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Ascodinium fissilum
Ascodinium fissilum, Harding, 1990b, p. 16–17; pl.2, figs.1–8 ex Harding in Williams et al., 1998, p. 57.
This name was not validly published in Harding (1990b) since the lodgement of the holotype was not specified (I.C.N. Article 40.7).
Holotype: Harding, 1990, pl.2, fig.6
Locus typicus: O. Gott Claypitt, Lower Saxony, Germany
Stratum typicum: Barremian
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Original description: [Harding, 1990, p. 16-17]:
Diagnosis:
Shape: Ambitus subcircular to peridinioid. Apical and left antapical horns developed to variable extent. Epicyst and hypocyst of approximately equal length. Greatest width in postcingular region. Strong to moderate dorso-ventral compression.
Phragma: Periphragm extremely thin (ca. 0.1-0.2 µm thick), laevigate. Scattered intratabular tubercles (ca. 0.75 µm dia.) indicate paratabulation. Endophragm ca. 1 µm thick, surface densely microgranulate. Pericoels restricted to cornucavation of apical and left antapical horns, though occasionally absent.
Paratabulation: As shown by intratabular tubercles and paraplate splitting: ortho-hexa peridinioid. ?pr, 4", 3a, 7", ?6c, 5""", 2"""", 2s.
Archaeopyle: Type (4A3I), paraplates lost as a free, simple polyplacoid operculum - both periphragm and endophragm remain adpressed and are effectively lost as a single operculum.
Paracingulum: Slightly indented, generally unsculptured or with few tubercles near paraplate margins.
Parasulcus: Suppression of parasutures has reduced paratabulation to two distinguishable regions presumably representing a large as and ps+rs+ls.
Dimensions:
Length (59) 52.0 (43) µm. Width (51) 44.1 (39) µm. Specimens = 14 (9).
Affinities:
This species differs from all the previously described species of the genus by its much reduced cavation and the character of the wall layers. A. hesperum (Davey) Helenes 1983 differs in having a much thicker endophragm and being larger. A strange characteristic displayed by specimens of this species is that of partial disintegration along all paraplate boundaries. This feature is not believed due to processing techniques.
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Supplemental description:
Heilmann-Clausen and Thomsen 1995, p. 290:
Description:
The cyst-shape is bi-conical or sphaerical. Folds or ridges in the phragma apparently indicate the parasulcus and paracingulum. The endophragm is of variable thickness, often very thick. The periphragm is thin and flexible. A very small pericoel can usually be seen both apically and antapically. Sometimes the phragma subdivide into several thin layers at the antapex, e.g. Pl. 3, figs. 7, 13.
The free operculum is of type 4A3I, and the shape of the archaeopyle conforms with this. However, the position of the archaeopyle sometimes seems to overlap with the precingular or even the paracingular zone (e.g. Pl. 3, figs. 7, 9 and Text-fig. 12).
Remarks:
The three excysted specimens drawn in Text-fig. 12 show archaeopyles with shape indicating the 4A31-type. The several free opercula, which have been recovered, conforms with this archaeopyle type. It is therefore remarkable, that the archaeopyle suture between paraplate 2a and what should be paraplate 4" in one specimen (Text-fig. 12E) coincides with the anterior paracingular ridge, and in another (Text-fig. 12C) with the posterior paracingular ridge.
A possible explanation of this paradox is that the "paracingular ridges" do not always occur in a genuine paracingular position of the cyst, but may be offset to a precingular, or even intercalary position. This interpretation is contrary to the experience of the general constancy of paratabular features on dinoflagellate cysts.
None of the specimens in the rich material showed the partial disintegration of paraplates mentioned by HARDING (1990).
This name was not validly published in Harding (1990b) since the lodgement of the holotype was not specified (I.C.N. Article 40.7).
Holotype: Harding, 1990, pl.2, fig.6
Locus typicus: O. Gott Claypitt, Lower Saxony, Germany
Stratum typicum: Barremian
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Original description: [Harding, 1990, p. 16-17]:
Diagnosis:
Shape: Ambitus subcircular to peridinioid. Apical and left antapical horns developed to variable extent. Epicyst and hypocyst of approximately equal length. Greatest width in postcingular region. Strong to moderate dorso-ventral compression.
Phragma: Periphragm extremely thin (ca. 0.1-0.2 µm thick), laevigate. Scattered intratabular tubercles (ca. 0.75 µm dia.) indicate paratabulation. Endophragm ca. 1 µm thick, surface densely microgranulate. Pericoels restricted to cornucavation of apical and left antapical horns, though occasionally absent.
Paratabulation: As shown by intratabular tubercles and paraplate splitting: ortho-hexa peridinioid. ?pr, 4", 3a, 7", ?6c, 5""", 2"""", 2s.
Archaeopyle: Type (4A3I), paraplates lost as a free, simple polyplacoid operculum - both periphragm and endophragm remain adpressed and are effectively lost as a single operculum.
Paracingulum: Slightly indented, generally unsculptured or with few tubercles near paraplate margins.
Parasulcus: Suppression of parasutures has reduced paratabulation to two distinguishable regions presumably representing a large as and ps+rs+ls.
Dimensions:
Length (59) 52.0 (43) µm. Width (51) 44.1 (39) µm. Specimens = 14 (9).
Affinities:
This species differs from all the previously described species of the genus by its much reduced cavation and the character of the wall layers. A. hesperum (Davey) Helenes 1983 differs in having a much thicker endophragm and being larger. A strange characteristic displayed by specimens of this species is that of partial disintegration along all paraplate boundaries. This feature is not believed due to processing techniques.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Supplemental description:
Heilmann-Clausen and Thomsen 1995, p. 290:
Description:
The cyst-shape is bi-conical or sphaerical. Folds or ridges in the phragma apparently indicate the parasulcus and paracingulum. The endophragm is of variable thickness, often very thick. The periphragm is thin and flexible. A very small pericoel can usually be seen both apically and antapically. Sometimes the phragma subdivide into several thin layers at the antapex, e.g. Pl. 3, figs. 7, 13.
The free operculum is of type 4A3I, and the shape of the archaeopyle conforms with this. However, the position of the archaeopyle sometimes seems to overlap with the precingular or even the paracingular zone (e.g. Pl. 3, figs. 7, 9 and Text-fig. 12).
Remarks:
The three excysted specimens drawn in Text-fig. 12 show archaeopyles with shape indicating the 4A31-type. The several free opercula, which have been recovered, conforms with this archaeopyle type. It is therefore remarkable, that the archaeopyle suture between paraplate 2a and what should be paraplate 4" in one specimen (Text-fig. 12E) coincides with the anterior paracingular ridge, and in another (Text-fig. 12C) with the posterior paracingular ridge.
A possible explanation of this paradox is that the "paracingular ridges" do not always occur in a genuine paracingular position of the cyst, but may be offset to a precingular, or even intercalary position. This interpretation is contrary to the experience of the general constancy of paratabular features on dinoflagellate cysts.
None of the specimens in the rich material showed the partial disintegration of paraplates mentioned by HARDING (1990).