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Hystrichosphaeropsis tenerum
Original description: [Vieira et al., 2024, p. 8-9]:
Hystrichosphaeropsis tenerum sp. nov.
Plate 3, figures 1–12; Figure 6
Derivation of name. Named after the Latin tenera – thin, soft, tender – as relates to the cyst membrane.
Holotype. Plate 3, figures 1–4. Slide R43/55A, coordinates 103 x 16. Volgograd Volga region, Ravninnaya R43 borehole, sample 55, collection number IGG-2/23-06. Specimen dimensions: total length 54 μm, total width 40 mm, endocyst length 21 μm, endocyst width 32 μm, cingulum width 7–14 μm.
Paratype. Plate 3, figures 9, 10. Slide R43/55B, coordinates 94 x 8. Volgograd Volga region, borehole R-43, sample 55, coll. number IGG-2/23-07. Specimen dimensions: total length 47 μm, total width 36 μm, endocyst length 14 μm, endocyst width 28 μm, cingulum width 7–10 μm.
Type locality. The village of Ravninny in the vicinity of Volgograd, Volgograd region, Russia.
Type stratum. Keresta Formation, Volga–Caspian subregion.
Diagnosis. Gonyaulacoid bicavate proximate cysts without apical horn. Epicyst and hypocyst contacting the endocyst in cingular zone. Endocyst is flattened-oval. Tabulation is marked by thin ribs on the pericyst and by a precingular archaeopyle.
Description. Cysts of Hystrichosphaeropsis oval, angular-trapezoid in shape, without apical horn. Epicyst and hypocyst are ovaltrapezoid. Pericyst is in contact with endocyst in cingular zone. Periphragm is thin, smooth, transparent, and generally bears distinct sutures and short simple gonal spikes (Plate 3, figures 3, 8). Sutures are occasionally ornamented with low delicate septa with denticulate edges. Endocyst is oval, highly flattened (Plate 3, figure 6). Endophragm is extremely thin, frequently crumpled into creases. Precingular archaeopyle, type P (3ʺ). Operculum is generally free. Cingulum is formed by plates rectangular in shape and fringed by distinct sutures (Plate 3, figures 4, 10). Sulcus is limited by ribs, comprises a large pore (Plate 3, figure 5). Tabulation is marked by sutures, archaeopyle, sulcus position: 4', 6ʺ, 6c, 5"', 1p, 1ʺʺ (Figure 6). A large aperture (claustrum) is generally observed on the periphragm in the antapical zone on the ventral surface (Plate 3, figures 1, 7, 9, 11).
Dimensions. Cyst total length 43 (50) 54 mm, cyst total width 36 (39) 47 mm, endocyst length 14 (20) 25 mm, endocyst width 27 (32) 36 mm. Number of specimens measured: 10.
Comparison. Hystrichosphaeropsis tenerum sp. nov. differs from other species of Hystrichosphaeropsis in its small size, comparatively similar epicyst and hypocyst shape, lack of apical horn, distinct sutural ribs on the pericyst and gonal spines, clearly discernible on cingulum. The species Hystrichosphaeropsis rectangularis is characterised by well-developed sutural ribs on the pericyst and the absence of an apical horn, which is also characteristic of the new species described here. However, Hystrichosphaeropsis rectangularis has a more spherical endocyst, angular apical horn and antapical pericoel (Bujak et al. 1980), which distinguishes it from Hystrichosphaeropsis tenerum sp. nov. The new species is comparable to Hystrichosphaeropsis complanata in the development of a thin and delicate periphragm, often forming folds on the pericyst, but the latter species differs in the presence of a distinct oval endocyst, the formation of an apical horn, and a well-defined pericoel in the area of the cingulum (Eisenack 1965).
Remarks. Epicyst and hypocyst are occasionally shifted (inclined) towards the dorsal surface (Plate 3, figure 12). The epicyst and hypocyst shapes vary from trapezoid to oval. Cyst phragm is so thin and transparent that tabulation details are not always discernible, especially on the ventral
surface, and remain vague (Plate 3, figure 11).
Occurrence. The south of the Russian Platform, Keresta Formation, in the Ravninnaya R43 borehole, Volgograd Volga region; Keresta-1 borehole, the Yergeni Hills. Middle Eocene, Lutetian stage.
Associated taxa. The new dinoflagellate cyst species were found in an assemblage of organic-walled dinoflagellate cysts from the Keresta Formation, which includes the following taxa: Areosphaeridium diktyoplokum, Cleistosphaeridium diversispinosum, Cordosphaeridium cantharellum, Corrudinium incompositum, Costacysta bucina, Dinopterigium cladoides, Diphyes colligerum, Diphyes pseudoficusoides, Distatodinium ellipticum, Elytrocysta brevis, Enneadocysta arcuata, Heteraulacacyata porosa, Melitasphaeridium pseudorecurvatum, Microdinium reticulatum, Operculodinium centrocarpum, Operculodinium nanaconulum, Pentadinium goniferum, Pentadinium laticinctum, Pentadinium aff. lophophorum, Rottnestia borussica, Samlandia chlamydophora, Tectatodinium pellitum, Thalassiphora spinifera, Wilsonidium compactum, and Wilsonidium echinosuturatum.
Hystrichosphaeropsis tenerum sp. nov.
Plate 3, figures 1–12; Figure 6
Derivation of name. Named after the Latin tenera – thin, soft, tender – as relates to the cyst membrane.
Holotype. Plate 3, figures 1–4. Slide R43/55A, coordinates 103 x 16. Volgograd Volga region, Ravninnaya R43 borehole, sample 55, collection number IGG-2/23-06. Specimen dimensions: total length 54 μm, total width 40 mm, endocyst length 21 μm, endocyst width 32 μm, cingulum width 7–14 μm.
Paratype. Plate 3, figures 9, 10. Slide R43/55B, coordinates 94 x 8. Volgograd Volga region, borehole R-43, sample 55, coll. number IGG-2/23-07. Specimen dimensions: total length 47 μm, total width 36 μm, endocyst length 14 μm, endocyst width 28 μm, cingulum width 7–10 μm.
Type locality. The village of Ravninny in the vicinity of Volgograd, Volgograd region, Russia.
Type stratum. Keresta Formation, Volga–Caspian subregion.
Diagnosis. Gonyaulacoid bicavate proximate cysts without apical horn. Epicyst and hypocyst contacting the endocyst in cingular zone. Endocyst is flattened-oval. Tabulation is marked by thin ribs on the pericyst and by a precingular archaeopyle.
Description. Cysts of Hystrichosphaeropsis oval, angular-trapezoid in shape, without apical horn. Epicyst and hypocyst are ovaltrapezoid. Pericyst is in contact with endocyst in cingular zone. Periphragm is thin, smooth, transparent, and generally bears distinct sutures and short simple gonal spikes (Plate 3, figures 3, 8). Sutures are occasionally ornamented with low delicate septa with denticulate edges. Endocyst is oval, highly flattened (Plate 3, figure 6). Endophragm is extremely thin, frequently crumpled into creases. Precingular archaeopyle, type P (3ʺ). Operculum is generally free. Cingulum is formed by plates rectangular in shape and fringed by distinct sutures (Plate 3, figures 4, 10). Sulcus is limited by ribs, comprises a large pore (Plate 3, figure 5). Tabulation is marked by sutures, archaeopyle, sulcus position: 4', 6ʺ, 6c, 5"', 1p, 1ʺʺ (Figure 6). A large aperture (claustrum) is generally observed on the periphragm in the antapical zone on the ventral surface (Plate 3, figures 1, 7, 9, 11).
Dimensions. Cyst total length 43 (50) 54 mm, cyst total width 36 (39) 47 mm, endocyst length 14 (20) 25 mm, endocyst width 27 (32) 36 mm. Number of specimens measured: 10.
Comparison. Hystrichosphaeropsis tenerum sp. nov. differs from other species of Hystrichosphaeropsis in its small size, comparatively similar epicyst and hypocyst shape, lack of apical horn, distinct sutural ribs on the pericyst and gonal spines, clearly discernible on cingulum. The species Hystrichosphaeropsis rectangularis is characterised by well-developed sutural ribs on the pericyst and the absence of an apical horn, which is also characteristic of the new species described here. However, Hystrichosphaeropsis rectangularis has a more spherical endocyst, angular apical horn and antapical pericoel (Bujak et al. 1980), which distinguishes it from Hystrichosphaeropsis tenerum sp. nov. The new species is comparable to Hystrichosphaeropsis complanata in the development of a thin and delicate periphragm, often forming folds on the pericyst, but the latter species differs in the presence of a distinct oval endocyst, the formation of an apical horn, and a well-defined pericoel in the area of the cingulum (Eisenack 1965).
Remarks. Epicyst and hypocyst are occasionally shifted (inclined) towards the dorsal surface (Plate 3, figure 12). The epicyst and hypocyst shapes vary from trapezoid to oval. Cyst phragm is so thin and transparent that tabulation details are not always discernible, especially on the ventral
surface, and remain vague (Plate 3, figure 11).
Occurrence. The south of the Russian Platform, Keresta Formation, in the Ravninnaya R43 borehole, Volgograd Volga region; Keresta-1 borehole, the Yergeni Hills. Middle Eocene, Lutetian stage.
Associated taxa. The new dinoflagellate cyst species were found in an assemblage of organic-walled dinoflagellate cysts from the Keresta Formation, which includes the following taxa: Areosphaeridium diktyoplokum, Cleistosphaeridium diversispinosum, Cordosphaeridium cantharellum, Corrudinium incompositum, Costacysta bucina, Dinopterigium cladoides, Diphyes colligerum, Diphyes pseudoficusoides, Distatodinium ellipticum, Elytrocysta brevis, Enneadocysta arcuata, Heteraulacacyata porosa, Melitasphaeridium pseudorecurvatum, Microdinium reticulatum, Operculodinium centrocarpum, Operculodinium nanaconulum, Pentadinium goniferum, Pentadinium laticinctum, Pentadinium aff. lophophorum, Rottnestia borussica, Samlandia chlamydophora, Tectatodinium pellitum, Thalassiphora spinifera, Wilsonidium compactum, and Wilsonidium echinosuturatum.