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Ovoidinium striatum
Ovoidinium striatum Riding and Helby, 2001h, p.232–233, figs.4A–P,5A–D.
Holotype: Riding and Helby, 2001h, figs.4I–K.
Age: late Aptian.
Original description (Riding & Helby, 2001h):
Description. An intermediate to large species of Ovoidinium with an elongate subpentagonal biconal outline. The cyst is slightly dorsoventrally flattened and the narrow parasulcus is deeply indented. A long apical horn, which is blunt and rounded distally, is present on the epicyst. On the hypocyst, a single, shorter, antapical horn, which is normally pointed distally, is present. This antapical horn is markedly offset to the left. The endocyst is ellipsoidal, with or without short (2-3µm long) apical and antapical bosses or protuberances. Endophragm smooth, varying from 0.5 to 1 µm in thickness. The majority of the endocyst lies within the epicyst which is longer that hypocyst. The periphragm is relatively thin (<0.5µm) and is smooth to irregularly microgranulate or granulate. Distally smooth, nontabular, parallel longitudinal folds, 1-2µm high, are present on the periphragm. These folds may be slightly sinuous and are inserted regularly every 1-2.5µm. Most of the folds are interrupted by the prominent paracingular crests which may have undulose to serrate distal rims. Longitudinal ridges are also present within the parasulcus. Occasionally the periphragmal ridges may be locally discontinuous. The paracingulum is relatively short (i.e. narrow) and laevorotatory with a marked midventral offset. Archaeopyle combination apical/anterior intercalary , type (4A3I), operculum simple, generally free.
Dimensions (µm; n=25): Min. (Mean) Max.
Length of cyst: 81 (98) 116
Length of epicyst (measured ventrally): 43 (53) 64
Length of paracingulum: 4 (6) 9
Length of hypocyst (measured ventrally): 28 (39) 49
Length of apical horn: 9 (17) 25
Length of epipericoel: 12 (19) 27
Length of antapical horn: 4 (12) 20
Length of hypopericoel: 12 (19) 28
Equatorial width of cyst: 43 (52) 67
The measured specimens are from a sidewall core sample in Gwydion-1 well at 738.10m
Comments. This distinctive species of Ovoidinium has polar horns which are both variable in size. The antapical horn, in particular, may be significantly reduced. The longitudinal folds impart a characteristic striate appearance to the periphragm. In some assemblages, the periphragmal folds/ridges may be discontinuous and are separated into irregular elements such as grana.
Comparison. Ovoidinium striatum differs from the other species of the genus in bearing a prominent, dense cover of longitudinal folds on the periphragm. None of the other 13 species of this genus (Williams et al., 1998, p. 447-449) have linear ornamentation on either the periphragm or endophragm. Ovoidinium cinctum (Cookson & Eisenack 1958) Davey 1970 is similar in overall morphology but has unaligned grana. The type, Ovoidinium verrucosum, as the name suggests, is verrucate. Others have either smooth (for example O. ovale [Cookson & Eisenack 1985] Lentin & Williams 1976) or granulate (e.g. O. scabrosum [Cookson & Hughes 1964] Davey 1970) periphragms. Ovoidinium incomptum Duxbury 1983 and O. incorporeum Duxbury 1983 also lack linear ornamentation and are both thin-walled forms.
Derivation of name. From the Latin, stria meaning line, furrow or stripe and referring to the characteristic dense longitudinal folds in the periphragm.
Holotype and type locality. Figures 4I-K, specimen CPC 35791, from a sidewall core sample at 738.10m in the Gwydion-1 well.
Stratigraphical distribution. Ovoidinium striatum characterizes the late Aptian and is confined to the middle to upper Diconodinium davidii Zone (subzones 2biiib-2biia) (Foster, this volume; Helby & Partridge, in prep.) in offshore north-western Australia and Papua New Guinea. It commonly occurs in acme proportions in the median subzone (2biiib).
Holotype: Riding and Helby, 2001h, figs.4I–K.
Age: late Aptian.
Original description (Riding & Helby, 2001h):
Description. An intermediate to large species of Ovoidinium with an elongate subpentagonal biconal outline. The cyst is slightly dorsoventrally flattened and the narrow parasulcus is deeply indented. A long apical horn, which is blunt and rounded distally, is present on the epicyst. On the hypocyst, a single, shorter, antapical horn, which is normally pointed distally, is present. This antapical horn is markedly offset to the left. The endocyst is ellipsoidal, with or without short (2-3µm long) apical and antapical bosses or protuberances. Endophragm smooth, varying from 0.5 to 1 µm in thickness. The majority of the endocyst lies within the epicyst which is longer that hypocyst. The periphragm is relatively thin (<0.5µm) and is smooth to irregularly microgranulate or granulate. Distally smooth, nontabular, parallel longitudinal folds, 1-2µm high, are present on the periphragm. These folds may be slightly sinuous and are inserted regularly every 1-2.5µm. Most of the folds are interrupted by the prominent paracingular crests which may have undulose to serrate distal rims. Longitudinal ridges are also present within the parasulcus. Occasionally the periphragmal ridges may be locally discontinuous. The paracingulum is relatively short (i.e. narrow) and laevorotatory with a marked midventral offset. Archaeopyle combination apical/anterior intercalary , type (4A3I), operculum simple, generally free.
Dimensions (µm; n=25): Min. (Mean) Max.
Length of cyst: 81 (98) 116
Length of epicyst (measured ventrally): 43 (53) 64
Length of paracingulum: 4 (6) 9
Length of hypocyst (measured ventrally): 28 (39) 49
Length of apical horn: 9 (17) 25
Length of epipericoel: 12 (19) 27
Length of antapical horn: 4 (12) 20
Length of hypopericoel: 12 (19) 28
Equatorial width of cyst: 43 (52) 67
The measured specimens are from a sidewall core sample in Gwydion-1 well at 738.10m
Comments. This distinctive species of Ovoidinium has polar horns which are both variable in size. The antapical horn, in particular, may be significantly reduced. The longitudinal folds impart a characteristic striate appearance to the periphragm. In some assemblages, the periphragmal folds/ridges may be discontinuous and are separated into irregular elements such as grana.
Comparison. Ovoidinium striatum differs from the other species of the genus in bearing a prominent, dense cover of longitudinal folds on the periphragm. None of the other 13 species of this genus (Williams et al., 1998, p. 447-449) have linear ornamentation on either the periphragm or endophragm. Ovoidinium cinctum (Cookson & Eisenack 1958) Davey 1970 is similar in overall morphology but has unaligned grana. The type, Ovoidinium verrucosum, as the name suggests, is verrucate. Others have either smooth (for example O. ovale [Cookson & Eisenack 1985] Lentin & Williams 1976) or granulate (e.g. O. scabrosum [Cookson & Hughes 1964] Davey 1970) periphragms. Ovoidinium incomptum Duxbury 1983 and O. incorporeum Duxbury 1983 also lack linear ornamentation and are both thin-walled forms.
Derivation of name. From the Latin, stria meaning line, furrow or stripe and referring to the characteristic dense longitudinal folds in the periphragm.
Holotype and type locality. Figures 4I-K, specimen CPC 35791, from a sidewall core sample at 738.10m in the Gwydion-1 well.
Stratigraphical distribution. Ovoidinium striatum characterizes the late Aptian and is confined to the middle to upper Diconodinium davidii Zone (subzones 2biiib-2biia) (Foster, this volume; Helby & Partridge, in prep.) in offshore north-western Australia and Papua New Guinea. It commonly occurs in acme proportions in the median subzone (2biiib).