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Microdinium jurassicum
Microdinium jurassicum Riding and Helby, 2001e, p.120,122–123, figs.6A–T.
Taxonomic junior synonym: Microdinium oxfordense (name not validly published), according to Riding and Helby (2001e, p.120).
Holotype: Riding and Helby, 2001e, figs.6M–O.
Age: Callovian–Oxfordian.
Original description (Rding and Helby, 2001e):
Microdinium jurassicum sp.nov. (Figs 6A-T)
1988 Microdinium sp. A (M.P. 619) ; Helby et al., fig. 10G-H.
1996 Microdinium ornatu, auct. Non Cookcson & Eisenack 1960 ; Burger, pl. 7, figs O-Q.
Previous Australian usage :
Microdinium ocfordensis – Ott (1970, pl. 8, figs 26-28)
M.P. 619 – Helby.
Microdinium jurassicum – Helby.
Description: A small species of Microdinium, elongate ellipsoidal to rounded subquadrangular/subpentagonal in dorsoventral or lateral view and may be slightly flattened. Autophragm relatively thick (1-2 µm), microreticulate. The fenestrate of the microreticulum are subcircular or irregularly subovoidal, narrow (<0.5-1 µm in maximum diameter) and of variable density. They are normally more densely spaced close to the parasutures. Occasionally , on , rarely two, large (up to 2.5 µm high and 1 µm wide) intratabular tubercles are present on the precingular and postcingular paraplates. A similar tubercule or protuberance ,ay be present at the apex of the cyst. Parasutures marked by prominent (1-2 µm high), distally-smooth to slightly undulate parasutural rests which may also be irregularly microreticulate. Apical archaeopyle, operculum simple, generally free.
Dimensions (µm, n=49); Min.(Mean)Max.
Overall length of the entire cysts: 25(37)49
Overall length of the loistocyst: 27(36)49
Overall width of cyst body: 20(33)47
The measured specimens are from conventional cores from Jabiru-1 well at 1642.50m and Jabiru-3 well at 1604.00m to 1604.13m. and sidewall cores from Arunta-1 well at 2135.00m, Challis-11 ST1 well at 1552.50m, Challis-11 ST2 well at 1842.00m, Rowan-1ST well at 3181.00m and Taltarni-1 well at 2950.00m.
Microdinium jurassicum has a wide size range. The sidewall core material from Challis-11 ST1 well at 1552.50m has yielded the smallest specimens. In the three specimens available, the height of a single entire cyst as 25 µm, the height of loistocysts carried from 27 to 28 µm and the width varied between 20 and 22 µm. By contrast, the specimens from side wall cores in Arunta-1 well at 2135.00m, Challis-11 ST2 well at 1842.00, Rowan-1 ST well at 3181.00m and Taltarni-1 well at 2950.00 proved of intermediate size. The measured of 37 specimens from these four wells have the following data:
Dimensions: (µm, n=37). Min.(Mean)Max.
Overall length of the entire cyst: 30(36)41
Overall length of loistocyst: 28(34)42
Overall width of cyst body: 20(31)42
However, by far the largest ,material came from conventional cores in Jaribu-2 well at 1642.50m and Jaribu-1 well at 1604.00m to 1604.13m. Data from 9 specimens produced the following:
Dimensions: (µm, n=9). Min.(Mean)Max.
Overall length of entire cyst: 46(48)49
Overall length of loistocyst: 36(42)49
Overall width of cyst body: (35(40)47
All examined material is unequivocally considered to be Microdinium jurassicum. The smallest morphotypes consistently occur in the Callovian Rigaudella aemula Zone and this may be stratigraphically significant (N. G. Marshall, personal communication, 1998).
Comments: This is the first species of Microdinium described fro Jurassic strata; most are Cretaceous and/or Paleogene (Williams et al., 1998, p. 399-404). Microdinium? ovatum Horowitz 1975, from the Triassic of Israel, was considered allochtoneous by Williams et al, 1998. Previously, the oldest in situ material of this genus was the spinose Microdinium? fibratum Batten & Lister 1988 from Barremian of south-east Engeland (Batten & Lister, 1988). Species of Microdinium have occasionally been reported, by not convincingly illustrated, from the Jurassic. The principle Jurassic partiform gonyaulacalean genus is Jansonia, which seems confined to the Mid Jurassic (Pocock 1972; Rauscher & Schmitt 1990; Riding et al. 1991; Martínez et al. 1999). The most characteristic features of Microdinium jurassicum are the microreticulate autophragm and the prominent, distally smooth to slightly irregular (undulate) parasutural crests, which may also be microreticulate. In rare cases, the crests may be denticulate. The precinglar paraplate series is extremely short and the paracingulam is normally higher than the precingulars. The species is very variable in size. There are several other species in the Callovian and Oxfordian of Australia with microreticulate autophragm, such as Durotrigia magna Riding & Helby (this volume) and Yalkalpodinium elangiana Riding & Helby (this volume). Therefore, it is possible that this feature may be preservational. However, the micrreticulation is considered to be a primary feature in Microdinium jurassicum because a wide range of preservational styles in material from seven wells has been studied and the autophragm is consistently microreticulate.
Comparison: Microdinium jurassicum closely resembles the genotype, M. ornatum Cookson & Eisenack, 1960. The autophragm of the latter is smooth, it is slightly smaller and its parasutural ridges/crests may comprise aligned, discrete elements. It also lacks the intratabular tubercles, which characterize some suites of M. jurassicum. The overwhelming majority of previously described species of Microdinium are generally characterized by nontabular, low relief, ornamentation, rather than being microreticulate. These include M. balteus (Below 1987) Lentin & Williams 1989, which is verrucate, M. carpentieriae Slimani 1994 and M. dentatum Vozzhenikova 1967, which are granulate, and M. reticulatum Vozzhenikova 1967 which has reticulate autophragm. Many forms have denticulate or spinose parasutural crests, for example M. angulare (Below 1987) Lentin & Williams 1989, M. carpentieriae Slimani 1994, M/. dentatum Vozzhenikova 1967 and M. setosum Sarjeant 1966. Slimani 1994 described several species from Campanian to Danian of Belgium and The Netherlands which have penitabular ornamentation, including M. bensonii Slimani 1994, M. marheineckii Slimani 1994 and M. mariae Slimani 1994. Some species have negative ornamentation: Microdinium? alatum Conrad 1941 ex Sarjeant 1967 is densely areolate and has high, striate parasutural crests. The paraplates of M. carinatum (Below 1987) Lentin & Williams 1989 have large perforations largely arranged in penitabuar position; the perforations represent refelected trichocyst pores according to Below (1987b). Microdinium? Horridum (Below 1987) Lentin & Williams 1989 has an autophragm which is irregularly perforate (Below 1987b, Pl. 17, figs 1-6). This form is readily distinguishes from M. jurassicum in having extremely prominent gonal spines formed by extensions of the parasutural crests. The autophragm in Microdinium? Reteinvolvatum (Below 1987) Lentin & Williams 1989 is densely and widely perforate. In addition to this form having larger and more dense perforations than M. jurassicum, it also has low parasutural ridges.
Derivation of name: From the occurrence of this species in the Jurassic.
Holotype and type locality: Figures 6M-O, CPC 35560, Taltarni-1 well, sidewall core at 2950.00m
Startigrapical distirbutaion: Microdinium jurassicum ranges from the Callovian lower Rigaudella aemula Zone (7aiibi) to the Oxfordian lower part of the upper Wanaea spectabilis Zone (6cib) in the Timor Sea region (Foster, this volume; Helby & Partridge, in prep. The species occurs consistently from the upper part of the Lower Rigaudella aemula Zone (7aiia) to the mid Wanaea spectabilis Zone (6ciib).
Taxonomic junior synonym: Microdinium oxfordense (name not validly published), according to Riding and Helby (2001e, p.120).
Holotype: Riding and Helby, 2001e, figs.6M–O.
Age: Callovian–Oxfordian.
Original description (Rding and Helby, 2001e):
Microdinium jurassicum sp.nov. (Figs 6A-T)
1988 Microdinium sp. A (M.P. 619) ; Helby et al., fig. 10G-H.
1996 Microdinium ornatu, auct. Non Cookcson & Eisenack 1960 ; Burger, pl. 7, figs O-Q.
Previous Australian usage :
Microdinium ocfordensis – Ott (1970, pl. 8, figs 26-28)
M.P. 619 – Helby.
Microdinium jurassicum – Helby.
Description: A small species of Microdinium, elongate ellipsoidal to rounded subquadrangular/subpentagonal in dorsoventral or lateral view and may be slightly flattened. Autophragm relatively thick (1-2 µm), microreticulate. The fenestrate of the microreticulum are subcircular or irregularly subovoidal, narrow (<0.5-1 µm in maximum diameter) and of variable density. They are normally more densely spaced close to the parasutures. Occasionally , on , rarely two, large (up to 2.5 µm high and 1 µm wide) intratabular tubercles are present on the precingular and postcingular paraplates. A similar tubercule or protuberance ,ay be present at the apex of the cyst. Parasutures marked by prominent (1-2 µm high), distally-smooth to slightly undulate parasutural rests which may also be irregularly microreticulate. Apical archaeopyle, operculum simple, generally free.
Dimensions (µm, n=49); Min.(Mean)Max.
Overall length of the entire cysts: 25(37)49
Overall length of the loistocyst: 27(36)49
Overall width of cyst body: 20(33)47
The measured specimens are from conventional cores from Jabiru-1 well at 1642.50m and Jabiru-3 well at 1604.00m to 1604.13m. and sidewall cores from Arunta-1 well at 2135.00m, Challis-11 ST1 well at 1552.50m, Challis-11 ST2 well at 1842.00m, Rowan-1ST well at 3181.00m and Taltarni-1 well at 2950.00m.
Microdinium jurassicum has a wide size range. The sidewall core material from Challis-11 ST1 well at 1552.50m has yielded the smallest specimens. In the three specimens available, the height of a single entire cyst as 25 µm, the height of loistocysts carried from 27 to 28 µm and the width varied between 20 and 22 µm. By contrast, the specimens from side wall cores in Arunta-1 well at 2135.00m, Challis-11 ST2 well at 1842.00, Rowan-1 ST well at 3181.00m and Taltarni-1 well at 2950.00 proved of intermediate size. The measured of 37 specimens from these four wells have the following data:
Dimensions: (µm, n=37). Min.(Mean)Max.
Overall length of the entire cyst: 30(36)41
Overall length of loistocyst: 28(34)42
Overall width of cyst body: 20(31)42
However, by far the largest ,material came from conventional cores in Jaribu-2 well at 1642.50m and Jaribu-1 well at 1604.00m to 1604.13m. Data from 9 specimens produced the following:
Dimensions: (µm, n=9). Min.(Mean)Max.
Overall length of entire cyst: 46(48)49
Overall length of loistocyst: 36(42)49
Overall width of cyst body: (35(40)47
All examined material is unequivocally considered to be Microdinium jurassicum. The smallest morphotypes consistently occur in the Callovian Rigaudella aemula Zone and this may be stratigraphically significant (N. G. Marshall, personal communication, 1998).
Comments: This is the first species of Microdinium described fro Jurassic strata; most are Cretaceous and/or Paleogene (Williams et al., 1998, p. 399-404). Microdinium? ovatum Horowitz 1975, from the Triassic of Israel, was considered allochtoneous by Williams et al, 1998. Previously, the oldest in situ material of this genus was the spinose Microdinium? fibratum Batten & Lister 1988 from Barremian of south-east Engeland (Batten & Lister, 1988). Species of Microdinium have occasionally been reported, by not convincingly illustrated, from the Jurassic. The principle Jurassic partiform gonyaulacalean genus is Jansonia, which seems confined to the Mid Jurassic (Pocock 1972; Rauscher & Schmitt 1990; Riding et al. 1991; Martínez et al. 1999). The most characteristic features of Microdinium jurassicum are the microreticulate autophragm and the prominent, distally smooth to slightly irregular (undulate) parasutural crests, which may also be microreticulate. In rare cases, the crests may be denticulate. The precinglar paraplate series is extremely short and the paracingulam is normally higher than the precingulars. The species is very variable in size. There are several other species in the Callovian and Oxfordian of Australia with microreticulate autophragm, such as Durotrigia magna Riding & Helby (this volume) and Yalkalpodinium elangiana Riding & Helby (this volume). Therefore, it is possible that this feature may be preservational. However, the micrreticulation is considered to be a primary feature in Microdinium jurassicum because a wide range of preservational styles in material from seven wells has been studied and the autophragm is consistently microreticulate.
Comparison: Microdinium jurassicum closely resembles the genotype, M. ornatum Cookson & Eisenack, 1960. The autophragm of the latter is smooth, it is slightly smaller and its parasutural ridges/crests may comprise aligned, discrete elements. It also lacks the intratabular tubercles, which characterize some suites of M. jurassicum. The overwhelming majority of previously described species of Microdinium are generally characterized by nontabular, low relief, ornamentation, rather than being microreticulate. These include M. balteus (Below 1987) Lentin & Williams 1989, which is verrucate, M. carpentieriae Slimani 1994 and M. dentatum Vozzhenikova 1967, which are granulate, and M. reticulatum Vozzhenikova 1967 which has reticulate autophragm. Many forms have denticulate or spinose parasutural crests, for example M. angulare (Below 1987) Lentin & Williams 1989, M. carpentieriae Slimani 1994, M/. dentatum Vozzhenikova 1967 and M. setosum Sarjeant 1966. Slimani 1994 described several species from Campanian to Danian of Belgium and The Netherlands which have penitabular ornamentation, including M. bensonii Slimani 1994, M. marheineckii Slimani 1994 and M. mariae Slimani 1994. Some species have negative ornamentation: Microdinium? alatum Conrad 1941 ex Sarjeant 1967 is densely areolate and has high, striate parasutural crests. The paraplates of M. carinatum (Below 1987) Lentin & Williams 1989 have large perforations largely arranged in penitabuar position; the perforations represent refelected trichocyst pores according to Below (1987b). Microdinium? Horridum (Below 1987) Lentin & Williams 1989 has an autophragm which is irregularly perforate (Below 1987b, Pl. 17, figs 1-6). This form is readily distinguishes from M. jurassicum in having extremely prominent gonal spines formed by extensions of the parasutural crests. The autophragm in Microdinium? Reteinvolvatum (Below 1987) Lentin & Williams 1989 is densely and widely perforate. In addition to this form having larger and more dense perforations than M. jurassicum, it also has low parasutural ridges.
Derivation of name: From the occurrence of this species in the Jurassic.
Holotype and type locality: Figures 6M-O, CPC 35560, Taltarni-1 well, sidewall core at 2950.00m
Startigrapical distirbutaion: Microdinium jurassicum ranges from the Callovian lower Rigaudella aemula Zone (7aiibi) to the Oxfordian lower part of the upper Wanaea spectabilis Zone (6cib) in the Timor Sea region (Foster, this volume; Helby & Partridge, in prep. The species occurs consistently from the upper part of the Lower Rigaudella aemula Zone (7aiia) to the mid Wanaea spectabilis Zone (6ciib).