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Dollidinium sinuosum
Dollidinium sinuosum (Cookson and Eisenack, 1960) emend. Helby and Stover, 1987
Originally Palaeostomocystis, subsequently (and now) Dollidinium.
Helby and Stover, 1987, provided a revised description for this species, which Lentin and Williams, 1993, considered to be an emendation.
Holotype: Cookson and Eisenack, 1960, pl.38, fig.16 (lost according to Helby and Stover, 1987)
Lectotype (considered as a neotype): Helby and Stover, 1987, figs.4A-C; designated by Helby and Stover, 1987)
Locus typicus: Canning Basin, Australia
Stratum typicum: Tithonian-Berriasian
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G.L. Williams short notes on species, Mesozoic-Cenozoic dinocyst course, Urbino, Italy, May 17-22, 1999 - LPP VIEWER CD-ROM 99.5.
Dollidinium sinuosum (Cookson and Eisenack, 1960b) Helby and Stover, 1987b, has a tabulation of 6', 6", 6c, 6"’, 1p, 2"”. It is hexalobate, dorso-ventrally compressed, and can have low parasutural ridges revealing all or nearly all of the paratabulation. The archeopyle is eccentrically placed, subapical, oval to subcircular, opening on the left side of the dorsal surface and formed from the loss of most of paraplates 3' and 4'. Note both 3" and 4" are peaked.
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Original description: Cookson and Eisenack, 1960, p. 258: Palaeostomocystis sinuosa
Shell small, rather flat, outline squarish and +/- deeply embayed with four to seven rounded prominences; a clearly marked circular opening occupies the greater part of one of the 'bays'. The shell-membrane is thin and finely granular, sometimes more pronouncedly so over the ends of the prominences.
Dimensions: Range 38-54 Ám, opening 12-24 Ám.
Revised description: Helby and Stover, 1987, p. 160-162
Proximate cysts, typically dorsoventrally compressed, hexalobate with two lateral lobes and a polar lobe on both the epicyst and hypocyst. Marginal equatorial area between the anterior and posterior lateral lobes generally narrower and more deeply concave than the concavities between the lateral and polar lobes.
Cyst wall smooth to finely granular, about 1.5 Ám thick, may split locally, especially at or near the ends of the lateral lobes. Surface features varying from faint, transverse, linear markings delimiting only the paracingulum to low, parasutural ridges revealing all or nearly all of the paratabulation. Paratabulation formula 0-2pr, 6', Oa, 6'', 6c, 6''', 1p, 2'''', Xs (Figs 1,3). Archeopyle encompassing most of paraplates 3' and 4', consisting of an eccentrically placed, subapical, oval to subcircular, evenly margined, opening on the anterior left side of the dorsal surface. Right, left, and posterior margins of the archeopyle within the borders of the two apical paraplates, whereas anterior archeopyle margin apparently coincident with that of paraplates 3' and 4' . All specimens lacking opercula and isolated opercular pieces not observed. Paracingulum indicated by the lateral equatorial indentations only, or by those plus faint, transverse, linear markings, or by the indentations and low, parasutural ridges. In the latter case, six subrectangular paraplates may be discernible. Parasulcus generally not indicated, occasionally it may be expressed by a low, parasutural, transverse ridge which subdivides the midventral hypocystal area.
Variability: Variability is evident in size, degree of wall separation and expression of tabulation. Most populations have a relatively narrow size range with the mean height extending from 44-52Ám. The distinctively paratabulate specimens encountered in Resolution- (Fig.4G-K) are much smaller than the smallest form previously recorded. However, these specimens are identical in all other respects to other populations of Dollidinium sinuosum. Separation of the wall layers at the extremities of the lateral lobes may be inconsistently developed, even within a single specimen. The major variations in tabulation are discussed below. However, even on relatively smooth forms, some parasutural junctions on the lobes are expressed as small, pointed nodes (Fig.4E,F). The occurrence of these nodes often| creates a bracket-like outline on the extremities of the lobes where prominent granules may be concentrated.
Paratabulation. According to our interpretation of paratabulation of D. sinuosum (Figs 1,3) there are at least 14 paraplates on the epicyst. The two, minute, pre-apical paraplates, of which the left appears to be slightly larger than the right, are not always discernible; we do not regard their presence or absence as diagnostic. In the apical series, paraplates 1', 2', 5', and 6' are generally clearly expressed, and the archeopyle comprises most of paraplates 3' and 4' . The relatively large, longitudinally elongate first apical paraplate is tapered anteriorly, and occupies most of the midventral area. Paraplates 2' and 6' are similarly shaped, and 6' is larger than 2'. On the dorsal surface, the geniculate margins of paraplates 3' and 4' and the 4' /5' boundary are usually evident. However, the 3' /4' ridge may be missing entirely (when the archeopyle margin touches 3''), or only the posterior part of the ridge is visible (when the archeopyle margin fails to contact 3'').
In the precingular series, paraplates 1" and 5" appear to be larger than the others, 1" and 6'' are flat topped, whereas 2'' to 5'' are gabled. On some specimens, the peak on 3" is centred, and that on 4" is offset to the right (Fig. 5A), on others the opposite situation exists (Fig.5B). Six, subrectangular paraplates define a relatively broad paracingulum. Paraplate 6c is evidently much narrower than the others, and the exact shape of the paraplates at the ends of the paracingulum is uncertain.
On the hypocyst, most of the paraplates are defined by parasutural ridges, especially on the dorsal surface. The paratabulation is much less clear in and around the ventral area where faint, incomplete ridges and minor folds obscure outlines of the paraplates. There are at least 6, 4-sided postcingular paraplates, 2 on the ventral and 4 on the dorsal surface, which do not differ greatly in size and shape. These paraplates, together with the 2, large, 6-sided antapical paraplates, are readily identifiable as to paraplate series. We are much less certain about the identity of the small, polygonal(?), paraplate designated lp, which alternatively, could be a sulcal paraplate or an additional poorly developed postcingular paraplate. The presence of the middorsal ridge separating the third and fourth postcingular paraplates is noteworthy. Removal of this ridge would result in the hypocyst having a standard peridiniacean paratabulation pattern. Subdivision of the parasulcus, except for the delineation of a posterior sulcal paraplate on some specimens, is not demonstrable.
Discussion. Neither the paratabulation pattern nor the archeopyle type of D. sinuosum occur on any known genus. Significant features of the paratabulation include 1, the presence of 6 apical paraplates instead of the more customary 4, which in itself, is unusual, but not unique; 2, the geniculate condition of paraplates 3' -5', which results in the subjacent precingular paraplates being gabled; and 3, the occurrence on the hypocyst of a longitudinal, essentially middorsal, parasutural ridge which separates paraplates 3'''and 1'''' on the left from 4''' and 2'''' on the right. The ridge imparts a symmetry to the hypocyst not known on any other dinoflagellate genus having two antapical paraplates. As far as we know, D. sinuosum is the only species whose archeopyle is apical (not in the sense of position, but in that it involves apical paraplates); is located on the anterior left dorsal surface; has an oval to subcircular outline and an even margin and is composed of parts of paraplates 3' and 4' . The archeopyle on Cladopyxidium McLean 1972 is similar in that its archeopyle is apical, involves 2 paraplates, and has a circular to nearly circular outline but it is distinct in being more or less centrally located at the apex of the cyst, rather than on the left laterodorsal surface, and in involving paraplates 2' and 3', not 3' and 4'. The only other genus possessing an offset, dorsal archeopyle is Selenopemphix Benedek emend. Lentin & Williams 1976 (see Stover & Evitt 1978, p.l22), but its archeopyle is intercalary. Besides the difference in archeopyles, Cladopyxidium and Selenopemphix possess other features which readily separate each from Dollidinium.
Horologinella, in common with Dollidinium, evidently has 6 apical paraplates, one or more of which are lost to form a circular, midapical archeopyle (J. Backhouse, pers. comm.). While the size and distribution of precingular paraplates are similar, Horologinella lacks the gabling of the anterior margin of the dorsal precingular paraplates and also lacks the middorsal hypocystal symmetry of Dollidinium. In particular, there is distinct gabling of the posterior boundary of 3''' adjacent to a substantial contact with 1'''' in Horologinella.
Originally Palaeostomocystis, subsequently (and now) Dollidinium.
Helby and Stover, 1987, provided a revised description for this species, which Lentin and Williams, 1993, considered to be an emendation.
Holotype: Cookson and Eisenack, 1960, pl.38, fig.16 (lost according to Helby and Stover, 1987)
Lectotype (considered as a neotype): Helby and Stover, 1987, figs.4A-C; designated by Helby and Stover, 1987)
Locus typicus: Canning Basin, Australia
Stratum typicum: Tithonian-Berriasian
--------------------------------------------------
G.L. Williams short notes on species, Mesozoic-Cenozoic dinocyst course, Urbino, Italy, May 17-22, 1999 - LPP VIEWER CD-ROM 99.5.
Dollidinium sinuosum (Cookson and Eisenack, 1960b) Helby and Stover, 1987b, has a tabulation of 6', 6", 6c, 6"’, 1p, 2"”. It is hexalobate, dorso-ventrally compressed, and can have low parasutural ridges revealing all or nearly all of the paratabulation. The archeopyle is eccentrically placed, subapical, oval to subcircular, opening on the left side of the dorsal surface and formed from the loss of most of paraplates 3' and 4'. Note both 3" and 4" are peaked.
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Original description: Cookson and Eisenack, 1960, p. 258: Palaeostomocystis sinuosa
Shell small, rather flat, outline squarish and +/- deeply embayed with four to seven rounded prominences; a clearly marked circular opening occupies the greater part of one of the 'bays'. The shell-membrane is thin and finely granular, sometimes more pronouncedly so over the ends of the prominences.
Dimensions: Range 38-54 Ám, opening 12-24 Ám.
Revised description: Helby and Stover, 1987, p. 160-162
Proximate cysts, typically dorsoventrally compressed, hexalobate with two lateral lobes and a polar lobe on both the epicyst and hypocyst. Marginal equatorial area between the anterior and posterior lateral lobes generally narrower and more deeply concave than the concavities between the lateral and polar lobes.
Cyst wall smooth to finely granular, about 1.5 Ám thick, may split locally, especially at or near the ends of the lateral lobes. Surface features varying from faint, transverse, linear markings delimiting only the paracingulum to low, parasutural ridges revealing all or nearly all of the paratabulation. Paratabulation formula 0-2pr, 6', Oa, 6'', 6c, 6''', 1p, 2'''', Xs (Figs 1,3). Archeopyle encompassing most of paraplates 3' and 4', consisting of an eccentrically placed, subapical, oval to subcircular, evenly margined, opening on the anterior left side of the dorsal surface. Right, left, and posterior margins of the archeopyle within the borders of the two apical paraplates, whereas anterior archeopyle margin apparently coincident with that of paraplates 3' and 4' . All specimens lacking opercula and isolated opercular pieces not observed. Paracingulum indicated by the lateral equatorial indentations only, or by those plus faint, transverse, linear markings, or by the indentations and low, parasutural ridges. In the latter case, six subrectangular paraplates may be discernible. Parasulcus generally not indicated, occasionally it may be expressed by a low, parasutural, transverse ridge which subdivides the midventral hypocystal area.
Variability: Variability is evident in size, degree of wall separation and expression of tabulation. Most populations have a relatively narrow size range with the mean height extending from 44-52Ám. The distinctively paratabulate specimens encountered in Resolution- (Fig.4G-K) are much smaller than the smallest form previously recorded. However, these specimens are identical in all other respects to other populations of Dollidinium sinuosum. Separation of the wall layers at the extremities of the lateral lobes may be inconsistently developed, even within a single specimen. The major variations in tabulation are discussed below. However, even on relatively smooth forms, some parasutural junctions on the lobes are expressed as small, pointed nodes (Fig.4E,F). The occurrence of these nodes often| creates a bracket-like outline on the extremities of the lobes where prominent granules may be concentrated.
Paratabulation. According to our interpretation of paratabulation of D. sinuosum (Figs 1,3) there are at least 14 paraplates on the epicyst. The two, minute, pre-apical paraplates, of which the left appears to be slightly larger than the right, are not always discernible; we do not regard their presence or absence as diagnostic. In the apical series, paraplates 1', 2', 5', and 6' are generally clearly expressed, and the archeopyle comprises most of paraplates 3' and 4' . The relatively large, longitudinally elongate first apical paraplate is tapered anteriorly, and occupies most of the midventral area. Paraplates 2' and 6' are similarly shaped, and 6' is larger than 2'. On the dorsal surface, the geniculate margins of paraplates 3' and 4' and the 4' /5' boundary are usually evident. However, the 3' /4' ridge may be missing entirely (when the archeopyle margin touches 3''), or only the posterior part of the ridge is visible (when the archeopyle margin fails to contact 3'').
In the precingular series, paraplates 1" and 5" appear to be larger than the others, 1" and 6'' are flat topped, whereas 2'' to 5'' are gabled. On some specimens, the peak on 3" is centred, and that on 4" is offset to the right (Fig. 5A), on others the opposite situation exists (Fig.5B). Six, subrectangular paraplates define a relatively broad paracingulum. Paraplate 6c is evidently much narrower than the others, and the exact shape of the paraplates at the ends of the paracingulum is uncertain.
On the hypocyst, most of the paraplates are defined by parasutural ridges, especially on the dorsal surface. The paratabulation is much less clear in and around the ventral area where faint, incomplete ridges and minor folds obscure outlines of the paraplates. There are at least 6, 4-sided postcingular paraplates, 2 on the ventral and 4 on the dorsal surface, which do not differ greatly in size and shape. These paraplates, together with the 2, large, 6-sided antapical paraplates, are readily identifiable as to paraplate series. We are much less certain about the identity of the small, polygonal(?), paraplate designated lp, which alternatively, could be a sulcal paraplate or an additional poorly developed postcingular paraplate. The presence of the middorsal ridge separating the third and fourth postcingular paraplates is noteworthy. Removal of this ridge would result in the hypocyst having a standard peridiniacean paratabulation pattern. Subdivision of the parasulcus, except for the delineation of a posterior sulcal paraplate on some specimens, is not demonstrable.
Discussion. Neither the paratabulation pattern nor the archeopyle type of D. sinuosum occur on any known genus. Significant features of the paratabulation include 1, the presence of 6 apical paraplates instead of the more customary 4, which in itself, is unusual, but not unique; 2, the geniculate condition of paraplates 3' -5', which results in the subjacent precingular paraplates being gabled; and 3, the occurrence on the hypocyst of a longitudinal, essentially middorsal, parasutural ridge which separates paraplates 3'''and 1'''' on the left from 4''' and 2'''' on the right. The ridge imparts a symmetry to the hypocyst not known on any other dinoflagellate genus having two antapical paraplates. As far as we know, D. sinuosum is the only species whose archeopyle is apical (not in the sense of position, but in that it involves apical paraplates); is located on the anterior left dorsal surface; has an oval to subcircular outline and an even margin and is composed of parts of paraplates 3' and 4' . The archeopyle on Cladopyxidium McLean 1972 is similar in that its archeopyle is apical, involves 2 paraplates, and has a circular to nearly circular outline but it is distinct in being more or less centrally located at the apex of the cyst, rather than on the left laterodorsal surface, and in involving paraplates 2' and 3', not 3' and 4'. The only other genus possessing an offset, dorsal archeopyle is Selenopemphix Benedek emend. Lentin & Williams 1976 (see Stover & Evitt 1978, p.l22), but its archeopyle is intercalary. Besides the difference in archeopyles, Cladopyxidium and Selenopemphix possess other features which readily separate each from Dollidinium.
Horologinella, in common with Dollidinium, evidently has 6 apical paraplates, one or more of which are lost to form a circular, midapical archeopyle (J. Backhouse, pers. comm.). While the size and distribution of precingular paraplates are similar, Horologinella lacks the gabling of the anterior margin of the dorsal precingular paraplates and also lacks the middorsal hypocystal symmetry of Dollidinium. In particular, there is distinct gabling of the posterior boundary of 3''' adjacent to a substantial contact with 1'''' in Horologinella.