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Nyktericysta sibirica
Etymology. From Siberia.
Holotype. GEOR, no. K22 23/136 1; southwest of Western Siberia, borehole A1, sample 136; 849.45 m of depth; upper part of the Vikulovo Formation; designated herein; Pl. 12, fig. 5.
Diagnosis. Intermediate ceratioid cyst with four well developed horns: one apical, one lateral, and two antapical horns; cyst two walled, cornucavate; endocyst smooth, pericyst scabrous, irregularly wrinkled; paratabulation not indicated; archeopyle apical (type tA).
Description (Figs. 2; 3a). Intermediate, dorsoventrally compressed ceratioid cysts with apical, one lateral, and two antapical horns. The size and shape of the apical horn varies from short triangular with a broad base to long, conical; the distal end is rounded (Figs. 2a, 2b). The lateral horns are uneven. The right lateral horn is strongly reduced to broad rounded bulge. The left lateral horn is short, with small, or usually absent postcingular extension. The characteristically vertical or subvertical proximal process in the precingular zone is well developed. This process is broadly triangular, with a rounded distal termination. The antapical horns are short, triangular, and even. A characteristic asymmetry is often present: the apical horn is shifted from the conventional vertical axis and located above the right antapical horn, whereas the left
lateral horn is elongated almost in parallel with the apical horn and lies above the left antapical horn (Pl. 12, figs. 3, 7, 9; Fig. 3a).
The cyst is two walled. The endocyst has the shape of the pericyst and is not tightly contacting the latter, forming small spaces (pericoel) between the horns of the endocyst and pericyst. The apical pericoel is larger than the others. The distal ends of the antapical horns can be solid, but can have a visible pericoel.
The endocyst is smooth; the pericyst is scabrous or covered with small, widely spaced granules; both walls are very thin. It is often difficult to discern fine ornamentation of the external wall, because it is usually strongly folded. The paratabulation is not developed. The cingulum is sometimes discernible by the presence of lateral indentations and slightly or clearly visible thin transverse fold; the sulcus is not visible. The archeopyle is apical, type tA. The operculum is adnate ventrally, rarely broken off.
D i m e n s i o n s, μm. Cyst length, 72 (83) 98; cyst width, 38 (48) 56; apical horn length, 23 (34) 46; lateral horn length, 10 (17) 22; commensurate antapical horns, length, 10 (15) 20; apical pericoel length, 7–10.
Variability. The shape of the apical horn can be of two types, short triangular with a broad base or longiconical; the length varies widely and depends on the shape; the length of the antapical horn, as the length of the precingular process, varies only slightly within a few microns.
Comparison. N. sibirica sp. nov. differs from other species of the subgenus Hastodinium in the shape and position of the lateral horn, which is extended laterally, whereas in N. (H.) beierensis Wan et Zhang, 1990 and N. (H.) ramuliformis Wan et Zhang, 1990, this process lies closer to the central cyst axis. It is also distinguished by the shagreen ornamentation of the periphragm, compared to the coarse grained surface of N. (H.) beierensis and spinate ornamentation of N. (H.) ramuliformis. The external wall of the cyst in the new species is loosely attached to the internal wall, leaving uneven spaces along the entire perimeter, whereas in the two other species of the subgenus, both walls are tightly pressed to each other everywhere, except the distal end of the horns. The asymmetry of the cysts of N. (Hastodinium) sibirica sp. nov. expressed
in the shift of the apical horn from the central axis toward the right antapical horn distinguishes it from all known species of the genus Nyktericysta.
Remarks. The length of the horns varies, although the main features, such as the location, number, and shape of the horns, ornamentation, relationships between the outer and inner walls of the cyst remain stable. Most likely, the morphological variability is an indication of adaptation to unstable environment of habitat of dinoflagellates producing the cysts. All the studied specimens come from the same locality, hence, the separation of several species or subspecies seems unnecessary.
Material. Twenty five well preserved specimens; Russia, southwest of Western Siberia; upper part of the Vikulovo Formation, Middle–Upper Aptian; borehole A1, interval 893.02–825.07 m; borehole A3, interval 899.63–854.52 m; borehole B4, interval 868.4–835.1 m; borehole B3, interval 886.25–845.3 m.
Holotype. GEOR, no. K22 23/136 1; southwest of Western Siberia, borehole A1, sample 136; 849.45 m of depth; upper part of the Vikulovo Formation; designated herein; Pl. 12, fig. 5.
Diagnosis. Intermediate ceratioid cyst with four well developed horns: one apical, one lateral, and two antapical horns; cyst two walled, cornucavate; endocyst smooth, pericyst scabrous, irregularly wrinkled; paratabulation not indicated; archeopyle apical (type tA).
Description (Figs. 2; 3a). Intermediate, dorsoventrally compressed ceratioid cysts with apical, one lateral, and two antapical horns. The size and shape of the apical horn varies from short triangular with a broad base to long, conical; the distal end is rounded (Figs. 2a, 2b). The lateral horns are uneven. The right lateral horn is strongly reduced to broad rounded bulge. The left lateral horn is short, with small, or usually absent postcingular extension. The characteristically vertical or subvertical proximal process in the precingular zone is well developed. This process is broadly triangular, with a rounded distal termination. The antapical horns are short, triangular, and even. A characteristic asymmetry is often present: the apical horn is shifted from the conventional vertical axis and located above the right antapical horn, whereas the left
lateral horn is elongated almost in parallel with the apical horn and lies above the left antapical horn (Pl. 12, figs. 3, 7, 9; Fig. 3a).
The cyst is two walled. The endocyst has the shape of the pericyst and is not tightly contacting the latter, forming small spaces (pericoel) between the horns of the endocyst and pericyst. The apical pericoel is larger than the others. The distal ends of the antapical horns can be solid, but can have a visible pericoel.
The endocyst is smooth; the pericyst is scabrous or covered with small, widely spaced granules; both walls are very thin. It is often difficult to discern fine ornamentation of the external wall, because it is usually strongly folded. The paratabulation is not developed. The cingulum is sometimes discernible by the presence of lateral indentations and slightly or clearly visible thin transverse fold; the sulcus is not visible. The archeopyle is apical, type tA. The operculum is adnate ventrally, rarely broken off.
D i m e n s i o n s, μm. Cyst length, 72 (83) 98; cyst width, 38 (48) 56; apical horn length, 23 (34) 46; lateral horn length, 10 (17) 22; commensurate antapical horns, length, 10 (15) 20; apical pericoel length, 7–10.
Variability. The shape of the apical horn can be of two types, short triangular with a broad base or longiconical; the length varies widely and depends on the shape; the length of the antapical horn, as the length of the precingular process, varies only slightly within a few microns.
Comparison. N. sibirica sp. nov. differs from other species of the subgenus Hastodinium in the shape and position of the lateral horn, which is extended laterally, whereas in N. (H.) beierensis Wan et Zhang, 1990 and N. (H.) ramuliformis Wan et Zhang, 1990, this process lies closer to the central cyst axis. It is also distinguished by the shagreen ornamentation of the periphragm, compared to the coarse grained surface of N. (H.) beierensis and spinate ornamentation of N. (H.) ramuliformis. The external wall of the cyst in the new species is loosely attached to the internal wall, leaving uneven spaces along the entire perimeter, whereas in the two other species of the subgenus, both walls are tightly pressed to each other everywhere, except the distal end of the horns. The asymmetry of the cysts of N. (Hastodinium) sibirica sp. nov. expressed
in the shift of the apical horn from the central axis toward the right antapical horn distinguishes it from all known species of the genus Nyktericysta.
Remarks. The length of the horns varies, although the main features, such as the location, number, and shape of the horns, ornamentation, relationships between the outer and inner walls of the cyst remain stable. Most likely, the morphological variability is an indication of adaptation to unstable environment of habitat of dinoflagellates producing the cysts. All the studied specimens come from the same locality, hence, the separation of several species or subspecies seems unnecessary.
Material. Twenty five well preserved specimens; Russia, southwest of Western Siberia; upper part of the Vikulovo Formation, Middle–Upper Aptian; borehole A1, interval 893.02–825.07 m; borehole A3, interval 899.63–854.52 m; borehole B4, interval 868.4–835.1 m; borehole B3, interval 886.25–845.3 m.