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Jansonia manifesta
Jansonia manifesta Riding and Walton in Riding et al., 1991
Holotype: Riding et al., 1991, pl.8, fig.1
Locus typicus: Section at Lon Ostatoin, Skye (NG 406 729), Scotland
Stratum typicum: Bathonian
Original diagnosis: Riding and Walton, 1991, p.148
Ovoidal to angular dinoflagellate cysts, subovoidal in dorsoventral outline. Paratabulation partiform gonyaulacacean, indicated by low, smooth ridges including and below the precingular series. The prominent, laevorotatory paracingulum is displaced by up to its entire height at the parasulcus. Autophragm relatively thick, dark brown; ornamented by low, smooth, randomly arcuate, discontinuous, branching ridges forming an irregular reticulum.
Original description: Riding and Walton, 1991, p.148
A species of Jansonia ovoidal in ambitus, elongate longitudinally. The dorso-ventral outline may rarely be subangular polygonal (Plate 8, fig. 11). A well developed partiform gonyaulacacean paratabulation is expressed, at and below the precingular series, by low-relief, smooth entire parasutural ridges. The Kofoidian paratabulation formula is: ?pr, ?4", ?4a, 7", 7c, 6""", 1p, 1"""", ?Ss. The paracingulum is relatively high representing, on average, 16% of the cyst length. It is laevorotatory and is displaced by up to its entire height ventrally (Plate 8, fig. 2). Rarely the paracingular area may be incised (Plate 8, fig. 12). Paraplate 7" is narrow and is geniculate anteriorly and posteriorly. Due to the sagittal position of the 1""""/1p parasuture, the ps paraplate is offset towards the right lateral side. The species is markedly dorso-ventrally flattened, which causes the 1"""" paraplate to be relatively wide (Text-Figure 4E, F). The longitudinal intraparacingular parasutures are reduced in height and may be suppressed. The archeopyle is combination, apical/anterior intercalary with a ventrally adnate, simple operculum. In the majority of specimens the operculum is attached ventrally at the as paraplate. This adnate situation is interpreted as being the norm; mechanical damage is invoked for specimens where the operculum and loisthocyst are separated (e.g. Plate 8, fig. 12). The autophragm above the principal archeopyle suture is thin and not paratabulate; the precise number and topology of the apical and anterior intercalary paraplates consequently is unclear.
The autophragm of the loisthocyst is relatively thick and dark brown in color, having a distinctive leathery appearance (Jansonius, 1986). It has an irregular reticulum formed by smooth, arcuate, anastomosing ridges which are about 1 Ám high and wide (Text-Figure 4G). This ornamentation may be reduced in the paracingulum and/or parasulcus.
Affinities:
Riding and Walton, 1991, p.148: Jansonia manifesta differs from the type species, J. jurassica, in that the loisthocyst is distinctly paratabulate and is smaller and more elongate. The authors have had the opportunity to study topotype material of J. jurassica for comparative purposes, generously provided by J. Jansonius (Esso Resources Canada Limited). The operculum of J. manifesta is usually attached ventrally but may be detached from the loisthocyst, presumably by mechanical damage (Plate 8, figs. 9-12, 14, 16). When present, however, the operculum is relatively thin and is not paratabulate; the configuration of the precingular paraplates strongly suggest that this species has four apical and four anterior intercalary paraplates (compare Evitt, 1985, text-fig. 5.18K; Below,1987, text-fig. 10).
Jansonia manifesta seems to have preferred brackish-water situations; it is confined to the Duntulm Formation in the Hebrides Basin, with a possible reworked record at the base of the Upper Ostrea Member (W.W., personal observation). The species was recovered in extremely high abundance at Lon Ostatoin immediately above a freshwater intercalation, suggesting that it was an opportunistic taxon thriving when brackish-water paleoenvironments were reestablished. The closely related J. jurassica was also found in a Middle Jurassic, fresh to brackish water depositional regime in western Canada by Pocock (1972). Small numbers of J. manifesta have also been recovered from the Bathonian of southwest England (J.B .R., personal observation) and a possible specimen from the Callovian of northeast France, figured as J. jurassica, was noted by Rauscher and Schmitt (1990, pl. 11, fig. 17).
Dimensions:MinimumMeanMaximum
Overall length:23.5 Ám29.2 Ám38.4 Ám
Overall breadth:18.9 Ám23.8 Ám34.1 Ám
Height of paracingulum:4.4 Ám5.3 Ám7.8 Ám
(60 specimens measured)
Holotype: Riding et al., 1991, pl.8, fig.1
Locus typicus: Section at Lon Ostatoin, Skye (NG 406 729), Scotland
Stratum typicum: Bathonian
Original diagnosis: Riding and Walton, 1991, p.148
Ovoidal to angular dinoflagellate cysts, subovoidal in dorsoventral outline. Paratabulation partiform gonyaulacacean, indicated by low, smooth ridges including and below the precingular series. The prominent, laevorotatory paracingulum is displaced by up to its entire height at the parasulcus. Autophragm relatively thick, dark brown; ornamented by low, smooth, randomly arcuate, discontinuous, branching ridges forming an irregular reticulum.
Original description: Riding and Walton, 1991, p.148
A species of Jansonia ovoidal in ambitus, elongate longitudinally. The dorso-ventral outline may rarely be subangular polygonal (Plate 8, fig. 11). A well developed partiform gonyaulacacean paratabulation is expressed, at and below the precingular series, by low-relief, smooth entire parasutural ridges. The Kofoidian paratabulation formula is: ?pr, ?4", ?4a, 7", 7c, 6""", 1p, 1"""", ?Ss. The paracingulum is relatively high representing, on average, 16% of the cyst length. It is laevorotatory and is displaced by up to its entire height ventrally (Plate 8, fig. 2). Rarely the paracingular area may be incised (Plate 8, fig. 12). Paraplate 7" is narrow and is geniculate anteriorly and posteriorly. Due to the sagittal position of the 1""""/1p parasuture, the ps paraplate is offset towards the right lateral side. The species is markedly dorso-ventrally flattened, which causes the 1"""" paraplate to be relatively wide (Text-Figure 4E, F). The longitudinal intraparacingular parasutures are reduced in height and may be suppressed. The archeopyle is combination, apical/anterior intercalary with a ventrally adnate, simple operculum. In the majority of specimens the operculum is attached ventrally at the as paraplate. This adnate situation is interpreted as being the norm; mechanical damage is invoked for specimens where the operculum and loisthocyst are separated (e.g. Plate 8, fig. 12). The autophragm above the principal archeopyle suture is thin and not paratabulate; the precise number and topology of the apical and anterior intercalary paraplates consequently is unclear.
The autophragm of the loisthocyst is relatively thick and dark brown in color, having a distinctive leathery appearance (Jansonius, 1986). It has an irregular reticulum formed by smooth, arcuate, anastomosing ridges which are about 1 Ám high and wide (Text-Figure 4G). This ornamentation may be reduced in the paracingulum and/or parasulcus.
Affinities:
Riding and Walton, 1991, p.148: Jansonia manifesta differs from the type species, J. jurassica, in that the loisthocyst is distinctly paratabulate and is smaller and more elongate. The authors have had the opportunity to study topotype material of J. jurassica for comparative purposes, generously provided by J. Jansonius (Esso Resources Canada Limited). The operculum of J. manifesta is usually attached ventrally but may be detached from the loisthocyst, presumably by mechanical damage (Plate 8, figs. 9-12, 14, 16). When present, however, the operculum is relatively thin and is not paratabulate; the configuration of the precingular paraplates strongly suggest that this species has four apical and four anterior intercalary paraplates (compare Evitt, 1985, text-fig. 5.18K; Below,1987, text-fig. 10).
Jansonia manifesta seems to have preferred brackish-water situations; it is confined to the Duntulm Formation in the Hebrides Basin, with a possible reworked record at the base of the Upper Ostrea Member (W.W., personal observation). The species was recovered in extremely high abundance at Lon Ostatoin immediately above a freshwater intercalation, suggesting that it was an opportunistic taxon thriving when brackish-water paleoenvironments were reestablished. The closely related J. jurassica was also found in a Middle Jurassic, fresh to brackish water depositional regime in western Canada by Pocock (1972). Small numbers of J. manifesta have also been recovered from the Bathonian of southwest England (J.B .R., personal observation) and a possible specimen from the Callovian of northeast France, figured as J. jurassica, was noted by Rauscher and Schmitt (1990, pl. 11, fig. 17).
Dimensions:MinimumMeanMaximum
Overall length:23.5 Ám29.2 Ám38.4 Ám
Overall breadth:18.9 Ám23.8 Ám34.1 Ám
Height of paracingulum:4.4 Ám5.3 Ám7.8 Ám
(60 specimens measured)