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Moorodinium tesselatum
Moorodinium tessellatum Riding and Helby, 2001a, p.9,11,13, figs.6A–P. Holotype: Riding and Helby, 2001a, fig.6J. Age: Toarcian.
Description. Proximate, acavate, longitudinally elongate dinoflagellate cysts belonging to the genus Moorodinium which are intermediate in size. The outline in dorsoventral view is highly variable around a broad subpentagonal theme. There are two basic varieties; firstly, forms with a broad, bulbous epicyst and a subconical hypocyst normally with a single antapical horn. Secondly, individuals with rounded subtriangular epicysts with an apical horn or protuberance and three-sided hypocysts with a straight or concave antapex. The latter variety may have up to four antapical horns or protuberances may also occur. The epicyst is normally longer than the hypocyst. A reticulum is formed by low smooth ridges which normally define hexagonal and quadrate fenestrae; the autophragm is otherwise smooth. Paratabulation presumably gonyaulacalean, indicated only by the paracingulum, archaeopyle and the presumed gonal position of the antapical horns or proturbances, where developed. The paracingular parasutures are marked by smooth ridges or low crests. Parasulcus indented. Archaeopyle probably epicystal.
Dimensions (µm, n=33): Min. (Mean) Max.
Length of autocyst: 63 (82) 102
Width of autocyst: 46 (59) 75
Length of epicyst: 26 (47) 62)
Length of hypocyst: 25 (36) 52
Epicyst as % of overall length: 37 (56) 69
Length of apical horns: 3 (6) 10
Length of antapical horns: 3 (8) 18
Diameter of fenestrate: 4 (8) 12
The measured specimens are all from a sidewall core sample in Skua-6 well at 2385.00m.
Comments. Moorodinium tessellatum is a relatively large cyst (see Dimensions above), however using the criteria of Stover & Evitt (1978, p. 5), it is mainly within the intermediate range. The species has an extremely variable outline around a broad, subpentangular theme. The highly variable number of horns or protuberances are of similarly variable sizes and shapes. They are normally subconical with distally rounded terminations but may have bluntly pointed extremities. The epicyst is commonly bulbous but may be subconical and these latter forms, which comprise the minority, are informally termed ‘deltoid morphotypes’. Most have a bulbous epicyst and typically have a single antapical horn which is assumed to be inserted at, or close to, the centre of the antapical (1””) paraplate. These are informally referred to as ‘bulbous morphotypes’. Dinoflagellate cysts with a single horn are present on the hypocyst are extremely rare, another Jurassic example being Wanaea Cookson & Eisenack 1958. Up to four antapical horns/protuberances may be present. When four are present, they are presumably inserted at the gonal points on the 1””paraplate. Rarely, lateral horns or protuberances may also be present. The horns or protuberances may be broken off in poorly-preserved material. The autophragm is relatively thin and readily folded. A relatively large, brown, subcircular accumulation body is present close to the paracingulum in many specimens. Commonly, the only indication of paratabulation is the paracingulum, which is low in height and is weakly laevorotatory; it is indicated by prominent, distally smooth ridges or crests. The paracingulum is typically low in position on the cyst and normally appears to offset or cut the mosaic-like reticulate pattern which is developed over all the autophragm. Rarely, the paracingulum is either not developed or suppressed. In one specimen, the fields forming the reticulum are rectangular to square on either side of a split which may be the principal aracheopyle suture. These four-sided fields may be paracingular paraplates. The lack of a consistently developed epicystal aracheopyle is intriguing. However, some specimens have a split apparently immediately anterior to , or within, the paracinguum. The reticular units of Moorodinium tessellatum are overwhelmingly six-sided. The effect of the superpositioning of the ventral and dorsal surfaces often tends to obscure this feature. This species is assumed to have a gonyaulacalean paratabulation. The characteristic regular reticulum is reminiscent of gymnodinioid and suessioid forms. If Moorodinium tessellatum was gymnodinioid, each polygonal field defined by the reticulum would represent an amphiesmal vesicle in the parent theca (Fensome et al.,1993). The species is not considered likely to have a suessioid affinity because paraplates arranged in latitudinal series are generally lacking and the paracingulum in Moorodinium tessellatum is relatively narrow. This may indicated that the reticulum is a feature of ornamentation, perhaps to give strength to the cyst wall, and that the fields do not necessarily represent paraplates. However, in Fig 6M, there appears to be a paracingulum indicated by two equatorial rows of four-sided fields, which may represent paraplates. This broadly resembles Suessia Morbey 1975 (see Morbey 1975, figs. 12-15), which has an equatorial paracingulum comprising many paraplates , most of which are four-sided. However, both Below (1987, figs. 43,53) and Stover & Helby (1987, figs 21A, B) illustrated suessioid paratabulation patterns in which the paracingular paraplates are only four-sided. Should these rectangular and square areas represent the paracingulum, it would mean that the surrounding hexagonal fields are also reflected thecal plates. As this phenomenon in M. tessellatum is only observed on a single specimen, its suessioid affinity must be deemed equivocal at present. When first descriped by Blackhouse (1988), the genus was deemed to be an index for lacustrine or estuarine deposition.
Comparison. Moorodinium tessellatum is superficially similar to suessioid dinoflagellate cysts in having an autophragm which is subdivided into numerous fields. Moorodinium, however, is significantly longer than the five suessioid genera, apparently has an epicystal archaeopyle and the fields are more numerous and possibly do not reflect paratabulation (Bucefalo Palliani & Riding, 1997). Moorodinium tessellatum has some similarities with representatives of Cassiculosphaeridia Davey 1969 and Valensiella Eisenack 1963, which may have fine and irregularly reticulate autophragms, are ovoidal and have apical archeopyles. Scrinioccassis Gocht 1964 emend. Preuss 1989 also has a reticulate autophragm, yet is ovoidal, has much thicker autophragm and has a combination apical/precingular archaeopyle.
Derivation of name. From the Latin tessellatus, meaning mosaic.
Holotype and type locality. Fig 6J, CPC 35143, Skua-6 well, sidewall coring sample at 2385.00m
Stratigraphical distribution. See Appendix 1 and Fig. 12.
Description. Proximate, acavate, longitudinally elongate dinoflagellate cysts belonging to the genus Moorodinium which are intermediate in size. The outline in dorsoventral view is highly variable around a broad subpentagonal theme. There are two basic varieties; firstly, forms with a broad, bulbous epicyst and a subconical hypocyst normally with a single antapical horn. Secondly, individuals with rounded subtriangular epicysts with an apical horn or protuberance and three-sided hypocysts with a straight or concave antapex. The latter variety may have up to four antapical horns or protuberances may also occur. The epicyst is normally longer than the hypocyst. A reticulum is formed by low smooth ridges which normally define hexagonal and quadrate fenestrae; the autophragm is otherwise smooth. Paratabulation presumably gonyaulacalean, indicated only by the paracingulum, archaeopyle and the presumed gonal position of the antapical horns or proturbances, where developed. The paracingular parasutures are marked by smooth ridges or low crests. Parasulcus indented. Archaeopyle probably epicystal.
Dimensions (µm, n=33): Min. (Mean) Max.
Length of autocyst: 63 (82) 102
Width of autocyst: 46 (59) 75
Length of epicyst: 26 (47) 62)
Length of hypocyst: 25 (36) 52
Epicyst as % of overall length: 37 (56) 69
Length of apical horns: 3 (6) 10
Length of antapical horns: 3 (8) 18
Diameter of fenestrate: 4 (8) 12
The measured specimens are all from a sidewall core sample in Skua-6 well at 2385.00m.
Comments. Moorodinium tessellatum is a relatively large cyst (see Dimensions above), however using the criteria of Stover & Evitt (1978, p. 5), it is mainly within the intermediate range. The species has an extremely variable outline around a broad, subpentangular theme. The highly variable number of horns or protuberances are of similarly variable sizes and shapes. They are normally subconical with distally rounded terminations but may have bluntly pointed extremities. The epicyst is commonly bulbous but may be subconical and these latter forms, which comprise the minority, are informally termed ‘deltoid morphotypes’. Most have a bulbous epicyst and typically have a single antapical horn which is assumed to be inserted at, or close to, the centre of the antapical (1””) paraplate. These are informally referred to as ‘bulbous morphotypes’. Dinoflagellate cysts with a single horn are present on the hypocyst are extremely rare, another Jurassic example being Wanaea Cookson & Eisenack 1958. Up to four antapical horns/protuberances may be present. When four are present, they are presumably inserted at the gonal points on the 1””paraplate. Rarely, lateral horns or protuberances may also be present. The horns or protuberances may be broken off in poorly-preserved material. The autophragm is relatively thin and readily folded. A relatively large, brown, subcircular accumulation body is present close to the paracingulum in many specimens. Commonly, the only indication of paratabulation is the paracingulum, which is low in height and is weakly laevorotatory; it is indicated by prominent, distally smooth ridges or crests. The paracingulum is typically low in position on the cyst and normally appears to offset or cut the mosaic-like reticulate pattern which is developed over all the autophragm. Rarely, the paracingulum is either not developed or suppressed. In one specimen, the fields forming the reticulum are rectangular to square on either side of a split which may be the principal aracheopyle suture. These four-sided fields may be paracingular paraplates. The lack of a consistently developed epicystal aracheopyle is intriguing. However, some specimens have a split apparently immediately anterior to , or within, the paracinguum. The reticular units of Moorodinium tessellatum are overwhelmingly six-sided. The effect of the superpositioning of the ventral and dorsal surfaces often tends to obscure this feature. This species is assumed to have a gonyaulacalean paratabulation. The characteristic regular reticulum is reminiscent of gymnodinioid and suessioid forms. If Moorodinium tessellatum was gymnodinioid, each polygonal field defined by the reticulum would represent an amphiesmal vesicle in the parent theca (Fensome et al.,1993). The species is not considered likely to have a suessioid affinity because paraplates arranged in latitudinal series are generally lacking and the paracingulum in Moorodinium tessellatum is relatively narrow. This may indicated that the reticulum is a feature of ornamentation, perhaps to give strength to the cyst wall, and that the fields do not necessarily represent paraplates. However, in Fig 6M, there appears to be a paracingulum indicated by two equatorial rows of four-sided fields, which may represent paraplates. This broadly resembles Suessia Morbey 1975 (see Morbey 1975, figs. 12-15), which has an equatorial paracingulum comprising many paraplates , most of which are four-sided. However, both Below (1987, figs. 43,53) and Stover & Helby (1987, figs 21A, B) illustrated suessioid paratabulation patterns in which the paracingular paraplates are only four-sided. Should these rectangular and square areas represent the paracingulum, it would mean that the surrounding hexagonal fields are also reflected thecal plates. As this phenomenon in M. tessellatum is only observed on a single specimen, its suessioid affinity must be deemed equivocal at present. When first descriped by Blackhouse (1988), the genus was deemed to be an index for lacustrine or estuarine deposition.
Comparison. Moorodinium tessellatum is superficially similar to suessioid dinoflagellate cysts in having an autophragm which is subdivided into numerous fields. Moorodinium, however, is significantly longer than the five suessioid genera, apparently has an epicystal archaeopyle and the fields are more numerous and possibly do not reflect paratabulation (Bucefalo Palliani & Riding, 1997). Moorodinium tessellatum has some similarities with representatives of Cassiculosphaeridia Davey 1969 and Valensiella Eisenack 1963, which may have fine and irregularly reticulate autophragms, are ovoidal and have apical archeopyles. Scrinioccassis Gocht 1964 emend. Preuss 1989 also has a reticulate autophragm, yet is ovoidal, has much thicker autophragm and has a combination apical/precingular archaeopyle.
Derivation of name. From the Latin tessellatus, meaning mosaic.
Holotype and type locality. Fig 6J, CPC 35143, Skua-6 well, sidewall coring sample at 2385.00m
Stratigraphical distribution. See Appendix 1 and Fig. 12.