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Susadinium australe
Susadinium australe Riding and Helby, 2001a, p.19,22, figs.11A–P.
Questionable assignment: Riding and Helby (2001a, p.19).
Holotype: Riding and Helby, 2001a, figs.11M–O.
Age: early Toarcian.
Original description (Riding & Helby, 2001a):
Description. Small, with a rounded pentagonal to subovoidal outline. Autophragm relatively thick, smooth, scabrate to microreticulate, frequently with low, smooth, anastomosing ridges. Prominent intratabular, rounded, commonly inflated protuberances are present in the precingular, postcingular and antapical paraplate series. These protuberances are surmounted by rounded to quadrate/rectangular periphragmal extensions which may be low ridges or relatively high and septate in form. The distal edges of the ridges are smooth to echinate and their height varies between specimens and also within individuals. There is a marked tendency for the protuberances and ridges/septa to be best developed in the precingular and postcingular paraplate series. Furthermore, they are typically located close to the paracingulum. Occasionally, low relief or negative ornamentation in the paracingulum partially indicate the paracingular paraplate boundaries. Parasulcus apparently not subdivided. Archaeopyle combination, apparently involving the apical and anterior intercalary paraplate series; compound operculum.
Dimensions (µm, n=31): Min. (Mean) Max.
Length incl. operculum: 38 (45) 53
Length excl. operculum: 31 (44) 53
Width: 31 (42) 56
Maximum height of protuberances: 2 (4) 7
The measured specimens are from sidewall core samples in Skua-3 (2402.50m) and Skua-4 (2366.00m) wells.
Comments. The most striking morphological feature of this species is the rounded, intratabular protuberances surmounted by ridges or septate periphragmal extensions. These are best developed in the precingular and postcingular paraplate series. However, low, reduced protuberances and ridges/septa may be present in the antapical series. These vary markedly in shape according to the size of the paraplate. In smaller paraplates, such as 1”, the protuberances may be suppressed. The apparent separation of the periphragm and endophragm where the ridges and/or septa emerge on the protuberances means that the protuberances are floored by autophragm proximally. Normally, the ridges/septa are echinate (and open) distally. However, they may also be distally smooth. In the precingular and postcingular paraplate series, the protuberances and ridges/septa are present immediately adjacent to the paracingulum. This leaves up to 50% of the paraplate entirely free of major ornamentation. Such eccentricity of intratabular features extremely unusual among dinoflagellate cysts; normally features like these are located in the centers of paraplates. In the precingular paraplate series, the periphragmal extensions may be inserted close together, forming a row. The ridges/septa vary in height within assemblages, specimens and in single paraplates, giving a dissected and incomplete appearance. Frequently the side of the ridge/septal complex is reduced in height toward the paracingulum. This feature is comparable to the septa, which are open toward the paracingulum in Amphorula Dodekova, 1969. However, this ridge/septal variability in S? australis is not as regular as in Amphorula. In some specimens of S. australis, the periphragmal extensions may be highest toward the paracingulum. The protuberance on the 2”” paraplate may be significantly larger and more bulbous than the 1”” paraplate, engendering an asymmetrical outline in dorsoventral view (Fensome et al., 1993). This prominent feature can often be a useful orientational criterion. Furthermore, a crack close to the 1””/2”” parasuture is often present; this may be a result of mechanical damage along a line of weakness. The intratabular protuberances are largely parallel-sided in the Timor Sea specimens but are more rounded in the Carnarvon Basin specimens (Helby et al, 1987). Also the latter appear to have aligned, coalescing low relief elements rather than complete ridges/septa. Susadinium? Australis is tentatively attributed to Susadinium because the archaeopyle is a combination, apparently involving the apical and anterior intercalary paraplate series. Unequivocal representatives of Susadinium have single paraplate anterior intercalary archaeopyles (type I) (Fensome et al., 1993). Individuals of S? australis where the operculum is missing are typically rounded quadrangular. The outline is normally slightly longitudinally elongate, but may be equant or, rarely, wider than long. A detailed analysis of the principal archaopyle suture is problematic as the autophragm in this part of the cyst is thin and easily folded. The paratabulation formula is ?5’, ?3a, 7”, Xc, 5”’, 2””, Xs.
Comparison. Susadinium? Australis differs from other species of Susadinium in its combination archaeopyle and the rounded quadrangular/rectangular intratabular protuberances in the precingular and postcingular areas which are open distally. The antapical paraplates bear only slightly inflated protuberances, unlike Susadinium scrofoides. The six previously described species of the genus do not have protuberances surmounted by ridges or crests. They all have rounded, distally closed intratabular features exept Susadinium? pinna (Below 1987) Lentin & Williams 1989 and S? tabulatum (Below 1987) Lentin & Williams 1989, which have parasutural crests and negative penitabular ornamentation respectively. Susadinium delmense (Below 1987) Lentin & William 1989 and S. faustum (Bjaerke 1980( Lentin & Wiliams 1985 have granulate and baculate autophragm respectively. The thick-walled Susadinium knertense (Below 1987) Lentin & Williams 1989 is relatively elongate and subangular in outline. The Late Jurassic Tringadinium has an apical archaeopyle with a simple operculum and a different paratabulation pattern, with four apical, six precingular and six postcingular paraplates. Horologinella Cookson & Eisenack 1962 is similar to S? australis in shape and apparent archaeopyle style. However, Horologinella is much more deeply insiced equatorially, its principal archaeopyle suture is circular, not angular (Blackhouse, 1988, pl. 47, figs. 2, 4) and it lacks intratabular protusions.
Derivation of name. The specific name refers to the Southern Hemisphere location.
Holotype and type locality. Figs. 11M-O, CPC 35322, Skua-4 well, sidewall core at 2366.00m
Stratigraphical distribution. Susadinium? australis is recorded from the Toarcian of the Timor Sea and from Enderby-1 well in the Carnarvon Basin (Helby et al., 1987).
Questionable assignment: Riding and Helby (2001a, p.19).
Holotype: Riding and Helby, 2001a, figs.11M–O.
Age: early Toarcian.
Original description (Riding & Helby, 2001a):
Description. Small, with a rounded pentagonal to subovoidal outline. Autophragm relatively thick, smooth, scabrate to microreticulate, frequently with low, smooth, anastomosing ridges. Prominent intratabular, rounded, commonly inflated protuberances are present in the precingular, postcingular and antapical paraplate series. These protuberances are surmounted by rounded to quadrate/rectangular periphragmal extensions which may be low ridges or relatively high and septate in form. The distal edges of the ridges are smooth to echinate and their height varies between specimens and also within individuals. There is a marked tendency for the protuberances and ridges/septa to be best developed in the precingular and postcingular paraplate series. Furthermore, they are typically located close to the paracingulum. Occasionally, low relief or negative ornamentation in the paracingulum partially indicate the paracingular paraplate boundaries. Parasulcus apparently not subdivided. Archaeopyle combination, apparently involving the apical and anterior intercalary paraplate series; compound operculum.
Dimensions (µm, n=31): Min. (Mean) Max.
Length incl. operculum: 38 (45) 53
Length excl. operculum: 31 (44) 53
Width: 31 (42) 56
Maximum height of protuberances: 2 (4) 7
The measured specimens are from sidewall core samples in Skua-3 (2402.50m) and Skua-4 (2366.00m) wells.
Comments. The most striking morphological feature of this species is the rounded, intratabular protuberances surmounted by ridges or septate periphragmal extensions. These are best developed in the precingular and postcingular paraplate series. However, low, reduced protuberances and ridges/septa may be present in the antapical series. These vary markedly in shape according to the size of the paraplate. In smaller paraplates, such as 1”, the protuberances may be suppressed. The apparent separation of the periphragm and endophragm where the ridges and/or septa emerge on the protuberances means that the protuberances are floored by autophragm proximally. Normally, the ridges/septa are echinate (and open) distally. However, they may also be distally smooth. In the precingular and postcingular paraplate series, the protuberances and ridges/septa are present immediately adjacent to the paracingulum. This leaves up to 50% of the paraplate entirely free of major ornamentation. Such eccentricity of intratabular features extremely unusual among dinoflagellate cysts; normally features like these are located in the centers of paraplates. In the precingular paraplate series, the periphragmal extensions may be inserted close together, forming a row. The ridges/septa vary in height within assemblages, specimens and in single paraplates, giving a dissected and incomplete appearance. Frequently the side of the ridge/septal complex is reduced in height toward the paracingulum. This feature is comparable to the septa, which are open toward the paracingulum in Amphorula Dodekova, 1969. However, this ridge/septal variability in S? australis is not as regular as in Amphorula. In some specimens of S. australis, the periphragmal extensions may be highest toward the paracingulum. The protuberance on the 2”” paraplate may be significantly larger and more bulbous than the 1”” paraplate, engendering an asymmetrical outline in dorsoventral view (Fensome et al., 1993). This prominent feature can often be a useful orientational criterion. Furthermore, a crack close to the 1””/2”” parasuture is often present; this may be a result of mechanical damage along a line of weakness. The intratabular protuberances are largely parallel-sided in the Timor Sea specimens but are more rounded in the Carnarvon Basin specimens (Helby et al, 1987). Also the latter appear to have aligned, coalescing low relief elements rather than complete ridges/septa. Susadinium? Australis is tentatively attributed to Susadinium because the archaeopyle is a combination, apparently involving the apical and anterior intercalary paraplate series. Unequivocal representatives of Susadinium have single paraplate anterior intercalary archaeopyles (type I) (Fensome et al., 1993). Individuals of S? australis where the operculum is missing are typically rounded quadrangular. The outline is normally slightly longitudinally elongate, but may be equant or, rarely, wider than long. A detailed analysis of the principal archaopyle suture is problematic as the autophragm in this part of the cyst is thin and easily folded. The paratabulation formula is ?5’, ?3a, 7”, Xc, 5”’, 2””, Xs.
Comparison. Susadinium? Australis differs from other species of Susadinium in its combination archaeopyle and the rounded quadrangular/rectangular intratabular protuberances in the precingular and postcingular areas which are open distally. The antapical paraplates bear only slightly inflated protuberances, unlike Susadinium scrofoides. The six previously described species of the genus do not have protuberances surmounted by ridges or crests. They all have rounded, distally closed intratabular features exept Susadinium? pinna (Below 1987) Lentin & Williams 1989 and S? tabulatum (Below 1987) Lentin & Williams 1989, which have parasutural crests and negative penitabular ornamentation respectively. Susadinium delmense (Below 1987) Lentin & William 1989 and S. faustum (Bjaerke 1980( Lentin & Wiliams 1985 have granulate and baculate autophragm respectively. The thick-walled Susadinium knertense (Below 1987) Lentin & Williams 1989 is relatively elongate and subangular in outline. The Late Jurassic Tringadinium has an apical archaeopyle with a simple operculum and a different paratabulation pattern, with four apical, six precingular and six postcingular paraplates. Horologinella Cookson & Eisenack 1962 is similar to S? australis in shape and apparent archaeopyle style. However, Horologinella is much more deeply insiced equatorially, its principal archaeopyle suture is circular, not angular (Blackhouse, 1988, pl. 47, figs. 2, 4) and it lacks intratabular protusions.
Derivation of name. The specific name refers to the Southern Hemisphere location.
Holotype and type locality. Figs. 11M-O, CPC 35322, Skua-4 well, sidewall core at 2366.00m
Stratigraphical distribution. Susadinium? australis is recorded from the Toarcian of the Timor Sea and from Enderby-1 well in the Carnarvon Basin (Helby et al., 1987).