area is broad and open; no central structures have yet been observed. The LM image is dark, although the collar-cycle is brighter. Element curvature is dextrogyre in the distal shield and suture obliquity is broadly dextral (clockwise). In the proximal shield these orientations are the same when viewed proximally. Suture curvature in the collar-cycle is laevogyre when viewed proximally; element imbrication is anticlockwise (for T. verenae). The crystallography of these coccoliths has not yet been determined.

1.2b. Imbricating placoliths (R-unit dominated)

Order WATZNAUERIALES Bown, 1987

Remarks: Includes placoliths (or modified placoliths) with shields formed from elements which display imbrication and in which the V-unit is usually reduced or vestigial, resulting in a high birefringence LM image. Element curvature is laevogyre in the distal shield with broadly sinistral obliquity, and the same in the proximal shield when viewed proximally. Imbrication is clockwise in the distal shield, anticlockwise in the proximal shield, and clockwise in the V-unit cycle when developed (e.g. Bussonius).

Family WATZNAUERIACEAE Rood, Hay & Barnard, 1971

Description: Imbricating placoliths with two shields and a central-area which is usually closed or narrow and devoid of central structures; or filled by a plug, spanned by bars, axial cross, or grill. The distal shield is superficially tricyclic, composed of a broad outer-cycle of imbricating elements (R-unit) joined along kinked sutures; a narrow, median-cycle of peg-like elements (V-unit) and a narrow inner cycle (R-units). The two shields are actually formed from single R-unit elements into which fit the narrow cycle of peg-like V-units (Young & Bown, 1991). The dominance of the R-units creates a highly-birefringent LM image, in which the V-unit cycle appears only as a thin, dark line. The typical rim morphology is modified in a number of genera, listed below (see also Figure 6).

A. Genera with Watznaueria type rim
B. Genera with modified Watznaueria-type rim

1.2c. Other placolith-like groups

Order ARKHANGELSKIALES Bown & Hampton 1997 (in Bown & Young 1997)

Description: Tiered 'placoliths' (see comments), with 3-5 closely appressed 'shields'. Central-area structures include transverse bars with proximal net; axial or near-axial crosses with proximal net; and perforate plates crossed by axial or near-axial sutures. LM images vary significantly, from predominantly dark in the Kamptneriaceae, to predominantly bright in the Arkhangelskiellaceae.

Comments: The Kamptneriaceae appears to have originated from loxolith coccoliths, and the tiered placolith rim structure is actually a modified loxolith construction (Hampton et al. , in prep.). The earliest representatives of the family are only slightly modified loxolith coccoliths. Such an evolutionary history has not yet been established for the Arkhangelskiellaceae.

Family ARKHANGELSKIELLACEAE Bukry, 1969 emend. Bown & Hampton

Description: Tiered 'placolith' coccoliths with central-areas spanned by axial crosses and grills, or filled by a perforate plate divided by axial sutures. The 'shields' are typically bright in cross polarised light (rim dominated by R-unit) but bicyclic images are also observed.

Family KAMPTNERIACEAE Bown & Hampton 1997 (in Bown & Young 1997)
Type genus: Kamptnerius Deflandre, 1959.

Description: Modified loxolith coccoliths, with distinctive LM images consisting of a narrow to moderately-broad rim with a narrow, dark outer cycle; a diagnostic, bright median cycle; and a dark, inner cycle. Central-area structures are generally dark in LM, and may be a transverse bar, crossbars or plate, usually perforate; proximally-situated fine nets may be seen when preservation is good. Rim structure varies from apparently typical loxolith (e.g. Thierstein, 1974: pl.4, figs 1, 9, 12) to placolith-like, with at least three (pseudo) 'shields' (e.g. Thierstein, 1974: pl.7, figs 6, 8). The tiered nature is produced by lateral protrusions from the outer cycle of the loxolith wall. The V-unit cycle dominates the wall, but is penetrated by a thin cycle of R-units, seen as a peg-like cycle in proximal view, and a median cycle in distal view, and it is this cycle which creates the distinctive LM image.

1.3. Heterococcoliths of uncertain affinities

1.3a. Muroliths

1.3b. Placoliths


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