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

MESOZOIC (Bown & Young 1997)

1.1a. Imbricating muroliths (loxoliths)
Order EIFFELLITHALES - (Fig. 1)
• Family CHIASTOZYGACEAE
• Family EIFFELLITHACEAE
• Family RHAGODISCACEAE
1.1b. Non-imbricating muroliths (protoliths)
Order STEPHANOLITHIALES - (Fig. 2)
• ?Family CALCIOSOLENIACEAE
• Family PARHABDOLITHACEAE
• Family STEPHANOLITHIACEAE
1.2. Placolith coccoliths
1.2a. Non-imbricating (or radial) placoliths and related taxa
Order PODORHABDALES
• Family AXOPODORHABDACEAE - (Fig. 3)
• Family BISCUTACEAE - (Fig. 4)
• Family CALYCULACEAE
• Family PREDISCOSPHAERACEAE
• Family CRETARHABDACEAE - (Fig. 5)
• Family MAZAGANELLACEAE
• Family TUBODISCACEAE
1.2b. Imbricating placoliths (R-unit dominated)
Order WATZNAUERIALES
• Family WATZNAUERIACEAE - (Fig. 6)
1.2c. Other placolith-like groups
Order ARKHANGELSKIALES
• Family ARKHANGELSKIELLACEAE
• Family KAMPTNERIACEAE
• Genera incertae sedis
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).

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

• Ahmuellerella Reinhardt, 1964 (= ActinozygusGartner, 1968) {complex axial cross structure or plate, includes 8 axial or radial 'bars'}
• ?Bownia Varol & Girgis, 1994 {type (and possibly only) species, B. mutterlosei, is bicyclic; both cycles highly birefringent}
• Bukrylithus Black, 1971 {unicyclic (LM) with broad, fibrous, tapering axial crossbars}
• Diadorhombus Worsley, 1971 {type species, D. rectus , has a square-shaped loxolith rim and is probably not related to similarly-shaped Stephanolithiaceae coccoliths, e.g.Rhombolithion }
• Heteromarginatus Bukry, 1969 {bicyclic, additional small bars in the central-area}
• Misceomarginatus Wind & Wise in Wise & Wind, 1977 (?= Monomarginatus Wind & Wise in Wise & Wind, 1977) {narrow, bicyclic rim; wide central-area with axial crossbars and perforate plate}
• Monomarginatus Wind & Wise in Wise & Wind, 1977 {narrow, unicyclic rim; wide central-area with axial, concentric and lateral bars}
• Rhabdophidites Manivit, 1971 emend. Lambert, 1987 (= Rhabdolekiskus Hill, 1976) {small basal coccolith with axial cross supporting very tall spine}
• Staurolithites Caratini, 1963 (= Staurorhabdus Noel, 1973; Vekshinella Loeblich & Tappan, 1963; Vagalapilla Bukry, 1969; Haslingfieldia Black, 1973; ?Pontilithus Gartner, 1968) {practically a form-genus/genera for loxoliths with a simple axial cross}
• Vaucherauvillius Goy, 1979 {axial, lateral and concentric bars, span the central-area}

B. Central-area with transverse bar

• ? Archaeozygodiscus Bown, 1985 {Triassic genus, bicyclic (LM) with a birefringent, spine-bearing bar. Distal/outer cycle displays anticlockwise imbrication, unlike all other Mesozoic loxoliths}
• Amphizygus Bukry, 1969 (= Bipodorhabdus Noel, 1970) {bicyclic rim; tranverse bar formed from laths which continue around the inner edge of the rim, delineating the two circular perforations. No spine}
• Gorkaea Varol & Girgis, 1994 {bicyclic (LM), bright inner cyclic is broad; robust, birefringent, transverse bar; ?junior synonym of Zeugrhabdotus}
• ?Genus Placozygus Hoffman, 1970 {distal/outer cycle shows little or no imbrication; rim exhibits spiral interference pattern. The common species, P. sigmoides, is best classified as Zeugrhabdotus}
• Zeugrhabdotus sigmoides (Bramlette & Sullivan, 1961) Bown & Young 1997
• Reinhardtites Perch-Nielsen, 1968 {unicyclic, very broad rim, with narrow central-area spanned/filled by a bar}
• Tranolithus Stover, 1966 (?= Pontilithus Gartner, 1968) {central-area spanned by 2-4 broad, disjunct platelets. In T. orionatus, 4 platelets constitute a transverse bar, and there is a proximal net of lateral bars}
• Tubirhabdus Prins ex Rood, Hay & Barnard, 1973 {central-area structure supports broad, hollow, flaring spine}
• Zeugrhabdotus Reinhardt, 1965 (= GlaukolithusReinhardt, 1964; ?Gorkaea Varol & Girgis, 1994; Lordia Varol & Girgis, 1994; Rectapontis Varol & Jakubowski, 1989; Zygolithites Black, 1972; Barringtonella Black, 1973) {uni- and bicyclic forms, with variably constructed transverse bar}

C. Central-area with diagonal cross

• Chiastozygus Gartner, 1968 {includes uni- and bicyclic forms with variably constructed diagonal crossbars}

D. Central-area closed or open with variable bars/grills

• Crepidolithus Noel, 1965 (?= Millbrookia Medd, 1979) {broad, high rim with central-area vacant, closed or spanned by bars or grill}
• Neocrepidolithus Romein, 1979 {broad, high rim with narrow or closed central-area which may be spanned by bars}

E. Central-area with net or vacant

• Loxolithus Noel, 1965 {broad, open central-area, probably spanned by a rarely preserved, finely perforate net, see Lambert, 1993, pl. 5, figs 1 and 2 (named Millbrookia perforatatherein)}

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.

• ?Diloma Wind & Cepek, 1979 {?tricyclic - dark, narrow, inner and outer cycles; bright, broad median cycle; central-area spanned by axial or near-axial cross, with or without lateral bars}
• Eiffellithus Reinhardt, 1965 {crossbars generally fibrous and spine-bearing; relationship between Neocomian forms, and Albian and younger representatives uncertain; see Rothia}
• Helicolithus Noel, 1970 {narrow central-area, filled by broad, lath-formed crossbars}
• Rothia Varol & Girgis, 1994 {alternative name for Neocomian Eiffellithus representatives}
• Tegumentum Thierstein in Roth & Thierstein, 1972 {similar to Eiffellithus, however, the inner rim cycle is strongly imbricate and the crossbars are lath-formed rather than fibrous}

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.

• Calcicalathina Thierstein, 1971 {central-area filled by a large, domed, granular mass}
• Percivalia Bukry, 1969 {concentric multicyclic inner-rim construction with granular plate or bar}
• Rhagodiscus Reinhardt, 1967 (?= Viminites Black, 1975) {central-area filled by a granular plate which may be perforate and spine-bearing}

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.

?Family CALCIOSOLENIACEAE Kamptner, 1937

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

• Calciosolenia Gran, 1912 (= Acanthosolenia Bernard, 1939; Scapholithus Deflandre, 1954) {small, rhombic muroliths with numerous, parallel transverse bars}

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.

• Bucanthus Bown, 1987 {bicyclic (LM), with offset central cross}
• Crucirhabdus Prins ex Rood, Hay & Barnard, 1973 (= Apertius Goy, 1979) {spine-bearing axial cross, with or without additional lateral bars}
• Diductius Goy, 1979 {bicyclic (LM), with central-area grill}
• Mitrolithus Deflandre, 1954 {central-area filled by massive boss/spine}
• Parhabdolithus Deflandre, 1952 {central-area filled or part-filled by broad, often tall, spines}
• Saeptella Goy, 1979 {central-area axial cross and grill}
• ?Thurmannolithion Grun & Zweili, 1980 {axial cross, lateral bars, and minor longitudinal bars forming a grill}
• ?Timorella Bown, 1987 {modified protoliths, with tapering, cup-like rim and central-area plate}
• ?Umbria Bralower & Thierstein in Bralower et al., 1989 {bicyclic (LM) with granular central-area plate}

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).

• Corollithion Stradner, 1962 {bicyclic, polygonal, usually hexagonal, rim with 4-6 radial bars in the central-area}
• [Cylindralithus Bramlette & Martini, 1964] {see Watznaueriaceae}
• [Diadorhombus Worsley, 1971] {type species, D. rectus , has a loxolith rim; see Chiastozygaceae}
• Rectilius Goy, 1979 {central-area grill}
• Rhombolithion Black, 1973 {diamond-shaped rim; arguably a junior synonym of Stradnerlithus}
• Rotelapillus Noel, 1973 {high, bicyclic (LM), circular rim with lateral rim spines and 8 radial central-area bars}
• Stephanolithion Deflandre, 1939 {bicyclic (LM), elliptical to geometric rim with lateral rim spines and 1-8 central-area bars}
• Stoverius Perch-Nielsen, 1984 {broadly-elliptical to circular, bicyclic (LM) rim with central-area cross}
• Stradnerlithus Black, 1971 (= Diadozygus Rood, Hay & Barnard, 1971; Nodosella Rood, Hay & Barnard, 1973) {inconspicuous (LM), elliptical or polygonal rim with 4 or more central-area bars}
• Truncatoscaphus Rood, Hay & Barnard, 1971 {elongate, subhexagonal rim with 6 or more central-area bars}

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.

• Axopodorhabdus Wind & Wise in Wise & Wind, 1977 {tall-spine-bearing axial cross}
• Cleistorhabdus Black, 1972 {central-area plate with one large perforation/spine base}
• ?Cribrosphaerella Deflandre in Piveteau, 1952 (= Psyktosphaera Pospichal & Wise, 1990) {elliptical to subrectangular rim; multiperforate central-area net}
• ?Cribrocorona Perch-Nielsen, 1973 {high, subcircular rim; narrow central-area with grill}
• Dekapodorhabdus Medd, 1979 {10 central-area perforations}
• Dodekapodorhabdus Perch-Nielsen, 1968 {12 central-area perforations}
• Ethmorhabdus Noel, 1965 {multiperforate central-area net with or without axial cross and spine}
• Hemipodorhabdus Black, 1971 {spine-bearing transverse bar}
• Hexapodorhabdus Noel, 1965 {bars delineate 6 large perforations, including one at each end of the central-area }
• ?Nephrolithus Gorka, 1957 {reniform rim; central-area net with 2 to numerous pores}
• Octopodorhabdus No‘l, 1965 {bars delineate 8 large perforations, including one at each end of the central-area}
• Octocyclus Black, 1972 {bars delineate 8 large perforations, lying at the sides of a longitudinal bar}
• Perrisocyclus Black, 1971 (= Teichorhabdus Wind & Wise in Wise & Wind, 1977; Duplexipodorhabdus Varol & Girgis, 1992) {one or two cycles of central-area perforations}
• [?Podorhabdus Noel, 1965] {granular central structure with two marginal perforations, supports tall, hollow, flaring spine; see discussion of family}
• Teichorhabdus Wind & Wise in Wise & Wind, 1977 {two cycles of small central-area pores}
• Tetrapodorhabdus Black, 1971 {diagonal or four offset cross bars}

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.

• Biscutum Black in Black & Barnes, 1959 (= Palaeopontosphaera Noel, 1965) {broadly elliptical-elliptical shields with or without a tube-cycle; the central-area may be imperforate or narrow and vacant or spanned by a simple structure (cross or bar)}
• Boletuvelum Wind & Wise in Wise & Wind, 1977 {Biscutum-like coccoliths with large, hollow, flaring spines, closed distally by a domed covering; may simply represent well-preserved Biscutum coccoliths (see Hattner & Wise, 1980, pl.4, fig.6; Lambert, 1993, pl.13, fig.1)}
• Crucibiscutum Jakubowski, 1986 {Biscutum-like coccoliths with prominent, birefringent, axial cross}
• Discorhabdus Noel, 1965 (= Bidiscus Bukry, 1969) {circular rim, generally no central-area structure, but may (Jurassic forms) or may not (Cretaceous forms) bear a spine}
• Gaarderella Black, 1973 {broad shields with wide granular central-area plate; rarely reported}
• Gephyrobiscutum Wise, 1988 {bicyclic rim; narrow central-area spanned by an oblique transverse bar; rarely reported}
• Seribiscutum Filewicz et al. in Wise & Wind, 1977 {central-area spanned by broad platelets}
• Similiscutum de Kaenel & Bergen, 1993 {subcircular-elliptical shields with smooth, grey appearance in LM and a narrow, bright inner-cycle (not a tube cycle); central-area narrow and may be empty or spanned by an axial cross}
• Sollasites Black, 1967 (= Costacentrum Bukry, 1969; Noellithina Grun & Zweili, 1974 in Grun et al., 1974) {elliptical, usually bicyclic rim; distinct central-area structure composed of axial cross, multiple longitudinal bars and/or concentric bars. Classified in a separate family or subfamily by some authors}

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).

• Calyculus Noel, 1973 (= Proculithus Medd, 1979; Incerniculum, Vikosphaera, Catillus Goy, 1979) {see description of family}
• Carinolithus Prins in Grun, Prins & Zweili, 1974 {extremely modified; trumpet-shaped coccoliths}

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.

• ?Petrarhabdus Wind & Wise in Wise, 1983 {massive, short, blocky spine}
• Prediscosphaera Vekshina, 1959 (= DeflandriusBramlette & Martini, 1964) {see description of family}

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.

• Cretarhabdus Bramlette & Martini, 1964 (= Allemannites Grun in Grun & Allemann, 1975) {axial cross and net}
• Cruciellipsis Thierstein, 1971 (= MiravetesinaGrun in Grun & Allemann, 1975 {broad shields with broad, tapering, birefringent axial cross bars; lateral bars have been reported}
• ?Flabellites Thierstein, 1973 {central-area spanned by small, blocky diagonal cross; outline may be asymmetric}
• Grantarhabdus Black, 1971 (= GephyrorhabdusHill, 1976) {diagonal cross bars}
• Helenea Worsley, 1971 (= Microstaurus Black, 1971) {distal shield inner cycle displays distinct suture obliquity and ?imbrication; narrow central-area spanned by crossbars which may bifurcate at their ends and usually support a short, blocky spine}
• [Microstaurus Black, 1971] {commonly used genus, but a junior synonym of Helenea}
• Mirevetesina Grun in Grun & Allemann, 1975 {broad shields, weak axial cross, net; ?early Cruciellipsis }
• ? Pickelhaube Applegate, Covington & Wise, 1987 {large, broad, strongly concavo-convex ?proximal shield; narrow ?distal shield; central-area axial cross and lateral bars}
• Polypodorhabdus Noel, 1965 {distal shield slopes in to the central-area; axial cross and numerous lateral bars}
• Retecapsa Black, 1971 (= Allemannites Grun in Grun & Allemann, 1975) {axial cross with lateral bars; some authors include these forms within Cretarhabdus }
• Speetonia Black, 1971 {single transverse bar}
• [Stradneria Reinhardt, 1964] {holotype drawing shows a weak axial cross and a solid intergrowth of radial laths; name not commonly used}

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.

• Mazaganella Bown, 1987 {low to moderately high rim, dark shields (bright inner cycle in M. protensa); central-area axial cross, which is broad and plate-like in M. pulla}
• Triscutum Dockerill, 1987 {distal shield is elevated; variable central-area structures, mainly grills and nets}

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.

• Manivitella Thierstein, 1971 {low proximal collar-cycle; large rim and wide ?vacant central-area}
• Tubodiscus Thierstein, 1973 {high and birefringent proximal collar-cycle; wide ?vacant central-area}

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

• Cyclagelosphaera Noel, 1965 {circular shields; narrow or closed central-area}
• Lotharingius Noel, 1973 (= Bennocyclus Zweili & Grun, 1974 in Grun et al., 1974) {central-area axial cross, with or without additional lateral bars}
• Watznaueria Reinhardt, 1964 (= Ellipsagelosphaera Noel, 1965; Caterella Black, 1971; CalolithusNoel, 1965; Coptolithus Black, 1973; MargolatusForchheimer, 1972; Actinosphaera Noel, 1965) {central-area closed or narrow but may be spanned by transverse bar, bars or grill}

B. Genera with modified Watznaueria-type rim

• Ansulasphaera Grun & Zweili, 1980 {high, narrow, cylindrical proximal shield (anticlockwise imbrication); narrow, vacant central-area}
• ?Genus Bibreviconus Rahman & Roth, 1991 {high, cylindrical morphology with a peg-like distal cycle; possibly the isolated central part of Cyclagelosphaera deflandreispecimens which have lost the shields}
• Bussonius Goy, 1979 {V-unit cycle high and broad, forms a third, uppermost shield; central-area axial cross and lateral bars}
• ?Genus Cylindralithus Bramlette & Martini, 1964 {earliest species, C. nudus, has watznaueriacean construction, but the shields are high (cf. Ansulasphaera) and differentiation between them is reduced (clockwise distal shield imbrication, anticlockwise proximal shield imbrication; central-area vacant or with cross bars}
• ?Genus Diazomatolithus Noel, 1965 {subcircular-circular with broad, vacant central-area; distal shield is monocyclic with radial sutures, proximal shield may be high and tapering with laevogyre element curvature, broadly sinistral obliquity and anticlockwise imbrication; low birefringence due to the expanded V-unit which forms the distal shield}
• [Darwinilithus Watkins in Watkins & Bowdler, 1984] {possible junior synonym of Cylindralithus but tube-cycle elements protrude distally}

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.

• Acaenolithus Black, 1973 {2-3 cycles form the distal shield, the inner cycle is broadest; bicyclic LM image, broad, bright inner cycle and narrow, dark outer cycle; broad central-area axial cross and grills, support boss or spine}
• Arkhangelskiella Vekshina, 1959 {1-2 distal shield cycles; bright, unicyclic LM image although darker towards outer edge; central-area perforate plate with axial sutures}
• Aspidolithus Noel, 1969 {2-3 cycles form the distal shield, the inner cycle is broadest; indistinct bicyclic LM image, broad, bright inner cycle and narrow, darker outer cycle; central-area perforate plate with axial sutures, with no boss or spine. Considered a junior synonym of Broinsonia by some}
• Broinsonia Bukry, 1969 (?= Aspidolithus Noel, 1969) {as for Aspidolithus but if both genera are used, it is restricted to forms with broad central-area axial crosses and grills}
• Thiersteinia Wise & Watkins in Wise, 1983 {as for Aspidolithus with perforate plate, axial struts and spine}

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.

• [Cribricatillus Black, 1973] {modified loxolith rim with Gartnerago-like LM image, central-area axial cross and net or bars; ?junior synonym of Gartnerago}
• ?Crucicribrum Black, 1973 {small ?tiered placoliths, with perforate central-area plate and axial cross and/or sutures}
• Gartnerago Bukry, 1969 (= ?Cribricatillus Black, 1973; Laffittius Noel, 1969) {see family description. central-area structures - tranverse bar, broad axial cross, or perforate plate with axial or near axial sutures; additional proximal nets or grills}
• Kamptnerius Deflandre, 1959 {as for Gartneragowith an asymmetric rim flange forming a wing; reduced central-area plate}

1.3. Heterococcoliths of uncertain affinities

1.3a. Muroliths

• Clepsilithus Crux, 1987 {loxolith with 8 or more broad bars in the central-area}
• Laguncula Black, 1971 {?loxoliths with bulbous/spherical, hollow 'spines'}
• Paralithella Lambert, 1993 {protoliths with central-area axial cross and longitudinal bars}
• Rectocorona Lambert, 1987 {protoliths with short, flaring distal-process}
• Tortolithus Crux, 1982 {muroliths with central-area closed by overlapping plates}
• ?Angulofenestrellithus Bukry, 1969 {narrow, bicyclic rim with broad perforate central-area plate (?3 cycles of holes)}

1.3b. Placoliths

• Boletuvelum Wind & Wise in Wise & Wind, 1977 {large, hollow, closed, flaring distal spine - see Biscutaceae}
• Chiastella Lambert, 1993 {?tricyclic distal shield, diagonal cross}
• Diazomatolithus Noel, 1965 {subcircular-circular with broad, vacant central-area; distal shield is monocyclic with radial sutures, proximal shield may be high with dextrally imbricating elements; low birefringence - see Watznaueriaceae}
• Haqius Roth, 1978 {(?elliptical)-circular with monocyclic distal shield formed from numerous dextrally imbricate elements, low birefringence LM image; narrow or closed central-area}
• Markalius Bramlette & Martini, 1964 {moderately birefringent interference figure with a bright tube-cycle; central-area narrow or closed}
• Prolatipatella Gartner, 1968 {narrow, ?tiered rim; thin, imperforate plate across wide central-area}
• Repagulum Forchheimer, 1972 {imbricating placoliths; monocyclic distal shield with numerous imbricate elements, distinctive but inconspicuous, low birefringence, 'flaring' LM image; central-area spanned by ~16 radial bars}

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.