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Dinogymnium acuminatum
Dinogymnium acuminatum Evitt et al., 1967, p.8-16, pls.1-2; pl.3, figs.1-8,10,12,20; text-figs.11-23.
Taxonomic junior synonyms: Gymnodinium (subsequently Dinogymnium) kasachstanicum and Dinogymnium microgranulosum, both according to Lentin and Vozzhennikova (1990, p.15).
Holotype: Evitt et al., 1967, pl.1, figs.21-23; pl.2, fig.5; text-figs.16-18.
Locus typicus: Del Puerto Canyon, Stanislaus County, Califonia, USA
Age: Maastrichtian
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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.
Dinogymnium acuminatum Evitt et al., 1967. Test essentially biconical with prominent nearly equatorial cingulum, mean CI 53.5. Antapex usually terminated by a blunt but distinct point (occasionally produced and acute). Surface granulate. Size: length 48-127 µm, width 27-86 µm.
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Original description: Evitt et al., 1967, p. 9, 11, 13-16
Test essentially biconical with prominent, nearly equatorial cingulum; mean Cl = 53.5 (observed range 47-60). Right end of cingulum offset toward the antapex about one full cingulum width with respect to the left end. Cingulum bordered on the epitract by a ridge-like thickening of the wall; wall thickness essentially unmodified in corresponding position on hypotract. Sulcus prominent, restrictcd to the hypotract; discernible about three-quarters the distance to the antapex. Epitract and hypotract with numerous longitudinal folds. The longest folds extend from the cingulum nearly to the poles, leaving two unfolded areas, one at the apex, including the operculum and a narrow zone about the archcopyle, and the other at the antapex, including the antapical point and immediately surrounding area. Toward the cingulum on the hypotract additional, successively shorter folds tend to be located between the longer ones, leading to the kind of heterocostate pattern of ridges that has been described by Deflandre in D. heterocostatum (Deflandre, 1936). On the epitract only very short folds are usually present at the cingulum between the main ones that extend nearly to the apex.
Apex slightly crested and, therefore, more narrowly rounded in dorsal or ventral view than in lateral view. Archeopyle relatively narrow and long (length about twice the width), aligned with the apical "crest" and acutely pointed at dorsal and ventral extremities. The two sutures commonly fail to meet at one or both extremities, leaving the operculum attached by one or two narrow connections.
Antapex usually terminated by a blunt but distinct point (occasionally produced and acute), somewhat inclined toward the ventral direction and commonly more conspicuous in lateral or oblique views than in direct dorsal or ventral views. Wall thickness approximately uniform except for the thickened ridge along the cingulum and a local thickening at the apex where the operculum is commonly about twice as thick as the wall a short distance outside the archeopyle suture. Wall thickness varies among different specimens, reaching a maximum of 1.5-1.8 Ám.
A distinctive granulation, which is well shown in the illustrations, covers the entire exterior surface of the test except for small areas at the apex and the antapex (the antapical "point,"). Blind-ended wall-canals, which open on the interior of the test only, are associated with most of the granules, whereas through-going wall-canals occur chiefly in the granule-free polar areas. These features of the surface ornament and wall structure are described in more detail in a separate section.
Variability:
Essentially constant features of this species appear to include: the location of the cingulum slightly below the equator, the thickened margin along the apical edge of the cingulum, the character and distribution of surface granules and wall-canals, the elongate archeopyle, and the rather numerous longitudinal folds that show a tendency toward a heterocostate pattern in the hypotract. The "crested" apex and pointed antapex are typical features easily recognizable in many specimens but also easily modified by distortion or obscured by unfavorable orientation. The precise configuration ot the antapical "point" is moderately variable as suggested by the illustrations. The greatest variable in the species is size. Dimensions and other quantitative relationships are summarized in the next section. Linear measurements for the smallest and largest specimens differ by a factor of about 3 (48 x 27 Ám and 127 X 86 Ám). This range in size, which is unusually great for fossil dinoflagellates, indicates a volumetric difference of about 27-fold between smallest and largest individuals. Several characters vary with size. for example with an increase in size, the number of longitudinal folds increases and the heterocostate arrangement of those on the hypotract becomes more marked; also the typical characters of the apex and antapex become more apparent.
In addition to varying in size, specimens of D. acuminatum, like other species of Dinogymnium with longitudinal folds and a deep cingulum, differ markedly in shape. Distortion by compression, extension, or general "inflation" produces specimens with conspicuously different superficial appearances. In analyzing, defining, and identifying species of Dinogymnium with folded walls, care must be taken to make adequate allowance for these distortional effects.
Supplemental description: Mehrotra and Sarjeant, 1987, p. 169
Autoblast biconical in shape; apical end showing a small opening, antapex having a blunt termination. Longitudinal folds well developed, extending from poles to cingulum; phragma otherwise smooth. Cingulum indicated by a subequatorial depression bounded by thick ridges, moderately helicoid. Sulcus not seen, owing to lateral cyst flattening.
Affinities:
Evitt et al., 1967
Among previously described species only D. heterocostatum (Deflandre) closely resembles D. acuminatum. Critical comparison of the two species on all important points is not possible, partly because the range of intraspecific variability in D. heterocostatum is not adequately shown by the few specimens that constitute the original material studied by Deflandre, and partly because some features of D. heterocostatum cannot be seen on Deflandre's specimens embedded in chert.
Nevertheless, it appears that the two species are similar in the following respects:
1. Overall size and shape are comparable, as far as they can be judged, and the cingulum is situated slightly below the equator.
2. In both species the apex is broadly rounded in lateral view, more narrowly rounded in ventral or dorsal view. Deflandre, 1936 infers "une sorte de carene" at the apex in ventral view.
3. Although the archeopyle cannot be clearly seen in any of Deflandre's specimens, the traces of it which can be detected in the side view of the holotype suggest a structure like that in D. acuminatum.
4. The apex of the holotype of D. heterocostatum as seen in lateral view terminates in a rounded angle directed somewhat ventrally. The configuration is similar to those specimens of D. acuminatum in which the antapical point is not strongly developed.
5. Both species have a granulate surface, although this feature is not mentioned by Deflandre and the full distribution of granules cannot be traced on his specimens.
6. Longitudinal folds of several lengths, which suggested the specific epithet "heterocostatum," occur in many specimens of D. acuminatum.
Comparison of the two species is not possible with regard to two critical points:
1. From Deflandre's material it is not clear whether the cingulum of D. heterocostatum is granulate or smooth. The consistent presence of granules over the surface of the cingulum is characteristic of D. acuminatum and serves to distinguish it from some similar undescribed species.
2. The presence of wall-canals -not to mention their structure and distribution- cannot be ascertained for D. heterocostatum. The distinctive character of these canals in D. acuminatum has already been described.
In summary, it is possible that the specimens here described as D. acuminatum are conspecific with D. heterocostatum. It is also possible that, while similar, the two species are distinct. Circumstances of preservation and the number of specimens in the original material prevent resolution of this uncertainty. Since the holotype of D. heterocostatum is from a chert pebble of uncertain age (indicated as "?senonien" by Deflandre, 1936) not collected in place, a search for more material from which matrix-free specimens might be extracted would be difficult and promises no decisive results. In view of these considerations, we believe it best to describe D. acuminatum as a new species. If future work should demonstrate that these two names are synonymous, it will then be appropriate that D. heterocostatum one of the first two fossil species referred to Gymnodinium (the other being G. cretaceum), will stand as the type species of Dinogymnium.
Taxonomic junior synonyms: Gymnodinium (subsequently Dinogymnium) kasachstanicum and Dinogymnium microgranulosum, both according to Lentin and Vozzhennikova (1990, p.15).
Holotype: Evitt et al., 1967, pl.1, figs.21-23; pl.2, fig.5; text-figs.16-18.
Locus typicus: Del Puerto Canyon, Stanislaus County, Califonia, USA
Age: Maastrichtian
--------------------------------------------------
G.L. Williams short notes on species, Mesozoic-Cenozoic dinocyst course, Urbino, Italy, May 17-22, 1999 - LPP VIEWER CD-ROM 99.5.
Dinogymnium acuminatum Evitt et al., 1967. Test essentially biconical with prominent nearly equatorial cingulum, mean CI 53.5. Antapex usually terminated by a blunt but distinct point (occasionally produced and acute). Surface granulate. Size: length 48-127 µm, width 27-86 µm.
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Original description: Evitt et al., 1967, p. 9, 11, 13-16
Test essentially biconical with prominent, nearly equatorial cingulum; mean Cl = 53.5 (observed range 47-60). Right end of cingulum offset toward the antapex about one full cingulum width with respect to the left end. Cingulum bordered on the epitract by a ridge-like thickening of the wall; wall thickness essentially unmodified in corresponding position on hypotract. Sulcus prominent, restrictcd to the hypotract; discernible about three-quarters the distance to the antapex. Epitract and hypotract with numerous longitudinal folds. The longest folds extend from the cingulum nearly to the poles, leaving two unfolded areas, one at the apex, including the operculum and a narrow zone about the archcopyle, and the other at the antapex, including the antapical point and immediately surrounding area. Toward the cingulum on the hypotract additional, successively shorter folds tend to be located between the longer ones, leading to the kind of heterocostate pattern of ridges that has been described by Deflandre in D. heterocostatum (Deflandre, 1936). On the epitract only very short folds are usually present at the cingulum between the main ones that extend nearly to the apex.
Apex slightly crested and, therefore, more narrowly rounded in dorsal or ventral view than in lateral view. Archeopyle relatively narrow and long (length about twice the width), aligned with the apical "crest" and acutely pointed at dorsal and ventral extremities. The two sutures commonly fail to meet at one or both extremities, leaving the operculum attached by one or two narrow connections.
Antapex usually terminated by a blunt but distinct point (occasionally produced and acute), somewhat inclined toward the ventral direction and commonly more conspicuous in lateral or oblique views than in direct dorsal or ventral views. Wall thickness approximately uniform except for the thickened ridge along the cingulum and a local thickening at the apex where the operculum is commonly about twice as thick as the wall a short distance outside the archeopyle suture. Wall thickness varies among different specimens, reaching a maximum of 1.5-1.8 Ám.
A distinctive granulation, which is well shown in the illustrations, covers the entire exterior surface of the test except for small areas at the apex and the antapex (the antapical "point,"). Blind-ended wall-canals, which open on the interior of the test only, are associated with most of the granules, whereas through-going wall-canals occur chiefly in the granule-free polar areas. These features of the surface ornament and wall structure are described in more detail in a separate section.
Variability:
Essentially constant features of this species appear to include: the location of the cingulum slightly below the equator, the thickened margin along the apical edge of the cingulum, the character and distribution of surface granules and wall-canals, the elongate archeopyle, and the rather numerous longitudinal folds that show a tendency toward a heterocostate pattern in the hypotract. The "crested" apex and pointed antapex are typical features easily recognizable in many specimens but also easily modified by distortion or obscured by unfavorable orientation. The precise configuration ot the antapical "point" is moderately variable as suggested by the illustrations. The greatest variable in the species is size. Dimensions and other quantitative relationships are summarized in the next section. Linear measurements for the smallest and largest specimens differ by a factor of about 3 (48 x 27 Ám and 127 X 86 Ám). This range in size, which is unusually great for fossil dinoflagellates, indicates a volumetric difference of about 27-fold between smallest and largest individuals. Several characters vary with size. for example with an increase in size, the number of longitudinal folds increases and the heterocostate arrangement of those on the hypotract becomes more marked; also the typical characters of the apex and antapex become more apparent.
In addition to varying in size, specimens of D. acuminatum, like other species of Dinogymnium with longitudinal folds and a deep cingulum, differ markedly in shape. Distortion by compression, extension, or general "inflation" produces specimens with conspicuously different superficial appearances. In analyzing, defining, and identifying species of Dinogymnium with folded walls, care must be taken to make adequate allowance for these distortional effects.
Supplemental description: Mehrotra and Sarjeant, 1987, p. 169
Autoblast biconical in shape; apical end showing a small opening, antapex having a blunt termination. Longitudinal folds well developed, extending from poles to cingulum; phragma otherwise smooth. Cingulum indicated by a subequatorial depression bounded by thick ridges, moderately helicoid. Sulcus not seen, owing to lateral cyst flattening.
Affinities:
Evitt et al., 1967
Among previously described species only D. heterocostatum (Deflandre) closely resembles D. acuminatum. Critical comparison of the two species on all important points is not possible, partly because the range of intraspecific variability in D. heterocostatum is not adequately shown by the few specimens that constitute the original material studied by Deflandre, and partly because some features of D. heterocostatum cannot be seen on Deflandre's specimens embedded in chert.
Nevertheless, it appears that the two species are similar in the following respects:
1. Overall size and shape are comparable, as far as they can be judged, and the cingulum is situated slightly below the equator.
2. In both species the apex is broadly rounded in lateral view, more narrowly rounded in ventral or dorsal view. Deflandre, 1936 infers "une sorte de carene" at the apex in ventral view.
3. Although the archeopyle cannot be clearly seen in any of Deflandre's specimens, the traces of it which can be detected in the side view of the holotype suggest a structure like that in D. acuminatum.
4. The apex of the holotype of D. heterocostatum as seen in lateral view terminates in a rounded angle directed somewhat ventrally. The configuration is similar to those specimens of D. acuminatum in which the antapical point is not strongly developed.
5. Both species have a granulate surface, although this feature is not mentioned by Deflandre and the full distribution of granules cannot be traced on his specimens.
6. Longitudinal folds of several lengths, which suggested the specific epithet "heterocostatum," occur in many specimens of D. acuminatum.
Comparison of the two species is not possible with regard to two critical points:
1. From Deflandre's material it is not clear whether the cingulum of D. heterocostatum is granulate or smooth. The consistent presence of granules over the surface of the cingulum is characteristic of D. acuminatum and serves to distinguish it from some similar undescribed species.
2. The presence of wall-canals -not to mention their structure and distribution- cannot be ascertained for D. heterocostatum. The distinctive character of these canals in D. acuminatum has already been described.
In summary, it is possible that the specimens here described as D. acuminatum are conspecific with D. heterocostatum. It is also possible that, while similar, the two species are distinct. Circumstances of preservation and the number of specimens in the original material prevent resolution of this uncertainty. Since the holotype of D. heterocostatum is from a chert pebble of uncertain age (indicated as "?senonien" by Deflandre, 1936) not collected in place, a search for more material from which matrix-free specimens might be extracted would be difficult and promises no decisive results. In view of these considerations, we believe it best to describe D. acuminatum as a new species. If future work should demonstrate that these two names are synonymous, it will then be appropriate that D. heterocostatum one of the first two fossil species referred to Gymnodinium (the other being G. cretaceum), will stand as the type species of Dinogymnium.