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Isabelidinium variabile
Isabelidinium variabile Marshall, 1988
Holotype: Marshall, 1988, figs.13A–E; Fensome et al., 1996, figs.3–7 — p.2419.
Locus typicus: Gippsland Basin, SE Australia
Stratum typicum: Santonian
Original diagnosis: Marshall, 1988, p. 206-207
Cysts weakly compressed ventrodorsally, length usually slightly greater than width, ventrodorsal outline subcircular to subpentagonal. Each lateral margin usually evenly convex, occasionally modified by a weak equatorial bulge. Apex bearing a rounded knob or subconical horn with a truncate tip. Antapical margin nearly straight to convex, usually with 1-2 faint horns indicated by rounded bulges or subconical projections. Cysts cornucavate, bicavate, or circumcavate with a narrow pericoel along lateral margins. Ventrodorsal endocyst outline subcircular to ovoidal with length exceeding width. Periphragm c. 0.2-0.5 Ám thick, surface smooth, often bearing numerous fine perforations c. 0.1-1.0 Ám in diameter. Parasutural ornament on periphragm absent or indicated by rows of low ridges and/or pores. Paraplates defined: 1-4", 1-3a, 1-7", 1-4""", 1"""", 2"""" (Fig.10). Endophragm c. 0.75-1.5 Ám thick, surface scabrate, granulate, verrucate, or rugulate; sculpture nontabulate. Paracingulum faint to distinct, indicated on periphragm by 2 parallel ridges or rows of pores 5-8 Ám apart. Ridges usually with a partite arrangement, appearing almost continuous on dorsal and edges of ventral surfaces. Archeopyle Type la/I. Periarcheopyle isodeltaform to isothetaform; operculum apparently attached posteriorly, not discernible on many specimens. Endo-operculum often in place with sutures apparent along all boundaries; additional sutures also frequently evident on endophragm along paraplate boundaries surrounding operculum. Dimensions. Pericyst length 53 (65) 90 Ám, width 44 (61) 84 Ám; endocyst length 48 (55) 64 Ám, width 43 (60) 74 Ám (45 specimens).
Affinities:
Marshall, 1988, p. 206: This group of cysts is difficult to circumscribe because of extreme variation.Common attributes are: 1, fairly consistent thicknesses and dimensions of the wall layers; 2, cysts weakly compressed ventrodorsally; 3, endocyst occupying much of the pericyst; 4, periphragm usually finely perforate; and 5, partite paracingulum usually indicated. Four intergrading morphotypes (subspp. A-D) are based on differing wall layers.
Holotype: Marshall, 1988, figs.13A–E; Fensome et al., 1996, figs.3–7 — p.2419.
Locus typicus: Gippsland Basin, SE Australia
Stratum typicum: Santonian
Original diagnosis: Marshall, 1988, p. 206-207
Cysts weakly compressed ventrodorsally, length usually slightly greater than width, ventrodorsal outline subcircular to subpentagonal. Each lateral margin usually evenly convex, occasionally modified by a weak equatorial bulge. Apex bearing a rounded knob or subconical horn with a truncate tip. Antapical margin nearly straight to convex, usually with 1-2 faint horns indicated by rounded bulges or subconical projections. Cysts cornucavate, bicavate, or circumcavate with a narrow pericoel along lateral margins. Ventrodorsal endocyst outline subcircular to ovoidal with length exceeding width. Periphragm c. 0.2-0.5 Ám thick, surface smooth, often bearing numerous fine perforations c. 0.1-1.0 Ám in diameter. Parasutural ornament on periphragm absent or indicated by rows of low ridges and/or pores. Paraplates defined: 1-4", 1-3a, 1-7", 1-4""", 1"""", 2"""" (Fig.10). Endophragm c. 0.75-1.5 Ám thick, surface scabrate, granulate, verrucate, or rugulate; sculpture nontabulate. Paracingulum faint to distinct, indicated on periphragm by 2 parallel ridges or rows of pores 5-8 Ám apart. Ridges usually with a partite arrangement, appearing almost continuous on dorsal and edges of ventral surfaces. Archeopyle Type la/I. Periarcheopyle isodeltaform to isothetaform; operculum apparently attached posteriorly, not discernible on many specimens. Endo-operculum often in place with sutures apparent along all boundaries; additional sutures also frequently evident on endophragm along paraplate boundaries surrounding operculum. Dimensions. Pericyst length 53 (65) 90 Ám, width 44 (61) 84 Ám; endocyst length 48 (55) 64 Ám, width 43 (60) 74 Ám (45 specimens).
Affinities:
Marshall, 1988, p. 206: This group of cysts is difficult to circumscribe because of extreme variation.Common attributes are: 1, fairly consistent thicknesses and dimensions of the wall layers; 2, cysts weakly compressed ventrodorsally; 3, endocyst occupying much of the pericyst; 4, periphragm usually finely perforate; and 5, partite paracingulum usually indicated. Four intergrading morphotypes (subspp. A-D) are based on differing wall layers.