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Meiourogonyaulax diaphana
Meiourogonyaulax diphanis Stevens, 1987
Originally (and now) Meiourogonyaulax, subsequently Lithodinia?. This species is here retained in Meiourogonyaulax following the retention of the genus by Riding and Helby (2001d, p.81,83).
Holotype: Stevens, 1987, figs.4A-D,6A-B; Fensome et al., 1996, figs.1-3,9-10 - p.2111.
Age: Early Berriasian.
Original description: Stevens, 1987, p. 191-192
Cysts proximate, elongate polygonal to rounded polygonal in outline, with short rounded, apical endo- and perihorns (3-6 µm and 6-12 µm in length, respectively). Cysts commonly compressed laterally (Fig.4A-D) and less commonly dorsoventrally (Fig.4J). Endophragm and periphragm diaphanous, both less than 1 µm thick, separated over most of the cyst by a pericoel up to 2 µm wide, but appressed at the parasutural folds or ridges (Fig.4F, lower right), and over most of the paracingulum (Fig.4C, left). Archeopyle apical, type [tA]; principal suture zigzag with a relatively deep sulcal notch and short accessory sutures. Operculum may be free but generally attached at the sulcal notch (Figs 4A, 6A), i.e. the posterior end of the 1" paraplate and the anterior end of the parasulcus clearly delineated. Operculum less commonly broken away due to compression of the cyst. Paratabulation absent or expressed only on the periphragm by low, narrow folds (less than 1 Ám wide and high, Fig.4A-H) and/or by striate, perforate ridges (2.5-3 µm high and less than 1 µm wide) with denticulate crests (Fig.4I,J). Paratabulation, when expressed, never complete, gonyaulacacean, 1pr, 4", 6"", 6c, 5""", 1p, 1"""", Os (Fig.6A,B). Preapical paraplate indicated on the right lateral side of the holotype (Fig.4A-D) with unclear boundary (dashed on Fig.6A) on the left lateral side. Long 1" paraplate characteristic due to the relatively deep sulcal notch (l~lg.4B,G,H,K). Six cingular paraplates interpreted, but parasutural features not evident between all cingular paraplates (dashed on Fig.6A,B). Five postcingular paraplates interpreted as 2""" to 6""", with a large 1p paraplate. Paracingulum 2.5-5 µm wide, indicated on nontabulate forms by an equatorial appression of the endo- and periphragm and/or by an indentation in the periphragm. On tabulate forms an equatorial appression of the wall layers delineated by parasutural folds or ridges. Parasulcus indicated where appressed wall layers form a shadow, linear expression extending from the sulcal notch almost to the antapex, apparently unsubdivided although commonly obscured and distorted due to the lateral compression of most cysts. Holotype (Fig.4A-D) the best preserved of specimens studied, with almost complete paratabulation (Fig.6A,B) characteristic of specimens with mostly parasutural folds; however, holotype with a striate, perforate parasutural ridge between the 5""" and 1"""" paraplates.
Measurements. Cyst length (dehisced) 41(48)54 µm, cyst width 32(41)52 µm (53 specimens).
Variability. The species is fairly constant in size, but is highly variable in outline, separation of the wall layers, and parasutural features. The cyst outline in lateral compression varies from rounded polygonal to elongate polygonal for nontabulate and tabulate forms respectively (Fig.7A-D), the overall shape apparently varying with the development of parasutural features and compressional folds and tears. A pericoel is always present at the apical horn (Fig.4C,K), however it is commonly obscured by folding and compression of the cyst walls. Paratabulation is expressed on the periphragm by folds (Fig.4D,E), and/or ridges (Fig.41,J), but is apparently never complete. A complete gradation from nontabulate to tabulate forms is observed (Fig.7A-D).
Affinities:
Stevens, 1987, p. 191: Kallosphaeridium de Coninck 1969 and Meiourogonyaulax Sarjeant 1966 are differentiated by the attached operculum and lack of paratabulation in the former and the presence of paratabulation and a free operculum in the latter. Both are described as having an autophragm. Apart from having two layers, the tabulate forms described above conform to Meiourogonyaulax and the nontabulate forms agree with Kallosphaeridium. The presence of two wall layers is not considered generically diagnostic nor does it preclude the assignment to either of these genera. As a complete gradation in the above features is exhibited by the specimens studied, it is preferable to group the variants as one species. The species is tentatively referred to Meiourogonyaulax, differing from other species in having two wall layers and an operculum which is commonly adherent. Meiourogonyaulax diaphanis differs from Cernieysta helbyi (Morgan) Stover & Helby 1987 in having its two wall layers appressed rather than separate at the parasutural ridges.
Originally (and now) Meiourogonyaulax, subsequently Lithodinia?. This species is here retained in Meiourogonyaulax following the retention of the genus by Riding and Helby (2001d, p.81,83).
Holotype: Stevens, 1987, figs.4A-D,6A-B; Fensome et al., 1996, figs.1-3,9-10 - p.2111.
Age: Early Berriasian.
Original description: Stevens, 1987, p. 191-192
Cysts proximate, elongate polygonal to rounded polygonal in outline, with short rounded, apical endo- and perihorns (3-6 µm and 6-12 µm in length, respectively). Cysts commonly compressed laterally (Fig.4A-D) and less commonly dorsoventrally (Fig.4J). Endophragm and periphragm diaphanous, both less than 1 µm thick, separated over most of the cyst by a pericoel up to 2 µm wide, but appressed at the parasutural folds or ridges (Fig.4F, lower right), and over most of the paracingulum (Fig.4C, left). Archeopyle apical, type [tA]; principal suture zigzag with a relatively deep sulcal notch and short accessory sutures. Operculum may be free but generally attached at the sulcal notch (Figs 4A, 6A), i.e. the posterior end of the 1" paraplate and the anterior end of the parasulcus clearly delineated. Operculum less commonly broken away due to compression of the cyst. Paratabulation absent or expressed only on the periphragm by low, narrow folds (less than 1 Ám wide and high, Fig.4A-H) and/or by striate, perforate ridges (2.5-3 µm high and less than 1 µm wide) with denticulate crests (Fig.4I,J). Paratabulation, when expressed, never complete, gonyaulacacean, 1pr, 4", 6"", 6c, 5""", 1p, 1"""", Os (Fig.6A,B). Preapical paraplate indicated on the right lateral side of the holotype (Fig.4A-D) with unclear boundary (dashed on Fig.6A) on the left lateral side. Long 1" paraplate characteristic due to the relatively deep sulcal notch (l~lg.4B,G,H,K). Six cingular paraplates interpreted, but parasutural features not evident between all cingular paraplates (dashed on Fig.6A,B). Five postcingular paraplates interpreted as 2""" to 6""", with a large 1p paraplate. Paracingulum 2.5-5 µm wide, indicated on nontabulate forms by an equatorial appression of the endo- and periphragm and/or by an indentation in the periphragm. On tabulate forms an equatorial appression of the wall layers delineated by parasutural folds or ridges. Parasulcus indicated where appressed wall layers form a shadow, linear expression extending from the sulcal notch almost to the antapex, apparently unsubdivided although commonly obscured and distorted due to the lateral compression of most cysts. Holotype (Fig.4A-D) the best preserved of specimens studied, with almost complete paratabulation (Fig.6A,B) characteristic of specimens with mostly parasutural folds; however, holotype with a striate, perforate parasutural ridge between the 5""" and 1"""" paraplates.
Measurements. Cyst length (dehisced) 41(48)54 µm, cyst width 32(41)52 µm (53 specimens).
Variability. The species is fairly constant in size, but is highly variable in outline, separation of the wall layers, and parasutural features. The cyst outline in lateral compression varies from rounded polygonal to elongate polygonal for nontabulate and tabulate forms respectively (Fig.7A-D), the overall shape apparently varying with the development of parasutural features and compressional folds and tears. A pericoel is always present at the apical horn (Fig.4C,K), however it is commonly obscured by folding and compression of the cyst walls. Paratabulation is expressed on the periphragm by folds (Fig.4D,E), and/or ridges (Fig.41,J), but is apparently never complete. A complete gradation from nontabulate to tabulate forms is observed (Fig.7A-D).
Affinities:
Stevens, 1987, p. 191: Kallosphaeridium de Coninck 1969 and Meiourogonyaulax Sarjeant 1966 are differentiated by the attached operculum and lack of paratabulation in the former and the presence of paratabulation and a free operculum in the latter. Both are described as having an autophragm. Apart from having two layers, the tabulate forms described above conform to Meiourogonyaulax and the nontabulate forms agree with Kallosphaeridium. The presence of two wall layers is not considered generically diagnostic nor does it preclude the assignment to either of these genera. As a complete gradation in the above features is exhibited by the specimens studied, it is preferable to group the variants as one species. The species is tentatively referred to Meiourogonyaulax, differing from other species in having two wall layers and an operculum which is commonly adherent. Meiourogonyaulax diaphanis differs from Cernieysta helbyi (Morgan) Stover & Helby 1987 in having its two wall layers appressed rather than separate at the parasutural ridges.