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Impagidinium reductum
Impagidinium reductum Stover and Helby, 1987
Holotype: Stover and Helby, 1987, fig.15I-K
Locus typicus: Exmouth Plateau, W Austalia
Stratum typicum: Hauterivian
Original description: Stover and Helby, 1987, p. 241-242
Cysts proximate, ellipsoidal, with an apical horn having a truncate tip. Endophragm and periphragm appressed and virtually indistinguishable between parasutural septa; total wall thickness about 3-5µm. Areas between parasutural features smooth or faintly ornamented, commonly bearing granules, verrucae, processes, and/or discontinuous ridges. Antapical process single, intratabular, always present (Figs 14C,D, 15C,O), as high as surrounding septa, with rounded or truncate end; other processes generally shorter than the antapical process. Parasutural septa variable in height from 5-10µm, with smooth or finely and unevenly denticulate crests, solid or hollow proximally, i.e., with minute pericoels at their bases. Paratabulation 3-4", 4-6"", Xc, 5""", 1p 1"""", reduced ventrally and cingularly (Fig.i3A,B). Anterior cingular septum absent; posterior one may be reduced ventrally as well. Septa adjacent to the parasulcus present only occasionally (Fig. 13C). The archeopyle precingular, 3"" only, and reduced (Fig. 15I); lateral and anterior margin of the free operculum bearing solid spines with bluntly rounded tips and may have a short ridge in addition to the spines (Fig.15H,I,M). Paracingulum indicated by a single posterior, transverse septum and by a septum-free band between the posterior margins of the precingular paraplates and the transverse septum (Figs 14A,B, 15A,H). Ends of the paracingulum, when discernible, offset by approximately three times the width of the paracingulum. Undivided and sinuous parasulcus either not delimited or delimited by usually incomplete parasutural septa of the bordering paraplates. Specimens 80 (92) 102 µm long, 60 (68) 76 µm wide; 18 specimens measured.
Comment: Although cingular paraplates are not indicated and the margins of many paraplates are expressed incompletely, the paratabulation seemingly follows the standard gonyaulacacean pattern. Aside from differences in size, variability is expressed by the distribution and type of ornamentation, and by the development (or lack of development) of the septa. Exclusive of the single intratabular process on the antapical paraplate, other paraplates (but not necessarily all of them) are ornamented to some extent. On some paraplates, a single intratabular projection is present; on others, several projections are clustered near the centre of the paraplate, and on still others several widely scattered isolated or coalesced projections occupy the paraplate. Impagidinium reductum is characterized by reduced paratabulation, in particular the consistent absence of an anterior cingular septum, presence of a single, antapical, intratabular process and ornamentation on at least some paraplates other than 1"""".
Affinities:
Stover and Helby, 1987, p.242-243: Other species of Impagidinium with reduced paratabulation include I. elegans (Cookson & Eisenack) Stover & Evitt 1978, I. patulum (Wall) Stover & Evitt 1978, and I. victorianum (Cookson & Eisenack) Stover & Evitt 1978. Each of these species lacks projections between parasutural features and has an anterior cingular septum developed laterally and dorsally. Impagidinium phlyctaena Stover & Helby (1987) has intratabular ornamentation on most paraplates and complete rather than reduced parasutural features.
Holotype: Stover and Helby, 1987, fig.15I-K
Locus typicus: Exmouth Plateau, W Austalia
Stratum typicum: Hauterivian
Original description: Stover and Helby, 1987, p. 241-242
Cysts proximate, ellipsoidal, with an apical horn having a truncate tip. Endophragm and periphragm appressed and virtually indistinguishable between parasutural septa; total wall thickness about 3-5µm. Areas between parasutural features smooth or faintly ornamented, commonly bearing granules, verrucae, processes, and/or discontinuous ridges. Antapical process single, intratabular, always present (Figs 14C,D, 15C,O), as high as surrounding septa, with rounded or truncate end; other processes generally shorter than the antapical process. Parasutural septa variable in height from 5-10µm, with smooth or finely and unevenly denticulate crests, solid or hollow proximally, i.e., with minute pericoels at their bases. Paratabulation 3-4", 4-6"", Xc, 5""", 1p 1"""", reduced ventrally and cingularly (Fig.i3A,B). Anterior cingular septum absent; posterior one may be reduced ventrally as well. Septa adjacent to the parasulcus present only occasionally (Fig. 13C). The archeopyle precingular, 3"" only, and reduced (Fig. 15I); lateral and anterior margin of the free operculum bearing solid spines with bluntly rounded tips and may have a short ridge in addition to the spines (Fig.15H,I,M). Paracingulum indicated by a single posterior, transverse septum and by a septum-free band between the posterior margins of the precingular paraplates and the transverse septum (Figs 14A,B, 15A,H). Ends of the paracingulum, when discernible, offset by approximately three times the width of the paracingulum. Undivided and sinuous parasulcus either not delimited or delimited by usually incomplete parasutural septa of the bordering paraplates. Specimens 80 (92) 102 µm long, 60 (68) 76 µm wide; 18 specimens measured.
Comment: Although cingular paraplates are not indicated and the margins of many paraplates are expressed incompletely, the paratabulation seemingly follows the standard gonyaulacacean pattern. Aside from differences in size, variability is expressed by the distribution and type of ornamentation, and by the development (or lack of development) of the septa. Exclusive of the single intratabular process on the antapical paraplate, other paraplates (but not necessarily all of them) are ornamented to some extent. On some paraplates, a single intratabular projection is present; on others, several projections are clustered near the centre of the paraplate, and on still others several widely scattered isolated or coalesced projections occupy the paraplate. Impagidinium reductum is characterized by reduced paratabulation, in particular the consistent absence of an anterior cingular septum, presence of a single, antapical, intratabular process and ornamentation on at least some paraplates other than 1"""".
Affinities:
Stover and Helby, 1987, p.242-243: Other species of Impagidinium with reduced paratabulation include I. elegans (Cookson & Eisenack) Stover & Evitt 1978, I. patulum (Wall) Stover & Evitt 1978, and I. victorianum (Cookson & Eisenack) Stover & Evitt 1978. Each of these species lacks projections between parasutural features and has an anterior cingular septum developed laterally and dorsally. Impagidinium phlyctaena Stover & Helby (1987) has intratabular ornamentation on most paraplates and complete rather than reduced parasutural features.