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Exiguisphaera
From Fensome et al., 2019:
Exiguisphaera Duxbury, 1979a, p.198–199.
Emendation: Jan du Chêne et al., 1986b, p.11. ; Duxbury, 2023
Type: Duxbury, 1979a, pl.2, figs.2–3; text-figs.1A–B, as Exiguisphaera phragma.
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Original description: [Duxbury, 1979]:
Description:
Small, spheroidal dinoflagellate cysts having a minutely perforate body wall. The reflected tabulation pattern, ?a.cl., 4`, 6", ?c, 6```, 1p, ?1p.v., 1````, is outlined by rows of short, simple spines distally connected by trabeculae, which give each crest a fencelike appearance. A few small, intratabular spines may be present. The archeopyle is precingular, formed by detachment of plates 3" and 4````.
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Modified description:
Stover and Williams, 1987, p. 93:
Synopsis:
Cysts proximochorate, subspherical; gonyaulacacean paratabulation indicated by fence-like (perforate?) septa; paratabulation formula: 1pr, 3`,1a, 6", Xc, 6```,1p, 1````, ?1s; precingular 2P archeopyle formed by loss of paraplates 3" and 4", operculum free.
Description:
Shape: Subspherical.
Wall relationships: Autophragm only.
Wall features: Parasutural features consist of fencelike (perforate?) septa; spines may be present between parasutural features.
Archaeopyle: Precingular type 2P, (3" and 4"); opercular pieces free.
Paratabulation: Indicated by parasutural features; gonyaulacacean, formula: 1pr, 3`,1a, 6", Xc, 6```, 1p, 1````, and ?ps.
Paracingulum: Indicated by parallel, transverse septa; undivided.
Parasulcus: Longitudinally elongate area mainly on the hypocyst and bounded by parasutural features.
Size: Small to intermediate; about 40 to 55 µm.
Affinities:
Exiguisphaera differs from Occisucysta Gitmez 1970, which also has a precingular 2P archeopyle, by having paraplates 3" and 4" released during archeopyle formation rather than 2" and 3". The 2P archeopyle also serves to separate Exiguisphaera from several other genera (e.g. Leptodinium Klement 1960 emended Stover and Evitt 1978, and Rhynchodiniopsis Deflandre 1935 emended Sarjeant, 1982), with a gonyaulacacean paratabulation expressed by parasutural features, and which have type P (3" only) archeopyles.
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Emended description:
Jan du Chêne et al., 1986: (Translation: Fensome et al., 1991, p. 213-214):
Description:
Shape: Cyst proximate to proximochorate, subspherical in shape, without a prominent apical protrusion.
Cyst wall: Autophragm only.
Characters of the wall: The autophragm is often vacuolated. The parasutures are well marked by alignments of arches linked to each other distally. Intratabular ornament is absent or consists of elements of low relief, regularly or irregularly distributed, such as spines, baculae, verrucae, etc.
Paratabulation: Indicated by parasutural septa which delimit the following paraplates: 1-?pr, 4`, 6``, 6c, 5-6s, 6```, 1p, 1````. The scheme formed by these paraplates is of the gonyaulacoid type.
Archaeopyle: 2P (3``+4``). The operculum is free, compound.
Paracingulum: Composed of 6 cingular paraplates, it is generally well delineated by parasutural septa.
Parasulcus: Oriented longitudinally, it comprises five or six paraplates.
Affinities:
Exiguisphaera differs from Diacanthum Habib, 1972; emend. Habib and Drugg (1987) in the structure of the parasutural septa, typically formed by the alignment of arches. Furthermore, the archaeopyle in Exiguisphaera is formed by the loss of paraplates 3`` and 4`` only.
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Type: Duxbury 1979a, pl. 2, figs. 2–3; text-figs.1A–B, as
Exiguisphaera phragma.
Emended diagnosis: Small to intermediate, spheroidal dinoflagellate cysts with a sexiform gonyaulacacean tabulation pattern 4', 6''', 6c, 5-6s, 6''', 1p, 1''''. Sutural crests invariably present, formed by solid elements which may furcate and anastamose and may be connected distally by a trabeculum. The archeopyle is precingular, type 2P, formed by the loss of plates 3" and 4". Where present, intratabular ornament ranges from coarse granules to spines.
Remarks: The genus Exiguisphaera Duxbury 1979a is emended here to include irregular sutural crests, as well as the distally trabeculate “fence-like” structures observed in Exiguisphaera phragma Duxbury 1979a, E. plectilis Duxbury 1980 and E. asketa Duxbury 2018.
In addition to the type species, Exiguisphaera phragma Duxbury 1979a, four further species of Exiguisphaera were recorded in the present study, Exiguisphaera asketa Duxbury 2018, Exiguisphaera lasia Duxbury n. sp., Exiguisphaera lita Duxbury n. sp. and Exiguisphaera trichota Duxbury n. sp.
In his remarks regarding E. phragma, Harding (1990, p. 30) described a, “free, compound biplacoid operculum”, and Jan du Chêne et al. (1986b) extended that concept to the genus Exiguisphaera, as “L’opercule est libre, composé”. This does not appear to be the case, however, as specimens of Exiguisphaera lita, E. phragma and E. trichota have been recorded with individual opercular plates within the cyst cavity (see Plate 29).
Small, spheroidal cysts with a two-plate (3"+4") archeopyle were recorded throughout the current study and these displayed considerable variation in onament type, height and distribution. Many of these fall within Exiguisphaera and Nexosispinum Davey 1979b, which appear to differ only because Exiguisphaera possesses sutural crests, whereas in Nexosispinum, “Although occasionally some process alignment may be present, this cannot be related to a cyst tabulation” (Davey 1979b, p. 558). Further discussion of these genera is included under individual taxa below.
Towards the base of the Hauterivian members of both Exiguisphaera and Nexosispinum tend to be particularly “simple”, in having very low surface features; these persist to higher levels, associated with more complex forms. Where sutural lineations were observed in these “simple” forms, they have been included in Exiguisphaera lita n. sp., and several specimens are illustrated in Plate 29, Figs. 2, 6, 10, 12, 16. Similar forms but without sutural lineations are discussed under the genus Nexosispinum, below.
The essential similarity of the genera Exiguisphaera and Nexosispinum was recognised by Prössl (1990, p. 104), although he suggested that details of the tabulation might only be revealed by SEM analysis, the only distinguishing characteristic again being sutural alignment in Exiguisphaera, but not in Nexosispinum.
----------------------------------------------------------------------------------------------------------------
Notes:
G.L. Williams short notes on species, Mesozoic-Cenozoic dinocyst course, Urbino, Italy, May 17-22, 1999 - LPP VIEWER CD-ROM 99.5.
Exiguisphaera Duxbury, 1979a, emend. Jan du Chêne et al., 1986b, is a proximate, subspherical cyst, with the tabulation indicated by fence-like (perforate) septa. The gonyaulacoid tabulation is ?1-2pr, 4`, 6", 6c, 6"', 1p, 1"'', xs. The precingular archeopyle is formed from the loss of plates 3" and 4". The operculum is solvate, secate.
Exiguisphaera Duxbury, 1979a, p.198–199.
Emendation: Jan du Chêne et al., 1986b, p.11. ; Duxbury, 2023
Type: Duxbury, 1979a, pl.2, figs.2–3; text-figs.1A–B, as Exiguisphaera phragma.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Original description: [Duxbury, 1979]:
Description:
Small, spheroidal dinoflagellate cysts having a minutely perforate body wall. The reflected tabulation pattern, ?a.cl., 4`, 6", ?c, 6```, 1p, ?1p.v., 1````, is outlined by rows of short, simple spines distally connected by trabeculae, which give each crest a fencelike appearance. A few small, intratabular spines may be present. The archeopyle is precingular, formed by detachment of plates 3" and 4````.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Modified description:
Stover and Williams, 1987, p. 93:
Synopsis:
Cysts proximochorate, subspherical; gonyaulacacean paratabulation indicated by fence-like (perforate?) septa; paratabulation formula: 1pr, 3`,1a, 6", Xc, 6```,1p, 1````, ?1s; precingular 2P archeopyle formed by loss of paraplates 3" and 4", operculum free.
Description:
Shape: Subspherical.
Wall relationships: Autophragm only.
Wall features: Parasutural features consist of fencelike (perforate?) septa; spines may be present between parasutural features.
Archaeopyle: Precingular type 2P, (3" and 4"); opercular pieces free.
Paratabulation: Indicated by parasutural features; gonyaulacacean, formula: 1pr, 3`,1a, 6", Xc, 6```, 1p, 1````, and ?ps.
Paracingulum: Indicated by parallel, transverse septa; undivided.
Parasulcus: Longitudinally elongate area mainly on the hypocyst and bounded by parasutural features.
Size: Small to intermediate; about 40 to 55 µm.
Affinities:
Exiguisphaera differs from Occisucysta Gitmez 1970, which also has a precingular 2P archeopyle, by having paraplates 3" and 4" released during archeopyle formation rather than 2" and 3". The 2P archeopyle also serves to separate Exiguisphaera from several other genera (e.g. Leptodinium Klement 1960 emended Stover and Evitt 1978, and Rhynchodiniopsis Deflandre 1935 emended Sarjeant, 1982), with a gonyaulacacean paratabulation expressed by parasutural features, and which have type P (3" only) archeopyles.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Emended description:
Jan du Chêne et al., 1986: (Translation: Fensome et al., 1991, p. 213-214):
Description:
Shape: Cyst proximate to proximochorate, subspherical in shape, without a prominent apical protrusion.
Cyst wall: Autophragm only.
Characters of the wall: The autophragm is often vacuolated. The parasutures are well marked by alignments of arches linked to each other distally. Intratabular ornament is absent or consists of elements of low relief, regularly or irregularly distributed, such as spines, baculae, verrucae, etc.
Paratabulation: Indicated by parasutural septa which delimit the following paraplates: 1-?pr, 4`, 6``, 6c, 5-6s, 6```, 1p, 1````. The scheme formed by these paraplates is of the gonyaulacoid type.
Archaeopyle: 2P (3``+4``). The operculum is free, compound.
Paracingulum: Composed of 6 cingular paraplates, it is generally well delineated by parasutural septa.
Parasulcus: Oriented longitudinally, it comprises five or six paraplates.
Affinities:
Exiguisphaera differs from Diacanthum Habib, 1972; emend. Habib and Drugg (1987) in the structure of the parasutural septa, typically formed by the alignment of arches. Furthermore, the archaeopyle in Exiguisphaera is formed by the loss of paraplates 3`` and 4`` only.
---------------------------------------------------------------------------------------------------
Type: Duxbury 1979a, pl. 2, figs. 2–3; text-figs.1A–B, as
Exiguisphaera phragma.
Emended diagnosis: Small to intermediate, spheroidal dinoflagellate cysts with a sexiform gonyaulacacean tabulation pattern 4', 6''', 6c, 5-6s, 6''', 1p, 1''''. Sutural crests invariably present, formed by solid elements which may furcate and anastamose and may be connected distally by a trabeculum. The archeopyle is precingular, type 2P, formed by the loss of plates 3" and 4". Where present, intratabular ornament ranges from coarse granules to spines.
Remarks: The genus Exiguisphaera Duxbury 1979a is emended here to include irregular sutural crests, as well as the distally trabeculate “fence-like” structures observed in Exiguisphaera phragma Duxbury 1979a, E. plectilis Duxbury 1980 and E. asketa Duxbury 2018.
In addition to the type species, Exiguisphaera phragma Duxbury 1979a, four further species of Exiguisphaera were recorded in the present study, Exiguisphaera asketa Duxbury 2018, Exiguisphaera lasia Duxbury n. sp., Exiguisphaera lita Duxbury n. sp. and Exiguisphaera trichota Duxbury n. sp.
In his remarks regarding E. phragma, Harding (1990, p. 30) described a, “free, compound biplacoid operculum”, and Jan du Chêne et al. (1986b) extended that concept to the genus Exiguisphaera, as “L’opercule est libre, composé”. This does not appear to be the case, however, as specimens of Exiguisphaera lita, E. phragma and E. trichota have been recorded with individual opercular plates within the cyst cavity (see Plate 29).
Small, spheroidal cysts with a two-plate (3"+4") archeopyle were recorded throughout the current study and these displayed considerable variation in onament type, height and distribution. Many of these fall within Exiguisphaera and Nexosispinum Davey 1979b, which appear to differ only because Exiguisphaera possesses sutural crests, whereas in Nexosispinum, “Although occasionally some process alignment may be present, this cannot be related to a cyst tabulation” (Davey 1979b, p. 558). Further discussion of these genera is included under individual taxa below.
Towards the base of the Hauterivian members of both Exiguisphaera and Nexosispinum tend to be particularly “simple”, in having very low surface features; these persist to higher levels, associated with more complex forms. Where sutural lineations were observed in these “simple” forms, they have been included in Exiguisphaera lita n. sp., and several specimens are illustrated in Plate 29, Figs. 2, 6, 10, 12, 16. Similar forms but without sutural lineations are discussed under the genus Nexosispinum, below.
The essential similarity of the genera Exiguisphaera and Nexosispinum was recognised by Prössl (1990, p. 104), although he suggested that details of the tabulation might only be revealed by SEM analysis, the only distinguishing characteristic again being sutural alignment in Exiguisphaera, but not in Nexosispinum.
----------------------------------------------------------------------------------------------------------------
Notes:
G.L. Williams short notes on species, Mesozoic-Cenozoic dinocyst course, Urbino, Italy, May 17-22, 1999 - LPP VIEWER CD-ROM 99.5.
Exiguisphaera Duxbury, 1979a, emend. Jan du Chêne et al., 1986b, is a proximate, subspherical cyst, with the tabulation indicated by fence-like (perforate) septa. The gonyaulacoid tabulation is ?1-2pr, 4`, 6", 6c, 6"', 1p, 1"'', xs. The precingular archeopyle is formed from the loss of plates 3" and 4". The operculum is solvate, secate.