Back
Meiourogonyaulax penitabulata
Meiourogonyaulax penitabulata Riding and Helby, 2001d, p.83,85,87, figs.11A-M,12A-I.
Holotype: Riding and Helby, 2001d, figs.11I-J.
Age: late Callovian.
Original description (Riding & Helby, 2001d):
Meiourogonyaulax penitabulata sp.nov. (Figs. 11A-M, 12A-I)
Previus Australian usage:
M.P. 745 – Helby.
Meiourogonyaulax sp. 745 – Helby.
Meiourogonyaulax ‘penitabular’ – Morgan.
Description: An ellipsoidal species of Meiourogonyaulax. The major paraplate series on the loistocyst bear prominent, low penitabular ridges or crests. The ridges or crests on the apical series (the operculum) are, however, parasutural. The forms with low, broad, robust ridges have short, solid denticles or spines surmounting the ridges. These elements are irregularly distributed and may be subconical, distally bifid or buccinate and may be joined by lender trabeculae. These trabeculate denticles/spines, when regularly inserted, may appear to be a fenestrate crest. The sporadic nature and variability of the ornamentation engenders an irregular outline. Individuals bearing entire or perforate crests have a more regular outline as the crests are smooth distally. The crests are normally narrow and may appear to have formed via the coalescing of densely inserted processes, which surmount a penitablar ridge. Normally the crests are perforate to occasionally fenestrate, giving them a spongeous appearance. The parasulcus is devoid of ornamentation and the ridges//crests bordering the paracingulum may frequently be parasutural, not penitabular. Autophragm is relatively thick, microreticulate to occasionally microgranulate. One or two dark, subspherical accumulation bodies are often present inside the cyst. Apical archeopyle; free, simple opercula common.
Dimensions (µm; n=42): Min.(Mean)Max.
Length of cyst incl. operculum: 66(74)85
Length of cysts excl. operculum: 48(64)76
Width of cyst: 50(67)85
Height of penitabular ridges/crests: 2(4)8
The average dimensions of several isolated opercula observed are 30 µmx 35 µm. the measured specimens are from sidewall core samples in Challis-11 ST1 well are 1552.50,, Challis-11 ST2 well ate 1842.00m and Rowan-1ST well at 3181.00m and 3183.00m.
Comments: Meiourogonyaulax penitabulata is an extremely distinctive taxon due to the prominent penitabular crests and/or ridges on all the major paraplate series on the loistocyst. The ridges and/or crests on the apical paraplate series, however, are parasutural. These features clearly indicate a standard gonyaulacalean paratabulation pattern. A preapical paraplate is present. Penitabular elements are relatively rare in the Mid Jurassic, being much more common in the Late Jurassic, Cretaceous and Palaeogene. The morphology of the penitabular ornamentation is somewhat variable. It varies from having regular, entire/perforate, distally smooth, relatively narrow crests (the ‘spongy’ morphology), to somewhat discontinuous robust ridges which are irregularly surmounted by short, bifid, buccinate, subconical or trabeculate processes (the ‘irregular’ morphology). The crests may appear to be forms by the dense coalescing of processes and they are occasionally fenestrae or, in extreme cases, somewhat trabeculate. The crests and the ridges/processes are markedly higher on the antapical (1’’’) paraplate. Both the distinctive ‘irregular’ and ‘spongy’ forms of Meiourogonyaulax penitabulata have exactly the same Stratigraphical range, being present in the late Callovian of the Timor Sea area. Moreover, they commonly occurs together, often in approximately similar relative propositions. Therefore, they are considered to be a single species and are not formally subdivided into subspecies or varieties here. The paracingulum is apparently not subdivided into paraplates and is the only major paraplate series on the loisthocyst which has parasutural, as opposed to penitabular, ridges or crests. The parasutural nature of the paracingular crests/ridges is far more common in the ‘irregular’ morphotype. The parasulcus is not subdivided. The penitabular nature of the ornamentation on the ventral paraplates makes the parasulcal area appear anomalously wide. The operculum is normally lost and the loistocysts are usually not markedly elongate. They may be equant and are frequently wider that long. The Autophragm is thick and microreticulate and/or occasionally microgranulate. The species is slightly dorsoventrally flattened.
Comparison: Meiourogonyaulax penitabulata differs from most other species of Meiourogonyaulax and Lithodinia in having penitabular crests or ridges in the major paraplate series. Some Tithonian specimens of M. bulloidea (Cookson & Eisenack 1960) Sarjeant 1969 emend. Riding & Helby (this volume) exhibit intratabular ridges and bosses, which have a resemblance to M. penitabulata, but differs in lacking consistent penitabular ridges/crests. Meiourogonyaulax viriosa sp. nov. lacks intratabular and penitabular ridges, while Lithodinia protothymosa exhibits randomly inserted intratabular elements, but lacks penitabular ridges. Meiourogonyaulax penitabulata is also relatively large for the genus; the majority of the validly published species are significantly smaller than this new species. The Bathonian Meiourogonyaulax insulofigurata Dodekova 1975 is the most similar in morphology, albeit superficially. It has thick, spongy autophragm in the intratabular areas and immediately adjacent to the parasutures (Dodekova 1975). However, M. insulofigurata is smaller than M. penitabulata and appears to have parasutural ornamentation. The majority of the previously published species of this characteristically Middle Jurassic genus have penitabular crests similar in the form to the penitabular crests of Meiourogonyaulax penitabulata. These include, M. amlasis Below 1981, M. borealis Sarjeant 1980, M. bulloidea, M. caytonensis (Sarjeant 1959) Sarjeant 1969, M. cristulata (Sarjeant 1959) Sarjeant 1969, M. ghermanii Neju 1971, M. pertusa, M. planoseptata Riding 1987 and M. stoveri. In particular, the specimens of M. bulloidea illustrated by Backhouse (1988) resemble Meiourogonyaulax penitabulata in that the relatively high parasutural crests appear in some cases to be penitabular. Meiourogonyaulax penitabulata is also similar to Atlantodinium jurassicum Zotto et al., 1987, however, the latter is fully penitabular and has penitabular crests in all six paracingular paraplates (Zotto et al., 1987). Likewise, all species of the characteristically Tethyan Jurassic genera Amphorula and Histiophora are fully penitabular and lack any parasutural features.
Derivation of name: From the characteristic penitabular crests and ridges.
Holotype and type locality: Figures 11I-J, CPC 35424, Challis-11 ST1 well, sidewall core samples at 1552.50m.
Stratigraphical distribution: Meiourogonyaulax penitanbulata has been recorded from the late Callovian strata of the Timor Sea region, Australia, where it is confined to the lower Rigeudella aemula Zone (subzones 7aiib-7aiia) (Foster, this volume, Helby & Partridge, in prep.). It is most prominent, and characteristic of, subzones 7aiibia to 7aiiaii.
Holotype: Riding and Helby, 2001d, figs.11I-J.
Age: late Callovian.
Original description (Riding & Helby, 2001d):
Meiourogonyaulax penitabulata sp.nov. (Figs. 11A-M, 12A-I)
Previus Australian usage:
M.P. 745 – Helby.
Meiourogonyaulax sp. 745 – Helby.
Meiourogonyaulax ‘penitabular’ – Morgan.
Description: An ellipsoidal species of Meiourogonyaulax. The major paraplate series on the loistocyst bear prominent, low penitabular ridges or crests. The ridges or crests on the apical series (the operculum) are, however, parasutural. The forms with low, broad, robust ridges have short, solid denticles or spines surmounting the ridges. These elements are irregularly distributed and may be subconical, distally bifid or buccinate and may be joined by lender trabeculae. These trabeculate denticles/spines, when regularly inserted, may appear to be a fenestrate crest. The sporadic nature and variability of the ornamentation engenders an irregular outline. Individuals bearing entire or perforate crests have a more regular outline as the crests are smooth distally. The crests are normally narrow and may appear to have formed via the coalescing of densely inserted processes, which surmount a penitablar ridge. Normally the crests are perforate to occasionally fenestrate, giving them a spongeous appearance. The parasulcus is devoid of ornamentation and the ridges//crests bordering the paracingulum may frequently be parasutural, not penitabular. Autophragm is relatively thick, microreticulate to occasionally microgranulate. One or two dark, subspherical accumulation bodies are often present inside the cyst. Apical archeopyle; free, simple opercula common.
Dimensions (µm; n=42): Min.(Mean)Max.
Length of cyst incl. operculum: 66(74)85
Length of cysts excl. operculum: 48(64)76
Width of cyst: 50(67)85
Height of penitabular ridges/crests: 2(4)8
The average dimensions of several isolated opercula observed are 30 µmx 35 µm. the measured specimens are from sidewall core samples in Challis-11 ST1 well are 1552.50,, Challis-11 ST2 well ate 1842.00m and Rowan-1ST well at 3181.00m and 3183.00m.
Comments: Meiourogonyaulax penitabulata is an extremely distinctive taxon due to the prominent penitabular crests and/or ridges on all the major paraplate series on the loistocyst. The ridges and/or crests on the apical paraplate series, however, are parasutural. These features clearly indicate a standard gonyaulacalean paratabulation pattern. A preapical paraplate is present. Penitabular elements are relatively rare in the Mid Jurassic, being much more common in the Late Jurassic, Cretaceous and Palaeogene. The morphology of the penitabular ornamentation is somewhat variable. It varies from having regular, entire/perforate, distally smooth, relatively narrow crests (the ‘spongy’ morphology), to somewhat discontinuous robust ridges which are irregularly surmounted by short, bifid, buccinate, subconical or trabeculate processes (the ‘irregular’ morphology). The crests may appear to be forms by the dense coalescing of processes and they are occasionally fenestrae or, in extreme cases, somewhat trabeculate. The crests and the ridges/processes are markedly higher on the antapical (1’’’) paraplate. Both the distinctive ‘irregular’ and ‘spongy’ forms of Meiourogonyaulax penitabulata have exactly the same Stratigraphical range, being present in the late Callovian of the Timor Sea area. Moreover, they commonly occurs together, often in approximately similar relative propositions. Therefore, they are considered to be a single species and are not formally subdivided into subspecies or varieties here. The paracingulum is apparently not subdivided into paraplates and is the only major paraplate series on the loisthocyst which has parasutural, as opposed to penitabular, ridges or crests. The parasutural nature of the paracingular crests/ridges is far more common in the ‘irregular’ morphotype. The parasulcus is not subdivided. The penitabular nature of the ornamentation on the ventral paraplates makes the parasulcal area appear anomalously wide. The operculum is normally lost and the loistocysts are usually not markedly elongate. They may be equant and are frequently wider that long. The Autophragm is thick and microreticulate and/or occasionally microgranulate. The species is slightly dorsoventrally flattened.
Comparison: Meiourogonyaulax penitabulata differs from most other species of Meiourogonyaulax and Lithodinia in having penitabular crests or ridges in the major paraplate series. Some Tithonian specimens of M. bulloidea (Cookson & Eisenack 1960) Sarjeant 1969 emend. Riding & Helby (this volume) exhibit intratabular ridges and bosses, which have a resemblance to M. penitabulata, but differs in lacking consistent penitabular ridges/crests. Meiourogonyaulax viriosa sp. nov. lacks intratabular and penitabular ridges, while Lithodinia protothymosa exhibits randomly inserted intratabular elements, but lacks penitabular ridges. Meiourogonyaulax penitabulata is also relatively large for the genus; the majority of the validly published species are significantly smaller than this new species. The Bathonian Meiourogonyaulax insulofigurata Dodekova 1975 is the most similar in morphology, albeit superficially. It has thick, spongy autophragm in the intratabular areas and immediately adjacent to the parasutures (Dodekova 1975). However, M. insulofigurata is smaller than M. penitabulata and appears to have parasutural ornamentation. The majority of the previously published species of this characteristically Middle Jurassic genus have penitabular crests similar in the form to the penitabular crests of Meiourogonyaulax penitabulata. These include, M. amlasis Below 1981, M. borealis Sarjeant 1980, M. bulloidea, M. caytonensis (Sarjeant 1959) Sarjeant 1969, M. cristulata (Sarjeant 1959) Sarjeant 1969, M. ghermanii Neju 1971, M. pertusa, M. planoseptata Riding 1987 and M. stoveri. In particular, the specimens of M. bulloidea illustrated by Backhouse (1988) resemble Meiourogonyaulax penitabulata in that the relatively high parasutural crests appear in some cases to be penitabular. Meiourogonyaulax penitabulata is also similar to Atlantodinium jurassicum Zotto et al., 1987, however, the latter is fully penitabular and has penitabular crests in all six paracingular paraplates (Zotto et al., 1987). Likewise, all species of the characteristically Tethyan Jurassic genera Amphorula and Histiophora are fully penitabular and lack any parasutural features.
Derivation of name: From the characteristic penitabular crests and ridges.
Holotype and type locality: Figures 11I-J, CPC 35424, Challis-11 ST1 well, sidewall core samples at 1552.50m.
Stratigraphical distribution: Meiourogonyaulax penitanbulata has been recorded from the late Callovian strata of the Timor Sea region, Australia, where it is confined to the lower Rigeudella aemula Zone (subzones 7aiib-7aiia) (Foster, this volume, Helby & Partridge, in prep.). It is most prominent, and characteristic of, subzones 7aiibia to 7aiiaii.