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Oligosphaeridium swanense
Oligosphaeridium swanense Riding and Helby, 2001f, p.163,165–166, figs.12A–F,13A–C. Holotype: Riding and Helby, 2001f, fig.12E. Age: Oxfordian–Kimmeridgian.
Original description (Riding & Helby, 2001f)
A large species of Oligosphaeridium with long, tubiform intratabular (paraplate-centered) processes. The processes are formed by the periphragm only, vary considerably in width, are open distally and are deeply indented as a result of branching of up to three times. Branching typically first occurs at th mid-point of the process, where the tubular process bifurcates, or occasionally trifurcates. The resultant branches further bifurcate in a distal direction and there may be a final bifurcation at the distal extremities. The distal parts of the processes are sharply pointed and simple; trabeculation is not developed. This branching causes the distal protion of the processes to be significantly expanded. The long, rounded subtriangular notches caused by the branching may be deeper on the hypocyst than on the epicyst. Epicystal processes are also generally slightly smaller and possibly more slender, however the variation of process length on individual specimens is minor. The width of the processes varies significantly however; the majority of these elements are between 5 and 11 mm. The processes are also slightly expanded proximally, where they are thickened, markedly striate and fibrous. Occasionally, processes are entirely longitudinally striated. Parasulcal processes are not normally developed, which deeply subdivide the precingular paraplates.
Dimensions (µm;n=31): Min. (Mean) Max.
Length of entire cyst incl. processes: 107 (131) 153
Length of entire cyst excl. processes: 48 (62) 72
Length of loisthocyst incl. processes: 87 (130) 163
Length of loisthocyst excl. processes: 40 (63) 86
Width of cyst incl. processes: 103 (129) 168
Width of cyst excl. processes: 48 (63) 84
Length of processes: 28 (43) 60
Width of processes: 3 (6) 18
The measured specimens are from conventional core samples at 3315.95m and 2219.95m from Alaria-1 well and sidewall core samples from Buang-1 well at 3505.20 m and Tenacious-1 well at 3002.00m.
Comments. The processes, particularly their branching, in Oligosphaeridium swanense are highly distinctive. Normally each process branches three times, the initial furcation generally occuring close to the central portion, although this is relatively variable. The final branching is close to the distal extremity and the median furcation being approximately intermediate in position. This species is distinctive in comparison to others in the genus because the initial, most proximal, branching is relatively close to the cyst body, within the median portion of the process. In some cases, the first furcation is only one third the distance from the cyst body to the distal tip of the process. In a very small number of forms, which may be transitional to other taxa, there is some re-connection of the distal elements of the processes. The majority of the processes are largely subparallel, but are expanded both proximally and distally. Occasionally the processes may be irregularly microreticulate, although the cyst wall is generally microscabrate. Their length is normally relatively constant in an individual specimen, however the width varies significantly. The apical processes are consistently the narrowest. Orientation is generally straightforward to interpret due to both the archaeopyle and the wide equatorial region, which is devoid of processes. The cyst is slightly dorsoventrally flattened. The operculum is consistently free and accessory archaeopyle sutures are often present. causing the precingular paraplates to be separated along their longitudinal sides. The accesory archeopyle sutures are typically relatively deep and chemical/mechanical damage to the precingular paraplate series may loosen individual paraplates. Representatives of other species of the genus occur in the same samples as Oligosphaeridium swanense, although as minor components.
Comparison. Oligosphaeridium swanense differs from the other 37 validly published species of Oligosphaeridium (see Williams et al., 1998 p. 434-438) in its large size and the extremely deep branching of the intratabular processes. Normally species of Oligosphaeridium are less than 100 µm in maximum diameter and any branching, or the incertion of narrow distal elements, occurs close to the distal end of the processes, The Early Cretaceous species Oligosphaeridium diluculum Davey 1982 and O. dividum Williams 1978 also have relatively deeply furcate, tubiform processes. However, both of these are smaller than O. swanense, have slender processes, the distal branching of which is not as deep as in the latter. Oligosphaeridium porosum Lentin & Williams 1981, from the Albian of southern India, resembles O. swanense in having broad processes which branch deeply. However, Oligosphaeridium swanense has longer processes which vary considerably in width and it is significantly larger than O. porosum. Two species also with broad intratabular processes which are distally branched are Oligosphaeridium pulcherrimum (Deflandre & Cookson 1955) Davey & Williams 1966 and O. patulum Riding & Thomas 1988. The branching in these species is not as deep as that in O, swanense (see Riding & Thomas 1988, fig. 9) and again they are slightly smaller than the latter. Oligosphaeridium fenestratum Duxbury 1980 and O. perforatum (Gocht 1959) Davey & Williams 1969 have processes which branch relatively deeply distally. Both these Early Cretaceous forms, however, are smaller than O. swanense and have fenestrae in the distal portions of the processes of Systematophora palmula Davey 1982 are similar in that they may be deeple branched (Davey, 1982, pl.1, figs. 1-3)
Derivation of name. From the Dingodinium swanense Zone.
Holotype and type locality Figure 12E, SPS 35721, Alaria-1 well, conventional core at 3319.95m
Stratigraphical distribution. Oligosphaeridium swanense ranges from the Oxfordian Wanaea spectabilis Zone (6c) to the Kimmeridgian upper Dingodinium swanense Zone (6aiia)
Original description (Riding & Helby, 2001f)
A large species of Oligosphaeridium with long, tubiform intratabular (paraplate-centered) processes. The processes are formed by the periphragm only, vary considerably in width, are open distally and are deeply indented as a result of branching of up to three times. Branching typically first occurs at th mid-point of the process, where the tubular process bifurcates, or occasionally trifurcates. The resultant branches further bifurcate in a distal direction and there may be a final bifurcation at the distal extremities. The distal parts of the processes are sharply pointed and simple; trabeculation is not developed. This branching causes the distal protion of the processes to be significantly expanded. The long, rounded subtriangular notches caused by the branching may be deeper on the hypocyst than on the epicyst. Epicystal processes are also generally slightly smaller and possibly more slender, however the variation of process length on individual specimens is minor. The width of the processes varies significantly however; the majority of these elements are between 5 and 11 mm. The processes are also slightly expanded proximally, where they are thickened, markedly striate and fibrous. Occasionally, processes are entirely longitudinally striated. Parasulcal processes are not normally developed, which deeply subdivide the precingular paraplates.
Dimensions (µm;n=31): Min. (Mean) Max.
Length of entire cyst incl. processes: 107 (131) 153
Length of entire cyst excl. processes: 48 (62) 72
Length of loisthocyst incl. processes: 87 (130) 163
Length of loisthocyst excl. processes: 40 (63) 86
Width of cyst incl. processes: 103 (129) 168
Width of cyst excl. processes: 48 (63) 84
Length of processes: 28 (43) 60
Width of processes: 3 (6) 18
The measured specimens are from conventional core samples at 3315.95m and 2219.95m from Alaria-1 well and sidewall core samples from Buang-1 well at 3505.20 m and Tenacious-1 well at 3002.00m.
Comments. The processes, particularly their branching, in Oligosphaeridium swanense are highly distinctive. Normally each process branches three times, the initial furcation generally occuring close to the central portion, although this is relatively variable. The final branching is close to the distal extremity and the median furcation being approximately intermediate in position. This species is distinctive in comparison to others in the genus because the initial, most proximal, branching is relatively close to the cyst body, within the median portion of the process. In some cases, the first furcation is only one third the distance from the cyst body to the distal tip of the process. In a very small number of forms, which may be transitional to other taxa, there is some re-connection of the distal elements of the processes. The majority of the processes are largely subparallel, but are expanded both proximally and distally. Occasionally the processes may be irregularly microreticulate, although the cyst wall is generally microscabrate. Their length is normally relatively constant in an individual specimen, however the width varies significantly. The apical processes are consistently the narrowest. Orientation is generally straightforward to interpret due to both the archaeopyle and the wide equatorial region, which is devoid of processes. The cyst is slightly dorsoventrally flattened. The operculum is consistently free and accessory archaeopyle sutures are often present. causing the precingular paraplates to be separated along their longitudinal sides. The accesory archeopyle sutures are typically relatively deep and chemical/mechanical damage to the precingular paraplate series may loosen individual paraplates. Representatives of other species of the genus occur in the same samples as Oligosphaeridium swanense, although as minor components.
Comparison. Oligosphaeridium swanense differs from the other 37 validly published species of Oligosphaeridium (see Williams et al., 1998 p. 434-438) in its large size and the extremely deep branching of the intratabular processes. Normally species of Oligosphaeridium are less than 100 µm in maximum diameter and any branching, or the incertion of narrow distal elements, occurs close to the distal end of the processes, The Early Cretaceous species Oligosphaeridium diluculum Davey 1982 and O. dividum Williams 1978 also have relatively deeply furcate, tubiform processes. However, both of these are smaller than O. swanense, have slender processes, the distal branching of which is not as deep as in the latter. Oligosphaeridium porosum Lentin & Williams 1981, from the Albian of southern India, resembles O. swanense in having broad processes which branch deeply. However, Oligosphaeridium swanense has longer processes which vary considerably in width and it is significantly larger than O. porosum. Two species also with broad intratabular processes which are distally branched are Oligosphaeridium pulcherrimum (Deflandre & Cookson 1955) Davey & Williams 1966 and O. patulum Riding & Thomas 1988. The branching in these species is not as deep as that in O, swanense (see Riding & Thomas 1988, fig. 9) and again they are slightly smaller than the latter. Oligosphaeridium fenestratum Duxbury 1980 and O. perforatum (Gocht 1959) Davey & Williams 1969 have processes which branch relatively deeply distally. Both these Early Cretaceous forms, however, are smaller than O. swanense and have fenestrae in the distal portions of the processes of Systematophora palmula Davey 1982 are similar in that they may be deeple branched (Davey, 1982, pl.1, figs. 1-3)
Derivation of name. From the Dingodinium swanense Zone.
Holotype and type locality Figure 12E, SPS 35721, Alaria-1 well, conventional core at 3319.95m
Stratigraphical distribution. Oligosphaeridium swanense ranges from the Oxfordian Wanaea spectabilis Zone (6c) to the Kimmeridgian upper Dingodinium swanense Zone (6aiia)