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Rimosicysta concava
Rimosicysta concava Marshall 1989
Holotype. Plate 4, figs. 12-14.
Locus typicus: Gippsland Basin, Southeastern Australia
Stratum typicum: Late Cretaceous
Description. Marshall 1989, p. 48
Cysts with a weak to distinct middorsal bulge; generally equidimensional in dorsoventral view. Each lateral margin extended equatorially to form a broad, flared projection that is either convex distally or bears two convexities separated by a medial, rounded notch. Notches occasionally associated with a weak lateral groove, ca. 1.5 µm wide, defining a possible paracingulum across the dorsal surface of the cyst. Corners of distal margins of equatorial projections have a rounded subconical shape; those on hypocyst occasionally extended posteriorally to form short horns (e.g., see Text-Figure 13C). Equatorial projections bordered by four pronounced concavities. Distance between concavities on adjacent flanks greatest on hypocyst. Apical margin marked by a shallow concavity, and bordered by two flared projections with truncated tips. Antapical margin with a shallow concavity or two concavities separated by a small rounded peak or bulge. Antapical margin bordered by two flared projections similar to those on apical margin. Corners of these apical and antapical projections rounded, occasionally extended to form short knobs. Autophragm smooth, strongly crumpled; usually 0.2-0.3 µm thick around periphery of cyst and frequently appears to thicken middorsally and midventrally. Autophragm thins markedly, and occasionally splits apart parallel to cyst outline, within concavities on apical, antapical, and lateral margins. Autophragm can have an irregular outline in these areas. Parasulcal region suggested on midventral surface by a shallow longitudinal groove; occasionally has a possible flagellar marking defined by a small (ca. 6 µm diameter), subcircular, shallow depression partly bordered by a crescentic ridge (Text-Figure 13D).
Size. Autocyst length 94(118)135 µm, width 90(121)146 µm; 14 specimens measured.
Discussion. Marshall 1989, p. 48
The autophragm has an extremely indistinct outline where it thins within the concavities around the periphery of the cyst, and it is not usually possible to detect if it is split apart. However, occasional specimens are split parallel to the cyst outline in these areas. It is often difficult to distinguish the apex from the antapex on this form because the epicyst and hypocyst are almost symmetrical. The two can be differentiated by the presence of a small peak on the antapical margin, and the greater distance between the concavities on the flanks of the hypocyst than on the epicyst.
Comparison. Marshall 1989, p. 48
The weak horns on the posterior margins of the equatorial projections of some specimens of Rimosicysta concava closely resemble the similarly positioned horns on R. cucullata. These species are easily distinguished by their different epicystal outlines. The dorsoventral outline of R. concava is most similar to that of R. eversa, but the former is more equidimensional and has more pronounced blunt-ended projections on the apical, antapical, and lateral margins.
Occurrence. Sunfish-l, 2482.7-2484.4 m.
Holotype. Plate 4, figs. 12-14.
Locus typicus: Gippsland Basin, Southeastern Australia
Stratum typicum: Late Cretaceous
Description. Marshall 1989, p. 48
Cysts with a weak to distinct middorsal bulge; generally equidimensional in dorsoventral view. Each lateral margin extended equatorially to form a broad, flared projection that is either convex distally or bears two convexities separated by a medial, rounded notch. Notches occasionally associated with a weak lateral groove, ca. 1.5 µm wide, defining a possible paracingulum across the dorsal surface of the cyst. Corners of distal margins of equatorial projections have a rounded subconical shape; those on hypocyst occasionally extended posteriorally to form short horns (e.g., see Text-Figure 13C). Equatorial projections bordered by four pronounced concavities. Distance between concavities on adjacent flanks greatest on hypocyst. Apical margin marked by a shallow concavity, and bordered by two flared projections with truncated tips. Antapical margin with a shallow concavity or two concavities separated by a small rounded peak or bulge. Antapical margin bordered by two flared projections similar to those on apical margin. Corners of these apical and antapical projections rounded, occasionally extended to form short knobs. Autophragm smooth, strongly crumpled; usually 0.2-0.3 µm thick around periphery of cyst and frequently appears to thicken middorsally and midventrally. Autophragm thins markedly, and occasionally splits apart parallel to cyst outline, within concavities on apical, antapical, and lateral margins. Autophragm can have an irregular outline in these areas. Parasulcal region suggested on midventral surface by a shallow longitudinal groove; occasionally has a possible flagellar marking defined by a small (ca. 6 µm diameter), subcircular, shallow depression partly bordered by a crescentic ridge (Text-Figure 13D).
Size. Autocyst length 94(118)135 µm, width 90(121)146 µm; 14 specimens measured.
Discussion. Marshall 1989, p. 48
The autophragm has an extremely indistinct outline where it thins within the concavities around the periphery of the cyst, and it is not usually possible to detect if it is split apart. However, occasional specimens are split parallel to the cyst outline in these areas. It is often difficult to distinguish the apex from the antapex on this form because the epicyst and hypocyst are almost symmetrical. The two can be differentiated by the presence of a small peak on the antapical margin, and the greater distance between the concavities on the flanks of the hypocyst than on the epicyst.
Comparison. Marshall 1989, p. 48
The weak horns on the posterior margins of the equatorial projections of some specimens of Rimosicysta concava closely resemble the similarly positioned horns on R. cucullata. These species are easily distinguished by their different epicystal outlines. The dorsoventral outline of R. concava is most similar to that of R. eversa, but the former is more equidimensional and has more pronounced blunt-ended projections on the apical, antapical, and lateral margins.
Occurrence. Sunfish-l, 2482.7-2484.4 m.