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Isabelidinium ponticum
Isabelidinium ponticum Marshall, 1988, p. 205
Holotype: Marshall, 1988, figs.8A,15A–F; Fensome et al., 1996, figs.1–5,13 — p.2287.
Locus typicus: Gippsland Basin, SE Australia
Stratum typicum: Santonian
Original diagnosis: Marshall, 1988, p. 205
Ventrodorsal outline subpentagonal, weakly to markedly elongate. Lateral margins on epicyst tapering towards apex or modified by weak, rounded shoulders adjacent to archeopyle. Apex bearing a rounded knob or subconical horn with a truncate tip. Lateral margins on hypocyst tapering towards 2 antapical horns: the left subconical, tip rounded; the right reduced, forming a rounded to subconical projection. Cysts bicavate or circumcavate with a narrow pericoel along lateral margins. Bicavate specimens frequently with isolated pericoels near equator, especially beneath paracingular ridges. Ventrodorsal endocyst outline subcircular to ovoidal, length exceeding width. Periphragm c. 0.4-0.5 Ám thick, surface smooth; bearing numerous closespaced perforations. Pores subcircular, 0.5-1.5 Ám in diameter, usually distributed irregularly, occasionally arranged in intratabular fields and separated by non-perforate pandasutural bands. Only fragments of paraplates evident. Paracingulum partite, defined by 2 parallel sets of ridges separated by a shallow groove on periphragm. Archeopyle Type la/?: periarcheopyle stenothetaform to isothetaform, operculum usually attached posteriorly. Endoarcheopyle often indicated by splits, type uncertain.
Dimensions: Pericyst length 59 (78) 90 Ám, width 46 (52) 63 Ám; endocyst length 41 (48) 53 Ám, width 40 (48) 59 Ám (29 specimens).
Affinities:
Marshall, 1988, p. 205: Variation exists in the ventrodorsal outline of the pericyst and in size and shape of the anterior and posterior pericoels (Fig.8). More elongate and prominently horned specimens appear similar to C. porosa, but differ in being smaller, with finer, more closely spaced perforations and weaker indications of a paracingulum. Rare specimens with the largest perforations seem almost transitional with C. porosa. Morphological gradation exists between the smaller examples of I. ponticum and I. variabile subsp. A; manifested by decrease in size of the perforations on the periphragm.
Holotype: Marshall, 1988, figs.8A,15A–F; Fensome et al., 1996, figs.1–5,13 — p.2287.
Locus typicus: Gippsland Basin, SE Australia
Stratum typicum: Santonian
Original diagnosis: Marshall, 1988, p. 205
Ventrodorsal outline subpentagonal, weakly to markedly elongate. Lateral margins on epicyst tapering towards apex or modified by weak, rounded shoulders adjacent to archeopyle. Apex bearing a rounded knob or subconical horn with a truncate tip. Lateral margins on hypocyst tapering towards 2 antapical horns: the left subconical, tip rounded; the right reduced, forming a rounded to subconical projection. Cysts bicavate or circumcavate with a narrow pericoel along lateral margins. Bicavate specimens frequently with isolated pericoels near equator, especially beneath paracingular ridges. Ventrodorsal endocyst outline subcircular to ovoidal, length exceeding width. Periphragm c. 0.4-0.5 Ám thick, surface smooth; bearing numerous closespaced perforations. Pores subcircular, 0.5-1.5 Ám in diameter, usually distributed irregularly, occasionally arranged in intratabular fields and separated by non-perforate pandasutural bands. Only fragments of paraplates evident. Paracingulum partite, defined by 2 parallel sets of ridges separated by a shallow groove on periphragm. Archeopyle Type la/?: periarcheopyle stenothetaform to isothetaform, operculum usually attached posteriorly. Endoarcheopyle often indicated by splits, type uncertain.
Dimensions: Pericyst length 59 (78) 90 Ám, width 46 (52) 63 Ám; endocyst length 41 (48) 53 Ám, width 40 (48) 59 Ám (29 specimens).
Affinities:
Marshall, 1988, p. 205: Variation exists in the ventrodorsal outline of the pericyst and in size and shape of the anterior and posterior pericoels (Fig.8). More elongate and prominently horned specimens appear similar to C. porosa, but differ in being smaller, with finer, more closely spaced perforations and weaker indications of a paracingulum. Rare specimens with the largest perforations seem almost transitional with C. porosa. Morphological gradation exists between the smaller examples of I. ponticum and I. variabile subsp. A; manifested by decrease in size of the perforations on the periphragm.