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Isabelidinium greenense
Isabelidinium greenense Marshall, 1990
Holotype: Marshall, 1990, figs.12A, 21F-I
Locus typicus: Southeastern Australia
Stratum typicum: Campanian
Original description: Marshall, 1990, p.24, 26
Ventrodorsal outline subcircular or longitudinally elongate and ranging from ovoidal to subpentagonal. Specimens with subpentagonal outlines modified by an apical horn and 1-2 antapical horns defining rounded subconical projections or rounded bulges. Lateral margins on either side of apex occasionally marked by rounded to slightly angular shoulders. Cysts usually bicavate, occasionally with narrow pericoels along parts of the lateral margins, rarely circumcavate. Ventrodorsal outline of endocyst subcircular to voidal and longitudinally elongate. Endophragm 0.4-0.5 Ám thick, surfaces adjacent to pericoels finely granulate or with fine, close spaced rods that can connect distally. Periphragm 0.5-1.0 Ám thick near contacts with endocyst, thickening in the polar areas with an irregular inner surface. Outer surface smooth and usually unsculptured, occasionally with scattered, isolated grana adjacent to pericoels. Grana with rounded outlines, height up to 0.5 Ám, width up to 1.0 Ám, usually concentrated in, or restricted to, areas adjacent to the lateral margins midway between the equator and polar areas. Areas surrounding horns marked by concentric wrinkles and fine, close spaced, circular perforations up to 1.0 Ám in diameter. Perforations occasionally also extend over much of the periphragm adjacent to pericoels. Specimens with circular to ovoidal ventrodorsal outlines have concentric wrinkles and perforations, often surrounding a knob or bulge, marking the positions of 1 apical and 2 antapical horns. Periphragm on rare variants marked by irregular clusters of rounded, solid projections of variable shape and distribution (e.g. Fig. 21Q, R). On specimens with highest concentration of projections, the outermost surface of these elements has a smooth, regular shape similar to that of the periphragm. These forms can also have solid tips on the horns. Paracingulum absent, indicated by faint rounded ridges, or defined by clear rounded ridges on outer periphragm surface and/or by a rounded groove on inner periphragm surface. Internal groove separated from underlying endophragm, producing pericoels along narrow bands. Paracingular markings 7-8 Ám apart, usually discontinuous and having a partite arrangement, rarely forming two almost continuous bands around dorsal surface and edges of ventral surface. Periarcheopyle Type I, outline variable, general shape ranging from stenodeltaform to eurydeltaform. Endoarcheopyle usually indicated by a transverse split between adjoining boundaries of paraplates 2-4 " and 1-3a, and frequently, by additional incomplete sutures between adjoining margins of paraplates 1-3a (Fig. 12A-C). Faint markings often outline positions of remaining boundaries of paraplates 1-3a and anterior margins of paraplates 3-5".
Dimensions. Pericyst length 81(105)206 Ám, width 66(74)85 Ám; endocyst length 56(67)78 Ám, width 67(71)80 Ám (20 specimens).
Type material. Holotype (Fig. 21F-I) pericyst length 116 Ám, width 84 Ám; endocyst length 71 Ám, width 80 Ám.
Affinities and remarks:
Marshall, 1990, p.26: Abundant exceptionally well preserved specimens were recovered from the Cook Cruise sample Cl/83 9DB. Through study of many hundreds of examples it became apparent that a number of distinct morphotypes could be recognised. However, there appeared to be a continuous gradation in morphology between these forms in what seems to be a single, highly variable population. Since these intergradational variants cannot be separated from one another stratigraphically in the material studied, they areall treated within the one species.
Within this assemblage there is an extreme range of variation in the length of the horns (Fig. 14), and other highly variable features are the development of the paracingulum, periphragm sculpture, the thickness of the periphragm, and the outline of the periarcheopyle. The wide range in periarcheopyle outline shown in Fig. 14, especially the asymmetry of some examples, may be due to compression of the curved dorsal surface of the pericyst. Despite this possibility, there is sufficient evidence to indicate a range in outlines from isodeltaform to eurydeltaform. The outlines of paraplates surrounding the actual rupture of the endoarcheopyle are often partially defined by faint lines on the surface of the endophragm.
The most common variants are similar to those in Fig. 14G-N, S-W, and Y-CC, and are characterised by a smooth periphragm, weakly to moderately developed horns, faint indications of a partite paracingulum, and being bicavate. Less common variants include specimens with (a) a sculptured periphragm (Fig. 21A-C), (b) ventrodorsal pericyst outlines ranging from subcircular to ovoidal, and lacking clear indications of horns; these forms are bicavate to circumcavate (Fig. 14A-F), (c) unusually long horns (Fig. 14EE-GG), and (d) irregular clusters of rounded, solid projections on the periphragm (Fig. 21Q, R).
Isabelidinium greenense differs from I. pellucidum (Cookson and Eisenack) Lentin and Williams 1977 in lacking, or having considerably weaker sculpture on the periphragm. It also differs in having an abundance of fine perforations surrounding the horns, a thickened periphragm with an irregular internal surface in the polar regions, and in the morphology of the paracingulum. However, it is difficult to assess the importance of these latter characters at a specific level.
Holotype: Marshall, 1990, figs.12A, 21F-I
Locus typicus: Southeastern Australia
Stratum typicum: Campanian
Original description: Marshall, 1990, p.24, 26
Ventrodorsal outline subcircular or longitudinally elongate and ranging from ovoidal to subpentagonal. Specimens with subpentagonal outlines modified by an apical horn and 1-2 antapical horns defining rounded subconical projections or rounded bulges. Lateral margins on either side of apex occasionally marked by rounded to slightly angular shoulders. Cysts usually bicavate, occasionally with narrow pericoels along parts of the lateral margins, rarely circumcavate. Ventrodorsal outline of endocyst subcircular to voidal and longitudinally elongate. Endophragm 0.4-0.5 Ám thick, surfaces adjacent to pericoels finely granulate or with fine, close spaced rods that can connect distally. Periphragm 0.5-1.0 Ám thick near contacts with endocyst, thickening in the polar areas with an irregular inner surface. Outer surface smooth and usually unsculptured, occasionally with scattered, isolated grana adjacent to pericoels. Grana with rounded outlines, height up to 0.5 Ám, width up to 1.0 Ám, usually concentrated in, or restricted to, areas adjacent to the lateral margins midway between the equator and polar areas. Areas surrounding horns marked by concentric wrinkles and fine, close spaced, circular perforations up to 1.0 Ám in diameter. Perforations occasionally also extend over much of the periphragm adjacent to pericoels. Specimens with circular to ovoidal ventrodorsal outlines have concentric wrinkles and perforations, often surrounding a knob or bulge, marking the positions of 1 apical and 2 antapical horns. Periphragm on rare variants marked by irregular clusters of rounded, solid projections of variable shape and distribution (e.g. Fig. 21Q, R). On specimens with highest concentration of projections, the outermost surface of these elements has a smooth, regular shape similar to that of the periphragm. These forms can also have solid tips on the horns. Paracingulum absent, indicated by faint rounded ridges, or defined by clear rounded ridges on outer periphragm surface and/or by a rounded groove on inner periphragm surface. Internal groove separated from underlying endophragm, producing pericoels along narrow bands. Paracingular markings 7-8 Ám apart, usually discontinuous and having a partite arrangement, rarely forming two almost continuous bands around dorsal surface and edges of ventral surface. Periarcheopyle Type I, outline variable, general shape ranging from stenodeltaform to eurydeltaform. Endoarcheopyle usually indicated by a transverse split between adjoining boundaries of paraplates 2-4 " and 1-3a, and frequently, by additional incomplete sutures between adjoining margins of paraplates 1-3a (Fig. 12A-C). Faint markings often outline positions of remaining boundaries of paraplates 1-3a and anterior margins of paraplates 3-5".
Dimensions. Pericyst length 81(105)206 Ám, width 66(74)85 Ám; endocyst length 56(67)78 Ám, width 67(71)80 Ám (20 specimens).
Type material. Holotype (Fig. 21F-I) pericyst length 116 Ám, width 84 Ám; endocyst length 71 Ám, width 80 Ám.
Affinities and remarks:
Marshall, 1990, p.26: Abundant exceptionally well preserved specimens were recovered from the Cook Cruise sample Cl/83 9DB. Through study of many hundreds of examples it became apparent that a number of distinct morphotypes could be recognised. However, there appeared to be a continuous gradation in morphology between these forms in what seems to be a single, highly variable population. Since these intergradational variants cannot be separated from one another stratigraphically in the material studied, they areall treated within the one species.
Within this assemblage there is an extreme range of variation in the length of the horns (Fig. 14), and other highly variable features are the development of the paracingulum, periphragm sculpture, the thickness of the periphragm, and the outline of the periarcheopyle. The wide range in periarcheopyle outline shown in Fig. 14, especially the asymmetry of some examples, may be due to compression of the curved dorsal surface of the pericyst. Despite this possibility, there is sufficient evidence to indicate a range in outlines from isodeltaform to eurydeltaform. The outlines of paraplates surrounding the actual rupture of the endoarcheopyle are often partially defined by faint lines on the surface of the endophragm.
The most common variants are similar to those in Fig. 14G-N, S-W, and Y-CC, and are characterised by a smooth periphragm, weakly to moderately developed horns, faint indications of a partite paracingulum, and being bicavate. Less common variants include specimens with (a) a sculptured periphragm (Fig. 21A-C), (b) ventrodorsal pericyst outlines ranging from subcircular to ovoidal, and lacking clear indications of horns; these forms are bicavate to circumcavate (Fig. 14A-F), (c) unusually long horns (Fig. 14EE-GG), and (d) irregular clusters of rounded, solid projections on the periphragm (Fig. 21Q, R).
Isabelidinium greenense differs from I. pellucidum (Cookson and Eisenack) Lentin and Williams 1977 in lacking, or having considerably weaker sculpture on the periphragm. It also differs in having an abundance of fine perforations surrounding the horns, a thickened periphragm with an irregular internal surface in the polar regions, and in the morphology of the paracingulum. However, it is difficult to assess the importance of these latter characters at a specific level.