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Rimosicysta
Rimosicysta, Marshall, 1989, p. 36-38, 40
Type species: Rimosicysta eversa, Marshall, 1989
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Original description: [Marshall, 1989]:
Synopsis:
Cysts compressed; with a middorsal bulge, and longitudinal markings along center of ventral surface defining a possible parasulcus. Autophragm smooth or weakly sculptured; dorso-ventral outline highly variable, equidimensional or elongate, widest near equator, and frequently modified by up to six blunt-ended extensions: two antapical, two lateral, and two apical. Autophragm thins markedly and may be split apart parallel to cyst outline along apical and antapical margins, and within concavities on epicystal and hypocystal flanks.
Description:
Shape: Cysts compressed dorsoventrally, frequently with a pronounced middorsal bulge which is often flattened to produce a transverse fold. Middle of ventral surface occasionally bearing a shallow longitudinal groove bordered by two ridges or two or more longitudinal folds; more commonly marked by a weak to pronounced longitudinal fold extending along much of the cyst length. When only one fold occurs, fold often considerably higher on hypocyst. Dorsoventral outline highly variable: equidimensional or longitudinally elongate; widest at equator; and frequently modified by rounded to subrectangular, blunt-ended extensions: one on the midlength of each of lateral margin, and two on each of the apical and antapical margins. Lateral margins on either side of equator frequently bearing pronounced concavities.
Wall Relationships: Autophragm only.
Wall Features: Surface smooth, often crumpled, and occasionally bears hollow, rounded to pointed projections around periphery. Projections form a serrated outline when arranged in groups. Autophragm becomes extremely thin and may be split apart parallel to cyst outline along apical and antapical margins, and within concavities on epicystal and hypocystal flanks. Autophragm often thickens slightly middorsally and midventrally.
Paracingulum: Possibly indicated by two parallel rows of ridges on dorsal surface.
Parasulcus: Possibly outlined by longitudinal markings on midventral surface.
Size: Large (after Stover and Evitt, 1978).
Discussion:
When the outline of the prominent lateral fold on the dorsal surface is superimposed on that of the longitudinal fold on the center of the ventral surface a distinctive cross-shaped image is produced. However, the longitudinal fold is not always evident and may alternately be expressed as two longitudinal folds and/or a shallow groove.
Affinities:
Rimosicysta and Limbicysta are similar genera and both have some dinoflagellate-like features (Text-Figures 4, 5). The unusual areas of thinning and associated splits around the periphery of the cyst distinguish these genera from any previously described dinoflagellate or acritarch. Rimosicysta differs mainly from Limbicysta in lacking elongate horns on the apical, antapical, and lateral margins. The apical margin on Rimosicysta is generally flattened and the outer wall thins and may be split apart parallel to the cyst outline in this area. The blunt-ended projections on the antapical and lateral margins of Rimosicysta resemble the similarly positioned horns on Limbicysta, but are considerably shorter.
Type species: Rimosicysta eversa, Marshall, 1989
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Original description: [Marshall, 1989]:
Synopsis:
Cysts compressed; with a middorsal bulge, and longitudinal markings along center of ventral surface defining a possible parasulcus. Autophragm smooth or weakly sculptured; dorso-ventral outline highly variable, equidimensional or elongate, widest near equator, and frequently modified by up to six blunt-ended extensions: two antapical, two lateral, and two apical. Autophragm thins markedly and may be split apart parallel to cyst outline along apical and antapical margins, and within concavities on epicystal and hypocystal flanks.
Description:
Shape: Cysts compressed dorsoventrally, frequently with a pronounced middorsal bulge which is often flattened to produce a transverse fold. Middle of ventral surface occasionally bearing a shallow longitudinal groove bordered by two ridges or two or more longitudinal folds; more commonly marked by a weak to pronounced longitudinal fold extending along much of the cyst length. When only one fold occurs, fold often considerably higher on hypocyst. Dorsoventral outline highly variable: equidimensional or longitudinally elongate; widest at equator; and frequently modified by rounded to subrectangular, blunt-ended extensions: one on the midlength of each of lateral margin, and two on each of the apical and antapical margins. Lateral margins on either side of equator frequently bearing pronounced concavities.
Wall Relationships: Autophragm only.
Wall Features: Surface smooth, often crumpled, and occasionally bears hollow, rounded to pointed projections around periphery. Projections form a serrated outline when arranged in groups. Autophragm becomes extremely thin and may be split apart parallel to cyst outline along apical and antapical margins, and within concavities on epicystal and hypocystal flanks. Autophragm often thickens slightly middorsally and midventrally.
Paracingulum: Possibly indicated by two parallel rows of ridges on dorsal surface.
Parasulcus: Possibly outlined by longitudinal markings on midventral surface.
Size: Large (after Stover and Evitt, 1978).
Discussion:
When the outline of the prominent lateral fold on the dorsal surface is superimposed on that of the longitudinal fold on the center of the ventral surface a distinctive cross-shaped image is produced. However, the longitudinal fold is not always evident and may alternately be expressed as two longitudinal folds and/or a shallow groove.
Affinities:
Rimosicysta and Limbicysta are similar genera and both have some dinoflagellate-like features (Text-Figures 4, 5). The unusual areas of thinning and associated splits around the periphery of the cyst distinguish these genera from any previously described dinoflagellate or acritarch. Rimosicysta differs mainly from Limbicysta in lacking elongate horns on the apical, antapical, and lateral margins. The apical margin on Rimosicysta is generally flattened and the outer wall thins and may be split apart parallel to the cyst outline in this area. The blunt-ended projections on the antapical and lateral margins of Rimosicysta resemble the similarly positioned horns on Limbicysta, but are considerably shorter.