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Dissiliodinium curiosum
Dissiliodinium curiosum Burger and Sarjeant, 1995, p.120-122,124, figs.3,4a-c,5a-b,6a-b,7a-b,8a-b,9a-b,10a-b,11a-c,12a-b,13a-b,14a-b.
Taxonomic senior synonym: Dissiliodinium globulus, according to Leereveld (1997, p.413) - however, Feist-Burkhardt and Monteil (2001, p.64,72) retained Dissiliodinium curiosum.
Holotype: Burger and Sarjeant, 1995, figs.4a-c.
Age: latest Jurassic-Early Cretaceous.
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Original description (Burger and Sarjeant, 1995):
Dissiliodinium curiosum n.sp. (figs. 3-15)
Derivation of name: Based on the variable mode of dehiscence (l. for weird, unusual).
Holotype: Barrow Group, Carnavon Terrace (offshore Carnarvon Basin), North West shelf; Barriasian to early Valanginian; Dredge sample 96DR05/6, palynological preparation number MFP9248-1, 1047/050; Figs. 4a-c.
Description: The complete cyst is spherical, oval or egg-shaped, without horns or other protrusions. The hypotract is normally complete, the epitract usually incomplete though loss of paraplates during excystment. The hypotract is hemispherical to cup-shaped, frequently folded; the epitract is somewhat smaller and hemispherical to broadly conical with rounded apex, and if preserved is usually partly detached. The autohphargm is extremely thin, with a cover of evenly distributed and sharply defined, discrete, large grana about 0.5 µm in diameter and on average set 0.5-1 µm apart. Excystment causes the loss of variable portions of the epitract, commonly including several or all paraplates of the precingular series. In some specimens two (usually 1’’ and 2’’; or 1’’ and 6’’) or three (usually 1’’, 2’’ and 6’’; 1’’-3’’; or 2’’-4’’) precingular paraplates are still attached; they give the archeopyle a zigzag ventral margin. Occasionally all six precingulars are still present. The apex may remain attached by long and narrow paraplates 1’and 4. In many specimens the epitract is almost wholly lost. The dorsal archeopyle margins may show tiny notches at the conjunctions of cingular paraplates; slightly opened hypotractal parasutures may also occur in the ventral and antapical areas. In several specimens pseudo-dehiscence removes large portions of the epitract, independently of paraplate boundaries. Occasional breakdown of the hypotract is also observed, with partial detachment of postcingular and sulcal paraplates.
Dimensions: cyst diameter in cingular plane (57 specimens) 47-(60)-83 µm. Apical dimensions (8 undehisced specimens) 57-64 µm. Cingular by apical dimensions of holotype 79 x 68 µµ, paratype a 57 x 50 µm; cingular by (dehisced) apical dimensions of paratype b 64 x 51 µm, paratype c 69 x 48 µm; cingular (dorsal/ventral) dimensions of paratype d 52 µm.
Distribution: The species was found in several (uppermost Jurassic to) Lower Cretaceous dredge samples of the Barrow Group and the Muderong Shale, with an abundance peak in the Berriasian (Table 2).
Comments: Despite manifestations of several apparent modes of dehiscence, this population clearly represents one species and is here assigned to the genus Dissiliodinium. The specimens are easily deformed and difficult to photograph, but are readily distinguished by regular and uniform wall ornament. During the progressive breakdown of the extremely fragile autophragm, the precingular paraplates are gradually lost, often with 1’’ and 2’’ and sometimes 6’’ the last to go. Archeopyle types formed at this stage range from 2P to 6P. In some specimens the apical cyst portion is lost at an early stage of breakdown, resulting in archeopyles of type tA, or types tA+1P to tA+3P. In the final stage of breakdown, the entire epitract is lost, with archeopyle type tA+tP. In addition, a few specimens display irregular pseudo-archepyles not bounded by paraplate margins. It cannot be ascertained whether these may represent a different species, for instance Leiosphaeridia? Perthensis Backhouse, 1988 (see below).
Comparison: The specimens described here differ from other representatives of the genus in Australia by their cyst ornament. Dissiliodinium caddaense (Filatoff, 1975) Stover & Helby, 1987, occurs in the Bajocian of Western Australia and is larger; it has a less regular and less uniform cyst ornament, develops an antapical nipple and lacks distinct apical parasutures. Dissiliodinium sp. of Helby et al., 1987 (see their p.27, Fig. 16E) from the early Middle Jurassic appears to have a less regular wall ornament. The type species of the genus D. globulum Drugg, 1978, fall within the lower size range of the Australian species but lacks visible post-cingular hypotract parasutures and has a less conspicuous surface ornament. D. willei Bailey & Partington, 1991, from the Middle Jurassic of the North Sea displays a less uniform ornament, ranging from low relief granulate to vermiculate and reticulate. Leiosphaeridia? Perthensis Backhouse, 1988, from the Neocomian of the Perth Basin, has comparable dimensions and a similar ornament, while the shell also fractures very easily. However, it lacks evidence of paratabulation and, although a portion of the wall is usually removed, Backhouse did not identify the aperture as a genuine archeopyle.
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Taxonomic senior synonym: Dissiliodinium globulus, according to Leereveld (1997, p.413) - however, Feist-Burkhardt and Monteil (2001, p.64,72) retained Dissiliodinium curiosum.
Holotype: Burger and Sarjeant, 1995, figs.4a-c.
Age: latest Jurassic-Early Cretaceous.
----------------------------------------------------------
Original description (Burger and Sarjeant, 1995):
Dissiliodinium curiosum n.sp. (figs. 3-15)
Derivation of name: Based on the variable mode of dehiscence (l. for weird, unusual).
Holotype: Barrow Group, Carnavon Terrace (offshore Carnarvon Basin), North West shelf; Barriasian to early Valanginian; Dredge sample 96DR05/6, palynological preparation number MFP9248-1, 1047/050; Figs. 4a-c.
Description: The complete cyst is spherical, oval or egg-shaped, without horns or other protrusions. The hypotract is normally complete, the epitract usually incomplete though loss of paraplates during excystment. The hypotract is hemispherical to cup-shaped, frequently folded; the epitract is somewhat smaller and hemispherical to broadly conical with rounded apex, and if preserved is usually partly detached. The autohphargm is extremely thin, with a cover of evenly distributed and sharply defined, discrete, large grana about 0.5 µm in diameter and on average set 0.5-1 µm apart. Excystment causes the loss of variable portions of the epitract, commonly including several or all paraplates of the precingular series. In some specimens two (usually 1’’ and 2’’; or 1’’ and 6’’) or three (usually 1’’, 2’’ and 6’’; 1’’-3’’; or 2’’-4’’) precingular paraplates are still attached; they give the archeopyle a zigzag ventral margin. Occasionally all six precingulars are still present. The apex may remain attached by long and narrow paraplates 1’and 4. In many specimens the epitract is almost wholly lost. The dorsal archeopyle margins may show tiny notches at the conjunctions of cingular paraplates; slightly opened hypotractal parasutures may also occur in the ventral and antapical areas. In several specimens pseudo-dehiscence removes large portions of the epitract, independently of paraplate boundaries. Occasional breakdown of the hypotract is also observed, with partial detachment of postcingular and sulcal paraplates.
Dimensions: cyst diameter in cingular plane (57 specimens) 47-(60)-83 µm. Apical dimensions (8 undehisced specimens) 57-64 µm. Cingular by apical dimensions of holotype 79 x 68 µµ, paratype a 57 x 50 µm; cingular by (dehisced) apical dimensions of paratype b 64 x 51 µm, paratype c 69 x 48 µm; cingular (dorsal/ventral) dimensions of paratype d 52 µm.
Distribution: The species was found in several (uppermost Jurassic to) Lower Cretaceous dredge samples of the Barrow Group and the Muderong Shale, with an abundance peak in the Berriasian (Table 2).
Comments: Despite manifestations of several apparent modes of dehiscence, this population clearly represents one species and is here assigned to the genus Dissiliodinium. The specimens are easily deformed and difficult to photograph, but are readily distinguished by regular and uniform wall ornament. During the progressive breakdown of the extremely fragile autophragm, the precingular paraplates are gradually lost, often with 1’’ and 2’’ and sometimes 6’’ the last to go. Archeopyle types formed at this stage range from 2P to 6P. In some specimens the apical cyst portion is lost at an early stage of breakdown, resulting in archeopyles of type tA, or types tA+1P to tA+3P. In the final stage of breakdown, the entire epitract is lost, with archeopyle type tA+tP. In addition, a few specimens display irregular pseudo-archepyles not bounded by paraplate margins. It cannot be ascertained whether these may represent a different species, for instance Leiosphaeridia? Perthensis Backhouse, 1988 (see below).
Comparison: The specimens described here differ from other representatives of the genus in Australia by their cyst ornament. Dissiliodinium caddaense (Filatoff, 1975) Stover & Helby, 1987, occurs in the Bajocian of Western Australia and is larger; it has a less regular and less uniform cyst ornament, develops an antapical nipple and lacks distinct apical parasutures. Dissiliodinium sp. of Helby et al., 1987 (see their p.27, Fig. 16E) from the early Middle Jurassic appears to have a less regular wall ornament. The type species of the genus D. globulum Drugg, 1978, fall within the lower size range of the Australian species but lacks visible post-cingular hypotract parasutures and has a less conspicuous surface ornament. D. willei Bailey & Partington, 1991, from the Middle Jurassic of the North Sea displays a less uniform ornament, ranging from low relief granulate to vermiculate and reticulate. Leiosphaeridia? Perthensis Backhouse, 1988, from the Neocomian of the Perth Basin, has comparable dimensions and a similar ornament, while the shell also fractures very easily. However, it lacks evidence of paratabulation and, although a portion of the wall is usually removed, Backhouse did not identify the aperture as a genuine archeopyle.
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