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Stephodinium europaicum
Stephodinium "europaicum" Cookson and Hughes, 1964, p.50, pl.8, figs.9–17.
Taxonomic senior synonym: Stephodinium coronatum, according to Clarke and Verdier (1967, p.67).
Holotype: Cookson and Hughes, 1964, pl.8, figs.9–12.
Age: late Albian.
Original description, Cookson & Hughes 1964: Stephodinium europaicum sp. nov. (Plate 8, figs. 9-17)
Diagnosis. Shell broadly oval with a definite surface tabulation composed of plates of varying sizes delimited by low, narrow ledges. An equatorial girdle incompletely encircles the shell leaving an untabulated area on the ventral surface which appears to represent a longitudinal furrow (Pl. 8, fig. 10). The girdle is bounded by two conspicuous flanges which extend from the shell in a plane at right angles to the longitudinal axis; the ledges of the pre- and post-equatorial plates adjacent to the girdle run radially across the flanges to their margins. The exact tabulation has not been determined. Almost certainly there is a single four-or ?five-sided antapical plate (Pl. 8, fig. 13) and probably one four or ?five-sided apical plate from which a short horn projects (Pl. 8, fig. 14). A large and strongly outlined plate, comparable in position with plate 3" of many other dinoflagellates, extends on the dorsal surface from the apical plate to the margin of the pre-equatorial flange (Pl. 8, figs. 11, 12, 15). This plate, which marks the position of the pylome, widens considerably towards the girdle. The shell-membrane is thin and smooth, except at the margins of the girdle-flanges which are minutely serrated.
Holotype. Plate 8, figs. 9-12; K5190, Cambridge Greensand (59/ 11 / 12).
Dimensions. Holotype body 59 μm, long, horn 5 μm, body breadth 75 μm, overall breadth 46 μm. Range, body 52- 60 μm long, 42-52 μm, broad, overall breadth 67- 78 μm.
Occurrence. Upper Gault, CGl, 59/ 11/14; Greensand, 59/ 11 /12, CG3, CG4, 59/ 11 /11, MDI; Chalk Marl, 59/11/7, CG8.
Comment: In its general form and size Stephodinium europaicum conforms to Deflandre's species S. coronatum. However, as the description of this species was based on a single specimen preserved in a flint ( ?Senonian) it is not possible to compare the Cambridge forms with it. Another species S. australicum, of comparable geological range, was recently described by Cookson and Eisenack (1962). All the available specimens of S. australicum were considerably flattened and in consequence difficult to interpret. However, in spite of some still existing uncertainties regarding the morphology of S. australicum there is no doubt that it is specifically distinct from S. europaicum. S. europaicum is of special interest in that it completely confirms the dinoflagellate affinities of Stephodinium originally suggested by Deflandre, and gives a more accurate idea of the orientation of the shell than was previously possible. Furthermore, it provides the first clear evidence of the existence of a system of tabulation in a member of the genus as well as the presence of a transverse girdle previously postulated by Deflandre. Unfortunately gaps in our knowledge still remain owing to the delicate nature of the shells and their consequent distortion and preferred orientation in permanent mounts. Most of the details and photographs had to be obtained from a few fully expanded examples that were removed for examination to hollow slides containing 50 per cent. glycerine solution, and later mounted in glycerine jelly.
Taxonomic senior synonym: Stephodinium coronatum, according to Clarke and Verdier (1967, p.67).
Holotype: Cookson and Hughes, 1964, pl.8, figs.9–12.
Age: late Albian.
Original description, Cookson & Hughes 1964: Stephodinium europaicum sp. nov. (Plate 8, figs. 9-17)
Diagnosis. Shell broadly oval with a definite surface tabulation composed of plates of varying sizes delimited by low, narrow ledges. An equatorial girdle incompletely encircles the shell leaving an untabulated area on the ventral surface which appears to represent a longitudinal furrow (Pl. 8, fig. 10). The girdle is bounded by two conspicuous flanges which extend from the shell in a plane at right angles to the longitudinal axis; the ledges of the pre- and post-equatorial plates adjacent to the girdle run radially across the flanges to their margins. The exact tabulation has not been determined. Almost certainly there is a single four-or ?five-sided antapical plate (Pl. 8, fig. 13) and probably one four or ?five-sided apical plate from which a short horn projects (Pl. 8, fig. 14). A large and strongly outlined plate, comparable in position with plate 3" of many other dinoflagellates, extends on the dorsal surface from the apical plate to the margin of the pre-equatorial flange (Pl. 8, figs. 11, 12, 15). This plate, which marks the position of the pylome, widens considerably towards the girdle. The shell-membrane is thin and smooth, except at the margins of the girdle-flanges which are minutely serrated.
Holotype. Plate 8, figs. 9-12; K5190, Cambridge Greensand (59/ 11 / 12).
Dimensions. Holotype body 59 μm, long, horn 5 μm, body breadth 75 μm, overall breadth 46 μm. Range, body 52- 60 μm long, 42-52 μm, broad, overall breadth 67- 78 μm.
Occurrence. Upper Gault, CGl, 59/ 11/14; Greensand, 59/ 11 /12, CG3, CG4, 59/ 11 /11, MDI; Chalk Marl, 59/11/7, CG8.
Comment: In its general form and size Stephodinium europaicum conforms to Deflandre's species S. coronatum. However, as the description of this species was based on a single specimen preserved in a flint ( ?Senonian) it is not possible to compare the Cambridge forms with it. Another species S. australicum, of comparable geological range, was recently described by Cookson and Eisenack (1962). All the available specimens of S. australicum were considerably flattened and in consequence difficult to interpret. However, in spite of some still existing uncertainties regarding the morphology of S. australicum there is no doubt that it is specifically distinct from S. europaicum. S. europaicum is of special interest in that it completely confirms the dinoflagellate affinities of Stephodinium originally suggested by Deflandre, and gives a more accurate idea of the orientation of the shell than was previously possible. Furthermore, it provides the first clear evidence of the existence of a system of tabulation in a member of the genus as well as the presence of a transverse girdle previously postulated by Deflandre. Unfortunately gaps in our knowledge still remain owing to the delicate nature of the shells and their consequent distortion and preferred orientation in permanent mounts. Most of the details and photographs had to be obtained from a few fully expanded examples that were removed for examination to hollow slides containing 50 per cent. glycerine solution, and later mounted in glycerine jelly.