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Gonyaulacysta fenestrata
Gonyaulacysta fenestrata Riding and Helby, 2001f, p.150-153, figs.5A-I,6A-B. Holotype: Riding and Helby, 2001f, fig.5D. See also the discussion under Gonyaulacysta oligodentata (name not validly published). Age: Kimmeridgian-Tithonian.
Original description (Riding and Helby, 2001f)
Description. A large, elongate species of Gonyaulacysta with a long, distally-blunt, slender apical horn. The cyst is primarily dorsoventrally flattened. The pericyst, excluding the apical horn, is elongate supellipsoidal to subpolygonal and is antapically truncate. The endocyst is elongate ellipsoidal, normally with a prominent apical protuberance. Bicavate cyst organisation; the epicyst may be epicavate or cornucavate. Paratabulation partially indicated by parasutural crests which are generally smooth distally and fenestrate. The fenestrae are subcircular, ellipsoidal or rectangluar; normally they are best developed in the midlateral and antapical areas and inserted close to the distal margin of the crest. Occassionally, the parasutural crests may be distally irregular or undulose. Parasutural features are generally entirely lacking, or profoundly reduced, midventrally in the parasulcal area and the middorsal hypocystal area. A prominent subcircular claustrum is present in the antapical parasulcal paraplate. Periphragm is smooth to microscabrate and may be irregurlarly microreticulate. The endophragm is markedly thicker than the periphragm and smooth. The perioperculum is absent; the large endoperculum is frequently displaced and visible. Both the perioperculum is absent; the large endoperculum is frequently displaced and visible. Both the periarcheopyle and the endoarchaeopyle is occasionally slightly enlarged posteriorly, at the 3"/3c-4c paraplate boundary.
Dimensions. (µm; n=35) Min. (Mean) Max.
Length of pericyst: 113 (133) 158
Length of apical horn: 14 (24) 31
Length of epipericyst (excl. paracingulum)*: 69 (82) 100
Length (height) of paracingulum: 3 (5) 8
Length of hypopericyst (excl. paracingulum): 34 (45) 55
Length of endocyst: 71 (86) 103
Width of pericyst at paracingulum**: 55 (70) 86
Width of endocyst at paracingulum: 48 (62) 79
Height of parasutural crests***: 1 (3) 7
Diameter of fenestrae: 1 (1.5) 3
* including the apical horn
** includes the parasutural crests at the paracingulum
*** measured midlaterally, within the precingular paraplate series
The measured specimens are from sidewall core samples in Buang-1 well at 3504.00m and 3505 m, Frigate-1 well at 1233.00 m and Tenacious-1 well at 3002.00m and a ditch cuttings sample from Hadrian-1 well between 3285.00m and 3300.00m.
Comments. Gonyaulacysta fenestrata is a unique species, being large and elongate with fenestrate parsutural crests. The subcircular to ellipsoidal fenestrae on the parasutural crests are diagnostic. The characteristic perforations exhibit some variability in their density of insertion. In some specimens the fenestrae are rectangular seperated by distinct rods or short processes. They are normally present in the midlateral and antapical areas of the cyst, closely adjacent to the distal margin of the crests. The antapical parasutural crest appear to be susceptible to mechanical damage. This may affect the distal part of the parasutural crest, giving rise to an irregurlarly denticulate margin due to 'bursting' of the fenestrae. Normally, the parasutural crests are distally smooth, but in occasional specimens, they may be slightly undulose distally. The lateral parasutural crests are the most prominent and consequently these may be the only crests to exhibit significant fenestrae. The pericyst is elongate subellipsoidal in outline with a truncated antapex and the parasutural crests are interrupted at the paracingulum, thereby imparting a distinctive equatorial profile. The relatively thin apical horn is highly variable in length. It is blunt distally due to the presence of the first preapical paraplate (1pr/P of Helenes & Lucas-Clark, 1997). A porichnon (Evitt, 1985) is present on the ventral side of the apical horn, at the 2pr/1'/4' paraplate triple junction. The hypopericoel is consistently prominent, with a relatively large cavity developed both antapically and laterally in the hypocyst. By contrast, the epipericoel is generally smaller and more variable, with occasional forms having small pericoels and being merely cornucavate. The endocyst is ellipsoidal and normally has an apical protuberance is present in the endocyst. The paratabulation of the species appears to be precisely that determined for G. dualis (Brideaux & Fisher 1976) Stover & Evitt 1978 by Helenes and Lucas-Clark (1997). However, the midventral paratabulation is not known in detail due to the significant surpression of parasutures around the parasulcus. This phenomenon was also noted in Gonyaulacysta jurrasica by Stover and Evitt (1978, p. 277). The material studied is strongly dorsoventrally flattened and the orientation, dorsal or ventral, may appear difficult to determine. This scenario is exacerbated by the suppression of the midventral parasutural crests. However, using the ventral claustrum in the antapical parasulcal paraplate and the dorsal archaeopyle, the orientation can be readily determined. This is a gonyaulacelean species where the absence of the perioperculum is clearly demonstrable (Eaton, 1984). However, the large endoperculum is frequently displaced and retained within the cyst. The two archaeopyles may be slightly enlarged along the paracingulum. Gonyaulacysta fenestrata is similar n overall morphology to G. jurrasica (see Comparison), differing only in size and the presence of fenestrate, largely smooth parasutural crests. Therefore, given that G. jurassica is currently separated into 3 subspecies (Williams et al, 1998, p. 251-252), the possibility of giving fenestrata subspecific status within G. jurrasica was considered. However, G. fenestrata is arguably more similar to G. dualis, and this is maintained as a seperate species. We consider G. jurrasica has too many subspecific and varietal subdivisions and the most important stratigraphical morphotypes should be elevated to specific status.
Comparison.
Gonyaulacysta fenestrata is similar in morphology, and identical in paratabulation, to G. dentata (Raynaud, 1978) Lentin & Vozzhennikova, 1990 G. dualis, G. eisenackii (Deflandre 1939) Dodekova 1967 and G. jurrasica susp. jurassica. It may have an enlarged archaeopyle at the paracingulum like G. jurrassica (see Williams et al., 1978). The unusually long epicyst (with respect to the hypocyst) in G. fenestrata, G. dualis and G. jurassica is especially striking. However, the criterion distinguishing this species from all species in this important genus are the characteristic fenestrate parasutural crests. Gonyaulacysta fenestrata is most similar in size to G. dentata en G. dualis, and furthermore the parasutural crests in the latter species are typically smooth. However the crests in G. dualis are not fenestrate and may also be relatively sparsely denticulate or spinose (Brideaux & Fisher, 1976). Gonyaulacysta dentata (Raynaud1978) Lentin & Vozzhennikova, 1990 and G. eisenackii (Deflandre 1939) Górka 1965 have epicysts and hypocysts of approximatey equal length and denticulate parasutural crests. The latter Late Jurassic subspecies, with densely denticulate parasutural crests, has smaller epipericoels and hypopericoels and is significantly smaller in overall size than G. fenestrata (see Deflandre, 1939). In terms of overall size and epicystal morphology, the variety of G. jurrassica which is closest to G. fenestrata is G. jurrassica subsp. adecta Sarjeant 1982 var, longicornis (Deflandre 1938) Downie & Sarjaent 1965. This large, elongate morphotype is characteristic of the mid Oxfordian in Europe (Riding & Thomas, 1997), but is epicavate and has strongly denticulate parasutural crests. An example of a denticulate parasutural crest of G. jurassica subsp. adecta is figured as Fig6E. Gonyaulacysta fenestrata differs from G. sp. cf. fenestrata in having smooth distal margins to the parasutural crests. Figure 6 illustrates the differences in parasutural crest morphology of G. fenestrata, G. sp. cf. G. fenestrata, G. dualis and G. jurassica subsp. adecta.
Derivation of name. From the Latin fenestra, meaning window, and referring to the window-like holes or perforations in the distal parts of the parasutural crests in this species.
Stratigraphical distribution.
Gonyaulacysta fenestrata ranges from the Kimmeridgian mid Dingodinium swanense Zone (6aiia) to the Thitonian Cribroperidinium perforans Zone (5d) in the Timon Sea region. Indentical forms were illustrated as G. dualis from the Chichali and Sembar formation of Pakistan (Beju 1979, 1980).
Previous Australian usage: Gonyaulacysta oligodentata (pars) - Helby
Original description (Riding and Helby, 2001f)
Description. A large, elongate species of Gonyaulacysta with a long, distally-blunt, slender apical horn. The cyst is primarily dorsoventrally flattened. The pericyst, excluding the apical horn, is elongate supellipsoidal to subpolygonal and is antapically truncate. The endocyst is elongate ellipsoidal, normally with a prominent apical protuberance. Bicavate cyst organisation; the epicyst may be epicavate or cornucavate. Paratabulation partially indicated by parasutural crests which are generally smooth distally and fenestrate. The fenestrae are subcircular, ellipsoidal or rectangluar; normally they are best developed in the midlateral and antapical areas and inserted close to the distal margin of the crest. Occassionally, the parasutural crests may be distally irregular or undulose. Parasutural features are generally entirely lacking, or profoundly reduced, midventrally in the parasulcal area and the middorsal hypocystal area. A prominent subcircular claustrum is present in the antapical parasulcal paraplate. Periphragm is smooth to microscabrate and may be irregurlarly microreticulate. The endophragm is markedly thicker than the periphragm and smooth. The perioperculum is absent; the large endoperculum is frequently displaced and visible. Both the perioperculum is absent; the large endoperculum is frequently displaced and visible. Both the periarcheopyle and the endoarchaeopyle is occasionally slightly enlarged posteriorly, at the 3"/3c-4c paraplate boundary.
Dimensions. (µm; n=35) Min. (Mean) Max.
Length of pericyst: 113 (133) 158
Length of apical horn: 14 (24) 31
Length of epipericyst (excl. paracingulum)*: 69 (82) 100
Length (height) of paracingulum: 3 (5) 8
Length of hypopericyst (excl. paracingulum): 34 (45) 55
Length of endocyst: 71 (86) 103
Width of pericyst at paracingulum**: 55 (70) 86
Width of endocyst at paracingulum: 48 (62) 79
Height of parasutural crests***: 1 (3) 7
Diameter of fenestrae: 1 (1.5) 3
* including the apical horn
** includes the parasutural crests at the paracingulum
*** measured midlaterally, within the precingular paraplate series
The measured specimens are from sidewall core samples in Buang-1 well at 3504.00m and 3505 m, Frigate-1 well at 1233.00 m and Tenacious-1 well at 3002.00m and a ditch cuttings sample from Hadrian-1 well between 3285.00m and 3300.00m.
Comments. Gonyaulacysta fenestrata is a unique species, being large and elongate with fenestrate parsutural crests. The subcircular to ellipsoidal fenestrae on the parasutural crests are diagnostic. The characteristic perforations exhibit some variability in their density of insertion. In some specimens the fenestrae are rectangular seperated by distinct rods or short processes. They are normally present in the midlateral and antapical areas of the cyst, closely adjacent to the distal margin of the crests. The antapical parasutural crest appear to be susceptible to mechanical damage. This may affect the distal part of the parasutural crest, giving rise to an irregurlarly denticulate margin due to 'bursting' of the fenestrae. Normally, the parasutural crests are distally smooth, but in occasional specimens, they may be slightly undulose distally. The lateral parasutural crests are the most prominent and consequently these may be the only crests to exhibit significant fenestrae. The pericyst is elongate subellipsoidal in outline with a truncated antapex and the parasutural crests are interrupted at the paracingulum, thereby imparting a distinctive equatorial profile. The relatively thin apical horn is highly variable in length. It is blunt distally due to the presence of the first preapical paraplate (1pr/P of Helenes & Lucas-Clark, 1997). A porichnon (Evitt, 1985) is present on the ventral side of the apical horn, at the 2pr/1'/4' paraplate triple junction. The hypopericoel is consistently prominent, with a relatively large cavity developed both antapically and laterally in the hypocyst. By contrast, the epipericoel is generally smaller and more variable, with occasional forms having small pericoels and being merely cornucavate. The endocyst is ellipsoidal and normally has an apical protuberance is present in the endocyst. The paratabulation of the species appears to be precisely that determined for G. dualis (Brideaux & Fisher 1976) Stover & Evitt 1978 by Helenes and Lucas-Clark (1997). However, the midventral paratabulation is not known in detail due to the significant surpression of parasutures around the parasulcus. This phenomenon was also noted in Gonyaulacysta jurrasica by Stover and Evitt (1978, p. 277). The material studied is strongly dorsoventrally flattened and the orientation, dorsal or ventral, may appear difficult to determine. This scenario is exacerbated by the suppression of the midventral parasutural crests. However, using the ventral claustrum in the antapical parasulcal paraplate and the dorsal archaeopyle, the orientation can be readily determined. This is a gonyaulacelean species where the absence of the perioperculum is clearly demonstrable (Eaton, 1984). However, the large endoperculum is frequently displaced and retained within the cyst. The two archaeopyles may be slightly enlarged along the paracingulum. Gonyaulacysta fenestrata is similar n overall morphology to G. jurrasica (see Comparison), differing only in size and the presence of fenestrate, largely smooth parasutural crests. Therefore, given that G. jurassica is currently separated into 3 subspecies (Williams et al, 1998, p. 251-252), the possibility of giving fenestrata subspecific status within G. jurrasica was considered. However, G. fenestrata is arguably more similar to G. dualis, and this is maintained as a seperate species. We consider G. jurrasica has too many subspecific and varietal subdivisions and the most important stratigraphical morphotypes should be elevated to specific status.
Comparison.
Gonyaulacysta fenestrata is similar in morphology, and identical in paratabulation, to G. dentata (Raynaud, 1978) Lentin & Vozzhennikova, 1990 G. dualis, G. eisenackii (Deflandre 1939) Dodekova 1967 and G. jurrasica susp. jurassica. It may have an enlarged archaeopyle at the paracingulum like G. jurrassica (see Williams et al., 1978). The unusually long epicyst (with respect to the hypocyst) in G. fenestrata, G. dualis and G. jurassica is especially striking. However, the criterion distinguishing this species from all species in this important genus are the characteristic fenestrate parasutural crests. Gonyaulacysta fenestrata is most similar in size to G. dentata en G. dualis, and furthermore the parasutural crests in the latter species are typically smooth. However the crests in G. dualis are not fenestrate and may also be relatively sparsely denticulate or spinose (Brideaux & Fisher, 1976). Gonyaulacysta dentata (Raynaud1978) Lentin & Vozzhennikova, 1990 and G. eisenackii (Deflandre 1939) Górka 1965 have epicysts and hypocysts of approximatey equal length and denticulate parasutural crests. The latter Late Jurassic subspecies, with densely denticulate parasutural crests, has smaller epipericoels and hypopericoels and is significantly smaller in overall size than G. fenestrata (see Deflandre, 1939). In terms of overall size and epicystal morphology, the variety of G. jurrassica which is closest to G. fenestrata is G. jurrassica subsp. adecta Sarjeant 1982 var, longicornis (Deflandre 1938) Downie & Sarjaent 1965. This large, elongate morphotype is characteristic of the mid Oxfordian in Europe (Riding & Thomas, 1997), but is epicavate and has strongly denticulate parasutural crests. An example of a denticulate parasutural crest of G. jurassica subsp. adecta is figured as Fig6E. Gonyaulacysta fenestrata differs from G. sp. cf. fenestrata in having smooth distal margins to the parasutural crests. Figure 6 illustrates the differences in parasutural crest morphology of G. fenestrata, G. sp. cf. G. fenestrata, G. dualis and G. jurassica subsp. adecta.
Derivation of name. From the Latin fenestra, meaning window, and referring to the window-like holes or perforations in the distal parts of the parasutural crests in this species.
Stratigraphical distribution.
Gonyaulacysta fenestrata ranges from the Kimmeridgian mid Dingodinium swanense Zone (6aiia) to the Thitonian Cribroperidinium perforans Zone (5d) in the Timon Sea region. Indentical forms were illustrated as G. dualis from the Chichali and Sembar formation of Pakistan (Beju 1979, 1980).
Previous Australian usage: Gonyaulacysta oligodentata (pars) - Helby